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August 23

the shape of heart

What is the geometric shape of the heart? Can I say that the heart is a truncated cone? (I do not think it is a truncated cone that is not really like that) — Preceding unsigned comment added by 46.210.149.99 (talk) 00:18, 23 August 2013 (UTC)[reply]

Asked and Answered on the Maths Desk

The following discussion has been closed. Please do not modify it.

For what purpose? In what context? μηδείς (talk) 00:28, 23 August 2013 (UTC)[reply]

Let's presume the context is the human heart. Why does the purpose matter? Perhaps it's so that the IP can make a Heart-Shaped Box. --Demiurge1000 (talk) 02:06, 23 August 2013 (UTC)[reply]

Kind of like a softball that's been hit a few too many times. ←Baseball Bugs ^{What's up, Doc?} carrots→ 03:12, 23 August 2013 (UTC)[reply]

Is softball played much in Israel? CambridgeBayWeather (talk) 04:55, 23 August 2013 (UTC)[reply]

Nah, they play strictly hardball. ←Baseball Bugs ^{What's up, Doc?} carrots→ 06:01, 23 August 2013 (UTC)[reply]

Breathing

If I inhale a mixture of gases called air which is 78.09% nitrogen, 20.95% oxygen, 1 % water vapour, 0.93% argon, 0.039% carbon dioxide, what mixture of gases do I exhale? Th4n3r (talk) 10:48, 23 August 2013 (UTC)[reply]

Amongst other things, it'll depend on your species, how long you held the breath, and the various physical conditions of your body. 86.141.186.4 (talk) 12:49, 23 August 2013 (UTC)[reply]

I think we can expect that the vast majority of those posting questions here are homo sapiens, and I might even go out on a limb and suggest that they are homo sapiens sapiens. StuRat (talk) 01:54, 25 August 2013 (UTC) [reply]

It depends on a great number of factors. Googling "rest oxygen consumption" returns a number of sites that give the oxygen consumption of adults as around 5% of what's available. It's converted to an equal volume of carbon dioxide. On this basis, the exhaled air will be 78.1 % nitrogen, 19.9% oxygen, 1.09% carbon dioxide, 1% water vapour, and 0.93% argon. Note that what you breaqth out is little changed from what you breath in, which is why mouth to mouth rescusitation works. Anything that increases metabolic rate will increase the conversion of oxygen to carbon dioxide - digesting a meal, physical exercise, thinking hard. Reacting to enviromental conditions will also increase it - for example shivering when cold. If you increase your body rate by either eating to much and getting fat, or building up muscle mass by exercise, your metabolic load increases but your lung volume capacity does not. (If you are quite obese, you lung capacity may be reduced, making you breath harder even at rest) Hence an increase in body weight will increase the percentage conversion of oxygen to carbon dioxide. Note that 1% water vapour in air represents 100% relative humidity at temperatures lower than 16 C, but only 20% rel humidity at 40 C. At the higher temperatures within the human comfort range, the body looses small amounts of water vapour to expired air. 1.122.214.154 (talk) 13:07, 23 August 2013 (UTC)[reply]

Don't forget that the water content will also increase; there's a lot of wet surface area inside the lungs. (The number above corresponds to only about 45% humidity at 20°C.) TenOfAllTrades(talk) 13:35, 23 August 2013 (UTC)[reply]

As is evident, especially in the winter, or when exhaling onto a glass surface. ←Baseball Bugs ^{What's up, Doc?} carrots→ 14:29, 23 August 2013 (UTC)[reply]

Alternative classification of Carnivora

Since not all Carnivora are in fact obligate carnivores and some are omnivores, is there some better and common (possibly unique) feature among them? Perhaps some taxonomists have proposed something on that? Brandmeister^talk 10:58, 23 August 2013 (UTC)[reply]

I would venture to suggest: their descent and relatedness. Linnean taxonomy used to be based on an ad-hoc mixture of visually obvious physical similarities, which taken in aggregate gave clues to organisms' actual relatedness (and were sometimes ambiguous or misleading), but nowadays it uses the much more rigorous methods of Cladistics, and can be corroborated by direct genetic comparisons. Characteristics that specific groups might have largely or uniquely in common are handy aide-memoires, but not ultimately definitive, because Evolution (if you'll excuse the trope). {The poster formerly known as 87.81.230.195} 212.95.237.92 (talk) 12:53, 23 August 2013 (UTC)[reply]

Yes, ultimately ancestry is the real basis of classification. Often there are physical features that are highly characteristic, but they may not be obvious -- for example the layout of the teeth is often one of the most informative things. So is the bone structure of the foot. I don't know enough about this group to say anything specific, though. Looie496 (talk) 14:59, 23 August 2013 (UTC)[reply]

Yes, not all Carnivorans are obligate carnivores -- so what? As Carnivora indicates, the extant Carnivorans form a monophyletic group, which is about as good as it gets for modern cladistics/systematics. I'm not sure what feature they would all share, because the pinnipeds are a rather different from the rest. Perhaps there is some shared skeletal feature, but it would be pretty opaque to a non-specialist. As .195 says above, what they share is common descent. Conceivably, we could change the name to "Foofles", while keeping all members the same, but that would cause its own headaches. The way the International_Code_of_Zoological_Nomenclature works, names should follow the Principle of priority, unless there is some other good reason to change them (e.g. splitting an old paraphyletic group into two or more new monophyletic ones). Finally, beware the etymological fallacy! Hope that helps, SemanticMantis (talk) 15:03, 23 August 2013 (UTC)[reply]

That is, in my opinion, naming that order because of dietary habits is problematic (and, possibly, in other cases too): several non-carnivorans also prey and eat meat as their staple food, thus also being eligible for the name Carnivora. Just found a source, which says in particular: "carnivorans never develop shearing dentition beyond the original P4/mi carnassial pair, and this combination of shearing and grinding dentition has served Carnivora well". Perhaps the International Commission on Zoological Nomenclature will someday reconsider :) Brandmeister^talk 17:51, 23 August 2013 (UTC)[reply]

I understand your objection to Carnivora, but it is actually very common for things to work out this way. It's just what happens when we prefer original names, but also want to use modern tools to make clades as large and inclusive as they can correctly be made. Does it bother you that not all hemiptera have "half-wings"? Or that not all Orthoptera have "straight wings"? It's a terrible system, but it's the best we've got :) SemanticMantis (talk) 18:15, 23 August 2013 (UTC)[reply]

See etymological fallacy. Come back if you have some questions about the relevance of that article to this problem. --Jayron32 14:56, 24 August 2013 (UTC)[reply]

Enemy locked onto heat signature; release flares

In the game Battlefield 3, one mission requires the player to occupy the gunner position in some sort of fighter aircraft. When the enemy locks onto the heat signature, a system of the aircraft occupied by the player warns the player and warns further when a missile is fired. The player is expected to release flares to thwart the enemy missile. How does the system recognise that the enemy aircraft has locked onto the heat signature? --89.241.229.123 (talk) 14:14, 23 August 2013 (UTC)[reply]

By what method does the attacker "lock on"? If it's via radio waves, those waves could be detected by the target vehicle's systems. ←Baseball Bugs ^{What's up, Doc?} carrots→ 14:28, 23 August 2013 (UTC)[reply]

Is the question about how the video game software operates this feature, or how its real-world counterpart works? Video games tend to use "omniscient software" that is aware of enemy moves, which simplifies the design and allows the creators to provide the desired user-experience.

In real life, a heat-seeking missile is not easy to detect. Because infrared missiles uses passive homing, they don't broadcast any type of strong signal. So the defending aircraft must use its own active RADAR to detect an unidentified object in the area; its onboard computer must calculate a trajectory and identify it as a potential missile; and then by deduction, it can be assumed to be a heat-seeker if it satisfies certain characteristics (size, velocity, RADAR signature - or lack thereof). This is an estimation problem that is prone to error and misidentification. Electronic warfare aircraft may escort fighter-aircraft in combat; these special airplanes carry more and better RADARs and computers (and other instruments) to help reduce error. Modern air doctrine emphasizes "Coordinated Command and Control" - enabling multiple airplanes, as well as ground and satellite facilities, to share combat information in real time, to help reduce errors in identification of unknown targets. Nimur (talk) 14:43, 23 August 2013 (UTC)[reply]

Here's a link to a Missile Launch Detector that Lockheed Martin makes [1]. It's a bit hard to tell, but it seems to work by detecting the IR flash of missile launch and then tracking the hot exhaust during flight. This discusses, briefly, the large dust and smoke plume from a Surface-to-air missile. Neither of these detection methods will work with a passive IR guided missile until the missile launches.Tobyc75 (talk) 18:18, 23 August 2013 (UTC)[reply]

Is there any condition in which two vectors can be divided?

Is there any condition in which two vectors can be divided? Concepts of Physics (talk) 14:52, 23 August 2013 (UTC)[reply]

This is not a common terminology for any commonplace operation in engineering or physics. You can define many mathematical operators that have qualitative similarities to division and accept vectors as inputs. But most people who use those operations prefer to use a different, more precise term, like computing an inverse or left-multiplication by the adjoint matrix. Nimur (talk) 14:57, 23 August 2013 (UTC)[reply]

This was discussed at length a month ago. Red Act (talk) 15:04, 23 August 2013 (UTC)[reply]

This page shows division of two vectors. Is this method correct? Concepts of Physics (talk) 15:28, 23 August 2013 (UTC)[reply]

That is a correct example for R (programming language) where the term "vector" is understood to be a one-dimensional array and where division is done element by element. This is a different use of "vector" than Vector (mathematics and physics). -- 200.7.90.57 (talk) 15:52, 23 August 2013 (UTC)[reply]

Division is the inverse of multiplication, so before answering that question you must chose some kind of multiplication to invert. The most common multiplication methods for vector dot product and cross product cannot be inverted. Dauto (talk) 18:20, 23 August 2013 (UTC)[reply]

If you want a fancy name for the operation described by 200.7.90.57, try Hadamard product. --Wrongfilter (talk) 21:05, 23 August 2013 (UTC)[reply]

Gap spanners on the International Space Station

What are the "gap spanners" that I read about astronauts installing on the International Space Station? The best I can find is that they are "used by spacewalkers to get from one module to another", but I can't find any details. Are they cables? Are they rods? What are they made out of? How are they attached? Are there any good photos of them online? -- 200.7.90.57 (talk) 15:30, 23 August 2013 (UTC)[reply]

Astronaut Jerry L. Ross, who flew on STS-88, explains gap spanners in this crew interview, available from spaceflight.nasa.gov:

“

Because the Unity node is so large in diameter we couldn't mount all the hardware on the outside of its structure that we wanted to, while it was still in the payload bay of the orbiter, and the launch environment that exists there. So, Jim and I will be installing a series of additional handrails, or handholds, on the outside of the station which we'll need for tasks, either on our flight or future ones. We'll also be putting some additional foot restraint sockets on the outside of the structure that we can plug our foot restraints into to do tasks on this EVA as well as others. We have a thing called a gap spanner, which is really nothing more than a long piece of strap that we're going to attach between a couple of handholds and cinch tight. That is a way of bridging a gap that was too long between handholds to provide a continuous translation path for crewmembers to operate on the station.

”

So, it seems like a gap spanner is a pretty mundane piece of cloth... until you realize that it's a festering breeding ground for orbital space-fungus! Here's a very high resolution photo from the same page of a ground technician swabbing the fungus off a gap spanner. Nothing is mundane or routine in manned space flight! Nimur (talk) 14:24, 24 August 2013 (UTC)[reply]

Thank you, Nimur. You're my hero! -- 200.7.90.57 (talk) 15:39, 24 August 2013 (UTC)[reply]

the haert rate in different bodies sizes

What is the reason that the big body is with low heart rate and a small body is with high heart rate? In example: addult from 10 years and above has heart rate of 60-100 per minute while an infant (until 90 days) has heart rate of 80-205 per minute! The same things are in animals. In example: the elphent has 25 HR per minut while the mouse has about 500 heart rate per minuts! So, in short: Why does as much as the body smaller the heart has high heart rate? 95.35.239.134 (talk) 16:36, 23 August 2013 (UTC)[reply]

It's partially related to one's activities. Many small animals are much more active than big ones, as such they require higher physical performance (particularly, better oxygen supply) which in turn requires high heart rate. Mice in particular are constantly vigilant to avoid predators, requiring higher physical performance and as such high heart rate. Brandmeister^talk 18:08, 23 August 2013 (UTC)[reply]

All right on mice you can say that they are active, but what do you say about babies compared to children? Children more active and yet they have only 60-100 beats per minute compared to babies who sleep most of the day and have between 85 -205 beats per minute.95.35.239.134 (talk) 19:56, 23 August 2013 (UTC)[reply]

My guess is that the size of the heart is also important, as a bigger heart can of course pump more blood than a smaller one if both were pumping at a similar speed. Since blood flow is limited by the elasticity of the blood vessels, I'm guessing a big heart pumping very quickly could create too much pressure and rupture the veins, especially if the diameter of the various types of veins does not increase proportionally to the size of the heart. I'm unable to confirm my guesses with reliable sources at the moment, however. Effovex (talk) 20:11, 23 August 2013 (UTC)[reply]

The typical operating speed (and presumably the most efficient speed) of mechanical devices, such as pumps, motors, and engines, trends to be inversely proportional to their size. Perhaps someone here with a mechanical engineering background can speak to this point. -- 200.7.90.57 (talk) 20:37, 23 August 2013 (UTC)[reply]

Indeed, though the reasons are somewhat different. The heart pumps by successive contractions. That means that at every beat it accelerates a small quantity of blood, which then gets decelerated in the blood vessels. The kinetic energy imparted to the blood depends on the ex-chamber spurt time, and the stroke/spurt distance. In a small animal the stroke distance (which is proportional to the length and to the width of the heart) is less, so to impart the same kinetic energy per unit volume of blood, and thus get the same blood velocity, the time per beat must be shortened. In animals of all sizes, the required blood velocity is much the same, as it is set by capilary wall thickness, gas and nutrient diffusion range, and tissue requirements. 1.122.171.86 (talk) 00:36, 24 August 2013 (UTC)[reply]

survive in a space station without air renewal

Suppose you are in the International Space Station, with destroyed air renewal systems and no other sources of oxygen.

You have plenty of food and water (but you cannot use it to produce oxygen). At the beginning the air is fresh.

How long can you survive there? Months? --Blacknight87 (talk) 17:27, 23 August 2013 (UTC)[reply]

Several days, certainly not reaching one month, I think (around one week at best) until the buildup of carbon dioxide reaches critical point. To extend the survival period it's possible to isolate every room by sealing hatch or whatever and once the CO2 level in one room reaches uncomfortable point, move to another. But still there should be some emergency backup system out there. Brandmeister^talk

There are Oxygen candles in the ISS. CO2 is usually deadly at about 10%. A human produces about 1 kg of CO2 a day. The molar mass of CO2 is 44 g, so 1 kg is about 23 moles or about 500 l of CO2 under normal condition. The ISS has a pressurized volume of 837 m³. So 10% concentration should be reached in 160 (person-)days. That is a lot longer than I would have expected. --Stephan Schulz (talk) 18:33, 23 August 2013 (UTC)[reply]

But is it enough oxygen for 160 days? --Zhitelew (talk) 18:57, 23 August 2013 (UTC)[reply]

Since one molecule of O2 makes one molecule of CO2, and both gasses have more-or-less (for small values of less!) the same volume under normal conditions, yes. Unless I made a stupid mistake ;-). --Stephan Schulz (talk) 19:40, 23 August 2013 (UTC)[reply]

I suspect that that 10% figure is rather too high. NIOSH (CDC) puts the IDLH level at 4% [2], noting that it produces symptoms of intoxication within 30 minutes at 5%, and unconsciousness after a few minutes at 7%. A study on submariners reported that extended exposure to 3% CO2 produced only mild symptoms, provided sufficient oxygen was provided. Using a more realistic 4% threshold, the lone astronaut on the ISS is good for perhaps 60 days—assuming that there are no other chemical or biochemical processes on board that produce additional carbon dioxide..... TenOfAllTrades(talk) 23:22, 23 August 2013 (UTC)[reply]

To look at it from another direction, the Lunar Excursion Module (LEM) on the ill-fated Apollo 13 mission had an internal volume of 6.7 cubic meters...less than 1% of the internal volume of the ISS. The oxygen and CO2 removal provided in the LEM was intended to be enough for 72 man/hours (2 men, one and a half days) - and had to suffice for 288 man/hours (three men, four days) to get the crew safely home. Notably, they didn't run even close to being short of oxygen - but when the CO2 removal system was rapidly overwhelmed, they ran to the brink of CO2 poisoning until the famous "square peg into a round hole" fix was developed. But you can see that even with only 1% of the volume, it took a considerable amount of time for the extra astronaut to overwhelm the LEM's filters to the point of danger. With over 100 times as much volume, that would take 100 times a long. This suggests that (a) Oxygen deprivation would not be an issue - CO2 would be the major problem and (b) that User:Stephan Schulz's calculations are probably about right. SteveBaker (talk) 21:16, 23 August 2013 (UTC)[reply]

Part of the reason why the CO2 level rose so 'slowly' in the LM was that the scrubber unit wasn't fully exhausted. As the scrubber gets to the end of its lifetime, the particles of lithium hydroxide in the canister acquire a coating of lithium carbonate, making them much less efficient absorbers of CO2 (as the gas has to diffuse through the inert carbonate to get to the active hydroxide). While the scrubber canister would normally be replaced at that point, if one is willing to tolerate an elevated partial pressure of CO2 in the cabin then one can run the canister much closer to exhaustion—let those little particles marinate in more concentrated CO2, consuming the lithium hydroxide more completely and absorbing more CO2 before their retirement.

In the case of the LM, its original canisters had a design capacity for efficient scrubbing of 60 man-hours, but could remain in service for twice that time at a cost of higher ambient CO2 levels if absolutely necessary. (On Apollo 13, they ran for 107 man-hours before the CM units could be connected.) In addition, there was a spare unit good for another 40 man-hours of normal use (80 man-hours in an emergency) if the square-peg-round-hole fix took longer than anticipated. [3] If the scrubbers were completely gone, using the 1 kg per day per person round figure Stephan employed above, the lunar module would have been out of luck in less than half a day. TenOfAllTrades(talk) 23:11, 23 August 2013 (UTC)[reply]

Thanks! That's great information. But I think the conclusion is the same - it's not running out of oxygen that's the problem. It's build-up of CO2. If CO2 could be perfectly removed, how long would it be before the astronauts have too little oxygen? My gut feel is that it's MUCH longer. SteveBaker (talk) 15:55, 24 August 2013 (UTC)[reply]

Do they have suicide pills on board so they don't have to suffocate to death? How long /would/ they last if the zombie apocalypse suddenly happened or something? Is it CO2 poisoning or is there something else needed from the ground that must happen before the air runs out? 108.27.81.195 (talk) 00:56, 24 August 2013 (UTC)[reply]

Under normal conditions, they keep a Soyuz capsule docked on the station that can be used as an emergency return vehicle if necessary. Assuming conditions on the Earth's surface are survivable there is a good chance that the crew would attempt to return to Earth before they ran out of air. However, I'm not entirely sure how easy or difficult it would be to manage a reentry if ground control had been destroyed. Dragons flight (talk) 04:23, 24 August 2013 (UTC)[reply]

They don't really need suicide pills - CO2 is commonly used for animal euthanasia, and of course they have a perfect "high altitude chamber" close at hand. Wnt (talk) 07:03, 25 August 2013 (UTC)[reply]

Also, until 2007, cosmonauts were issued with the TP-82 pistol - according to our article, they're still issued with "a regular semi-automatic pistol" - so they probably have at least one gun up there as well. Tevildo (talk) 17:35, 25 August 2013 (UTC)[reply]

Femininity facial analysis?

The Bradley/Chelsea Manning story has me wondering: with all the fancy facial recognition software, and research into reactions people have toward averaged faces, is there a program available that can analyze "femininity" in a face, either from a 2D picture or in person/3D? To me Manning's face always seemed a little bit female, and I was wondering if that is quantifiable. If so, does it actually predict some increase in odds of being transgendered or gay? Wnt (talk) 18:59, 23 August 2013 (UTC)[reply]

Here's a good list of refs, 101 of them about determining/sex gender of human faces via computer vision algorithms [4] (search is articles related to a seemingly influential paper on the topic [5]). There seems to have been a boom in the 1990-2006 range, but it's still being researched at a steady pace. It looks like even 10 year old routines can discriminate about as well as humans (~90% accuracy, hair removed or hidden), using things like support vector machines. Two articles jumped out as highly relevant to your question: "What's the difference between men and women? Evidence from facial measurement" [6], and "Boosting Sex Identification Performance" [7]. Some of these papers might have code online as supplementary material, or allow a (sufficiently savvy) reader to implement the algorithm. I didn't look for "off the shelf" software. SemanticMantis (talk) 21:01, 23 August 2013 (UTC)[reply]

Well, how does one DEFINE femininity? Seems to me like this is a highly subjective thing to evaluate. 24.23.196.85 (talk) 01:07, 24 August 2013 (UTC)[reply]

How about with science, like SemanticMantis demonstrated with references and logic, and a surprisng lack of caps. Shadowjams (talk) 06:05, 24 August 2013 (UTC)[reply]

Yes, defining femininity is problematic. Note also that the papers are mostly about determining sex, not gender. In this case "female-ness" might be a better word than femininity, when discussion how computer algorithms score faces. SemanticMantis (talk) 14:24, 24 August 2013 (UTC)[reply]

How about those physical traits that estrogen gives. For testosterone that would be square face, baldness, beard etc. Electron9 (talk) 17:29, 26 August 2013 (UTC)[reply]

And note that feminine men are only a subset of gay men ("twinks"). There are others who are ultra-butch ("bears"), and everything in-between. StuRat (talk) 01:46, 25 August 2013 (UTC)[reply]

I should thank SemanticMantis for some good sources ... though I haven't managed to access several of them in full as of yet. Do you know whether these 90% or so accurate algorithms misidentify the same people as being of the wrong sex, or is it different for each one? It didn't look like anyone was actually trying to identify "quantitative" variations in sex though. Wnt (talk) 07:00, 25 August 2013 (UTC)[reply]

Wnt, I think that quantitative "scoring" does happen inside the algorithms, but no idea on if one face fools all the algorithms, or each is tricked by different faces. I suppose there should be some "Standard face set", but not sure how many of those papers use it.

For the support vector machine approach, some finite list of quantified attributes is used to represent each face as a vector. The goal of the machine is to make the best hyperplane to separate the data so that females are on the "right", and males on the "left". In this case, the hyperplane would describe the the most androgynous faces. So you could at least come up with an "androgyny score", based on distance to the separating hyperplane, and then large distances would be either "very male" or "very female", depending on which side of the hyperplane the face is on. Anyway, if you want a specific paper, drop me a line on my talk page or email, I can probably send you pdf. SemanticMantis (talk) 18:13, 26 August 2013 (UTC)[reply]

Geology/Pleistocene/CLIMAP

During the 1970's, CLIMAP was created to draw a map of the world at the time of the glacial maximum, 13Kya, (or is it earlier?). Where can I find that map and where can I find maps of North America between then and now. I have a particular interest in Lake Bonneville and Lake Agassiz.Bobgustafson1 (talk) 19:19, 23 August 2013 (UTC)[reply]

You might start with out article on CLIMAP: Climate: Long range Investigation, Mapping, and Prediction. It contains a reference to "CLIMAP (1981). Seasonal reconstructions of the Earth’s surface at the last glacial maximum in Map Series, Technical Report MC-36. Boulder, Colorado: Geological Society of America.", which probably provides many more details, though you'd probably need a good research library to find that book. However, it would be a mistake to think of CLIMAP as aiming to "draw a map". Rather it was a collection of related research projects to document conditions as they existed on Earth about ~18 kya. There would be many maps, some regional, some global, each reflecting or summarizing different parts of the investigation. In addition, other results of the related studies are probably scattered through various academic journals from that time period. Since you asked, reading about Lake Agassiz and Lake Bonneville might also be useful. My impression though is that our understanding of Lake Agassiz and its changes through time have improved greatly since the 70s, so you might want to look at more recent reviews. Dragons flight (talk) 20:14, 23 August 2013 (UTC)[reply]

Adults

Do mle adults have XY chromosomes and female adult humans have XX chromosomes? If so, is it possible to change theses chromosomes? Pass a Method talk 22:07, 23 August 2013 (UTC)[reply]

See biological sex, gender, sex chromosomes, XY sex-determination system, aneuploidy, sex differences in humans. The short answer to the first question is "mostly". The short answer to the second is "no". 86.163.2.116 (talk) 22:17, 23 August 2013 (UTC)[reply]

Yes, mostly; no. See also, incomplete penetration. μηδείς (talk) 04:54, 24 August 2013 (UTC)[reply]

See also, hmmm, androgen insensitivity, testicular feminization, freemartin, intersex, Klinefelter syndrome Turner syndrome, XYY syndrome. Note that there is research underway to extend X inactivation to chromosome 21 so as to offer treatment to people with Down syndrome, for example, but this is still far off (no obvious research path to a successful, safe whole-body gene therapy of every cell to do it). But it wouldn't be much to modify that to a goal of changing the sex chromosomes. That said, wrong-sex gonads and other tissues in someone of the other sex hormonally can be cancer risks, so even if you make the change without incident the goal itself might have additional dangers. Wnt (talk) 19:42, 24 August 2013 (UTC)[reply]

August 24

World's biggest skyscraper

What's the world's biggest skyscraper by basically, uh, how big it is, that is the volume enclosed by the outside walls? I wonder how Shanghai Tower stacks up. 108.27.81.195 (talk) 00:45, 24 August 2013 (UTC)[reply]

See List of tallest buildings. Dismas|^(talk) 00:54, 24 August 2013 (UTC)[reply]

I think 108 IP is asking about the biggest building by VOLUME, not height. But I have no idea where to get this info -- can someone help? 24.23.196.85 (talk) 01:06, 24 August 2013 (UTC)[reply]

See List of largest buildings in the world, which answers that very question. WHAAOE. --Jayron32 01:07, 24 August 2013 (UTC)[reply]

Bah! I was close. Was getting called away to something else while replying, so I didn't get a chance to double check. Dismas|^(talk) 01:11, 24 August 2013 (UTC)[reply]

The Boeing building is by far the largest building by volume, but it's not much of a skyscraper. The two lists linked here would need to intersect. ←Baseball Bugs ^{What's up, Doc?} carrots→ 02:52, 24 August 2013 (UTC)[reply]

I believe there's a Wikipedia tool for that (in fact, I used it not too long ago to look for movies set in London during certain time periods -- you just enter "Films set in London" and, say, "Films set during the 1940s", or in your case, "Largest buildings" and "Tallest buildings") -- does anyone remember what's this tool called? 24.23.196.85 (talk) 04:08, 24 August 2013 (UTC)[reply]

Are you perhaps thinking of WP:CATSCAN? Dismas|^(talk) 04:10, 24 August 2013 (UTC)[reply]

Yep, that's the one! Thanks! 24.23.196.85 (talk) 04:14, 24 August 2013 (UTC)[reply]

Strongest known tornado outside of the United States; A question about the Fujita scale

I searched for this both online and in Wikipedia articles, but I could not find the answer. Basically, what is the strongest known or recorded tornado (in terms of intensity) outside of the United States? As a side question (although related), are either the Fujita scale or the Enhanced Fujita scale outside of the United States and Canada? Narutolovehinata5 ^tccsdnew 03:54, 24 August 2013 (UTC)[reply]

The article List of F5 and EF5 tornadoes has seven entries from Europe - see also List of European tornadoes and tornado outbreaks, List of Asian tornadoes and tornado outbreaks and List of Southern Hemisphere tornadoes and tornado outbreaks, although those articles generally don't have the intensities listed. The Enhanced Fujita scale is based on damage to typical American structures ("Manufactured Home – Double Wide", "Large, Isolated Retail Building [K-Mart, Wal-Mart]"), so I don't believe it's used outside the USA and Canada - our article doesn't say otherwise. Tevildo (talk) 11:53, 24 August 2013 (UTC)[reply]

According to this website, there were two strongest tornadoes in Europe, both T10-T11 on TORRO scale (one struck France on 19 August 1845 and the other one Italy on 24 July 1930). In terms of deaths the strongest one seems to be Daulatpur–Saturia tornado. Brandmeister^talk 13:57, 24 August 2013 (UTC)[reply]

Astigmatisms and autostereograms

I'm looking for solid references (preferably ones that pass WP:MEDRS) discussing astigmatisms and autostereograms, but so far all I've found are forums and similarly unreliable sites at which people chat about the subject. Basically looking for something explaining the effects (or lack thereof) of astigmatisms on astigmatics' ability to "do" autostereograms, but something talking about other aspects of the process of "doing" autostereograms for astigmatics would be helpful. A little context, in case that help — I've been trying fruitlessly to teach an astigmatic acquaintance how to "do" autostereograms, and I'd like to know if the literature suggests that it's impossible/harder/no different/whatever. Nyttend (talk) 12:38, 24 August 2013 (UTC)[reply]

Firstly - there are two very different uses of the term "astigmatism" - one form (which I have in one eye) is when the lens of one or both eyes is not spherical so the focal distance is different vertically and horizontally and things are perpetually blurry in one direction or the other. This is easily corrected with glasses or contact lenses - and even uncorrected, it does not prevent one from seeing autostereograms (I see them just fine).

The other use of the term is for people who's eyes don't move correctly to line up on the object being concentrated on. They may be cross-eyed or have a lazy eye. Those people cannot see autostereograms because they rely on correct positioning of the two eyes to produce a 3D image.

So we're only talking about the SECOND kind of astigmatism. But it's even more confusing than that.

The problem is that MANY people are born with slightly crossed eyes or a "lazy eye" (Astigmatism-of-the-second-kind). In almost every case, this fixes itself within a month or so of birth - and all is well. But for about 5% of people, it takes three or more months to "fix itself" and once the obvious problem goes away, it seems like the person can see just fine

However, what happens (without medical intervention) is that the circuitry in the brain that learns how to fuse two separate images into a single three-dimensional representation doesn't develop properly in those people because this part of the brain forms in the first few months after birth. This problem has only been properly recognized in the last 20 years or so - and it's only properly diagnosed and treated (eg with eye patches) in a few places in the world. This afflicts about 5% of the population (although that number is dropping as the problem is recognized).

If you're one of the 5% then both of your eyes work fine, you can have 20/20 vision and your eyes can track targets perfectly - but your depth perception is not as good as it should be - and things like 3D movies and virtual reality headsets don't seem any better than 2D movies and 2D computer graphics. These people are easily recognized because they say things like "I don't see what the big deal is with 3D movies - they don't look any different!" But the problem with autostereograms is worse because the "image" is only present as a 3D displacement - so for people in the 5%, they stand NO CHANCE of understanding them.

There have been some successes in treating this condition - it seems that brain plasticity is sufficient that the problem can be treated with special exercises (at least in some cases). People who have had the treatment are AMAZED at how much different the world looks afterwards.

SteveBaker (talk) 15:48, 24 August 2013 (UTC)[reply]

The "second kind" you describe is strabismus; I haven't heard it called astigmatism. While it is clear that strabismus would interfere with stereograms, I think it is conceivable that astigmatism would also prove something of a hindrance, because any distortions imposed by the lens would appear to go in different directions for the two images to be fused, making them more difficult to align. Wnt (talk) 16:37, 24 August 2013 (UTC)[reply]

Not trying to sound unappreciative, Steve, but I'm specifically looking for solid sources, e.g. something in an ophthalmology or optometry journal discussing the subject. Nyttend (talk) 17:03, 24 August 2013 (UTC)[reply]

Yeah - I understood that (I don't have such things) - but I did want to clarify that a mindless search for "astigmatism" would produce confusing results. Also, even people who's eyes do all the right things may fail at understanding autostereogram because of the brain development thing. It would be necessary to be sure that an astigmatic (type I) was failing to understand an autostereogram because of the astigmatism rather than because of happening to be one of the 5%. SteveBaker (talk) 22:58, 24 August 2013 (UTC)[reply]

What scientific problems have been proven impossible to determine?

--121.7.36.13 (talk) 13:12, 24 August 2013 (UTC)[reply]

None (by definition). Dauto (talk) 13:52, 24 August 2013 (UTC)[reply]

Science does not deal in absolutes like proofs. We can prove scientific facts only ever with respect to a given theory, and support for a scientific theory is always tentative in principle (although what a scientist considers "tentative" is often more certain than what we consider "facts" in everyday life). There are a number of problems in math (including theoretical computer science, mathematical logic, and similar disciplines) that are proven to be impossible to answer in general. Examples include the Halting problem, the Post correspondence problem, and the Entscheidungsproblem. --Stephan Schulz (talk) 13:56, 24 August 2013 (UTC)[reply]

Even proving you can't for instance solve the halting problem doesn't mean there can't be an oracle machine that always gives the correct answer. It is very highly improbable though! Dmcq (talk) 14:33, 24 August 2013 (UTC)[reply]

There are many concepts which seem, at first glance, to be simple to the uninitiated which are, not merely difficult to prove, but actually proven to be impossible to determine. Many of the concepts surrounding "quantum" fields are governed by the Uncertainty principle, for example, which doesn't merely state that somethings are outside our current technological grasp of determining, but that they are physically impossible to determine, for example certain pairs of properties, such as momentum and position, are impossible to know simultaneously for any sufficiently small particle. --Jayron32 14:54, 24 August 2013 (UTC)[reply]

No - the proof relating to the halting problem doesn't say that you can't solve it - only that a Turing Engine can't solve it. I could disagree though that the halting problem "has proven impossible to determine" though - we've looked at it - and we are 100% sure that you cannot write a computer program that looks at another computer program and tells you whether that program will halt or not. This isn't some kind of mysterious unsolved problem - it's that we have total and complete certainty that some particular determination cannot be made. It's like the laws of thermodynamics say that you can't make a perpetual motion machine - or that the uncertainty principle says that you can't simultaneously know the position and momentum of an electron - or that godels' theorem shows that there exist some class of mathematical theorems that can never be proved or disproved.

I think we should help the OP to refine the question a bit. There are many areas of doubt to consider:

Some things we know for 100% certainty cannot ever exist: A Perpetual motion machine or a computer program that can tell whether other computer programs will eventually halt or not, an engine that'll propel a spacecraft at the speed of light...that kind of thing. It's not just that we haven't discovered a way to make these things - it's that we know for 100% sure that they are impossible.
Some things we know for 100% certainty we cannot ever measure: The nature of light from a star that lies outside of the observable universe, the exact position and momentum of an electron. These are fundamentally unknowable things.
Some things we definitely don't know - but might one day know: What is dark matter? What caused the first living thing to appear on earth? More research is needed - but we can probably work out the answer somehow.
Some things are reasonably well known but not with sufficient accuracy: Are there planets orbiting other stars that are habitable by humans? (Almost certainly - but maybe not.) How much temperature rise will we experience if we continue to dump CO2 into the atmosphere at present rates? (Definitely too much...but we don't know exactly how much too much.)
Some things that we expect to be true but cannot yet prove: Can every even integer greater than 2 be expressed as the sum of two primes? (We're pretty sure that the answer is "Yes" - but we can't yet prove it.)
Some things that we thought were probably true for the longest time - but were finally disproved: Euler's sum of powers conjecture, for example.
Some things that nearly everyone thought were false for the longest time - but were finally proven true: Continental drift, for example.
Some things that we thought were true - but are only approximately true - or are only true in special cases: Newton's laws of motion, for example.
Some things are "unfalsifiable" - we can never prove them false, but they might one day turn out to be proven true: The existence of God, The "simulation hypothesis", for example.
Some pairs of ideas that both seem to be "proven" true - but are contradictory in nature so either we're missing something, or one or both of them must actually be slightly incorrect: Gravity and Quantum Theory, for example. Gravitational theory works great at large scales and Quantum theory works great at small scales - but when the two intersect (eg at the boundary of a black hole), they predict different outcomes - so we know there is a problem somewhere.
Some things that many non-scientists THINK scientists have trouble with - but which are actually well understood: How come a bee can fly with wings that seem to be too small? (Wingtip vortices are exploited by bees in a way that was not obvious to one writer - and everyone took that as "Scientists say that bees can't fly - look how stupid scientists are!")

I'm sure there are other categories of doubt and error that I haven't thought of...but to find concrete examples, our OP needs to be much more specific. SteveBaker (talk) 15:28, 24 August 2013 (UTC)[reply]

Good list, but you are confusing the two most distinct classes. The Halting problem is proven to be impossible to solve for a Turing machine (or, indeed, any kind of algorithmic machine - the diagonal proof does not really depend much on the specifics of the machine). A perpetuum mobile, on the other hand, is impossible under the laws of thermodynamics. But the laws of thermodynamic are just very well-supported assumptions. There is nothing logically impossible about them changing tomorrow (just assume that reality is a giant computer simulation, and tomorrow they put in a new patch). But there is no patch to reality that makes a the Halting problem for general Turing machines solvable. Of course, the Halting problem for real computes is trivially solvable (in theory), since they only have a finite number of states. --Stephan Schulz (talk) 16:26, 24 August 2013 (UTC)[reply]

Thermodynamics is a bit of a tricky case - see Constantin Carathéodory, who formulated a derivation of the second law based entirely on mathematics (and the concept of "heat", which may reconnect it to the physical world - I'm not a good enough mathematician to comment). It's on a lot firmer ground than special relativity, at least. Tevildo (talk) 16:39, 24 August 2013 (UTC)[reply]

(ec) A good answer, as always, but I'm not sure about all of the "100%" examples. I would say that we should distinguish between mathematical propositions (such as the halting problem), which can be conclusively proved, and scientific propositions (such as the inability for a spacecraft to reach c). The contradiction of such a statement will be inconsistent with our current understanding of the universe, but it's not _impossible_, in the way that a solution to the halting problem or a proof that 2 + 2 = 5 would be. (Incidentally, the non-observability of light from a star outside the observable universe is an analytic truth that doesn't even require mathematics to demonstrate - the fact that the observable universe is finite is a tentative scientific proposition that could (theoretically) be false). Tevildo (talk) 16:35, 24 August 2013 (UTC)[reply]

Why do the women have less blood than men?

←I've seen once (I don't remember where) a math formula of calculation for a blood quantity in the person. So, according to the formula, when you ask how much blood this person has, if he is a man you give 0.75 mili liter for every one kg of his body, and if she is a woman you give 0.65 for every kg of his body, and if it's baby you give 0.85 for every kg of his body. So, first of all, I would like to know if is it true. and second, according to this formula we can understand that the woman has less blood than man. In example, when a man is 60 kg, he has 4.5 liter of blood (60 times 075), when a woman is 60 kg she has 3.9 liter of blood, it's happen when both of them is in the same weight! It's about 13.5 percent less compared to a man. So, to sum up I have two questions, but the one depends in the another one: 1. is the formula true? (and if it's true, what is his origin). 2. why the women have less blood thatn men. 95.35.88.167 (talk) 18:18, 24 August 2013 (UTC)[reply]

Your units are wrong - those should be deciliters (dL) per kg, or you need to shift your decimal over a couple of places - but the values are approximately correct. The difference between men and women is primarily down to different body fat percentage. Adipose tissue (fat) contains less blood per kilogram of total weight than muscle or most organ tissues. The average amount of circulating blood per kilogram of body weight is lower in obese individuals (fat makes up more of their total body weight) and higher among young people. Infants can go higher than 100 mL of blood per kilogram: [8]. All these numbers are approximate, and will vary from person to person and study to study. This paper shows the distributions of values for typical children and adults at various stages of development. Note that figure 1B plots blood volume per kg of body weight, whereas figure 1A plots volume per kg of lean body mass—the values become statistically indistinguishable in adult males and females when you eliminate the contribution of body fat. TenOfAllTrades(talk) 20:39, 24 August 2013 (UTC)[reply]

No male is allowed to talk about it, but I have heard from a wise (dirty) ole man that it is something to do with the "monthlies"...Myles325a (talk) 07:02, 29 August 2013 (UTC)[reply]

metal mixture

I want to mixture of 50%copper+ 15%silver+12%cadmium+23%zinc but its breaking when I want make its its 37guage stripe. Why? You have any solution or metal which helpful to make its soft?? — Preceding unsigned comment added by 59.161.69.236 (talk) 19:57, 24 August 2013 (UTC)[reply]

A few points

The properties of a metal alloy may be very sensitive to the particular process used to make them.
even small amounts of impurities may have large effects.
how are you attempting to make the stripe? Hot or cold? Cutting or stretching? details do matter.

Dauto (talk) 21:21, 24 August 2013 (UTC)[reply]

Are you using Sheet metal#gauge measurement here - 37 gauge = 0.17 mm ??

Why waste good silver mixing it with cadmium and zinc? Cadmium in particular has sort of a mixed reputation. I suppose it's better than lead, but... well, anyway, our article on solder does mention "Sn-3.5Ag-0.74Cu-0.21Zn" (reading down I see there are several of these, but all 95% tin) Also "KappRad" Sn40Zn27Cd33. It looks like the solder recipes are meant to be near eutectic points for low melting temperature, which may not matter.

Point is: where did you get this recipe? If you're reinventing metallurgy on your own you'll have many disappointments. (I do wonder if you might find tin, due to its ductility, a component preferable to cadmium? Or maybe bismuth? But I am so not qualified to say any such thing!) Wnt (talk) 23:13, 24 August 2013 (UTC)[reply]

There are some alloys fairly close to the OP's mixture at List of brazing alloys, but everything we list has a larger Ag/Cu ratio. Tevildo (talk) 23:35, 24 August 2013 (UTC)[reply]

Yea, copper is quite ductile, and silver is fairly ductile, too. I'd suspect the problem is the cadmium and/or zinc, making it too brittle. Try lowering the ratio of one or both of those. If you need to keep that ratio, you will have to treat it more gently, like only work it a small amount, then reheat it and let it cool, to relieve the stress, before working it again. StuRat (talk) 01:36, 25 August 2013 (UTC)[reply]

A mineral that heats on contact

Is this really a special mineral with such characteristics, or just a normal hot rock? If it's the former, what's it called? Thanks, 84.109.248.221 (talk) 23:06, 24 August 2013 (UTC)[reply]

Looks like a typical internet fake to me. If the rock had been buried in the ground and emitted heat sufficient to char paper for thousands, perhaps millions of years until it was dug up - what would be the power source for such a thing? To my eyes, it looks like a lump of coke or even a barbeque briquette that someone heated up and then used to take the photos. We only have his word that it was hot when dug out of the ground. Yeah - maybe it could just maybe be radioactivity - but the amount of radiation it would take to make that much heat would probably have killed the guy who took the photos by now. SteveBaker (talk) 03:34, 25 August 2013 (UTC)[reply]

It's definitely not radioactive, it'd be quite lethal if so; most likely, it's a hoax. However, on the chance that it isn't: the outcomes look more like a chemical reaction (the glove doesn't look burned, but stained.) It could be a bunch of Iodine prills, or crystals, stuck together some way. While such a find would be exceedingly unlikely for any legit (or natural) reason- iodine can be used to manufacture meth, someone may have stumbled on a stash of it. At any rate, like I said, that's all quite unlikely, it's probably a scam.Phoenixia1177 (talk) 05:45, 25 August 2013 (UTC)[reply]

I also thought it looked like it could be iodine, both due to the black discoloration of paper and the brown discoloration of the glove. The "metallic" appearance isn't really that far off from iodine either. However, I assume the "rock" results from some industrial source, not meth manufacture. (the second is more newsworthy, but not really that big a chunk of the overall use of the element!) Wnt (talk) 08:05, 26 August 2013 (UTC)[reply]

It is quite clearly iodine crystals. I've seen this several times on the Internet recently, with different stories.

August 25

Fear and adrenaline

I read an article ( http://www.scientificamerican.com/article.cfm?id=scared-to-death-heart-attack) which claimed that fear can cause high levels of adrenaline which can lead to arrhythmia and eventually cardiac arrest, even in healthy people. But surely, this is very rare in people who don't have underlying health conditions. Even intense exercise, a popular activity performed by many, releases adrenaline and raises heart rate, and this is seen as beneficial to overall health. So how can fear be any different? I would argue that fear may also be beneficial, many people enjoy controlled fear such as roller coasters, which have the same effect and claim that this actually reduces stress. Clover345 (talk) 00:36, 25 August 2013 (UTC)[reply]

It normally only becomes a problem with long-term fear. Brief episodes of fear rarely cause heart attacks, despite it being a TV trope where the murderer scares somebody with a heart condition in order to kill them. StuRat (talk) 01:30, 25 August 2013 (UTC)[reply]

What is long term? Even people who have anxiety over an upcoming event whether its an exam or a speech they have to give to a large audience, rarely die from cardiac arrests. People who are uncomfortable with public speaking may for example be anxious and therefore have elevated heart rates for up to weeks before the event. Clover345 (talk) 10:41, 25 August 2013 (UTC)[reply]

Those roller coasters all have rules prohibiting people with heart problems - who also have to get clearance from their doctors to exercise. Martial54 (talk) 02:59, 25 August 2013 (UTC)[reply]

Nobody has to get clearance from doctors to exercise. Doctors can give "orders" to their patients, but there is no law requiring the patients to obey those orders. Looie496 (talk) 05:49, 25 August 2013 (UTC)[reply]

The legal authority of doctors over their patients has nothing to do with the effects of high heart rate on heart attacks which is the question asked. Rmhermen (talk) 13:30, 25 August 2013 (UTC)[reply]

You can be scared to death, but this isn't something I would be seriously concerned about. The purpose of the study was, probably, less "here's a way you can die" and more "let's see if this trope can actually happen". At any rate, if you're interested in the phenomena, some articles (related) you may have interest in are Parasympathetic rebound, Baskerville effect, and Takotsubo cardiomyopathy; and, less directly related, Fear, Panic attack, Acute stress reaction, Fight-or-flight response. The latter links are related only in so far as they involve stress responses due to fear/anxiety.Phoenixia1177 (talk) 03:53, 25 August 2013 (UTC)[reply]

For Op: stress increases your risk for heart disease, you may want to consider [9] for the general idea. The links I've given above discuss more acute cases of stress response. It is unlikely that a single given event will cause heart <Insert Fatal X> to happen, thus while the anxiety over public speaking may not cause you to just keel over on the spot, if you have to do it repeatedly and it is a stressor, it can cause you to be more at risk for heart related issues.Phoenixia1177 (talk) 10:54, 25 August 2013 (UTC)[reply]

But there is also conflicting research which suggests that stress has no direct link to heart disease or high blood pressure and only indirect links. How can this be explained?. Clover345 (talk) 00:03, 27 August 2013 (UTC)[reply]

I'm not sure what exactly you're asking. Whether stress directly physiologically impacts the heart or causes you to smoke/eat/etc. more, it's still the case that people under more stress are going to have more heart problems. Similarly, people who drink lots of alcohol are, generally, going to have more brain damage than the usual person; it doesnt matter that some of these are the result of driving drunk or falling (instead of just directly chemical), alcohol is still a contributing factor- same idea here.Phoenixia1177 (talk) 01:15, 27 August 2013 (UTC)[reply]

Not too bright...

I just bought a floor lamp with 5 light bulb sockets. Each says "To reduce the risk of fire, do not exceed use of a 60 watt incandescent or 13 watt CFL bulb". Now, a 13 watt CFL puts out the same amount of light as a 60 watt incandescent bulb. However, I don't see why I couldn't use 23 watt CFL bulbs, which are equivalent in lumens to 100 watt incandescent bulbs. As I understand it, there are three ways a high wattage bulb could cause a fire:

1) The electricity in the wires leading to the bulb could cause the wire to overheat. This should strictly be proportional to the wattage, shouldn't it ? That is, a 23 watt bulb should always create less heating of the wires than a 60 watt. Am I wrong here, and does the power factor play a role in heating ?

2) The bulb itself gets hot, and this is a result of inefficiency in conversion of electricity to light. However, the CFL is far more efficient, so I'd expect a 23 watt CFL to only convert some 5 watts to heat, while an incandescent 60 watt bulb converts something like 45 watts to heat.

3) A CFL does also have a ballast, and heat is produced there, too. Is heat here somehow worse than up in the bulb ? Maybe it can't dissipate as easily ?

So, is there any logic to the 13 watt CFL restriction, given that a 60 watt incandescent is acceptable ? StuRat (talk) 02:17, 25 August 2013 (UTC)[reply]

According to EnergyStar [10], you can use any bulb that is 60 watts, incandescent or cfl. These sites come to the same conclusion [11], [12]; but, they'r not as authoritative (though, it appears to be a common question.) As far as I can tell, the reason for the recommendation is that it should produce roughly the same amount of light; rather than prevent fire. (Just to point it out, since it is about fire risk, some people online say that there is a risk, but I haven't seen any clear explanation of why, nor have I seen this from any authoritative site.)Phoenixia1177 (talk) 03:10, 25 August 2013 (UTC)[reply]

Incandescent lamps convert about 5% of the electricity into light - and all the rest has to be waste. So 57 watts of waste. A CFL is about 5 times more efficient - about 25% of the energy turns into light. That's why a 13 watt CFL produces roughly the same amount of light as a 60 watt incandescent. But the CFL generates only about 10 watts of waste. Naively then, the 13 watt CFL ought to produce about five times less heat than the 60 watt incandescent...and I'd guess that heat is what matters in your floor lamp.

My best guess is that the people who printed that label had read that a 13 watt CFL is equivalent to a 60 watt incandescent - and mindlessly reasoned that they know that their appliance overheats with a more-than 60 watt bulb - and therefore it must also overheat with an "equivalent" CFL.

I think the label is wildly wrong! You ought to be able to use CFL's up to about 76 watts (which produce six times as much light - but only the same amount of waste heat as a 60 watt incandescent).

I suppose it's remotely possible that the inefficiencies in CFL's manifest themselves more dangerously than incandescents (eg producing more UV light or something) - and it's possible that the shape of the CFL blocks airflow and traps heat to a greater extent than an incandescent.

But the remarkable coincidence of the numbers they chose strongly suggest that the lamp manufacturers are idiots. SteveBaker (talk) 03:25, 25 August 2013 (UTC)[reply]

Yes, that was my guess, which explains the dual meaning of the title I chose. I just wanted to check with you guys to see if I was missing something. StuRat (talk) 03:37, 25 August 2013 (UTC)[reply]

Is the lamp certified by Underwriters Laboratories? μηδείς (talk) 17:31, 25 August 2013 (UTC)[reply]

Yes, but how is that relevant ? StuRat (talk) 07:57, 27 August 2013 (UTC)[reply]

Quick point. The risk of fire might come from the quality of the wiring in the fixture, rather than the accumulated heat output. So using a CFL of 76 watts would exceed the 60 watt rating of the wiring (a watt of electricity is a watt of electricity, regardless of if it's being converted to light or heat), even if it didn't exceed the 60-watt lightbulb heat equivalent. -- 205.175.124.72 (talk) 20:23, 25 August 2013 (UTC)[reply]

The wiring is almost certainly safe up to several hundred watts. It's actually amps that matter, and wiring that can carry less than an amp is rarely used for mains electricity. Dbfirs 06:49, 26 August 2013 (UTC)[reply]

If the OP is from a 120 V country, his 5 60 W bulbs will draw a total of 2.5 A. -- 200.7.90.57 (talk) 11:43, 26 August 2013 (UTC)[reply]

Apologies. Yes, you are correct. I'd missed the fact that there were five bulbs. 2.5 amps through thin wiring in a very narrow channel could generate significant heat. Dbfirs 13:40, 26 August 2013 (UTC)[reply]

Note also the power factor of Compact fluorescent lamp can be poor, so the demands on the wiring will be greater. Obviously they're not so bad as to make the 13W make sense but the point remains even if the wiring could support 5*76W incandescents (or a 380W heater) it doesn't mean it could support 5*76W CFLs. Nil Einne (talk) 15:30, 26 August 2013 (UTC)[reply]

Yes - but we're really talking about why they'd put a 13 watt limit on the CFL - it makes no sense since even a 60 watt CFL would produce less heat (and need the same wire thicknesses) compared to the 60 watt incandescent. The 76 watt number comes about from heat considerations alone - so technically, you're right and a 60 watt CFL ought to be the highest power you could be 100% sure would be safe. That said - it's pretty safe to assume that the wiring will have enough of a safety factor to support 76 watts without problems. SteveBaker (talk) 22:26, 25 August 2013 (UTC)[reply]

Perhaps it's just a bit of an "idiot proof" measure? If someone put in a 100W equivalent CFL bulb, the next person to swap it might assume that 100W is safe for the fixture, since they got used to the high light output. Vespine (talk) 23:16, 25 August 2013 (UTC)[reply]

I suppose that's possible...but then it takes a special kind of idiot to look closely at the equivalent wattage of the CFL yet completely ignore the warning on the floor lamp itself. The vastly more likely scenario is that some klutz might just think "Huh - this 60 watt (incandescent) bulb is kinda dim - I'll just stick a 100 watt one in there instead."...just ignoring the labeling entirely. SteveBaker (talk) 13:07, 26 August 2013 (UTC)[reply]

OK, thanks all. I will now feel safe using 23 watt (100 watt equivalent) CFL bulbs. StuRat (talk) 15:14, 26 August 2013 (UTC)[reply]

Resolved

StuRat (talk) 15:15, 26 August 2013 (UTC)[reply]

One point I did not see above: a plain old incandescent bulb can function well when it gets extremely hot (hot enough to bake little cakes in an Eazybake Oven, or while turned on in a kitchen oven. The glass envelope temperature of an incandescent bulb can be 200C to 400C, while a halogen bulb may be up to 1200C. [13]. Nothing in it is affected much by high temperature, within reason (eventually the glass will melt, or the solder will melt, or the adhesive holding the glass to the metal base will fail But a plain old inefficient light bulb has a very high permissible operating temperature, and the weak link might be paper insulation in the socket, the glass cover of the fixture, or the copper wire's insulation. A CFL, on the other hand, has all sorts of electronic devices in its base which are likely to fail at a far lower temperature. Some sources say to keep CFL's below 50C ambient. In a test I found online, a 10 watt CFL in a 3 liter container not really airtight, exceeded 58C inside 10 minutes.[14] A somewhat sealed enclosure with the same amount of waste heat dissipated by each type of bulb, should allow more waste heat from an incandescent bulb than from a CFL up to the point of thermal failure of the bulb. Heat shortens the life of CFLs. A hot CFL also produces less light and lower efficiency. [15]. (The latter ref says keep the ambient below 40C for CFLs. Edison (talk) 20:15, 26 August 2013 (UTC)[reply]

That is a good call, Edison. But in general, fittings for ordinary incandescent bulbs will have been designed so that the bulb is in free air, and the heat will drive convection air flow over it. The thermal resistance to ambient of a moderately hot object in free air can be roughly estimated by Pressman's formula:-

Rta = 7400 A^-07

where Rta is thermal resistance to ambient, K/W; A is surface area in mm².

So, the thermal resistance is roughly a constant for constant area, so we can estimate the surface temperature rise of a CFL by proportion. If a 60W incndescent dissipates approx 60 W and has a surface temperature of 200 C, or 175 K above room temperature, an equivalent CFL dissipating approx 13 W should have a surface temperature of 175 x 13/60 ie 38 K above room temperature, or a surface temperature around 63 C.

Pressman's formula is only a rough empirical guide, but this does confirm you are right - a light fitting designed for 60 W incandescents should be restricted to the equivalent CFL, and no attempt made to use the greater efficiency of CFL's to get more light. In practice, convection becomes somewhat non-linear as temperature increases, so the temperature rise of a CFL might be underestimated. But CFL's have a greater surface area, cancelling this out. 120.145.62.173 (talk) 01:02, 27 August 2013 (UTC)[reply]

Interesting. This particular lamp has 5 separate "goose necks" each with a bulb in a cone at the end, open to the air. I suppose I could remove the cones, but then I'd see the blinding bulbs. Maybe they needed to make fluorescent bulbs bigger, not smaller, so you could leave them uncovered and still not be blinded by them. (I've often thought this for cars, especially, where two blindingly bright lights is a bad idea, unless you think blinding the guy coming the other way as you drive over a hill is a good idea.) StuRat (talk) 07:54, 27 August 2013 (UTC)[reply]

I would not remove the cones. Apart from providing mechanical safety (they prevent broken glass should you knock the floor lamp over), removing them may not change the degree of air flow much. There are a variety of empirical formulae used by electronic and electrical engineers for estimating convection driven air flow, such as the following formula for the air flow rate through two apertures in an enclosure containing a hot object:-

V = C_d A [ 2 g H_d {(T1 - T2)²/(T1 + T2)} ]^0.5

where V is air flow rate; C_d is aperture discharge coefficient; A area of one aperture, g is accel due gravity; H_d is aperture vertical separation, T₁ is temperature outside enclosure; T₂ is tempertaure inside; all in standard SI units. This formula is known under the name of the Greek boffin who worked it out, but I can't recall his name right now. So, you can look these formulae up if you wish. I wouldn't bother though, as they give only rough estimates. With 5 bulbs you should have plenty of light. I agree with you about cars only to a certain extent. Headlights at full beam are designed to direct almost all light forward down the road a kilometer or so. That means a degree of focussing that may blind other drivers no matter how broad the source. Also, it is important that oncomming drivers can immediately tell whether there is one headlight or two, so they know whether they are passing a motor cycle or car on a narrow road. 124.178.180.228 (talk) 11:18, 27 August 2013 (UTC)[reply]

I'd think a wide "light bar" would be easier to distinguish from a motorcycle, where you'd still have a single headlight. Also, two headlights can sometimes mean a pair of motorcycles, causing a great deal of panic when they pass on either side of you. :-) StuRat (talk) 12:16, 28 August 2013 (UTC)[reply]

Crikey, Stu! Don't know about your country, but in mine, unless on a well lit 6-lane freeway, motorcyclists doing that would rightly attract a charge of Dangerous Driving (a very serious charge), and should an accident occur and the offending motor cyclist survive, possibly a charge of Dangerous Driving Causing Death, with a possible jail sentence - should any police patrol be in the vicinity, or the car driver reports it. In any case, such a motocyclist would certainly have a death wish. Not forgetting that motor cyclists are taught to ride near the centre line of their lane (for max visiblity to car drivers), and avoid riding abreast. 58.169.233.56 (talk) 16:24, 28 August 2013 (UTC)[reply]

It's probably illegal here too (unless some lobbyist for a company wanting to sell more underpants managed to legalize it), but motorcycle riders aren't known for being sticklers for the law. StuRat (talk) 05:14, 29 August 2013 (UTC)[reply]

The responses above miss an important issue: a large fraction of the energy wasted by an incandescent bulb is emitted as infrared radiation, which in most cases leaves the bulb and the fixture/lamp. CFLs emit almost no infrared; their waste heat load is much smaller, but it is nearly all generated inside the bulb and must be carried away by conduction through the base. The safe operating power for any given fixture will certainly be lower with a CFL installed than an incandescent bulb, particularly for enclosed fixtures. Using a 50 W CFL in an enclosed fixture designed for a 60 W incandescent bulb will most likely result in premature failure of the bulb, and possibly also damage to the fixture. The manufacturer has obviously taken the easy way out and not done engineering tests with CFLs to determine the safe limit, but rather just picked 13 W (60 W-equivalent) as a conservative safe value. --Srleffler (talk) 17:30, 27 August 2013 (UTC)[reply]

I question your assertion that incandescents loose a large fraction of their heat through radiation. The lamp stand "cones will reflect a lot of it back to the globe anyway. But let's assume it's true. That would mean that their surface temperature is lower than would be expected by Pressman's formula, as there is not the full 60W to drive up surface temperature. So my simple proportion estimate of CFL surface temperature above underestimates it. That makes it even more likely the lamp stand manufacturer was right to limit CFLs to 13 W. 58.169.233.56 (talk) 09:34, 28 August 2013 (UTC)[reply]

Pressman's formula is improperly applied in the discussion above. An incandescent light bulb is not a "moderately hot" object dissipating heat by convection into free air. The filament is intensely hot (thousands of degrees), and is in an enclosed space (the bulb). The filament is primarily cooled by radiation, not convection. From Luminous efficacy and this paper, a typical incandescent bulb's spectrum is roughly that of a 2800 K blackbody radiator. The emitted light itself has a luminous efficacy of radiation (LER) of about 15 lm/W. This indicates how much visible light there is, as a fraction of the total amount of radiation present. A typical 60 W incandescent bulb emits about 870 lm of light. At 15 lm/W LER the emitted radiant power is about 58 W. The bulb is about 96% efficient at converting electricity into radiation, but most of the radiation emitted is infrared. The remaining 4% (about 2.4 W) goes into heating the envelope of the bulb and its base.

I don't know offhand what the LER is for a CFL, so let's consider a hypothetical "ideal" daylight CFL that mimicks a 5800 K blackbody in the visible spectrum, but has no infrared or ultraviolet. The LER for that would be 251 lm/W (from the same paper referenced above). Assume the 13 W CFL emits 870 lm just like the 60 W incandescent bulb does. The radiant power is about 3.4 W. The remaining 9.5 W of power consumed by the CFL is waste heat that must be carried away from the bulb by convection or conduction through the base. In this worst-case comparison, the CFL dumps about four times more heat into the fixture than the equivalent incandescent bulb would.

In practice, not all of the infrared radiation makes it out of the fixture, so the incandescent bulb will dump more heat in the fixture than the above analysis suggests. Also, a real CFL probably has a lower LER than the best-case estimate above, and so emits more radiant power to produce the same luminous flux. The real CFL would dump less heat into the fixture than the above analysis indicates.

My point is that the lamp manufacturer is right to limit CFLs to a lower electrical power than the limit for incandescent bulbs. I doubt that they have put in the effort to determine the exact safe limit for CFLs. Rather, they would at best have done an experiment where they run 60 W-equivalent (13 W) CFLs in fixtures designed for 60 W incandescents, and verified that the fixtures do not burst into flames. They may well have also tried using a 25 W CFL and found that the fixture became too hot.--Srleffler (talk) 04:01, 29 August 2013 (UTC)[reply]

Srleffler, your amplification of your earlier post stating that most of the heat is lost via radiation is noted. But as I said before, the error thereby in starting with Pressman's Formula is such that the surface temperature of a CFL was underestimated. It is the bulb surface temperature (actually the bulb base temperature) that determines the thermal stress on the lamp fitting and wiring. The radiant heat from an incandescent bulb is shielded from the lamp socket by the bulb metal base. Quite a bit of the radiated heat would impinge on the cones, which are often enamelled mild steel or anodised aluminium and the like. A lot of this heat is reflected back to the bulb, raising the bulb glass temperature, reducing the error in starting with Pressman's Formula. The cone temperature is however, very much lower in temperature than the bulb glass, partly because of its much greater surface area and partly because of the air space between it and the bulb.

Incidentally, electronic engineers routinely calculate the surface temperature of metal parts ("heatsinks") on which hot transistors and other parts are bolted or clamped(using Pressman's F or some other method). I can tell you from much experience that 2.4 W (your estimate of dissipation in the bulb glass and base, and coincidentally of the same order as the power dissipated by transistors in a typical cheap stereo) would not raise the bulb surface temperature anywhere near the 200 C quoted, even if conduction out of the base was not possible. The diameter of a 60 W incandescent is about 55 mm. Area (4 pi r²) is thus 9500 mm² assuming it is a simple sphere, which of course will lead to significantly under estimating the surface area. Applying Pressman's Formula, the thermal resiatnce to atmosphere is then 12 K per watt. So, for 2.4 W dissipation, the temperature rise should be 29 K. You only need to touch the glass of a running 60 W globe to know that's way too low, so the dissipation must be very much greater than your 2.4 W figure.

The bulb base "intercepts" about 15 to 17% of the sphere surface. So 15 to 17% of the heat radiated by the filament is intercepted by the base and dissipated in it - about 9.8 W added to the 2.4 W. That indicates a surface temperature rise above ambient of 140 K, but that is an over-estimate as some heat will be lost via conduction into the lamp fitting. Still too low.

120.145.29.159 (talk) 04:46, 29 August 2013 (UTC)[reply]

A 60 W bulb in free space doesn't get as hot as 200°C. A quick look online suggests ~120°C as a better guess. I wouldn't have guessed quite that hot. I agree that 2.4 W doesn't seem like enough to reach this temperature. Interception of radiation by the base of the bulb is probably sufficient to explain it, however. This does raise the heat load of the 60 W incandescent a little bit above that of the 13 W CFL (12.2 W vs. about 9.5 W). Some of the CFL's radiation will also be absorbed in the lamp, of course. I don't think reflection of radiation back to the bulb is a big factor. The glass envelope of the bulb is transparent, so it does not efficiently absorb infrared. --Srleffler (talk) 18:02, 29 August 2013 (UTC)[reply]

This is a good discussion - because we both, and the OP, have all learnt something. However you you have not cited any reference to back up your claim that a 60 W incandescent only gets to ~120 C. I took the 200 C figure from Edison's post. He got his >200 C figure from a reference he cited which looks pretty authorative to me. The Wikipedia article on Incandescent lamps quotes the same figure, based on a 1964 GE pamplet on incandescents. I did a quick google and found a few websites giving >200 C and none giving ~120 C. Can you cite a reference?

The transparency of the glass envelope is not anywhere near as important as you may think. If it was truely transparent to infrared as well as visible and other wavelengths, a bulb in open free air would run at room temperature due to conduction to the surrounding air. It clearly does not run at room temperature, so there is some absorption. For a bulb in a typical cone fitting, the metal cone typically surrounds the bulb to about 80% or so. This largely means that any radiation fed back by the inside of the cone has nowhere to go expect back through the glass a few times, heating it a little bit more each time. It's a bit like a cavity radiator, where the radiation emitted through the cavity orifice appears as though the cavity is a near perfect black body surface even when it is far from being such a surface. Any small object suspended within a cavity radiator "sees" radiation corresponding to black body radiation.

120.145.29.159 (talk) 00:16, 30 August 2013 (UTC)[reply]

engineering

Good time, I am a ICSE student and i am interested in science. I wanted to know the fields in engineering. As in what what subjects are there, what types of engineering are there, if I like any 1 field in engineering then what to do for it. please help me in this matter. you would be very much thankful. 115.242.18.161 (talk) 03:34, 25 August 2013 (UTC)₵[reply]

See list of engineering branches, to start. StuRat (talk) 03:39, 25 August 2013 (UTC)[reply]

physics

what would happen to a seed if we planted them in a rotating wheel and provided them with all its needs. The question is how will that plant grow as we know that the roots grow towards the gravity and the shoot part of the plant grows towards the light in what manner will it grow in the rotating wheel??? — Preceding unsigned comment added by 115.242.66.252 (talk) 06:23, 25 August 2013 (UTC)[reply]

Excellent question. The spirit of science is all about inquiry. Try it and find out. Let us know your results. Dolphin (t) 07:15, 25 August 2013 (UTC)[reply]

Gravitropism is studied using clinostats as you suggest, and in space stations. In addition to gravity, plants use light (phototropism), water (hydrotropism), and touch (thigmotropism) to orient themselves.

In the absence of gravity, plant roots tend to seek out water and nutrients, and shy away from light. Leaves grow to maximize light collection. Many plants will grow just fine and look like they normally would. I don't know what something like a tree would do though. 88.112.41.6 (talk) 07:39, 25 August 2013 (UTC)[reply]

What size wheel and how fast? ←Baseball Bugs ^{What's up, Doc?} carrots→ 02:12, 26 August 2013 (UTC)[reply]

There was a Scientific American article about this in the 1970. In essence, you can simulate zero-G by a wheel that rotates fast enough (as I recall, on the order of minutes). There was a way to simulate the entire G range, from zero to arbitrarily high, depending on the design of your wheel and rotation rate. Edit: The article was in the "Amateur Scientist section of the June 1970 issue, page 141. Tdjewell (talk) 11:51, 26 August 2013 (UTC)[reply]

I don't see how you can simulate zero g with a rotating wheel on Earth. In a horizontal wheel that should always increase the g's, since now you have the vector sum of gravity and the apparent outward force. If it was spinning at 1 g, that would give you 1.44 g aimed outward and down. With a vertical wheel spinning at 1 g, you should get 0 g at the top, yes, but 2 g at the bottom. The only way I can see to get zero g constantly is if the wheel is a great circle around the Earth, and rotates at 1 g. StuRat (talk) 05:23, 29 August 2013 (UTC)[reply]

Stanford Torus / Elysium orbital habitat

Hullo. Having just seen the movie Elysium, I've been looking at some details of the Stanford Torus. The movie features an orbital habitat similar to a Stanford Torus, but it didn't appear to be rotating very fast, and had an open "top" on the actual habitat section!! Could someone please answer a couple of questions for me?

Your "Stanford Torus" article says that to get about 1G in the habitat ring, a structure 1.8km diameter needs to rotate at 1 rpm.

1. If the diameter was bigger would it need to rotate faster or slower to get the same G force?
2. If you are in a habitat that has 1 g at your feet level, but zero G at 900 meters above your head, how would the gravity gradient on your body affect you?
3. Would it actually be possible to have an open top on the habitat ring the way the movie does?

Thanks in advance 122.108.189.192 (talk) 07:55, 25 August 2013 (UTC)[reply]

You can read more about artificial gravity created by spinning here, or do the math yourself over here. Spoiler; larger diameters means you'll have to spin slower to maintain gravity. WegianWarrior (talk) 08:45, 25 August 2013 (UTC)[reply]

A difference of two parts in 900 in gravity from head to foot would not really be noticeable. An open top means that air will constantly be lost, but the rate of loss might not be a serious problem if there is a constant renewal (as on Earth). According to Atmosphere of Earth, the tropopause is about 12 kilometres up, and there are gases beyond that, so a mere 900m seems inadequate to retain a proper atmosphere. Perhaps someone can calculate the loss rate? Dbfirs 16:45, 25 August 2013 (UTC)[reply]

The Stanford torus is a mile in diameter, which is less than the scale height of gasses in the Earth's atmosphere (as Denver residents will attest). Therefore, it could not exist uncovered. I don't know if the movie uses a true Ringworld (or at least one around the Earth); if it does, and if it uses very, very high walls, it might work. Or maybe they have force fields? Actually just architectural invisibility might suffice, unless you see ships going through the 'empty' space at will - it shouldn't be that hard to make a metamaterial which is transparent with no refractive effect on light, when present as a nearly perfectly flat architectural element. Wnt (talk) 08:13, 26 August 2013 (UTC)[reply]

Actually, ships can apparently fly into the "interior" of the structure at will. (I haven't seen the film.) The science fiction writer Gary Westfahl has some remarks on this in the second half of the third paragraph in his review here. Deor (talk) 16:27, 26 August 2013 (UTC)[reply]

You have to spin faster if the ring is larger. Because we fall about 5m (16ft) in the first second on Earth, that 5m has to come from curvature if you want 1g from spin. you can draw a tangent to a ring 5m smaller in radius than the actual floor level; the ring has to spin fast enough to move from the tangent point to one of the intersections with the "floor level" in one second. That would grow with size, but less so than the size itself. (Stupid engineer's trick, does not work if the radius is not substantially larger than 5m) - ¡Ouch! (^{hurt me} / _{more pain}) 06:30, 28 August 2013 (UTC)[reply]

It can be this way or that way, if you are looking at meters per second, bigger is faster. If you compare rpm, smaller is faster. - ¡Ouch! (^{hurt me} / _{more pain}) 08:24, 28 August 2013 (UTC)[reply]

2) While that gradient isn't enough to notice, a larger gradient (from a smaller ring with higher RPMs) causes dizziness, disorientation, and nausea, particularly when standing up (you are literally "lightheaded"). If everyone was lying down (as in a sleeper ship) this would be less of an issue. StuRat (talk) 05:30, 29 August 2013 (UTC)[reply]

the heart realy in the size of the fist?

I saw in once forums someone that ask a doctor (cardiologist) if the heart in the size of the fist, and he answered that it's not exactly and it's not scientific to say such. So, my question is how is 'scientific' size of the heart? 176.13.1.78 (talk) 12:34, 25 August 2013 (UTC)[reply]

Our own article on heart also suggests that it is roughly fist-sized, but that is just a comparison to give lay people a rough idea of how large it is; there's no reason to think it's a particularly accurate measurement. You could very easily have an enlarged heart, but small hands, or vice versa. If an internal organ needs to be measured for size, you'd have to do some kind of medical imaging, such as a CT scan. Matt Deres (talk) 13:01, 25 August 2013 (UTC)[reply]

I looked briefly before and found that normal hearts were about 300 g, hypertensive hearts over 400. [16] The actual proportion with body mass wasn't given in that source. The density shouldn't be much more than 1 so that should be 300-400 ml. But I got bored and didn't look up the volume of a fist - if one of us happens to have a beaker handy we should push our hand in and see how much water is displaced. Wnt (talk) 08:16, 26 August 2013 (UTC)[reply]

This paper looks like it may have the data we need on the volume of a typical human hand - but it's behind a paywall. If anyone has JSTOR access, they might maybe take a peek at it for us. SteveBaker (talk) 13:02, 26 August 2013 (UTC)[reply]

From Table 1 of that paper: Men/Women, dominant hand volume(cm^3) is 431+/-53, 285+/-35, respectively. For non-dominant hand, we get 417+/-51, and 273+/-35. Largest hands were in men 40-49 years old, n=55 men, 45 women. I'll leave interpretation of this up to the others. SemanticMantis (talk) 14:47, 26 August 2013 (UTC)[reply]

Cool! So now we know! Hearts are around 300 to 400g - which, if their density is similar to water (like most of our bodies), would suggest a total (collapsed) volume of 300 to 400cc's - which is indeed pretty similar to the volume of a human hand at 280 to 430 cc's. Obviously, the beating heart will be larger because of all of the cavities - but then a fist encloses more volume than the hand itself...so probably this comparison is in the right ballpark. Of course the cardiologist is correct in saying that it's an inexact thing - but then how could it not be? Hearts and hands vary in size between people - and hearts and fists enclose different volumes depending on what state they are in - and just how far up your wrist is "hand" and when does it become "forearm"? But roughly - it's a pretty good analogy. SteveBaker (talk) 18:50, 26 August 2013 (UTC)[reply]

Hairs and nails

Why do our hairs and nails keep on growing even when they consist of dead cells? Concepts of Physics (talk) 14:24, 25 August 2013 (UTC)[reply]

Because the root isn't dead. See Human hair growth and Nail (anatomy)#Growth. AndyTheGrump (talk) 14:27, 25 August 2013 (UTC)[reply]

1) Because a multi-cellular organism can be dead without all of it's cells being dead. Those cells with a low metabolic rate, like hair follicles and nail bed cells, can survive for a long time just feeding off the blood, tissue, and air around them.

2) Much of the apparent growth after death is really due to the dehydrated body shrinking back more than the hair and nails. StuRat (talk) 15:01, 26 August 2013 (UTC)[reply]

That's not quite what the OP was asking, but it's a good point. ←Baseball Bugs ^{What's up, Doc?} carrots→ 16:00, 26 August 2013 (UTC)[reply]

...But a big ^{[citation needed]} on the 'low metabolic rate' claim—cells in the hair follicles are among the fastest-dividing in the human body. (Which is one of the reasons why cytotoxic chemotherapy makes patients' hair fall out.) For that matter, the fact that follicular cells may remain viable a long time after death doesn't necessarily mean that they are cranking out hair the whole time. Absent a supply of circulating, oxygenated blood – along with sufficient glucose – those cells aren't going to be secreting much keratin. TenOfAllTrades(talk) 01:46, 28 August 2013 (UTC)[reply]

Underwater train

Are there any trains/monorails that go underwater? ScienceApe (talk) 16:12, 25 August 2013 (UTC)[reply]

Do you mean without a tunnel? Dbfirs 16:20, 25 August 2013 (UTC)[reply]

Yes, without a tunnel. ScienceApe (talk) 18:00, 25 August 2013 (UTC)[reply]

The Brighton and Rottingdean Seashore Electric Railway had underwater tracks, with the cars supported on suitably ornate ironwork. I don't think there were any railways where the cars themsevles ran underwater - there would be insufficient force (due to the buoyancy of the air in the cars) to keep them on the track. Tevildo (talk) 16:29, 25 August 2013 (UTC)[reply]

Only if you filled the things with air. — Preceding unsigned comment added by 109.144.132.49 (talk) 19:27, 25 August 2013 (UTC)[reply]

True, of course, but the only sensible use for such a train would be for entertainment (as in the Brighton example), so it would need to carry passengers who could see out of it. It wouldn't be commercially viable to move goods by underwater railway rather than by boat. Tevildo (talk) 19:34, 25 August 2013 (UTC)[reply]

Fair point. The only thing I can suggest then is the rails that ships are dragged in and out of the water on at some shipyards. — Preceding unsigned comment added by 109.144.146.36 (talk) 19:40, 25 August 2013 (UTC)[reply]

There used to be a ride at Disney World based on 20,000 Leagues Under the Sea, where a mockup of the Nautilus ran around a submerged route on rails. Rojomoke (talk) 23:22, 25 August 2013 (UTC)[reply]

We have an article - 20,000 Leagues Under the Sea: Submarine Voyage. Not really an underwater train though: "The attraction vehicles were not actual submarines, but instead boats in which the guests sat below water level". 23:45, 25 August 2013 (UTC)

I remember going on that actually when I was a kid. Brings back memories. ScienceApe (talk) 04:33, 26 August 2013 (UTC)[reply]

Conceptually they could be useful for mining manganese nodules and methane clathrate and such, but in practice, the ready mobility and relocatability of shipping must vastly outweigh any leverage that could be obtained taking cars of ore up obliquely to the beach. Wnt (talk) 08:20, 26 August 2013 (UTC)[reply]

Another option, in the case of a underwater sightseeing train, would be a tunnel, but made of clear plastic. This would allow the passengers to view the sea life. StuRat (talk) 15:06, 26 August 2013 (UTC)[reply]

August 26

Which source is more trustworthy on masturbation?

The following two websites seem to contradict each other.

Source: http://www.babymed.com/getting-pregnant/male-masturbation-fertility-sperm-count

The first source suggests that masturbation decreases a man's fertility, because it advises that a man should conserve the ejaculations for intercourse in order to maintain a higher sperm count.

Source: http://www.mayoclinic.com/health/male-masturbation/AN01189

The second source suggests that masturbation has no effect on a man's fertility or sperm count.The two sources contradict each other, because masturbation can't affect and not affect the fertility at the same time. It has got to be one or the other, not both. Though, it may also be both but under different conditions. Sneazy (talk) 04:08, 26 August 2013 (UTC)[reply]

I don't think there is a contradiction. I believe the 1st source would be referring to immediate sperm count, where as the 2nd is referring to long term sperm count "in general". For example, someone who masturbates 5 times a week will not have a sperm count higer or lower then someone who doesn't masturbate at all. However, if you masturbate the day of your sperm count test then your sperm count will obviously be lower then someone who hasn't ejaculated at all in the last few days. In fact, I recently had some personal experience in precisely this regard and can tell you that the recommnedation is to not ejaculate at least three days before your sperm count test as this will provide an "optimal sample" for testing. So, I guess what it is saying is that it's possible that masturbating right before you have sex or even up to a day before could affect your fertility, but NOT that masturbating in general will affect your sperm count. Vespine (talk) 05:47, 26 August 2013 (UTC)[reply]

If it's not obvious, I'll add that sperm is produced in the testes at a rate far slower then most young males can expel it. It's is held in reserve in the epididymis until it is "required", but if it is ejaculated more then once every day or two you will be firing less then a full magazine, simply because you have not given enough time for the reserves to refill. In fact, the 1st source is less accurate because it is not masturbation that lowers the sperm count, it's the ejaculation, so even if you had intercourse with your mistress before having intercourse with your wife, your sperm count would be just as low on the 2nd occasion as if you masturbated. Vespine (talk) 05:58, 26 August 2013 (UTC)[reply]

It's worth saying that while having sex more than once within about 3 days (either with a female or solo - it doesn't matter which) will lower your sperm count - it'll never lower it to zero - so this is not an effective birth control mechanism! It's interesting to note that 18-29 year olds have sex an average of 112 times per year - which is the exact perfect rate for the full recharge of sperm. It seems that we've evolved a sex drive that closely matches our productivity. SteveBaker (talk) 12:58, 26 August 2013 (UTC)[reply]

...(either with a female or solo - it doesn't matter which). Does it matter if it's two males? CambridgeBayWeather (talk) 04:01, 27 August 2013 (UTC)[reply]

Good point - of course not. No matter how the semen gets out of the body - it's got to be replenished. SteveBaker (talk) 13:10, 27 August 2013 (UTC)[reply]

Hot Damn! Who was the stud machine going at it every 39 hours to balance out my celibacy? {The poster formerly known as 87.81.230.195} 90.213.246.168 (talk) 16:38, 26 August 2013 (UTC)[reply]

Boric acid

What is the pK_a1 for the deprotonation of boric acid? Yes, I know what I'm asking - I'm aware of the dominant hydrolysis reaction taking place under aqueous conditions. Plasmic Physics (talk) 07:53, 26 August 2013 (UTC)[reply]

What is the equilibrium constant for:

B(OH)₃ (s) + H₂O (l)

{\ce {<<=>}}

[B(OH)₃H₂O] (aq) ? Plasmic Physics (talk) 08:01, 26 August 2013 (UTC)[reply]

Found a likely ref but don't have time to check it until later today...Perelygin,, Yu. P.; Chistyakov, D. Yu. (2006). "Boric acid". Russian Journal of Applied Chemistry. 79 (12): 2041–2042.{{cite journal}}: CS1 maint: extra punctuation (link) DMacks (talk) 22:05, 27 August 2013 (UTC)[reply]

Thank you, I've used it correct the Boric acid article. It used the pK_a1 for the tetrahydroxyborate pK value. Plasmic Physics (talk) 03:24, 28 August 2013 (UTC)[reply]

Line on globe

On an old (ca 1915) globe, there is a great circle marked with a double line. It crosses the equator at 18°W and at 162°E, at an angle corresponding to the Earth's axial tilt, and is tangent to the tropics at 72°E (in India) and at 108°W (in the South Pacific). What might be the purpose of this line? 85.226.204.42 (talk) 08:53, 26 August 2013 (UTC)[reply]

Is it just marking the ecliptic plane? See pictures there, and at axial tilt. SemanticMantis (talk) 13:22, 26 August 2013 (UTC)[reply]

That's not possible, the ecliptic plane shifts with respect to the Earth as the Earth rotates. I think it is probably marking the apparent path of the first point of Aries, which is commonly used as the reference for celestial navigation. I could be wrong about that, but at any rate it's the path of some specific star or constellation. Looie496 (talk) 04:43, 27 August 2013 (UTC)[reply]

Sounds like a plausible explanation! Thank you very much for your help. 85.226.204.42 (talk) 10:03, 27 August 2013 (UTC)[reply]

That can't be it. Stars don't shift back and forth between hemispheres. It's probably the location of the ecliptic for some specific date and time. Dauto (talk) 14:24, 27 August 2013 (UTC)[reply]

Teflates

Does anyone know exactly why teflates (with OTeF⁻
₅) are such great oxidizing agents (They even oxidize Kr and Xe!)? Double sharp (talk) 11:16, 26 August 2013 (UTC)[reply]

Because the Te-F bond is weak ("hard" anion/"soft" cation -- i.e. the bonding orbitals are way out of line energy-wise), which makes it energetically favorable for the fluorine atoms to go somewhere else -- even onto a noble gas atom. FWIW 24.23.196.85 (talk) 21:52, 27 August 2013 (UTC)[reply]

Extensively drug-resistant bacteria

It seems that there's more and more news about bacterial diseases that are resistant to (almost) all available antibiotics. Are we doomed? Will there always be new antibiotics waiting to be discovered that can treat bacterial diseases, no matter how much the bacteria evolve? It seems that the odds are stacked against humans in this fight, because to be useful any new antibiotics not only need to kill bacteria, they also need to be safe for humans. Will we run out of safe compounds to try? --173.49.10.42 (talk) 11:50, 26 August 2013 (UTC)[reply]

Well, in a race between the evolution of bacterial resistance and human ingenuity, I'd back the latter every time. However, in many cases (such as in the over-use and mis-use of antibiotics) it's a race between evolution aided by human stupidity and human ingenuity - and that's not such an obvious slam-dunk!

My suspicion (without much evidence) is that while we may be losing the battle with antibiotics like penicillin, we may have huge success with nanotechnological, phage and plasmid approaches. Our Antibiotic resistance article also talks about phage therapy which looks like a promising alternative because it pits phages against bacteria and the phages should probe able to evolve to keep up with the bacteria.

Early antibiotics were discovered by looking at what nature did to eliminate bacterial infections - and we came across numerous microbes that seemed to excrete substances that were able to do that. That meant that we were in a continuous chase to find new "natural" substances without really understanding how they worked. In the modern world, we're increasingly able to examine the metabolic pathways and surface coats of bacteria and to design drugs that specifically attack them. That's an important change in tactics - and I believe that it does give us an edge here.

However, no matter what we do, we're going to have to get a lot smarter about how we use these drugs and not hand them out like candy as a kind of placebo treatment to patients with viral illnesses such as the common cold. We need a sharper delineation between drugs used to treat farm animals and those that we use for humans. We need less 'leakage' of antibiotic agents between patients in places like hospitals.

SteveBaker (talk) 12:50, 26 August 2013 (UTC)[reply]

Quibble: Not what nature did to eliminate "bacterial infections". That's the immune system. What we copied was the chemical warfare between different species of bacteria, or between molds and bacteria, that one sort of inglamorous life used against another in the struggle over resources. --Trovatore (talk) 19:09, 26 August 2013 (UTC)[reply]

Agreed, and antibacterial soaps are another culprit. Just wash the germs down the drain with regular soap and water. StuRat (talk) 15:10, 26 August 2013 (UTC)[reply]

Although it's not entirely obvious that repeatedly washing your hands doesn't cause the evolution of bacteria that stick to your skin better and resist being washed away. Ultimately, there may not be much that can be done beyond continually switching the strategies we use to get rid of them. Ultimately, what we need is a strategy whereby we don't kill bacteria - but instead encourage them to evolve such as to be harmless to us. Not killing them - but finding a drug that allowed them to reproduce more easily if they don't harm us than if they do - would evolve benign bacteria and resistance to such a hypothetical drug like that would not develop. SteveBaker (talk) 18:43, 26 August 2013 (UTC)[reply]

"The Borg Cube has adapted to our shield frequencies, switching to continuously changing frequencies now." StuRat (talk) 03:36, 27 August 2013 (UTC) [reply]

We certainly aren't doomed, because we can always temporarily stop using a given class of antibiotic. Generally speaking bacteria pay a price for drug resistance, and if it ceases to be forced on them, they rapidly evolve to lose it. Anyway no antibiotics at all were available for most of human history, and we somehow managed to pull through. Looie496 (talk) 02:24, 27 August 2013 (UTC)[reply]

Yes - and drug cocktails are another approach - forcing the bacteria to be simultaneously resistant to 10 or 100 different drugs might well overwhelm its ability to support all of the extra chemical pathways needed simultaneously. SteveBaker (talk) 13:08, 27 August 2013 (UTC)[reply]

Resting heart rate below 30 bpm not unusual?

I had long thought that Indurain's resting heart rate of 28 bmp was a "one in a million" unusual heart rate for well trained athletes. However, when searching for this in the net, I found that this is apparently not the case, there are many people who claim that they are fit healthy amateur athletes who have resting heart rates below 30 bpm. But then, I have myself a resting heart rate that can range from the low 40s to the high 30s and I know quite a few people with similar heart rates, so in that respect it doesn't seem unusual for people who train a lot harder than me to have heart rates in the high 20s. Count Iblis (talk) 12:22, 26 August 2013 (UTC)[reply]

We cannot offer medical advice - but we do have an article about this at Athletic heart syndrome and you might also want to read Bradycardia.

SteveBaker (talk) 12:34, 26 August 2013 (UTC)[reply]

Graph of Force with respect to acceleration

Suppose, we have all three quantities in the equation "Force = mass x acceleration" constant. Now, if I want to plot the graph of "F" and "a", in which "F" lies on y-axis and "a" lies on x-axis. What type of graph this would be? Concepts of Physics (talk) 16:31, 26 August 2013 (UTC)[reply]

If mass is constant, a straight line.--Gilderien Talk|List of good deeds 16:37, 26 August 2013 (UTC)[reply]

If it is a straight line, then it means force as well as acceleration are increasing with the same rate. But I am asking to plot the graph when neither force nor acceleration vary, both are constant (e.g. 5N and 8m/s/s). Concepts of Physics (talk) 17:22, 26 August 2013 (UTC)[reply]

Than your graph has just a single point in it (boring). Dauto (talk) 17:30, 26 August 2013 (UTC)[reply]

You only plot a graph where one thing is varying or unknown and the some other thing is calculated from it. So if you had a constant force, you could plot a graph to show how the amount of acceleration you'd get would change as you vary the amount of mass...or if the mass is constant, you can plot a graph to show how the acceleration changes depending on the amount of force you're applying. Basically, it's only really useful to plot a graph when you know only one of the three things. If you know two of them - then you can calculate the third...and if you know all three then you know all that there is to know and plotting a graph is essentially meaningless - and definitely quite useless! SteveBaker (talk) 18:37, 26 August 2013 (UTC)[reply]

Maybe a page like this might help? Vespine (talk) 03:52, 27 August 2013 (UTC)[reply]

Alcatraz modern day sewer

A minor reason often cited as a factor in the closure of Alcatraz prison was that the raw sewage went straight into San Francisco Bay. What happens to it now? The tourist load is heavy and I wager more sewage is generated today than when the prison was operating. Tarcil (talk) 17:54, 26 August 2013 (UTC)[reply]

Probably there is some kind of chemical treatment these days. But think about the numbers: At its peak, the prison had 300 inmates and about 200 staff - but the staff lived on the island, often with their families (there were 64 apartments on the island for that exact purpose) - so the total number of people on the island was probably closer to 600. That translates to about 600x24x365 = 5.2 million people-hours of sewage produced per year. Nowadays, there are 1.5 million people per year visiting the island - it's hard to know how long they each stay - we were there for three hours at least - so that's more like 4.5 million people-hours. I'd bet that most visitors take the "go before you leave" approach to toilets...so the current load is probably much less than it was at the peak of the prison - and since people aren't eating or drinking much while on the island, it's probably WAY less. SteveBaker (talk) 18:11, 26 August 2013 (UTC)[reply]

Wastewater is held in storage tanks and shipped to the mainland. Total wastewater output is about 6000 gallons per day. Details. TenOfAllTrades(talk) 21:03, 26 August 2013 (UTC)[reply]

So the typical 4,100 visitors each day produce about a gallon and a half of waste water each? A toilet takes between one and two gallons to flush - so it sounds very plausible.

A normal person in a house produces around 50 gallons of waste water per day - so the 600 or so 24/7 occupants of the island when it was a prison could easily have produced around 30.000 gallons per day - presumably too much for the converted landing craft that currently hauls sewage to the mainland each day.

SteveBaker (talk) 22:56, 26 August 2013 (UTC)[reply]

About Latitude and radiation

Hi,
By the article that I've read it's seem that the amount of radiation which hit the ground in some Latitude depends in the Latitude of the place.
I ask why; the Earth's orbit radius around the sun is way more bigger than the Earth radius.
So I would expect that the change in the amount of radiation will be minor around the globe.
Another think, in the true north the radiation direction should be vertical to the ground.
So I would expect that the amount of heat there will be far more less than it is(like Venus). Exx8 (talk) 22:22, 26 August 2013 (UTC)[reply]

You are right that the Earth's radius is small compared to radius of the orbit. And if that were the only effect, insolation would be more even around the globe. But we have axial tilt, and that means that the same radiation that strike 1m^2 at the equator is spread out over several m^2 at high latitudes. Flux is the general concept behind this. Imagine you hold a hoop in the rain, parallel to the ground. As you tilt the hoop, less and less rain falls through. SemanticMantis (talk) 22:30, 26 August 2013 (UTC)[reply]

But we would still have the same effect if the Earth had no axial tilt. In that case the solar radiation at the poles would still be less because the Sun's radiation hits at a shallow angle there, instead of straight on, as at the equator. Also, at the poles the Sun's radiation must travel through far more atmosphere before reaching the ground, which reduces some forms of radiation. However, the Earth's magnetic field does direct highly charged particles towards the poles. StuRat (talk) 07:39, 27 August 2013 (UTC)[reply]

Yes, you're right, on a given day, insolation would still drop off with latitude if there were no axial tilt. But it would be a slightly different pattern, and there would be no seasonal variation. I do think insolation would be "more even" without the tilt, but I don't have a ref for that :) I suppose flux is the most important part for the OP to focus on. SemanticMantis (talk) 14:35, 27 August 2013 (UTC)[reply]

August 27

mechanical seal

difference between balanced and unbalanced mechanical seal — Preceding unsigned comment added by 117.198.197.134 (talk) 05:34, 27 August 2013 (UTC)[reply]

Can you give us some context ? I doubt if we are talking about a mechanical marine mammal. StuRat (talk) 07:33, 27 August 2013 (UTC)[reply]

The terminology is well known to mechanical engineers, especially pump engineers, as googling "types of mechanical seals" will quickly show. I haven't actually answered this question as it sounds like a trade school homework question to me. 124.178.180.228 (talk) 08:53, 27 August 2013 (UTC)[reply]

Yes, and if this forum was only read by pump engineers, then omitting that they are talking about pumps might be acceptable. StuRat (talk) 12:06, 28 August 2013 (UTC) [reply]

A balanced mechanical seal stands up straight, while an unbalanced mechanical seal tends to tilt a bit. Hope that helps. :) ←Baseball Bugs ^{What's up, Doc?} carrots→ 13:19, 27 August 2013 (UTC)[reply]

I was really hoping to see those links piped to Paro (robot). Jessica Ryan (talk) 13:20, 27 August 2013 (UTC)[reply]

Fittingly, those are intended for the unbalanced. ←Baseball Bugs ^{What's up, Doc?} carrots→ 13:33, 27 August 2013 (UTC)[reply]

You rang? -- Jack of Oz ^{[pleasantries]} 08:30, 29 August 2013 (UTC) [reply]

Battery powered water boiler

Well I've been thinking for a while about something to use the spare batteries, so I think that this is a good one. No, I don't want to use open flame because this is too dangerous. So here is my questions:

Is it possible to power nichrome wire through a series of D batteries?
How much D batteries are needed to boil a normal steel bottle filled until half?
Will insulation help? What insulator are good for this (no, I don't want to use aerogel)
Is it practical? 140.0.229.26 (talk) 15:29, 27 August 2013 (UTC)[reply]

Probably not practical. It takes 4.18 Joules/gram to raise the temperature of water 1 deg C. That means it would take, say, 350 Joules to boil one gram of water from tap temperature (very approximate) or 350,000 joules for about a liter of water, a reasonable capacity for an electric kettle. It would take a LOT of D-cells to generate that many joules of heat, even without taking into account inefficiencies in turning all that chemical energy to heat. --Jayron32 15:47, 27 August 2013 (UTC)[reply]

The most energy I see for a D battery is around 75 kJ[17], so there is enough energy present in 5 of them for what Jayron32 needs for his liter of water. But how much energy can you actually get out and turn into heat that you can transfer to it? DMacks (talk) 16:35, 27 August 2013 (UTC)[reply]

If you have several dozen spare batteries, and nothing better to use them for, then yes, it's possible. I've got nichrome wire red hot with D cells, though for safety reasons you will probably want to adjust the current so that the wire gets less than red hot. One problem is that if you wrap the wire round the steel bottle, then the steel will short out the wire. The most efficient way of transferring heat is to immerse the wire in the fluid, but this can be very messy, and you are likely to get corrosion of the wire. Expanded polystyrene would be the best insulator, but the hot wire will melt it, so you might be better using glass fibre (rock wool). The numerous problems in implementing the project make it impractical, as Jayron said. I suggest that you find an alternative use for your batteries, or try connecting say four parallel banks of eight cells in series and use them to power a 12v flask heater sold for use in cars. Thirty-two batteries would at least warm the water in the flask, but I think it might take eight banks of eight to boil the water. Dbfirs 16:30, 27 August 2013 (UTC)[reply]

It would be more efficient to run a heat pump to boil the water but it is still a pretty poor use for batteries. You're basically degrading the energy. Dmcq (talk) 16:46, 27 August 2013 (UTC)[reply]

You could use the batteries to power a radio while you are riding a bicycle-generator or building a solar oven... DMacks (talk) 16:57, 27 August 2013 (UTC)[reply]

Don't forget it takes an additional 2260 J/g just to boil water at 100C. Enthalpy_of_vaporization#Other_common_substances. Saintrain (talk) 20:57, 28 August 2013 (UTC)[reply]

... that's to turn all the water into steam. ("Boil" has two meanings, I suppose.) Dbfirs 07:19, 29 August 2013 (UTC)[reply]

Wait. What? Doesn't everyone brew their tea in a still? Saintrain (talk) 20:23, 29 August 2013 (UTC)[reply]

Validity of employment tests

Hi all, I recently took some numeracy/reasoning tests for an application for a internal job application. I was quite surprised that I would have to take these tests due to the fact I have been doing a similar job for nearly 2 years and since I have a first class maths degree I feel I shouldn't really have to demonstrate that I am good at maths by jumping through some hoops. I was even more surprised to hear that they actually use these tests to gauge mathematical/reasoning ability (superceding qualifications). Ranting aside, I was wondering whether anyone has done proper scientific studies to gauge whether these tests actually accurately measure what they claim to? Or if they actually are in any way an accurate indicator of future job performance? I remember reading somewhere that Google had abandoned these tests since good scores were actually correlated with poor future job performance. Thanks! 80.254.147.164 (talk) 16:06, 27 August 2013 (UTC)[reply]

Here's three relevant science refs [18], [19], [20], ranging from 2008(first) to 1981 (last). The abstract of the most recent says:

“

The authors review criticisms commonly leveled against cognitively loaded tests used for employment and higher education admissions decisions, with a focus on large-scale databases and meta-analytic evidence. They conclude that (a) tests of developed abilities are generally valid for their intended uses in predicting a wide variety of aspects of short-term and long-term academic and job performance, (b) validity is not an artifact of socioeconomic status, (c) coaching is not a major determinant of test performance, (d) tests do not generally exhibit bias by underpredicting the performance of minority group members, and (e) test-taking motivational mechanisms are not major determinants of test performance in these high-stakes settings.

”

Here's some news/blog coverage of Google's thoughts on the topic [21], [22]. As you recall, Google VPs are pessimistic about the value of such tests. However, the authors of the paper above conclude that tests are generally valid predictors of performance. There are many scientists at Google, but I'm not sure how scientific their HR analysis is in this case. Hope that helps, SemanticMantis (talk) 20:46, 27 August 2013 (UTC)[reply]

The recent Google policy decision was that brain-teaser style questions - which are categorically not standardized tests - were poor predictors of candidate quality. Brain teaser questions test creativity, problem-solving, and out-of-the-box thinking; and candidates' ability to successfully answer such questions did not correlate to the candidate's ability to develop into a high quality, productive employee. On the other hand, standardized test questions - tests that ask questions about word-analogy and vocabulary, and tests that measure candidates' ability to correctly solve a large number of simple arithmetic calculations - correlate very well to later job performance. Perhaps this is surprising, perhaps not.

It is my opinion that many detractors do not like standardized testing because it is so objective; but it is not generally accurate to criticize standardized tests for being ineffective. These tests are specifically designed to correlate to specific performance capabilites. Modern tests are continually evolved by psychologists, using the scientific method, to improve the test's predictive power. Nimur (talk) 20:54, 27 August 2013 (UTC)[reply]

OP here. As usual the Wikipedia science reference desk has excelled itself. Referenced, scientific and complete answers, thankyou! 80.254.147.164 (talk) 08:59, 28 August 2013 (UTC)[reply]

As for using a math degree to predict success, the problems there are numerous, as standards may vary by institution, or within each institution over time and by instructor. So, you achieving a certain GPA by no means can be taken as evidence that somebody else with the same GPA in the same course of study possesses equivalent skills. StuRat (talk) 12:00, 28 August 2013 (UTC)[reply]

Google's style of interviewing with those "brain teasers" is one that we use where I work (and interview) when interviewing people where creativity and problem solving is a big part of the job. The idea isn't to predict anything using the score on the test. It's a way for a good interviewer to see how the candidate goes about solving them. We always start out the interview by explaining that to the candidate: "We aren't interested in whether you get the answer right - we're only interested in the approach you take."...and I think it's valuable. It's very instructive to see whether the candidate slowly and methodically works through all of the possibilities - or whether they take wild stabs and try things out - or whether they switch strategies when one doesn't produce results...whether they try to "game the system"...whether they ask questions of the interviewer. Some of out problems have loopholes and ambiguities built into them specifically to try to provoke those kinds of responses. They are tools to allow the interviewer to see the candidate work - they aren't test score numbers to punch into a spreadsheet.

A classic Google-style question is this one: "You've been magically shrunk until you're one centimeter tall and dumped inside a blender - the switch is going to be turned on in 30 seconds. QUICK! What do you do?!"...clearly there is no right answer here. One of ours (which we've now retired because too many people already know it) is: You have eight balls which look identical, one is slightly heavier than the others, using nothing but a balance beam with two scale pans, figure out which one it is using the least number of weighings. Most computer programmers jump to the answer '3' - we tell them that the correct answer is '2' and invite them to tell us how to do it. Remember - we don't care whether you solve it or not - it literally doesn't matter.

The problem (as I've heard it from Google employees) is not with the questions - but with the interviewers. It takes a certain amount of skill to extract the information about how creative a person is from those questions. If you have to interview a lot of people - it's tough to maintain a standard. Numerical score tests are easier to administer.

In our case, we use questions that have "right answers" but are not easy to solve under pressure. Many people get jobs with us after getting entirely the wrong answers - and some people who get the answers right don't get jobs here because we don't like the way they solved them. We also judge people on how well we think they'll fit into a particular team that we're putting together - maybe we already have a wild lateral-thinker in the team - and we want a methodical thinker instead...but maybe it's the other way around. Also, the "problem solving" part is only the second of three interviews - the first is a chatty "getting to know you" thing and the third is a more hard-core computer programming test (find the bugs in the following 10 programs - write me a program to solve some simple problem - describe an algorithm to do such-and-such...that kind of thing). SteveBaker (talk) 14:21, 28 August 2013 (UTC)[reply]

August 28

recharge standard batteries?

Can batteries that are not designed to be rechargable be effectively recharged in a recharger? Bubba73 ^{You talkin' to me?} 00:56, 28 August 2013 (UTC)[reply]

No. And it might be dangerous to try. Looie496 (talk) 02:13, 28 August 2013 (UTC)[reply]

Thank you. Bubba73 ^{You talkin' to me?} 02:48, 28 August 2013 (UTC)[reply]

Resolved

The above answer is incomplete. You can't recharge them with a standard charger, but it is possible with the right equipment. We have an article on this: Recharging alkaline batteries, although the information in it is a bit limited.--Srleffler (talk) 03:42, 28 August 2013 (UTC)[reply]

I suspect both our article and the one non peer reviewed source are being a bit liberal with the word "recharge". In general non rechargable batteries produce power using non reversible chemistry (the first step is reversible but the products are absorbed irreversibly by the carbon slush. I suspect what these chargers are doing (with a claim to get 25% of the original charge or similar once only) is not recharging but recovering more of the original charge. If you have a battery in an application which requires close to full power it stops working when it still contains energy. Putting a DC through it, or leaving it in the sun for an hour, might help with this but it is not technically "recharging". It also might be dangerous--BozMo talk 06:02, 28 August 2013 (UTC)[reply]

Yes, "recharge" is not the correct technical term for this process, but I don't remember the correct one. I think there is more to it than just recovering the original charge, but it has been many years since I learned about it and I don't remember the details. --Srleffler (talk) 04:13, 29 August 2013 (UTC)[reply]

To be clear - using a standard recharger (ie one designed to recharge NiCd or Lion batteries) puts energy into a non-rechargeable alkaline battery without recharging it. With no place to go, that energy turns into heat. The battery gets very hot, and pretty soon, the liquids inside boil. That produces an enormous pressure spike and the battery explodes - splattering sharp bits of metal casing and hot, nasty chemicals in all directions! So DON'T DO THAT!

Several dubious companies and a bunch of well-meaning amateur nut-jobs have come out with so-called "rechargers" for alkaline batteries. I'm not sure what chemistry they are relying on - but they don't work very well. They can't completely recharge the battery - and the battery only survives one or two of these "recharge" cycles before going dead forever. I strongly suspect (without evidence) that User:BozMo is correct about the process merely releasing the final charge that was already inside the battery. One could imagine that by creating heat inside that's enough to cause the contents of the battery to flow more easily (but not become hot enough to boil and explode)...then, perhaps convection currents would slowly stir up the chemicals and bring material that had not yet released it's charge into the main mixture - thereby resurrecting the battery for a little while longer. That's not "recharging" - but it might seem that way. But these rechargers need some sophistication - controlling the voltage and current over time and monitoring the temperature of the batteries. That makes them expensive (or dangerous!) and it would require a heck of a lot of slightly-recharged batteries to pay for the cost of the equipment. Certainly they take a long time to do what they do - and they don't do it very well.

Bottom line is that using regular rechargers with alkaline batteries is VERY DANGEROUS and doesn't work - and the dubious-quality specialist alkaline battery "rechargers" are expensive and don't work well enough to justify buying one. So the simplest answer to this question is "No!". SteveBaker (talk) 13:50, 28 August 2013 (UTC)[reply]

Thanks, I was asking about a standard battery charger designed to work on rechargable batteries. Bubba73 ^{You talkin' to me?} 14:10, 28 August 2013 (UTC)[reply]

Yeah - DON'T DO THAT! SteveBaker (talk) 03:30, 29 August 2013 (UTC)[reply]

I wonder how many of the posters above have actually tried using a charger specially designed for recharging alkalines. I have. It works. It works well. Batteries do not explode or get too hot in the charger. Why dont you people believe what our article says?86.181.30.8 (talk) 13:59, 29 August 2013 (UTC)[reply]

Probably because it isn't a good article. There is only one source cited actually on the topic - and that is a review of a single charger on a 'photography community' website. I'm wondering whether the article should be deleted... AndyTheGrump (talk) 14:11, 29 August 2013 (UTC)[reply]

How much would we suffer if Earth was overcast?

Change reality the minimum possible to make it cloudy 24/7, or at least 90 percent coverage. For the purposes of this I'll consider the sun never being uncomfortably bright to stare - or invisible, to be overcast. Sagittarian Milky Way (talk) 03:10, 28 August 2013 (UTC)[reply]

What if we could choose the type (stratus, cumulonimbus etc.), or even distribute those by latitude or even sun altitude (-90 to +90 degrees) Sagittarian Milky Way (talk) 03:19, 28 August 2013 (UTC)[reply]

See Year Without a Summer, except that your scenario would be quite a bit worse. Looie496 (talk) 03:35, 28 August 2013 (UTC)[reply]

Would it be as bad as Nuclear winter? 24.23.196.85 (talk) 06:49, 28 August 2013 (UTC)[reply]

I've actually considered this Q before. If the Earth were permanently overcast, we wouldn't know about stars or planets. We would have noticed day and night, though, and possibly noticed the cycles of the moon, not by observing it directly but by noticing the slight change in the light level of the overcast sky on certain nights, and then matching that to the tides. We also wouldn't have shadows cast by the Sun, making it much more difficult to calculate the diameter of the Earth. So, astronomy and geography would suffer quite a bit. We would eventually figure out the diameter of the Earth, after sailing around it, and would eventually see the stars, after we made planes that could fly above the clouds. Then we could begin forming all the astronomical models that in our world happened over thousands of years, based on high altitude observations. So, I think we'd eventually get to the same place we are now, it would just take a bit longer.

If you take it a bit further and imagine a world always covered in ground-level fog (ignoring how plants would get enough light to grow), then flying planes visually would be impossible, as would sailing ships. We wouldn't be able to get from one disconnected continent to another, and would have little reason to think the world was round. The invention of radar would finally allow for airplanes and sea voyages, and from there we would begin the discovery process described above. StuRat (talk) 11:48, 28 August 2013 (UTC)[reply]

Well, if we're talking historically then it wouldn't really matter. "Overcast" simply means that the sky is opaque to visible light via scattering of particles (water droplets, dust, soot, etc..) suspended/dispersed in the atmosphere. If for whatever reason the atmosphere were never transparent to visible light in the first place, It's likely we'd simply have wound up with eyes that were sensitive to wavelengths where the effect of scattering is minimal. That's my $0.02 anyway. Scattering intensity depends strongly on particle size and wavelength. (+)H₃N-Protein\Chemist-CO₂(-) 12:19, 28 August 2013 (UTC)[reply]

Overcast means opaque to visible light ? That would mean a black sky. More like translucent, I think, where visible light would still be useful. StuRat (talk) 12:23, 28 August 2013 (UTC)[reply]

Translucency is basically just light scattering. A thick enough layer of suspended particles in the atmosphere would substantially reduce the transmission of visible light via scattering, and thus make the sky more opaque (no need to invoke 100% opacity here). Since there's a strong angular dependence to the intensity of scattered light, and since you can't exactly change the scattering angle in this scenario, you'd be losing a fair amount of light. It depends somewhat on the size of the particles, but anything big enough to obscure light would work here, I'd expect absorbance to be a relatively minor component in all this. (+)H₃N-Protein\Chemist-CO₂(-) 03:35, 29 August 2013 (UTC)[reply]

I believe "opaque", when unqualified, means 100% opaque: [23]. It's like when you say a door is "closed". That means 100% closed, unless you qualify it with "partially closed", "50% closed", etc. Of course, in both cases, "100%" is actually "almost 100%". That is, a photon or two can pass through almost anything, and there are gaps around any door. StuRat (talk) 06:32, 29 August 2013 (UTC)[reply]

I don't really see why I would use the colloquial definition of "opaque" when I was clearly discussing optics. (+)H₃N-Protein\Chemist-CO₂(-) 10:27, 29 August 2013 (UTC)[reply]

Knowing how and why the earth is so overcast makes a big difference - we can't ignore it.

If it's surface-level water-vapor cloud - then the increase in albedo of the planet would turn it into an ice-ball pretty quickly - the oceans would freeze and life would either die out - or never develop. However, a surface level water-vapor cloud wouldn't exist in that kind of cold either - so the sky would clear up before that happened.
If it's a high-altitude water-vapor cloud then it would cause a rapid onset of very severe global warming because water vapor is a much nastier greenhouse gas than CO2 - and then the oceans would boil and the planet would turn into another Venus.
I suppose a very carefully balanced cloud layer somewhere between those two extremes might keep the planet habitable - but it would be a highly unstable equilibrium - so it would likely flip one way or the other within not too many years (eg A large volcano might add enough material to flip it one way or the other).

Of course you can imagine clouds made of other stuff - but things get complicated to imagine under those circumstances. SteveBaker (talk) 13:31, 28 August 2013 (UTC)[reply]

Article on Pharm D

Your outline of the Doctor of Pharmacy is misleading in its outline for education, It does not require 2-4 years + 4 years, The Pharm. D is the entry level for pharmacy requiring the completion of 6 years entered directly from high school. It replaced the Bachelors of Science in Pharmacy, a five year program, with a 6 year program, It is not a graduate degree it is a professional degree. — Preceding unsigned comment added by 76.19.174.161 (talk) 05:59, 28 August 2013 (UTC)[reply]

You can make these changes to the article yourself; please include citations to reliable sources OldTimeNESter (talk) 13:09, 28 August 2013 (UTC).[reply]

What determines a planet's angle of rotation?

Exactly as the question states. I'm curious as to what factors come into play for determining the angle of a planet's rotation, and why we can see such a wide spectrum of angles, as in the case of say Earth versus Uranus.

Martyk7 (talk) 07:00, 28 August 2013 (UTC)[reply]

Second paragraph at Planet#Rotation has some information which might keep you occupied until someone comes along wit more detail. 163.202.48.125 (talk) 08:13, 28 August 2013 (UTC)[reply]

In theory they should all start out rotating in the plane of the ecliptic. However, collisions with random sub-planets would soon change the rotational axis. In the case of Earth, for example, one theory of the Moon's formation holds that a giant impact knocked the material loose from Earth which then formed the Moon. Such an impact would also knock the Earth off it's original axis. You might think such impacts would be rare, but in the early solar system, they were probably quite common. They only became rare after most of those objects either impacted somewhere, fell into the Sun, fell into a stable orbit, or left the solar system. Looking at all the craters on the Moon, you can get an idea for how many impacts there have been. The Earth would have far more impacts, but erosion here covers most of them up. StuRat (talk) 11:35, 28 August 2013 (UTC)[reply]

Thanks, that's exactly what I was looking for. Martyk7 (talk) 21:53, 29 August 2013 (UTC)[reply]

Black hole question

Escape velocity is the speed at which an object can leave another's abject gravity WITHOUT further propulsion. So, if someone could go near a black hole with a rocket propulsion, especially if the black hole is a supermassive black hole with reasonable surface gravity, he could enter and get out from event horizon. So, why is ever written that is impossible to escape a black hole, while there is that possibility? I know that isn,t an easy question, but thanks for answering. Francesco. — Preceding unsigned comment added by 95.239.192.85 (talk) 10:00, 28 August 2013 (UTC)[reply]

The escape velocity from inside the event horizon is more than c. 163.202.48.126 (talk) 10:44, 28 August 2013 (UTC)[reply]

Other strange things happen at the event horizon, such as infinite gravitational red shift. This results in time slowing to a stop from the point of view of the outside world. This means that if you went to the event horizon and survived, you would never return in the life of the universe. And in case you did not get the point of 163.202.48.126, your rocket can never accelerate you to the speed of light, so you will never escape. Graeme Bartlett (talk) 10:58, 28 August 2013 (UTC)[reply]

Yes, but the rocket can, as happens with space aircraft, get me outside the event horizon where the escape velocity is less than c; if the surface gravity of a black hole is, for example 10 m/s" (as that of earth) ten meters under is howewer similar and this isn't difficult to get out; the surface gravity is infinite on the singularity, not on event horizon... — Preceding unsigned comment added by 95.239.192.85 (talk) 11:18, 28 August 2013 (UTC)[reply]

The gravity at the event horizon of a black hole is a heck of a lot more than that. However, a rocket flying outside the event horizon would be able to escape the black hole at distances where a rock with no propulsion, of the same mass, would fall in. StuRat (talk) 11:29, 28 August 2013 (UTC)[reply]

One of the many problems is (as others have pointed out) that relativistic time dilation is caused by gravitational fields - so as you approach the event horizon from outside, from your perspective, the rest of the universe would appear to go into "fast-forward" - and as the gravity increases, the rest of the universe starts to change faster and faster. At the exact boundary of the black hole, time in the rest of the universe is running infinitely fast - which means that whatever ultimate fate befalls the universe, happens instantly. You can't "escape" because from the perspective of the rest of the universe, time for you has stopped and you're frozen forever on the event horizon. SteveBaker (talk) 13:20, 28 August 2013 (UTC)[reply]

That is true only if you stay around very close to the event horizon. If you are free-falling through the horizon, the universe outside doesn't appear running infinitely fast. At least according to General Relativity (if it's true, nobody will ever be able to report to the outside, so everybody who wants to know has to fall into the black hole). Icek (talk) 14:31, 28 August 2013 (UTC)[reply]

I know, but my point is: as happens with space shuttle, that don't need earth's escape velocity of 11,2 km/s to escape earth's gravity but reaches with rocket engine higher quotas with less gravity and can escape with minor speed (generally 7-8 km/s), so a little under event horizon, far from singularity, the surface gravity is finite and so is possible WITH EXTERNAL PROPULSION get out from event horizon and reaches distance when is possible to escape. The escape velocity is a limit only without further propulsion; with constant pull each speed can be sufficient.95.239.192.85 (talk) 13:22, 28 August 2013 (UTC)[reply]

No. The event horizon is by definition, the point (actually boundary) of no return. As the article states, it is "the point at which the gravitational pull becomes so great as to make escape impossible." You could never go fast enough no matter how many rockets you had. Clarityfiend (talk) 13:35, 28 August 2013 (UTC)[reply]

Yes, but why? So, ten meters over the surface of a very supermassive black hole the surface gravity is reasonable (it can be even near that of earth if the black hole is very large) and ten meters under becames so strong to be impossible to resist it with even very strong aceleration? This seems to me very strange. The surface gravity on event horizon is finite or infinite?95.239.192.85 (talk) 13:46, 28 August 2013 (UTC)[reply]

Ten meters above the event horizon of a supermassive black hole, the effect of gravity is still extremely strong. A supermassive black hole does not at all have anything close to a "reasonable surface gravity", either a little above or a little below the event horizon. I'm not sure where you got the impression that a supermassive black hole can have surface gravity similar to that of Earth. A supermassive black hole can have a density that's less than that of Earth, but that's a different matter. Red Act (talk) 14:22, 28 August 2013 (UTC)[reply]

The gravitational acceleration can be made arbitrarily small by making the black hole large enough. Icek (talk) 14:31, 28 August 2013 (UTC)[reply]

Indeed; the concept of 'surface gravity' gets really weird when you're near a black hole's event horizon – see surface gravity#Surface gravity of a black hole – and actually varies inversely with the black hole's mass. TenOfAllTrades(talk) 14:39, 28 August 2013 (UTC)[reply]

If you approach the event horizon (from the outside of course) very slowly, braking with your rocket engine, then, as SteveBaker pointed out, you will see the universe running at a faster pace, which also means all kind of things will fall into the black hole and onto yourself within a short time (as seen from your perspective); it's more or less like you are traveling at a very large speed (close to the speed of light) relative to nearby galaxies while still being not in the immediate vicinity of large masses. And the equivalent to being at rest at a large distance from the black hole is more or less freely falling into the black hole.

In order to have a more complete picture, you have to consider space-time geometry (details are in the article Schwarzschild metric). Before anything else, you need to know what the coordinates actually mean: You can measure the circumference of a circle around the black hole at a constant distance from the center. The radial coordinate r is the radius that this circle would have in flat space. And the time coordinate is the time that an observer would measure very far away from the black hole. If you now look at the coefficients of the time coordinate differential squared (dt²) and the radial coordinate differential squared (dr²), you should notice that these coefficients switch signs at r = r_S (the Schwarzschild radius). That means that inside the event horizon, the radial direction effectively isn't a spatial direction anymore, but it's a time-like direction. I.e. it is as impossible to go to a larger r as it is impossible to travel to the past.

Icek (talk) 14:31, 28 August 2013 (UTC)[reply]

Question: So the only "direction" you can point your hpothetical rocket is the future - towards the singularity? 163.202.48.125 (talk) 14:51, 28 August 2013 (UTC)[reply]

Are you pointing rockets in a "future" direction on the surface of Earth? If you cannot do that, then you cannot do that inside a black hole either. Space there has 2 "circular" dimensions along the surface of an imagined sphere around the singularity, and 1 other dimension (corresponding to the Schwarzschild time coordinate). You can move in this 3-dimensional space. But note that the size of the 2 closed (circular) dimensions becomes smaller as time passes (proper time that is measured by your clock, as opposed to coordinate time). Icek (talk) 16:00, 28 August 2013 (UTC)[reply]

Your explanation is interesting. So the black hole can't be leaved because space-time. Howewer, to answer to other utents: what matters in escaping, if you have a rocket, isn't the speed but the acceleration you reach: if the surface gravity is 10 km/s^2 and your acceleration is 11 km/s^2 you can win gravity and escape (mantaining that acceleration, obviously) no matter how fast you go and how high could be escape velocity. For this reason i said that could be theoretically possible to escpape a black hole, especially if very large (10^10-10^12 solar masses) — Preceding unsigned comment added by 95.239.192.85 (talk) 14:55, 28 August 2013 (UTC)[reply]

No, you cannot. You need to get the energy for acceleration from somewhere. And the potential energy for lifting an object out of a black hole is greater than the mass-equivalent of the object. So even if you successively convert all your object into energy and use it to accelerate the rest, you will not be able to lift it out of the black hole. Indeed, whenever lift anything with a reaction drive out of a gravity field, you want to accelerate as fast as possible - otherwise you waste energy simply by lifting fuel. And if you drive that to the limit, you want to accelerate your ship immediately to c (you cannot do better), and we know that that is not enough to escape the black hole. --Stephan Schulz (talk) 15:38, 28 August 2013 (UTC)[reply]

Before thinking about acceleration, think about velocity (and let's stay on radial paths for the sake of simplicity). You might think of dr/dt (how much the radial coordinate of an object changes as its time coordinate changes by one (sufficiently small) unit) as the velocity, but that is only a kind of coordinate velocity. In the Schwarzschild metric, outside the event horizon, this coordinate velocity is limited to

c(1-{\frac {r_{S}}{r}})

in the same way that it is limited to c in flat spacetime.

And if you accelerate with your rocket engine near the event horizon, the acceleration you measured with the same rocket configuration in flat spacetime is now proper acceleration. Already in flat spacetime, proper acceleration a' of an object moving with respect to the observer is not equal to coordinate acceleration a (otherwise you could simply accelerate past the speed c), but it's

a'={\frac {a}{(1-{\frac {v^{2}}{c^{2}}})^{\frac {3}{2}}}}

(if the acceleration is along the same direction as the velocity v)

So I hope you can imagine (or even calculate by a few manipulations of the Schwarzschild metric equations) that an acceleration a bit larger than the gravitational acceleration isn't going to give you the speed you might have hoped for, and therefore the transition from above to below the event horizon isn't as discontinuous as it might have seemed.

Icek (talk) 16:00, 28 August 2013 (UTC)[reply]

What the OP needs to understand is that when you are inside of a black hole, even if you are moving outwards (so to speak) at the speed of light, you're are still moving towards the center. There is no path out. Dauto (talk) 16:05, 28 August 2013 (UTC)[reply]

Look at that Penrose diagram. The center of the black hole is the red line, the black hole is the triangle next to the red line and the rest of the diagram represents the whole universe outside of the black hole. Objects traveling at the speed of light follow diagonal lines at 45 degrees. If you're inside of the hole, no matter how fast you're going "outwards" you will still eventually reach the center of the hole. Dauto (talk) 16:16, 28 August 2013 (UTC)[reply]

As pointed out, the term "surface gravity" has problems in that I presume you're thinking of that term in the Newtonian sense, but the Newtonian concept of surface gravity becomes meaningless at a black hole's event horizon. As TenOfAllTrades pointed out, there does exist an alternative definition of the phrase "surface gravity", which is basically the Newtonian surface gravity renormalized by multiplying it by the gravitational redshift factor, and that renormalized value does go to zero as M increases. However, that's very different from there being something close to a Newtonian surface gravity that's close to zero close to an event horizon.

Avoiding the problematic term "surface gravity", and being super pedantic in order to be precise, the proper acceleration required to keep you at an r Schwarzchild coordinate that's ten meters greater than the black hole's Schwarzchild radius (which is at the event horizon) increases to infinity as the black hole's size increases to infinity, i.e., as the black hole's mass increases to infinity. Icek's statement at 14:31 that the acceleration can be made smaller by increasing the black hole's size is incorrect. An acceleration vastly greater than 10 m/s² is required to keep you at an r Schwarzchild coordinate that's ten meters greater than the Schwarzchild radius even with a black hole with a mass about that a star, and instead dealing with a supermassive black hole only makes the required acceleration much larger, not smaller. Red Act (talk) 16:47, 28 August 2013 (UTC)[reply]

Whoops. According to the top equation here and doing a little math, the proper acceleration required to stay a fixed δr above the event horizon only increases with increasing R_s up to R_s = 2 δr, beyond which contrary to my expectation it decreases. So I struck out my post, which apparently is incorrect. Red Act (talk) 18:54, 28 August 2013 (UTC)[reply]

So the surface gravity is infinite not only on the singularity, but even on the event horizon? Why (I believed only singularity had an infinite surface gravity)?95.239.192.85 ([[User talk: 95.239.192.85|talk]]) 16:58, 28 August 2013 (UTC)[reply]

What you said earlier was correct. The surface gravity is inversely proportional to the mass. But that doesn't mean that you can hover at the surface by accelerating outwards, and that's what Red Act was explaining. Your difficulty understanding the difference comes from the fact that you're imagining a black hole to be an object embedded in a Euclidean space (That's the most natural thing to do, but it is wrong). All it means is that if you fall into the black hole you will find yourself accelerating towards the center at that finite acceleration. If you try to hover by blasting your rocket engines, you will still find yourself approaching the center, though a bit more slowly. You cannot simply subtract one acceleration from the other since you are not in a Euclidean space. Dauto (talk) 17:32, 28 August 2013 (UTC)[reply]

It's worse than that; the longest (in proper time) worldlines there are the geodesics. Accelerating (even "outwards") gets you there faster, at least in your estimation. I'm not sure what it does in terms of "outside time" — whether, for instance, accelerating would allow you to rendezvous with a colleague who fell in shortly after you. --Tardis (talk) 13:07, 29 August 2013 (UTC)[reply]

What the OP needs to realize is that a speed limit is hard to break when you can't go any faster. You just simply can't get up to the speed to go out, even though your jets are blasting away - all they're really doing is increasing your relativistic mass and various other weird relativity parameters at that point. Wnt (talk) 18:55, 28 August 2013 (UTC)[reply]

The point of the question is that it is possible to leave a planet without ever reaching its scape velocity, so why can't one leave a B-hole without ever reaching c? Answer: The whole idea that c is the scape velocity of a B-hole is wrong to begin with. It's an oversimplification. Dauto (talk) 19:01, 28 August 2013 (UTC)[reply]

So, if I understood, the space time on the event horizon is so curved that you could only go into black hole and in every direction you move get into it. Another question: on object when reaches event horizon it's said that stops to esternal observer, but this is only an optical illusion, because black holes in reality increase their mass by absorbing other stars that even to esternal viewers as we are, fall into black holes (even to our proper time, black holes increase their mass, not only to objects that fall into).95.239.192.85 (talk) 00:51, 29 August 2013 (UTC)[reply]

Yes, all directions lead to the center, but the horizon itself doesn't have to be particularly curved. Curvature is proportional to surface gravity. Being a B-hole is a global property, not a local property. There is nothing particularly special about the horizon itself. We might be crossing some gigantic B-hole horizon right now as we speak (type) and not know it. Dauto (talk) 12:15, 29 August 2013 (UTC)[reply]

Enthalpy of formation

I'm trying to calculate the enthalpy of formation for molecular lithium hydride, and I'm coming up with an incorrect answer. What am I missing?

Molar enthalpy of fusion of lithium
Molar enthalpy of vaporisation of lithium
Half molar bond-dissociation enthalpy of hydrogen
Negative molar bond-dissociation enthalpy of lithium hydride

I should get ~141 kJ mol⁻¹, but my answer is >300 kJ mol⁻¹. Plasmic Physics (talk) 14:29, 28 August 2013 (UTC)[reply]

What is your source for your numbers so we can check the data? --Jayron32 14:48, 28 August 2013 (UTC)[reply]

Lithium
' '
Bond-dissociation energy
[24] (Table 2, D_e for X ¹Σ⁺) Plasmic Physics (talk) 23:09, 28 August 2013 (UTC)[reply]

It's been a long while since I've done somehing like this, but does the fact that lithium hydride is solid at STP change the answer at all? It seems like the enthalpy of fusion/vaporisation (which will have to be theoretical, since LiH decomposes before it vaporises) should "give some energy back" MChesterMC (talk) 08:29, 29 August 2013 (UTC)[reply]

I'm not trying to find the standard enthalpy of formation. Molecular LiH does actually exist. Plasmic Physics (talk) 22:24, 29 August 2013 (UTC)[reply]

Rope stretch

In rock climbing (a sport which I have recently taken up) there are two types of nylon kernmantle rope generally used. Dynamic rope has significant elasticity to act as a shock absorber and reduce peak impact forces during a fall (they will typically stretch around 30-40% of their length during a simulated fall of an 80kg load a distance 1.77x the length of the rope). "Static" rope on the other hand has much less stretch and is a better choice for abseiling (rappelling) and hauling loads since dynamic rope can end up rather annoyingly bouncy in these applications.

My question is: what is the difference between static and dynamic climbing rope which results in the difference in elasticity - is it different monomers, a different polymer chain length, a different weave pattern or something else? Thanks, Equisetum (talk | contributions) 15:11, 28 August 2013 (UTC).[reply]

It is perfectly possible to make a high-stretch dynamic rope and a low-stretch static rope out of the same materials. In the dynamic rope, the sheath is braided, and the core is filled with loosely twisted fibers, which are not themselves laid together, like this:[25], which allows them to slip against eachother, and against the sheath. After a drop, the sheath may be the only structural part remaining intact: the energy of the fall is absorbed in part by the friction of the core fibers pulling apart (this is why you don't use dynamic rope to bear human loads after it's been used to break a big fall). For a a static rope, (which may or may not have a braided sheath), the core is composed of fibers, laid into yarn, which is in turn laid into a solid, twisted cord, like this:[26], or in some cases a solid braid(see other pics in previous link). For actual rope products on the market, different fibers may be used, but the principle differences in performance of dynamic vs. static cordage are due to the structural properties of how the fibers are arranged. Of course, if you have spare sections of each, just do some dissecting! SemanticMantis (talk) 15:31, 28 August 2013 (UTC)[reply]

Update, if you read all of rope carefully, it says that dynamic kernmantle has the core fibers chopped into shorter sections, and the static kernmantle has both longer fibers, and less twist on the core. A conventionally laid or braided rope of the same fiber would have intermediate stretch, between the dynamic and static. SemanticMantis (talk) 15:37, 28 August 2013 (UTC)[reply]

Excellent answers, thank you. I should have done some dissection since I just retired my father's old dynamic rope and had some offcuts of static cord I used for re-slinging some nuts, but then, these were from different manufacturers and one was new and the other 20 years old (don't worry - it hasn't been used for over 10 years, I know that climbing ropes have limited lifespan), so I would hardly be controlling my variables very well! Equisetum (talk | contributions) 16:05, 28 August 2013 (UTC)[reply]

Homophobia

Is finding homosexual sex (or more broadly a kind of sex that does not fit one's sexuality) "icky" a learned behavior or is it an inherent part of human behavior. personally I don't find any kind of sex icky, including those kinds that don't fit my sexuality, though I don't find them attractive either. But many other people seem to genuinely disgusted by it even though they are not homophobes (or heterophobes?)--Irrational number (talk) 16:18, 28 August 2013 (UTC)[reply]

It's natural behavior, what happens is that a straight person will imagine him/herself to be in the position of the gay person and then judges that to be icky, obviously this is caused by this imagined scenario not being consistent with his/her own sexual preference. Count Iblis (talk) 16:36, 28 August 2013 (UTC)[reply]

Source for that? This is a complicated matter not helped by the fact that sexuality is something of a spectrum. 163.202.48.125 (talk) 16:41, 28 August 2013 (UTC)[reply]

Not to mention that homosexuality has been very differently treated in different societies (see e.g. the uber-macho Sacred Band of Thebes), and that at least in societies with general sexual taboos, many people often find all kind of sex "icky" - from kids to stereotypical "spinsters" to Lady Hillingdon of "Lie Back and Think of England" fame. --Stephan Schulz (talk) 17:43, 28 August 2013 (UTC)[reply]

The elephant in the room here is that when people think of straight sex, they tend to think of a penis penetrating a vagina, and when they think of gay male sex, they tend to think of a penis penetrating an anus. We know what comes out of anuses, and that's where the ickiness factor comes in. Well, just as penis-vagina contact is SO NOT the only kind of straight sex, penis-anus contact is SO NOT the only kind of gay male sex. Oh, I've had many discussions with people who say that these other activities are not really "sex", they're just foreplay or "fooling around" or whatever one does in the lead-up to actual sex. That is just so much garbage. So, we need to be really clear what we're talking about when we say "sex". -- Jack of Oz ^{[pleasantries]} 21:28, 28 August 2013 (UTC)[reply]

There's a certain amount of "ickiness" in sex in general, although "messiness" might be the better way to put it. It reminds me of an extremely old joke from Woody Allen: "Q: Is sex dirty? A: Only if you're doing it right." ←Baseball Bugs ^{What's up, Doc?} carrots→ 22:17, 28 August 2013 (UTC)[reply]

This may be of some interest: Disgust#Evolutionary_significance, also Evolution_of_morality#Disgust. If I recall, I think Richard Dawkins or similar did a program about where disgust comes from. The analogy used in it was that the thought of two ugly people having sex gives one a feeling of disgust but rationally one can see that the disgust mechanism doesn't indicate whether something is right or wrong, IRWolfie- (talk) 22:31, 28 August 2013 (UTC)[reply]

@ Jack above, a better solution than such clarity might be a boycott on answering any heterosexuals' questions on gay sex until they have tried it first. Kind of like the homework rule, above, "we'll get you over the stuck part". μηδείς (talk) 00:53, 29 August 2013 (UTC)[reply]

Yuck Medeis, did you have to choose those words? - ¡Ouch! (^{hurt me} / _{more pain}) 07:45, 29 August 2013 (UTC)[reply]

I thought you liked pain. Be careful of what you ask for. You'll get it, but it may not be exactly what you had in mind. -- Jack of Oz ^{[pleasantries]} 08:27, 29 August 2013 (UTC) [reply]

I don't have the answer, but I don't think "picturing it" explains it- even if picturing it causes some to feel disgust, it obviously doesn't in others. Moreover, lot's of men enjoy anal sex with a female partner and, yet, find the imagery of it with another male disgusting; and the same goes for other sex acts they would enjoy with a woman. Finally, I, personally, think it all seems pretty hot to picture, and to do, (either gender) and have felt this way since a young age- By numbers, I would definitely be labeled as straight, and, while this is all anecdotal, I can't imagine my perspective/experience is uncommon to the point of anomaly. In other words, "picturing it" causing disgust in some is what needs explained, not an explanation.Phoenixia1177 (talk) 03:44, 29 August 2013 (UTC)[reply]

I still don't have an answer for you, but you may find our articles: Homosexuality and psychology, Biology and sexual orientation, Societal attitudes toward homosexuality, Sexual norm, Heteronormativity, and Homophobia as a good place to start; especially the portions talking about societal attitudes. I'll be honest, this is one of those areas that's a giant morass of studies, politics, and sociology- in other words, it's gigantic and half of it's science, half of it's philosophy, half of it's pedantry, and half of it's bullshit (not exclusive halves:-) ). Because of my last sentence I'm not going to attempt any type of summation, better just to read through the articles and see if they takes you somewhere with a satisfying answer.Phoenixia1177 (talk) 09:40, 29 August 2013 (UTC)[reply]

Weather and sleep

So I live in Tehran and as you can tell it has a very polluted air. And I usually feel tired when I wake up, even when I wake up spontaneously (without an alarm clock) and I sleep in the normal time interval people sleep. And I found out that in the other cities that I've been to, although they are in very different locations of the country, I sleep very well, independent of the time interval, and I wake up feeling fresh, I wonder what is this related to... --Irrational number (talk) 16:23, 28 August 2013 (UTC)[reply]

Might be related to the fact that big cities suck? Dauto (talk) 16:28, 28 August 2013 (UTC)[reply]

Perhaps also noise perturbing sleep? Count Iblis (talk) 16:30, 28 August 2013 (UTC)[reply]

can't be it my room is quiet.--Irrational number (talk) 16:47, 28 August 2013 (UTC)[reply]

If your room isn't very dark, then that could also perturb your sleep. But if it really has to do with the polluted air, that would suggest that you may have apnea due to the air causing breathing problems that you may not notice when you are awake. Then it's best to see a doctor to get a sleep test done and test for allergies, asthma etc. Count Iblis (talk) 16:58, 28 August 2013 (UTC)[reply]

We may be straying into MedAdvice land here.. 163.202.48.125 (talk) 16:42, 28 August 2013 (UTC)[reply]

AW COME ON! let me ask it in a different way "does the weather of the place you sleep in affect the quality of your sleep in any way"? Is that better? I'm just curious about something!--Irrational number (talk) 16:47, 28 August 2013 (UTC)[reply]

From an anecdotal level (but confirmed by many), yes, absolutely. If it is too hot at night, I sleep badly. Somewhat surprisingly, heat during the day does not seem to be similarly bad (but maybe I'm just more tired because of little sleep at night ;-). --Stephan Schulz (talk) 17:45, 28 August 2013 (UTC)[reply]

The Med Advice police needs to chill out. We are just taking about a topic here. I still think the general suckyness of big cities is the culprit. Dauto (talk) 17:47, 28 August 2013 (UTC)[reply]

The line was crossed when Count Iblis suggested apnea as a medical cause - which could be construed as making a diagnosis - a clear violation of Kainaw's Criteria. SteveBaker (talk) 19:50, 28 August 2013 (UTC)[reply]

I think he said the best thing is to go see a doctor which doesn't break the rules. I think I'm gonna make a diagnosis as well: Half of the inhabitants of this forum are obsessed - You know who you are. Dauto (talk) 20:20, 28 August 2013 (UTC)[reply]

There's a big difference between:

"Go and see a doctor, who is the most appropriate source of advice about problems with the functioning of your body", and
"Go and see a doctor, because you may have apnea / cancer / a brain tumour / cholera / AIDS / the bubonic plague / an infestation of cockroaches in your guts / whatever" . -- Jack of Oz ^{[pleasantries]} 21:16, 28 August 2013 (UTC)[reply]

In the US we have lots of ads on TV that say "If you have symptom X, then you may have syndrome Y, so ask your doctor about it" (who will then prescribe their dangerous meds instead of sensibly suggesting lifestyle changes). We even have ads that say "You wouldn't want a doctor to do your job, so don't try to do his" (just take whatever crazy drugs he prescribes and shut up). I wish the makers of those ads could be convicted of practicing medicine without a license. StuRat (talk) 06:23, 29 August 2013 (UTC) [reply]

Some people have looked into a weather/sleep or air pollution/sleep connection. Plugging air pollution sleep into PubMed I get these hits:

Association between PM10 exposure and sleep of Egyptian school children ("There were statistically significant associations between particulate matter with an aerodynamic diameter <10 µm (PM10) exposure and DIMS and sleep hyperhidrosis (P?<?0.05)")
Is sleep apnea a winter disease?: meteorologic and sleep laboratory evidence collected over 1 decade ("More sleep-disordered breathing events were recorded in winter than in other seasons. This finding suggests that obstructive sleep apnea severity may be associated with other seasonal epidemiologic phenomena")
Association between exposure to secondhand smoke and sleep bruxism in children: a randomised control study ("High and moderate exposure to SHS is associated with sleep bruxism characterised by teeth grinding or clenching movements during sleep in children")

Perhaps the scientists here can tell you if these studies seem reasonable. None of them are a body of evidence, just single studies. 184.147.119.141 (talk) 00:29, 29 August 2013 (UTC)[reply]

Is there a physical barrier between the peritoneum and fallopian tube?

If not, what keeps all that stuff from just leaking out?

Thanks, Saintrain (talk) 20:41, 28 August 2013 (UTC)[reply]

Our internal organs don't just float; there is all manner of connective tissue holding things together. ←Baseball Bugs ^{What's up, Doc?} carrots→ 22:14, 28 August 2013 (UTC)[reply]

Sorry, wrong term. I meant "peritoneal cavity". (A little learning, something, something, spring.)

According to Ovary, "The ovaries are not attached to the fallopian tubes ..." and "The ovaries are uncovered in the peritoneal cavity ...", and, from Fallopian_tube#Anatomy, "The tubal ostium is the point where the tubal canal meets the peritoneal cavity ...". Since the fallopian tubes and ovaries have no physical (direct) connection, that implies that the tubes are open to the peritoneal cavity. So, why no leakage? Saintrain (talk) 22:52, 28 August 2013 (UTC)[reply]

I hope it's OK to quote non-WP sources :-), from The peritoneal environment in endometriosis,

"Fallopian tubes and ovaries are bathed in PF [peritoneal fluid]. Oocytes are

exposed to the peritoneal environment even after they are

captured by the fimbria because the Fallopian tube is a

conduit freely communicating with the peritoneal cavity.

Spermatozoa are exposed to PF factors in the Fallopian

tube before and during fertilization. The embryo undergoes

early development in the Fallopian tube. where it is also

potentially exposed to cellular and soluble components of

PF. There is evidence suggesting that at least some of the

uterine fluid may be of peritoneal origin (Casslen. 1986)."

Saintrain (talk) 23:09, 28 August 2013 (UTC)[reply]

Abdominal pregnancy can occur when an embryo floats from the fallopian tube into the abdomen and implants on internal organs. Based on this and on Saintrain's second source regarding PF in fallopian tubes, it appears that "leakage" does occur. 198.190.231.15 (talk) 16:54, 29 August 2013 (UTC)[reply]

This question always has freaked me out. In addition, I had to get a procedure where they stuck a tube only slightly thinner than a drinking straw into my abdomen to drain fluid after an operation, leaving a puncture shaped scar, yet when I asked the doctor said there was no need to suture it. μηδείς (talk) 00:50, 29 August 2013 (UTC)[reply]

August 29

Can't remember the name of a medical condition

An elderly acquaintance was recently diagnosed with a medical condition, and I can't remember its name. All I know is that it's causing severe pain in her back, and the condition's name begins with "arach", but the condition apparently doesn't have anything to do with spiders. Any clue what I could be thinking of? Nyttend (talk) 01:36, 29 August 2013 (UTC)[reply]

It isn't Sciatica is it? --Jayron32 01:38, 29 August 2013 (UTC)[reply]

Arachnoiditis Hot Stop talk-contribs 01:46, 29 August 2013 (UTC)[reply]

The name "arachnoiditis" sounds right, and the description in the article sounds a lot like the symptoms that she's experiencing. Thanks! Nyttend (talk) 02:15, 29 August 2013 (UTC)[reply]

Helpful advice: next time try typing in the words you know first for sure, "back pain", to google, and then start typing a...r...a...c...h... and it will fill in the answer for you. μηδείς (talk) 02:53, 29 August 2013 (UTC)[reply]

You just gave away my secret. Hot Stop talk-contribs 03:50, 29 August 2013 (UTC)[reply]

Didn't even think of that...Nyttend (talk) 03:54, 29 August 2013 (UTC)[reply]

That's why I get paid so much. μηδείς (talk) 05:34, 29 August 2013 (UTC)[reply]

And you are worth every cent we pay you, maybe even twice as much. StuRat (talk) 06:12, 29 August 2013 (UTC)[reply]

I can never seem to remember the name for the medical condition, aphasia. :-) StuRat (talk) 06:12, 29 August 2013 (UTC) [reply]

I'm sure I know it, but I usually just can't quite conjure up the name for presque vu. DMacks (talk) 06:17, 29 August 2013 (UTC)[reply]

Just read schizophrenia – great article, made me think twice. - ¡Ouch! (^{hurt me} / _{more pain}) 08:02, 29 August 2013 (UTC)[reply]

What are you telling us about our question, Ouch? Nyttend (talk) Nyttend (talk) 11:11, 29 August 2013 (UTC)[reply]

The condition's connection with spiders is only in the name, as "arachnoid" means "cobweb-like".[27] ←Baseball Bugs ^{What's up, Doc?} carrots→ 11:40, 29 August 2013 (UTC)[reply]

The reference is to the arachnoid mater, a thin and spiderweb-like layer of tissue that encloses the brain and spinal cord, between the dura mater and pia mater. Looie496 (talk) 22:09, 29 August 2013 (UTC)[reply]

Elysium – "open to space" space station?

In the recent movie “Elysium”, Elysium is a rotating space station orbiting Earth, designed as a luxurious gated suburb for the world’s wealthiest. These types of space stations mimic gravity by rotating giant rings in which people can reside (as made famous in the movie 2001). It appears that they could indeed work very much as depicted in that film. But in the movie Elysium, the station has a feature I have never seen depicted before, and I very much doubt could work. The rings of the space station are al fresco, that is, they have no covering at all. The idea is that the rotational velocity that provides the “gravity”, also keeps the air from expanding. (The depth of Elysium’s atmosphere appears to be no more than half a mile or so, at the most.) I suppose the designers would have considered:

"Earth is a rotating space station with gravity, and no hard covering between it and the vacuum of space, and Elysium is a rotating space station which mimics gravity, so why should it have a hard cover either?"

link

I wrote in an IMDB msg board that air pressure would immediately make the air boil away on Elysium, and that on Earth it takes miles of atmosphere to get the pressure we have at ground level. This blooper is not recognised as such in the “blooper list” of the movie, and I can only think that most people are not aware of how wrong it is. Unless, of course it is I who am wrong....Could that be possible? Myles325a (talk) 07:12, 29 August 2013 (UTC)[reply]

Air pressure on Earth is about 100,000 Pa, and that would be needed on the space station as well. One cubic meter of air weighs in at just above 1kg, so 500m of air would weigh 500kg per square, which is 5000N at 1.0g. So we have 5000 Pa, not 100,000 Pa. The factor of twenty means that the air would be gone quite fast, even by that ballpark estimate. - ¡Ouch! (^{hurt me} / _{more pain}) 07:50, 29 August 2013 (UTC)[reply]

OP myles325a back live. Thanks One.Ouch, but could you and succeeding posters spell out jargon properly? I don't know what a PA is; I gather it is an initialism. Anyway, I'm getting the drift, but a more accessible explanation would be appreciated. Myles325a (talk) 08:22, 29 August 2013 (UTC)[reply]

I presume Pa = Pascal (unit). I haven't seen the film (or even heard of it until now) but surely another problem is that it would be impossible to maintain a survivable temperature in the "open air". AndrewWTaylor (talk) 08:31, 29 August 2013 (UTC)[reply]

For Niven's Ringworld, the open architecture is plausible (even if the ringworld itself is not ;-). But then the "walls" of the Ringworld are 1000 miles high, not half a mile. At 1000 miles, the atmosphere is essentially all below you. --Stephan Schulz (talk) 09:12, 29 August 2013 (UTC)[reply]

Also see Orbitals, Halos and Bishop Rings. But these fictional/hypothetical structures all appear to be much larger than the Elysium space station, and depend on some form of unobtainium (such as Niven's scrith) to keep them in one piece. Gandalf61 (talk) 09:33, 29 August 2013 (UTC)[reply]

The Halos in your link make extensive use of hyper-dimensional architecture, metamaterials, nanomaterials, and exotic matter (to manipulate space-time), it does not rely on simple Newtonian mechanics to function. Plasmic Physics (talk) 12:26, 29 August 2013 (UTC)[reply]

Yes, that's the sort of thing I loosely filed under "unobtanium". Gandalf61 (talk) 16:25, 29 August 2013 (UTC)[reply]

In a thread elsewhere on this page, I linked to a review of the film by Gary Westfahl, in which he touches on this topic. Deor (talk) 13:02, 29 August 2013 (UTC)[reply]

The basic issue is that the scale height of an atmosphere doesn't permit this. Any atmosphere of Earthlike temperature and composition under Earthlike gravity is going to have a scale height of about 7.5 kilometers, which means that the pressure at the top of a 500-meter wall (1/15 of a scale height) is going to be around 95% of the surface pressure. If you want a 500-meter wall to retain an atmosphere, you'll need to reduce the scale height, either by using a denser gas (sulfur hexafluoride has a scale height of 1.5 km, still too much), or by increasing gravity (a surface gravity 70 times that of Earth's will drop the scale height to 107 meters, putting the top of the wall above 99% of the atmosphere). Reducing the temperature isn't an option, because by the time you get it low enough (3.5 kelvin), not even helium is a gas. --Carnildo (talk) 23:57, 29 August 2013 (UTC)[reply]

How was the moon created?

how was it made — Preceding unsigned comment added by 196.29.167.50 (talk) 08:47, 29 August 2013 (UTC)[reply]

See Moon#Formation and the articles linked from there. Rojomoke (talk) 08:55, 29 August 2013 (UTC)[reply]

A 2013 hypothesis suggests that the Moon formed from debris ejected from a cataclysmic thermonuclear detonation of the core of proto-Earth. Plasmic Physics (talk) 12:06, 29 August 2013 (UTC)[reply]

If our own articles aren't enough for you, could you supply a reference for your alternative? Rojomoke (talk) 12:13, 29 August 2013 (UTC)[reply]

[28]. Plasmic Physics (talk) 12:17, 29 August 2013 (UTC)[reply]

What would such a detonation look like, would the Earth vapourise, or will gaint asteroids be thrown out, or something in between? Plasmic Physics (talk) 01:40, 30 August 2013 (UTC)[reply]

"Despite what the revisionist historians tell you, there is no mention of the 'moon' anywhere in literature or historical documents -- anywhere -- before 1950. That is when it was initially launched." 178.48.114.143 (talk) 23:45, 29 August 2013 (UTC)[reply]

Antoher black hole question

Excuse me, I have another question about black holes: if they increase their mass by absorbing other stars even to us external observers, the fact about an object that stops on the event horizon is only an optical illusion (inother the gravitational time dilatation is valid even if object is on free fall?)? Thanks for your patience.95.234.63.70 (talk) 10:43, 29 August 2013 (UTC)[reply]

No, it is not an illusion, rather it is physical reality imposed by the principle of relativity: the notion that there is no universal frame of reference. In simplest terms, how you observe what happens at the event horizon depends on where you do the observing. To the person observing from the outside, the object falling into the black hole stops at the event horizon, while to that object itself, nothing special happens. They keep falling, without any special event that would even tell them they crossed it. The difference in observations between te two situation is nothing like an optical illusion. It's just that different observers in different frames of reference observe the event differently. That's what relativity means. --Jayron32 11:14, 29 August 2013 (UTC)[reply]

Both the external and internal points of view are real, hence the name relativity. Dauto (talk) 12:28, 29 August 2013 (UTC)[reply]

There is a complication here; unlike in Special Relativity there is no unambiguous way an observer can say when an event happened at a different location. This is not a problem in special relativity. So, the OP does have a point, you can't simply say that when the light signals arrive, that's when the event happens in your frame as that's not true in Special Relativity.

You can e.g. consider sending the infalling observer a message via light signals. What you find is that a message sent later than a certain time will never be received by the infalling observer. Count Iblis (talk) 13:10, 29 August 2013 (UTC)[reply]

Yes, but the problem is: we (external observer) see black holes absorbing other stars and so increasing their mass, we don,t see star stopping at horizon's surface (it's probably that the object stop its time but we see it equally falling into black hole). 95.234.63.70 (talk) 13:29, 29 August 2013 (UTC)[reply]

We would never see a star cross the event horizon, in practice we simply measure the mass of a black hole by considering how fast stars orbit it, so everything that is just outside the event horizon will then also be attributed to the mass of the black hole. Count Iblis (talk) 13:40, 29 August 2013 (UTC)[reply]

When we say that "the object stops at the event horizon" - we mean that each individual subatomic particle stops at the event horizon. So don't imagine a planet, buried forever, half inside and half outside of a black sphere. A better mental image is that as the planet (star, rock, spaceship) heads towards the black hole, tidal forces rip it apart - and eventually, rip even the atoms apart. From an outside observer's perspective, time for each individual quark (or whatever) slows right down and stops at the precise instant it hits the event horizon. The event horizon itself grows with each new addition of mass - so those objects do cross the event horizon and vanish - even from our perspective. But the idea that it's black at all is a bit misleading - these objects that are falling into the black hole are emitting immense amounts of radiation - some of which is visible from far away and is further smashing up the incoming stuff. The event horizon itself must have a fantastically small layer of sub-atomic mush that (to us) seems to be going slower and slower. But the actual appearance is even more confusing than that because gravitational red-shift is shifting the frequencies of light (and radio and X-rays, etc) so far off the chart that they'll be indetectable anyway. Roll in quantum effects and black-hole evaporation, hawking radiation...it's not a simple picture to imagine.

SteveBaker (talk) 13:41, 29 August 2013 (UTC)[reply]

This could be certainly possible for stellar black hole, but for very supermassive black holes with irrilevant tidal force and low surface gravity, a compact object wouldn't ripped apart. Inother, on a freefall is still valid the gravitational time dilatation? It isn't valid only if the object oppose resistance to gravity?80.116.228.11 (talk) 14:14, 29 August 2013 (UTC)[reply]

Gravitational time dilation still aplies for free falling objects plus there will also be some Doppler red shifting due to the motion of the object. The star certainly never reaches the horizon due to the (infinite) gravitational time dilation. Not only that, if quantum effects are included (as shown by Leonard Susskind), the individual particle orbitals enlarge until they wrap around the B-hole covering the whole surface. Off course, from the point of view of the falling object, none of that happens Dauto (talk) 15:21, 29 August 2013 (UTC)[reply]

Ageing

Is there an age where every organ of the body is at peak performance and nothing is growing or declining, since apparently growth and decline happens at different ages for different parts of the body？Clover345 (talk) 12:29, 29 August 2013 (UTC)[reply]

No. Life operates at the edge of chaos, and stasis is death (at least, according to Stuart Kauffman). See section 4.5.1 here [29], which summarizes some of Kauffman's work on gene expression in complex networks. Gene expression is basically what controls developmental biology, and also senescence. I'd also recommend "At Home in the Universe" [30], but you'd have to go to a library or buy it. SemanticMantis (talk) 14:51, 29 August 2013 (UTC)[reply]

Most aspects of physical and mental performance peak in the age range 20-25 for men, a bit younger for women, if I've grasped the data correctly. However, maximum strength for weightlifters peaks later, in the 30s, I believe. Looie496 (talk) 16:37, 29 August 2013 (UTC)[reply]

BMI

Is BMI an accurate measure of body faT? For instance would a body builder have a low BMI or a High one because he has very little fat but a lot of muscle?--86.181.30.8 (talk) 14:09, 29 August 2013 (UTC)[reply]

No it isn't very accurate and recent reseach points out to problems with using the MBI. The BMI is, however, a good general indicator, but it can be misleading. A body builder can have a high BMI while having an extremly low body fat percentage. People of Asian origin can have a low BMI while having a lot of fat, putting them at high risk of developing heart disease and diabetes. Count Iblis (talk) 14:59, 29 August 2013 (UTC)[reply]

(ec) No, it's certainly not accurate. Our Body mass index article has a section Body_mass_index#Limitations_and_shortcomings which you should probably read. It was always intended as a statistical means to estimate body fat over a large number of people - using it for an individual is problematic and has huge possibilities of error. However, it's extremely easy to measure - and it can provide a useful (if highly approximate) answer for some applications. SteveBaker (talk) 15:03, 29 August 2013 (UTC)[reply]

FAT

Does eating fat, on its own, make one fat?--86.181.30.8 (talk) 14:16, 29 August 2013 (UTC)[reply]

No, and not even a high total calory intake will on its own make one fat. Controlled experiments have consistently failed to demonstrate a link between diet and obesity. That doesn't mean that such a link over a long period does exist, just that it isn't as simple as eating too much for a while and putting on weight as a result. Count Iblis (talk) 15:02, 29 August 2013 (UTC)[reply]

I agree it's not as simple as that...but the ways in which it's not simple need to be explained very clearly before making such an extreme claim. You need to be VERY careful about how you phrase that - it's misleading in the extreme to say that your diet doesn't affect whether you're obese or not - and many, MANY sources would disagree with that. I think Count Iblis needs to provide us with some very good references for this rather extreme claim - and explain in considerably more detail what is meant by it. His preceding statement is horribly misleading! SteveBaker (talk) 15:11, 29 August 2013 (UTC)[reply]

No, if you kept your exercise schedule the same and instead of eating (say) 2,000 calories of carbohydrates each day (sugars, starches, etc) - you switched to eating 2,000 calories of fat instead - then you would be unlikely to change weight significantly as a result. Other health indicators such as cholesterol might go badly wrong - but the calorie intake is the same, it won't make you fatter. However, if you're eating a good diet and then add 2,000 calories of fat to your diet each day - then you'll put on weight rather rapidly! SteveBaker (talk) 15:11, 29 August 2013 (UTC)[reply]

~~Forgive me for not trusting the comments above, I had to look it up.~~

Here's a ref that is relevant to the question.

“

The percentage of energy from dietary fat is widely believed to be an important determinant of body fat, and several mechanisms have been proposed to account for such a relation. Comparisons of both diets and the prevalence of obesity between affluent and poor countries have been used to support a causal association, but these contrasts are seriously confounded by differences in physical activity and food availability. Within areas of similar economic development, regional intake of fat and prevalence of obesity have not been positively correlated. ... ... Moreover, within the United States, a substantial decline in the percentage of energy from fat consumed during the past two decades has corresponded with a massive increase in obesity. Diets high in fat do not appear to be the primary cause of the high prevalence of excess body fat in our society, and reductions in fat will not be a solution.

”

Emphasis added by me. From here [31], an article from The American Journal of Clinical nutrition, 1998. SemanticMantis (talk) 15:16, 29 August 2013 (UTC)[reply]

Yes - but that's not what Count Iblis said. He said (in essence) that calorie intake isn't the culprit and that diet doesn't matter - and without some pretty strong evidence, I have to call "bullshit" on that claim. The reason that decreasing fat intake doesn't reduce obesity is that people eat more carbohydrates (sugar, starch) to make up for it. Total calorie intake is the key - and it doesn't matter all that much where the calories come from. SteveBaker (talk) 17:40, 29 August 2013 (UTC)[reply]

Despite my phrasing, I wasn't really responding to you or Iblis, just providing a ref that I thought addressed the OP's question. I've stricken part of my post to make my intent more clear. SemanticMantis (talk) 17:55, 29 August 2013 (UTC)[reply]

See also Mediterranean diet, which is rather high in fat, and recently attracted lots of attention as a rather healthy diet (see refs in article). SemanticMantis (talk) 15:18, 29 August 2013 (UTC)[reply]

I don't know what you have in mind when you ask the question. If you are asking whether we accumulate the fat that we eat as fat in our body, then the answer is definitely no. Out bodies produce their own fat to store energy. But the fat that we eat has lots of calories, which make us fat. OsmanRF34 (talk) 17:28, 29 August 2013 (UTC)[reply]

The core determinant of weight is calories eaten vs. calories burned. If you eat a (say) 2000 calorie diet, and maintain the same activity schedule, the exact composition of the diet is of decidedly secondary consideration. But some diets are easier to maintain than others, and some (different ones, of course ;-) diets make it easier to exercise well. If you manage to eat a reasonable amount of your calories as fat without overeating in total, that's generally fine. --Stephan Schulz (talk) 17:36, 29 August 2013 (UTC)[reply]

Indeed, and a reasonable rule-of-thumb is that: weightGainedInPounds = (caloriesEaten - caloriesBurned)/3500

All diets that actually work have to play with how many calories your eat, or how many you burn, or some combination of the two. Mediterranean diets replace carbohydrates with fat - and hope that you'll be fooled into eating less calories because of it. But if you eat too much fat you'll put on weight despite not eating a single unit of carbohydrate. SteveBaker (talk) 17:52, 29 August 2013 (UTC)[reply]

Of course, one of the factors in overeating is the desire to eat itself. Different foods have a different affect on the sense of satiation; and foods that make you feel fuller with less calories are generally considered preferrable, if only because a person who is satiated (full) will eat less calories if their diet contains more foods that "fill you up" while providing less calories. Without going into exhaustive detail as to which foods tend to be more satiating; that has to be a factor in considering a healthy diet, from an "obesity-avoidance" perspective. People who are less hungry eat less food, so foods that fill you up and provide less calories in doing so are, on the balance, better for you. --Jayron32 18:11, 29 August 2013 (UTC)[reply]

I think the OP's question has been answered, but there is the question about what I wrote about total calory intake. What I'm saying is that it is not a universially valid statement to say that a big increase in calory intake will lead to a big weight gain. That doesn't mean that it will lead to weight gain in some people (paticularly if you are already obese, your weight will be more sensitive to the calory intake). But controlled experiments on people who are deliberately given way more to eat than they normally do (e.g. 5000 Kcal/day instead of 2500 Kcal/day) have debunked the simple calories in - calories out = weight gain model, see e.g. this documentary:

"In 1967, a medical researcher, Ethan Sims, carried out an experiment at Vermont state prison in the US. He recruited inmates to eat as much as they could to gain 25% of their body weight, in return for early release from prison.

Some of the volunteers could not reach the target however hard they tried, even though they were eating 10,000 calories a day. Sims's conclusion was that for some, obesity is nearly impossible.

It was with this in mind that 10 slim volunteers - who were not dieters - convened in more hospitable circumstances, for a recent experiment devised by the BBC's Horizon documentary. The 10 spent four weeks gorging on as much pizza, chips, ice cream and chocolate as they could, while doing no exercise, and severely limiting the amount they walked."

Count Iblis (talk) 18:37, 29 August 2013 (UTC)[reply]

The calories in vs. calories out model has not been debunked. More likely we just have a tendency to give wrong estimates of both variables. Calories in ones mouth may be consumed by gut flora or excreted intact. Calories out is impacted by basal metabolic rate, which may not be as constant as some would believe. Someguy1221 (talk) 21:01, 29 August 2013 (UTC)[reply]

Now what you said makes sense. Although I wouldn't dismiss Steve Baker's formula. In thermodynamic terms, it's true that 1 pound of fat equals 3600 kcal. But it appears that not all the calories that you intake can be stored. I would just expand the formula to include things about current weight and amount of extra kcal (the first 3600 extra kcal might give you an extra pound, but the same can't be true for the following groups of 3600 kcal). OsmanRF34 (talk) 18:46, 29 August 2013 (UTC)[reply]

Yes, the body will increase its metabolic rate to compensate for the increased calory intake. Without such a mechanism, eating slightly less would lead you to eventually starve to death. If you eat 2500 Kcal per day and you have a stable weight, then you won't starve to death if you start to eat 2400 Kcal per day and at the same time start to exercise a lot more. What happens is that your body will eventually start to burn 2400 Kcal per day. This is possible because the amount of calories used during heavy exercise isn't that much, the body can easily compensate for that by adjusting the metabolic rate.

Exercise is still important for maintaining a healthy weight, not because you burn a significant amount of calories, but because it enables this feedback mechanism to operate better. Count Iblis (talk) 20:28, 29 August 2013 (UTC)[reply]

But it's essential to note that the finding on calorie intake vs. calorie use only works in one direction. It violates no scientific principles for the body to eliminate excess calories without exercise, whether by increasing metabolic rate or by simply passing the excess through digestive tract without uptake, and simply allowing the gut flora to consume those calories. But if you start consuming fewer calories than your body intakes (or start burning more), you must be burning fat or something else to compensate. Someguy1221 (talk) 20:58, 29 August 2013 (UTC)[reply]

What Iblis has found is that sometimes, some people, under some conditions, can eat a lot of calories and not gain weight. Extrapolating the results of highly limited experiments is a suspicious way of making sweeping generalizations about all of humanity, doubly so when one merely cherry picks those specific experiments that happen to confirm the conclusion one had reached prior to searching for evidence in the first place. --Jayron32 22:43, 29 August 2013 (UTC)[reply]

Jack Sprat, for example. ←Baseball Bugs ^{What's up, Doc?} carrots→ 23:30, 29 August 2013 (UTC)[reply]

dog intelligence ranking

This is just a personal issue for me but Australian shepherds are listed as average intelligence while they and Border Collie rule the agility world, not the domain of dumb dogs. And to be truthful, most people pick agility dogs so they will be easy to train. Something is wrong with Coren's list. Maybe the new book about Chaser, the Border Collie, will be better. — Preceding unsigned comment added by Aikenite (talk • contribs) 22:52, 29 August 2013 (UTC)[reply]

Interesting, but this page is for asking questions, not to be used as a forum for expressing opinions. Looie496 (talk) 00:14, 30 August 2013 (UTC)[reply]

determining probabilities from rate laws

For something like radioactibe decay, the rate law is ${\frac {dN}{dt}}=-\lambda N$ . and for a general chemical reaction it is usually something like ${\frac {dN}{dt}}=kN^{x}$ (see rate law). Is it possible to deduce from the rate laws the probability that a reaction occurs after a particular elapsed time? 130.56.84.66 (talk) 02:20, 30 August 2013 (UTC)[reply]