The best answers address the question directly, and back up facts with wikilinks and links to sources. Do not edit others' comments and do not give any medical or legal advice.
let me guess...you just re read the three little pigs to your kid. Couldn't helpGeeBIGS (talk) 00:36, 7 December 2012 (UTC)[reply]
There are probably a great many reasons. Just one example — brick houses don't do well in earthquakes, so if you're building where earthquakes happen, brick is a bad choice. --Trovatore (talk) 00:37, 7 December 2012 (UTC)[reply]
Economics often has a lot to do with it. For example, the climate in the major Australian cities is not very different. However, in Perth double brick (brick outside, and rendered brick inside) construction completely dominates, but in other cities brick veneer (brick on the outside, frame&clad on the inside) dominates. The reason is an accident of history. In Perth a couple of clever chaps took advantage of local high quality clay deposits and devised their own factory machinery made from war surplus construction equipment - the result being that Perth was supplied with very cheap bricks. In other words in Perth double brick construction was cheaper but in other cities brick veneer was cheaper.
Double brick construction works very well in mediteranean climates such southern Australia, much of southern Europe. But in colder climates (ambient temperatures routinely below zero C) timber framed timber clad construction offers better thermal insulation, but is not maintenance free.
There are cultural aspects. For example, in many parts of Australia during the 1930's depression and at certain other times, people with low incomes got houses of timber frame asbestos/cement clad construction. Such houses were cheap to build and actually perform quite well in durability and in thermal properties, but ever since have had a cheap and nasty stigma. So, after the post- World War 2 ecomonic boom, anybody who didn't earn much usually stretched themselves and committed to an achievable dream - owning a brick house (even if that meant smaller rooms and paying off a loan for the next 30 years).
I imagine similar economic, environmental, and cultural considerations apply elsewhere in the world. Wickwack 124.182.154.215 (talk) 01:27, 7 December 2012 (UTC)[reply]
Traditionally houses were built in whatever building materials were available locally. I live in the UK and in my local area there are lots of sedimentary clays and sands and so, traditionally, the houses were built of brick. If you look at local maps from the 19th century there were brick fields and sandpits dotted all over the place. There is also some new red sandstone and some of the churches and public buildings are built of that. A few miles up the road you get into more rugged Millstone grit country and so all the houses, even basic workers' cottages, were built of stone. In other parts of the country, such as East Anglia houses were often built of flints set in lime mortar. As better and cheaper transport became available different building materials were used around the country and nowadays many new homes have timber frames with a brick veneer - quick to erect but still fit in with the traditionally built houses. The materials used today owe more to the speed and convenience of building rather than what is available locally. It is now very exensive to build using stone, whereas in some areas that was once the cheapest material. I don't know about other countries, but if you want to build a house in the UK it's generally easier to get planning permission if the building appears to fit in with the other buildings round about rather than being something that sticks out like a sore thumb. Also I think that most buyers prefer something that they expect to see in that area - unless they want to go for a big statement. Richerman(talk) 09:33, 8 December 2012 (UTC)[reply]
Good points. Here in Australia it is much the same with regard to getting the necessary Building Permits - Shires require that new dwellings and other buildings have an appearance that fits in with the existing buildings nearby. Some shires even specify what colours are to be used. Wickwack 120.145.28.146 (talk) 13:37, 8 December 2012 (UTC)[reply]
Interestingly (and embarrassingly) our article on building materials doesn't even mention bricks. Ghmyrtle (talk) 10:05, 8 December 2012 (UTC)[reply]
Venus in a small telescope
The other day I was looking at Venus with my 6.25" reflector. I was hoping to see it as a crescent. All I could see was a very bright spot, with all three eyepieces. Is Venus just too bright for that? Venustar84 (talk) 00:39, 7 December 2012 (UTC)[reply]
Estimate the current angular size: its diameter divided by its distance yields about 11 arc seconds. In principle, your telescope should be able to resolve about a half an arc-second in visible light. So, your minimum resolvable feature-size is good enough to image the structure of Venus' illumination pattern (its crescent); and because the planet appears bright enough that you should be able to magnify to fairly large size.
In practice, you're only about 10x above the resolution limit. So, any imperfections at all - atmospheric aberration, telescope alignment or mirror collimation issues, and so forth - will blur the image. How good is your scope's primary mirror, and how good are the optic(s) you're putting behind it? You can push it to its limits by measuring the circle of confusion of a bright star, who is well-approximated by a perfect point-source.
If you're collecting too much light - which happens when you're looking at planets - you can create an iris by cutting a hole of whatever size you want, out of cardboard or tinfoil, and shutter over some of your aperture. This dims the image and has the added bonus of improving your focus. Camera people call that process "stopping down the aperture." Telescope stores charge a lot of money for stops, but you can build one out of tinfoil or stiff card paper, without worrying too much about precision. Just try to be mostly circular, mostly centered, and mostly symmetrical. (Other experts recommend off-center stops, especially if your scope is Newtonian, for obvious reasons).
I have a 205 mm Newtonian scope, and lots of very good glass, a not-too-shabby set of cameras, and (formerly) a whole lot of human- and CPU-time to spend post-processing; but I have never successfully imaged any structure on Venus. I have had better luck with Jupiter clouds. Nimur (talk) 01:27, 7 December 2012 (UTC)[reply]
The crescent shape of Venus is visible using even moderately-good binoculars. Unless there's something seriously wrong with your telescope, you shouldn't have any difficulty resolving its shape. (Though if you do find it uncomfortably bright, you could partially obstruct the open end of your telescope aperture, or acquire a neutral density filter to take some of the edge off.)
That said, when exactly was 'the other day'? In recent weeks, Venus has been getting closer and closer to 'full' (see aspects of Venus for dates). Remember that Venus takes a lot longer than, say, the Moon to work through its phases. Based on this calculator widget, Venus was more than 80% illuminated on 1 November, and 88% illuminated on 1 December. Any time in November, Venus would only look a little bit out of round. Venus is also very nearly as far away from Earth as it can get (it is at its furthest when it is 'full', at superior conjunction on the opposite side of the Sun from Earth), and therefore very nearly at its smallest apparent size. At its furthest, its angular diameter is just under 10" (10 arc seconds); at its nearest, it swells to 66" across. TenOfAllTrades(talk) 01:35, 7 December 2012 (UTC)[reply]
As an amateur astronomer, I can clearly see Venus as a crescent (or some other phase) with my 60 mm (2.4 inch) telescope. I haven't tried recently, but if you're having trouble with your 6.25" reflector, and you're sure Venus wasn't near full that day, something is wrong. --140.180.249.232 (talk) 05:46, 7 December 2012 (UTC)[reply]
At 11", Venus must have been near-full (given that the angle for maximal distance (=full) is 10").
The theoretical limit of a 6" reflector is 0.75 arcsec (as a rule of thumb, it is 1 arcsec for the popular 4.5" tubes and inversely proportional to aperture.
Stopping the aperture is quite awful. It plays bloody hell with resolution. However, high-quality filters are a bit on the expensive side. If you want, you can make a simple cardboard stop or two (just cut a round hole and paint the cardboard black). You won't suffer much if you make a 2.25" (57mm) aperture - that's according to the rule above, still a 2-arcsec resolution, and if Venus is half or less lit, it'll be ~17 arcsec end to end.
From the data Nimur and Ten gave, it'll be about 9 months (calculated mentally) until Venus is down to half-lit. Hope that helps. - ¡Ouch! (hurt me / more pain) 09:56, 7 December 2012 (UTC)[reply]
Oh, and 6.25 is not "small." Those who laugh at it are brag-a-lots.
Wouldn't it be better to make a number of cutouts at the edge and keep your resolution? Filters for the moon and bright planets are rather cheap though, about $16 I think? Sagittarian Milky Way (talk) 15:30, 10 December 2012 (UTC)[reply]
As was pointed out in some TV segment I saw the other day, Galileo observed the different phases of Venus, whose size and shape demonstrated that Venus orbited the sun, not the earth. When it's at crescent phase it's closer to us and looks larger through a telescope. When it's at near-full phase it's farther away, so it looks small and roundish. But if you have doubts about the quality of your telescope, just aim it at some other objects, such as the moon, or one of the outer planets if they happen to be visible. Then you can tell if there's anything wrong with your telescope. ←Baseball BugsWhat's up, Doc?carrots→ 14:00, 8 December 2012 (UTC)[reply]
Speed of light, etc.
All our articles on relativity seem to be a little sparse in this regard, so I hope no one minds me asking some dumb questions. What I am trying to work out is how exactly two objects can have independent velocities that sum to c or greater, without actually being measured by each other as accelerating to the speed of light. I believe the easiest way to put this into simple terms is the following:
Two objects (A and B) accelerate in the same direction at the same speed (for our purposes, let's peg it at 51% the speed of light) relative to an observer in an inertial reference frame. This observer sees two objects, both accelerating toward him at 51% the speed of light. Nothing weird so far, but then, B changes direction. While A continues to accelerate toward our observer, B reverses direction, now accelerating away from the observer at 51% the speed of light.
The observer (as far as I can tell) will not notice anything out of the ordinary. Both objects continue to move at the same speed relative to him/her; one has merely changed direction. Object A, however, has a problem -- according to our observer, B is now accelerating away from A at slightly over the speed of light. Because of special relativity, however, A measures the velocity of B at slightly under the speed of light (I think this is correct). Therefore, if asked to describe B's location in time and space, Object A and the observer will give two completely different answers. Issues like length contraction aside, the object they are observing possesses the same physical qualities, but the two will not agree on its location. How, if it all, is this resolved? Evanh2008(talk|contribs) 04:08, 7 December 2012 (UTC)[reply]
These sort of apparent paradoxes show up all the time in special relativity problems. See the Ladder paradox for another one. The answer to your question is that both Object A and our observer are perfectly capable of calculating the location and velocity of B and both will arrive at the same answer so long as both work from the same set of data. If they don't get the same answers, then they have left something out of their calculations. It's the same basic problem with observing any far away object. Where we see a star now in the sky isn't where the star is now. An observer on the other side of the galaxy will see the same star in a different location, but given the right information about the star's position and velocity we can both calculate the star's actual position and come up with the same answer. --Jayron32 04:17, 7 December 2012 (UTC)[reply]
You need to think a lot more carefully about how your "observers" are measuring these things. With a radar gun? Then how do you convert a redshift into meters per second, and why? Or by timing how long it takes to go a given distance? Then how are you determining the elapsed time and the distance? The reason no one understood special relativity before Einstein's 1905 paper (even though all the math had been worked out before) is that they imagined that measuring speeds was something you could just do, and the details were irrelevant. Einstein carefully considered what it actually means to measure a speed, and showed that the specialness of the speed of light is quite natural. Unfortunately, most people still don't get it. They talk about special relativity in terms of "observers" who somehow just know the length and speed of everything (though apparently they don't really, since they all disagree with each other). Special relativity seems mysterious when described in this way. It isn't really.
You might want to read this translation of Einstein's paper. It's still one of the best introductions to special relativity. Note that what he calls "observers" in the paper are just scientists who look at things. The strange beast that's now called an "observer" is a later invention. -- BenRG (talk) 04:42, 7 December 2012 (UTC)[reply]
I have always taken that latter usage of "observer" to mean the God's eye view of things, on the assumption that God is not bound by the laws of physics and hence can observe anything. In the narrower intention that Einstein evidently meant, the "observer" would be more like a "measurer". Michelson, for example, didn't literally "observe" light traveling at the speed of light, but he invented a way to measure it. ←Baseball BugsWhat's up, Doc?carrots→ 13:53, 8 December 2012 (UTC)[reply]
Here are some diagrams that I've been meaning to make since forever, showing the Euclidean versions of velocity addition, length contraction, and (since someone mentioned it above) the barn-pole paradox.
Velocities are slopes (distance over time) and I hope the first diagram convinces you that there's no reason to expect simply adding the slopes to give the right answer. You can just add the angles, though (as long as the lines are in a common plane). The spacetime version of an angle is called rapidity and it's related to ordinary velocity v by v = tanh α (compare the Euclidean version: v = tan θ). In the length contraction case, the main point is that the two "observers" are making two different measurements, and it shouldn't be surprising that they get different results. It doesn't mean that reality is different for different people. The barn-pole paradox just illustrates a common beginner's mistake in applying the Lorentz transformation (which is a spacetime rotation).
I would like to add these diagrams, and the twin paradox one that I mentioned in another thread, to the articles someday, but I'm not sure how to deal with the likely claim of original research. It's hard for me to believe that no textbook in the last 107 years has used similar diagrams, but I haven't found one yet. -- BenRG (talk) 05:13, 10 December 2012 (UTC)[reply]
See the article on velocity addition (which is what the OP is attempting to apply here). Modocc (talk) 15:43, 8 December 2012 (UTC)[reply]
How much, or, In what ways, except from hormonal action, does Aging related to the Nervous system in compare to Genetic contorl?
In other words, How much does Aging affected by the Nervous system (and the endocrine system that it governs), In compare to Genetic control?— Preceding unsigned comment added by 109.66.11.190 (talk) 06:42, 7 December 2012 (UTC)[reply]
Genetics determines the form of the nervous system to a very detailed level. Aging only causes some minor deterioration, except in the case of diseases such as Alzheimer's disease. Looie496 (talk) 07:16, 7 December 2012 (UTC)[reply]
Hi. please note that I'M not asking how aging affects the NS. 109.66.11.190 (talk) 14:28, 7 December 2012 (UTC)[reply]
With all respect, the grammar of your question is so bad that it's almost impossible to understand what you are asking. I made a guess; apparently it was wrong. Can you try to make the question clearer? Looie496 (talk) 15:33, 7 December 2012 (UTC)[reply]
I'll try to translate:
In what manner and to what degree is aging related to the nervous system, as opposed to hormonal and genetic factors? In other words, how much is aging affected by the nervous system (and the endocrine system that it governs), in comparison to genetic control?
That's like asking how much driving is affected by cars, in comparison to roads. How could you answer a question like that? Looie496 (talk) 15:51, 7 December 2012 (UTC)[reply]
About not eating for 4 days straight, and subsisting only on fluids...
closed, weird question about washlets, asking for personal advise
The following discussion has been closed. Please do not modify it.
I only ask for nutritional advice, nothing medical, as no one is allowed to here.
I'm afraid that at the ski resort, there may be no washlet / bidet-like devices and the resort staff would likely reject my donation of a BioBidet attachment device to install on the toilet of the accommodation I'll stay in. (I'll still bring one in the remote off-chance that they consider my digestive needs and accept it after all.)
Being faced with undue hardship without access to a bidet, I would much rather subsist on fluids during the whole four days. Weighing 206 pounds currently, I would have 25 pounds to lose until reaching the normal-weight threshold anyway.
However, if the resultant hunger makes me ravenous, I may take yogurt in hopes that it's only urinated out, as yogurt is still more liquid than solid.
Therefore, the questions:
1. After explaining my digestive concerns for needing a bidet, would they let me install the bidet-attachment on their toilet in the room I'm in? (It's a donation that I'd like to have remain there after I return home with my ministry group.)
1a. Why might they not allow me to, when bidets clearly have better results than the "old way?" Wouldn't it be like rejecting the donation of a shower stall 100 years ago?
2. If I subsist strictly on fluids for the 4-5 days I'm away from home, how many pounds would I stand to lose? Also, what unpleasant nutritional side-effects would be bound to happen?
3. If the only things I eat are no tougher than yogurt, will I still be compelled to take the #2 before the 4 days, or will only the #1 happen?
4. What (else) can I eat that will not force me to take the #2, no matter how much of it I have? (Someone up to no good from AOLAnswers answered, "Hemlock. Try it." Needless to say, I have thus far, found no non-poisonous forms of hemlock, so it's seemingly not like mushrooms.)
I hope the answers prepare me well for this trip. --Mount Zynar (talk) 07:26, 7 December 2012 (UTC)[reply]
You're approaching this from a totally wrong angle. You should be asking what you can use in lieu of a bidet, and the answer is quite simple. Pop in the shower and wash yourself off down there. Like you, I like my nether regions to be clean, and when at home, I always take a partial shower afterwards. Just takes a minute or two, and it's a lot easier, and healthier, than starving yourself. Dominus Vobisdu (talk) 07:46, 7 December 2012 (UTC)[reply]
Do you want to step on what others leave behind (especially that which comes from "up there?") And do you ever want to stick your foot in the toilet bowl, even after it's flushed clean? Think about it. That's why I would not use that particular shower-option in place of bidets. --Mount Zynar (talk) 08:56, 7 December 2012 (UTC)[reply]
You could clean it, with bleach, before you leave. StuRat (talk) 09:16, 7 December 2012 (UTC)[reply]
Along those lines, request a handicapped access room, as they often have a movable shower hose, which would be a lot easier to use as a bidet. You could also ask if they have any rooms with bidets. If they have international tourist there, it's possible, although those rooms might cost more. StuRat (talk) 07:56, 7 December 2012 (UTC)[reply]
And, if forced to use a toilet, a bit of moisturizer squeezed onto the toilet paper can both assist in cleaning and prevent irritation. You could also just attach the device without asking them. You might want to remove it before the maid arrives each day, as there's a slight chance she might complain to the hotel management. But, most likely, they would just ignore it. I'd remove it when you go, though, or they might very well complain. I myself have made little fixes to hotel rooms I've stayed in, from putting bright CFLs in place of dim incandescent bulbs, to fixing a rattling A/C unit and a toilet that had a lever which stuck down. I never asked permission. StuRat (talk) 07:54, 7 December 2012 (UTC)[reply]
That all liquid diet might also result in diarrhea, so I'd avoid that. But, if you insist, all least do a dry run first (or is it a wet run ?), so you discover any problems it causes while you still have access to a bidet. StuRat (talk) 07:55, 7 December 2012 (UTC)[reply]
Stu, how are we sure that an all-liquid diet won't just cause frequent urination? --Mount Zynar (talk) 08:56, 7 December 2012 (UTC)[reply]
With pure water, you might be right. But other liquids are likely to contain some indigestable material. For example, pulp in orange juice. StuRat (talk) 09:14, 7 December 2012 (UTC)[reply]
You also misunderstand what faeces consist of: it is entirely possible to pass a stool without eating any solid food because of the dead cells the body has to eliminate, which it does in faecal matter. --TammyMoet (talk) 08:23, 7 December 2012 (UTC)[reply]
How long would that take to happen? On the night of New Years, I plan to take an hour or two committing multiple "enema washes" to empty out the bowels as much as I possibly can. If the process, involving only dead cells, would take more than the 4-5 days the trip lasts, and I don't eat any solids, then I don't think it's a problem. So how many days would it take? --Mount Zynar (talk) 08:56, 7 December 2012 (UTC)[reply]
Is it just me who finds this question weird? And although this is no medical question per se, doing outdoor sports like ski and not eating anything solid could have some health implications. OsmanRF34 (talk) 14:44, 7 December 2012 (UTC)[reply]
This is at least the fourth or fifth time the Refdesk has gotten hit with washlet-related spam. The idea that a ski resort shower won't work as a substitute for a final wash seems ridiculous. (Even if someone found that repulsive, the person before him didn't... unless he's avoiding the shower too) Note this is a brand new account. Prolonged fasting verges on a medical issue, especially in someone not accustomed to the practice who says he's going skiing, and enemas are definitely not nutritional. Even I can accept culling this one. Wnt (talk) 14:52, 7 December 2012 (UTC)[reply]
Agreed, furthermore, "Committing multiple enemas" sounds illegal in this context.165.212.189.187 (talk) 16:20, 7 December 2012 (UTC)[reply]
Wow, given that post-game analysis I am almost tempted to click "show" to see what I missed. μηδείς (talk) 17:09, 7 December 2012 (UTC)[reply]
Lung shot
Does getting shot in the lung feel like drowning?--Wrk678 (talk) 09:38, 7 December 2012 (UTC)[reply]
You have two lungs, and if only one fills with blood, you can still breathe, so it wouldn't be as bad as drowning. You would need to remain calm, though, as you might use more oxygen in an excited state than one lung can supply. StuRat (talk) 09:54, 7 December 2012 (UTC)[reply]
What you experience would depend on whether you are bleeding into the lung, which is not guaranteed even by being shot through it. If there is little bleeding and you've simply open an air passage between the lung and the chest cavity, our hypothetical victim has a pneumothorax. As you can read from that article, the symptoms don't sound anywhere close to the feeling of drowning. In the case of significant bleeding into the lungs, it would be called hemothorax, but the mortality caused by hemothorax is typically related to the loss of circulating blood rather than suffocation (although shortness of breath may be present). So in both cases, I suspect the answer is no. Someguy1221 (talk) 10:01, 7 December 2012 (UTC)[reply]
Only one example, but this 15-year-old said, "Do you ever get a side ache so bad you just want to lay down and give up?" Clarityfiend (talk) 21:19, 7 December 2012 (UTC)[reply]
Are German Turks darker than normal Turks?
I got this impression, but does
not have a source confirming or denying it. OsmanRF34 (talk) 14:28, 7 December 2012 (UTC)[reply]
I really meant it as a serious question. I have the impression that they immigrated from a region from Turkey where people are darker, and they didn't mixed too much with local Germans, who obviously have a fairer skin tone. OsmanRF34 (talk) 22:38, 7 December 2012 (UTC)[reply]
The people you are calling German Turks may be another ethnic group, such as the Romani people. StuRat (talk) 23:40, 7 December 2012 (UTC)[reply]
I have tutored two German Turks who emigrated to the US, and neither was particularly dark. I have met Spaniards and Italians darker than them. Both were also strongly Turk-nationalist, which makes me doubt they were of some minority ethnicity such as Gypsy. (I have also met blond Armenians from Turkey one would think were Poles.) μηδείς (talk) 02:45, 8 December 2012 (UTC)[reply]
The skin colour may vary due to regional differences in Turkey. The Turks in Germany came here mostly in the 1960s and 1970s to work in German industry as "Gastarbeiter" Italy, Spain and Greece were other places were workers were allowed to come to Germany. I do not have a chart from which places the most of the Turks came but I guess it was from the poor part which is the anatholian montains in the east. I look for a chart to ducument this. --Stone (talk) 13:51, 8 December 2012 (UTC)[reply]
In what ways, except from hormonal action, does Aging related to the Nervous system in compare to Genetic contorl?
Because this last question of mine became a Cynic charade, i decided to ask it again. Ben-Natan (talk) 16:44, 7 December 2012 (UTC)[reply]
So let's see if you understand this time. Your question, as you ask it, makes no sense. Do you mean "How is the Nervous system affected by the aging process, and how does this compare to the effects of genetic controls?" --TammyMoet (talk) 18:11, 7 December 2012 (UTC)[reply]
No man. is there anything extra, besides the neurologically-governed hormonal system, and genetic control, that affects aging?. 109.66.11.190 (talk) 19:07, 7 December 2012 (UTC)[reply]
Thank you. Why didn't you say that the first time? Anyway, diet, exercise and smoking all affect the aging process in humans. --TammyMoet (talk) 20:39, 7 December 2012 (UTC)[reply]
UV rays too, but that’s partly due to the genetic issues they tend to cause. Evanh2008(talk|contribs) 04:15, 8 December 2012 (UTC)[reply]
in what existing\potential methods we govern genetic control? (it seems to be the most affecting factor on aging).
I have absolutely no idea what you just said. Evanh2008(talk|contribs) 05:48, 10 December 2012 (UTC)[reply]
Nullification of time dilation effects via deceleration?
I wonder if time dilation effects might be nullified (or otherwise altered) by a deceleration equivalent to the rate of acceleration or some such? Or does relativity already preclude such things? — Preceding unsigned comment added by 66.87.127.23 (talk) 17:03, 7 December 2012 (UTC)[reply]
Nope... Dauto (talk) 01:18, 8 December 2012 (UTC)[reply]
Well, OK, not directly, I suppose ... but different acceleration histories will result in a difference in time. I agree that it's not quite the same as time dilation being "caused" by acceleration. Dbfirs 07:46, 8 December 2012 (UTC)[reply]
It's perfectly reasonable and accurate to say that time dilation is a direct consequence of acceleration. Consider the twin paradox, for example. The twin that ages least is the one that experiences the most acceleration. The way Einstein put it, at any given instant of acceleration the twin is momentarily in a different inertial reference frame (and thus stationary). In other words, the twin with the highest surplus of delta V stays the youngest.
No, it doesn't work that way. All the dilation is accountable from the integrated dilation due to relative motion. There is no correction factor or extra term due to acceleration. For instance, if one twin travels at speed v along a circle of radius R and the other twin travels at speed 2*v along a much wider circle (say a radius of 10*R),after the faster twin completes a turn (and the slower one completes 5 turns), the faster moving twin will have aged more slowly than the slow moving one even though his acceleration was smaller. Dauto (talk) 16:46, 8 December 2012 (UTC)[reply]
In that case, the faster object would initially be more dilated but would eventually be "overtaken" by the slower object due to the the cumulative effect of acceleration on the latter. Integrating relative motion is meaningless here because the process is asymmetrical. Otherwise, how could we differentiate one twin from the other in terms of say the classical (rectilinear) experiment?
Your understanding of the circular motion case is incorrect. No "overtaking" due to acceleration takes place. The faster moving twin becomes younger (By that I mean he ages more slowly) regardless of the amount of acceleration it has been subjected too. Note that in the rectilinear case the twin that accelerates must take into account the change in simultaneity that happens due to him changing reference frames when he accelerates. This is a separate effect from the time dilation effect. That's why it is always a good idea to have at least one observer remain un-accelerated and calculate all time dilation factors with respect to him. Dauto (talk) 19:57, 8 December 2012 (UTC)[reply]
Let's go back to the original thought experiment. Why does the traveling twin experience time dilation then?
Alright, may be an example is in order here. Let's call the twin that remains on Earth "Earl" and let's call the one that travels "Travis". Let's also say that they were both 20 years old when Travis started traveling to planet-X located 8 light-years away from Earth. In other words the proper distance between Earth and planet-X is , where cy stands for light-year. Lets split Travis motion into 5 steps
Step 1: Travis accelerates instantaneously to the speed . There is no time dilation associated with that acceleration. That acceleration is (off course) not realistic, but it makes the math simpler. We shall label that reference frame with a prime "", and Earl's reference frame is not primed "". The Lorentz factor for that speed is .
Step 2: Travis remains at that speed for 10 years - Earl's time - that is , where y stands for years. That's just enough time for Travis to reach planet-X: . Due to time dilation Earl observes Travis age only 6 years - that is
Step 3: Travis accelerates (again instantaneously) to the speed . Again, no time dilation applies here. We shall label that reference frame with a double prime "" "".
Step 4: Travis remains at that speed for 10 more years - Earl's time - that is , which is enough time to bring Travis back to Earth: . Again, due to time dilation Earl observes Travis age only 6 years - that is
Step 5: Travis accelerates instantaneously once more to Earth's rest frame without any time dilation.
Total travel time for Earl: .
Total travel time for Travis: .
That's what happened from Earl's point of view. Let's explore what happened from Travis point of view.
Step 1: Travis accelerates instantaneously to the speed . The distance between Earth and planet-X is Lorentz contracted to .
Step 2: Travis remains at that speed for 6 years - that is . That's just enough time for Travis to reach planet-X: . Due to time dilation Travis observes Earl age only 3.6 years - that is
Step 3: Travis accelerates instantaneously from to . There is no time dilation here but there is shift in simultaneity. There is no shift in step 1 (or step 5) because the distance between the twins is zero for those steps. The simultaneity shift is given by where is the relative speed between the reference frames and which is given by the Velocity-addition formula: . The Lorentz factor for that speed is . And we get the simultaneity shift . That means that in reference frame Travis turns 26 years old simultaneously with Earl turning 23.6 years old, but in reference frame the former happens years after the latter, which means that (taking into account time dilation) Earl has aged an extra
Step 4: Like in step 2 and
Step 5: No time elapses in step 5 for either Earl or Travis.
Could you explain (just for my peace of mind) how your mathematics, and the Gruber and Price examples, square with "This is because gravitational time dilation is manifested in accelerated frames of reference" in the article Gravitational time dilation, and how they square with the Equivalence principle? (I think I need to do a lot more reading on this before I understand it!) Dbfirs 21:22, 9 December 2012 (UTC)[reply]
That statement is misleading at best. The concept of gravitational time dilation itself is misleading and would be more aptly described as gravitational Doppler shift. If you send a light signal from the front end of a rocket to its back end, by the time the signal reaches its destination the rocket will have accelerated a bit, so the source is perceived to be moving towards the destination and a blue shift is observed. If the signal is going in the opposite direction, a redshift is observed instead. By the equivalence principle a similar Doppler shift must be observed by some one in the presence of a gravitational field. That shift is often called a gravitational time dilation but that causes a lot of confusion in my opinion. I prefer the term gravitational (Doppler) shift. Dauto (talk) 01:01, 10 December 2012 (UTC)[reply]
... so does "gravitational (Doppler) shift" explain why clocks at the top of a tower run faster (let's assume the tower is on a non-rotating planet to filter out differences in speed and centripetal acceleration)? (I suspect that if I understood the metric tensor of general relativity properly, then I wouldn't need to ask!) Dbfirs 12:35, 10 December 2012 (UTC)[reply]
Yes, Gravitational Doppler shift explains why a clock at the top of a tower moves faster. Dauto (talk) 14:52, 10 December 2012 (UTC)[reply]
Look, the twin paradox/effect is really very simple. It's closely analogous to the fact that a straight line is the shortest distance between two points. In spacetime it comes out the other way—the straight line is the longest distance (elapsed time) because of a sign change, but it's essentially the same thing. A straight line is a nonaccelerating worldline in flat spacetime. A curved path is an accelerating worldline. So it's true that if one twin accelerates and the other doesn't, the one that accelerates will end up younger. But it's a bit silly to say that the acceleration (bending) "causes" the extra distance, and ridiculous to say that the extra distance happens during the acceleration. The extra distance isn't in any particular place; it's just that the curve overall is longer. I could reproduce Dauto's long calculation in Euclidean geometry, and it would be almost the same aside from some sign changes. But there's no point; a simple diagram is sufficient.
In general relativity it's the same except that instead of a flat piece of paper you're drawing lines on a warped piece of paper (maybe it got wet and you dried it out). -- BenRG (talk) 03:03, 10 December 2012 (UTC)[reply]
On the right is a diagram I made ages ago showing the Euclidean version of a common presentation of the twin paradox. This diagram is far more complicated than the simple geometry of the triangle requires. There's no reason to slice the triangle horizontally or diagonally. Nobody would do it teaching Euclidean geometry. For some reason they do it when teaching the twin paradox. -- BenRG (talk) 03:09, 10 December 2012 (UTC)[reply]
What was the format of a single sample of audio recorded on a cassette tape?
The format for a single moment's worth of audio data on this, I suppose, is what I'm asking about
Say I hit record on a tape recorder and record for a fraction of a second the sound of me saying "AAHH." The smallest frame of the sampling rate, corresponding to one momentary sample (I assume signifying what the voltage caused by the fluctuating mirophone at that moment was), what format was THAT encoded in on the tape? Analog, I know, but how was the amplitude of the microphone at one moment encoded on the tape? Was the voltage fluctuation range of all microphones standard or did the recorder hardware somehow sense the microphone's range and normalize it to the standard range? So if the amplitude to encode onto the tape were, say, between -X and +X volts, how was A.BCD volts encoded on the tape? What was the format for encoding an analog value? 20.137.2.50 (talk) 19:56, 7 December 2012 (UTC)[reply]
You're overcomplicating things, perhaps. The degree of magnetization is proportional to the signal. There are no "frames", there is no "sampling rate", there are no "samples", there is no "encoding". There are no discrete "moments". --jpgordon::==( o ) 16:10, 8 December 2012 (UTC)[reply]
Then please expand upon what exactly "degree of magnetization means" as if I didn't understand at all what you meant by that. I thought a single spot on magnetic tape was either '1' or '0'.67.163.109.173 (talk) 16:17, 8 December 2012 (UTC)[reply]
Not if you're referring to analog sound recording. --jpgordon::==( o ) 16:28, 8 December 2012 (UTC)[reply]
If it was just 1s and 0s, it would be a digital recording. As for the "degree of magnification", I'm going to assume that what jpgordon refers to is the strength of the magnetic field on the tape as left by the recording head. WegianWarrior (talk) 16:39, 8 December 2012 (UTC)[reply]
OK, then maybe what I should be asking is for a link to a physical specification as to the corresponding between minimum and maximum magnetization for standard cassette tape and minimum and maximum microphone/speaker signal. 67.163.109.173 (talk) 17:02, 8 December 2012 (UTC)[reply]
Orders of magnitude (magnetic field) gives the magnetic field of a tape head (I think it means during writing) at 24 µT (240 mG). The write head is given a current calibrated so that the highest amplitudes will produce a field strength on that head of say 240 mG (and -240mG for the opposite peak). This writes an analog signal onto the tape, such that if one subsequently passes the read head over the tape, the magnetism of the tape induces a current in the coil around the read head (probably about an order of magnitude less than the write current) which is amplified to produce a listenable analog electronic signal. The designers of a given tape device know the performance of their read and write electromagnets and so calibrate their circuits so that the signal level they use in their electrical circuits maps to a full (but not overdriven) use of the available magnetic response of the tape. In an idealised view the tape is truly analog in both the imaging of the amplitude and the time resolution, and so the input signal is simply transduced as a magnetic image. But in reality the the oxide on the tape forms a series of tiny magnetic domains, which limits the spacial (and thus temporal) resolution of the tape (e.g. if you had a tape with very chunky domains, it would image high frequency sounds poorly). Also the tape head does not produce a perfectly point-sized magnetic field, which means the magnet is writing to a few adjacent domains simultaneously. Tim Hunkin has a lovely demonstration of how this works (it used to be on his website, but I think he's taken it down) where he makes magnetic tape by putting ground rust on some Sellotape, and then rubs it past a simple electromagnet driven off a microphone (into which he yells) - then he drags the tape back across the electromagnet and you can (just) hear his voice back from the microphone (which functions like a little speaker). Back to your plot, above - if you had a really teeny tiny magnetic compass, and you moved it slowly down the tape, its needle would be defected back and forth - the extent of its deflection would correspond to the extent of deflection in your plot from the zero centreline. -- Finlay McWalterჷTalk 17:16, 8 December 2012 (UTC)[reply]
Unfortunately this is one of those subjects where it is necessary to overcomplicate things. The magnetization on the tape is not a straightforward monotonic function of the signal, because of tape bias, especially AC bias. AC bias is used in audiocassette recording to overcome the hysteresis and nonlinearity of the recording medium.--Heron (talk) 03:40, 10 December 2012 (UTC)[reply]
There also exist Digital Audio cassettes whose standard is described in the linked article. SkylonS (talk) 09:59, 10 December 2012 (UTC)[reply]
December 8
The Hoverboard in just roughly 2 years, 1 month - how will Mattel prepare?
"The reference desk does not answer requests for opinions or predictions about future events" - which the OP is clearly asking for AndyTheGrump (talk) 02:25, 8 December 2012 (UTC)[reply]
Hello.
What obstacles must we discover / solve / overcome in order to build a practically-working hoverboard that is affordable enough for everyday, middle-class parents to buy for their children?
How likely is it that Mattel / any toy company will develop the Hoverboard in time?
Starting on January 1, 2015, Mattel's call center may get barraged by a massive queue of calls asking to order the Hoverboard. How might they deal with the sudden, massive influx of demand for an item that may or may not exist by then? If there is no hoverboard by 1-1-2015, how would Mattel deal with the overwhelming volume of calls asking for an item that hasn't yet been made (in time)?
If Hoverboards arrive after all, who might make models intended for adults? --Mount Zynar (talk) 22:23, 7 December 2012 (UTC)[reply]
They won't. It is a 'replica', it doesn't hover. [1]
Please do a little research first, rather than wasting our time with ridiculous questions. AndyTheGrump (talk) 23:25, 7 December 2012 (UTC)[reply]
I suppose you could make a real hoverboard, of a sort, based on a hovercraft:
A) Put a powerful downward fan at the front and back, with a rubber cowl around the bottom. You would stand in the center.
B) It could have a remote "dead man switch" the user would hold in his hand, so it would stop if the user let go.
C) I'd expect it to be somewhat unstable, so you'd need to balance on it, like a surf board.
D) It would be loud. Active noise reduction might help.
E) It would use up it's fuel supply quickly, especially if it ran on batteries. If it ran on gasoline or any other flammable fluid, then you'd have fumes to deal with and potential for fires.
F) I imagine it without any forward thrust provided. The user could roll downhill or kick off the ground, like with a skateboard. Leaning forward might also let some air escape out from under the back, and give a slight forward thrust that way.
G) Hopefully it would be light enough that it could be carried home when it runs out of charge/fuel. StuRat (talk) 23:32, 7 December 2012 (UTC)[reply]
Probably because there was no Pan Am ticket office to call. But Mattelmay still exist. --Mount Zynar (talk) 02:20, 8 December 2012 (UTC)[reply]
One other point:
H) You should put wheels on all 4 corners, to prevent damage from striking the ground, and to allow it to work as a normal skateboard when the fuel/charge is used up. StuRat (talk) 17:57, 8 December 2012 (UTC)[reply]
Isn't it obvious that the lack of Helium-3 on Terra is entirely the fault of Luna?
Earth originally had the same Helium ratio as any other rock in Sol system and then got knocked up to such an extent that she gave birth to the moon. In the resulting mix-mash, all the volatiles escaped. Since then oceans of Hydrogen have fallen from the sky while the Helium-4 was replaced through Alpha-decay. Ergo a Helium-3 deficiency. Hcobb (talk) 16:56, 8 December 2012 (UTC)[reply]
My apartment has far too many of these. Can anyone tell me what it is? I assume it will be helpful to say that I live in Cincinnati, Ohio. -FisherQueen (talk · contribs) 18:58, 8 December 2012 (UTC)[reply]
See Brown marmorated stink bug, and note that the article text says that the banding on the antennae is unique to this species. --Modocc (talk) 19:10, 8 December 2012 (UTC)[reply]
How did you manage to force the bug to pose for the picture near a penny? OsmanRF34 (talk) 20:09, 8 December 2012 (UTC)[reply]
Sometimes, Bugs, you are truly awful. Thank you. --jpgordon::==( o ) 01:16, 9 December 2012 (UTC)[reply]
Best...ref desk...pun...ever! μηδείς (talk) 03:57, 9 December 2012 (UTC)[reply]
They are a very common invasive species throughout much of the temperate USA. As they seek warmth in cold weather, they often find their way into houses and outbuildings. When found in late fall, they are very sluggish, and easily posed. SemanticMantis (talk) 04:33, 9 December 2012 (UTC)[reply]
They're pretty docile; not particularly afraid of people, or gentle swatting towards them, and they don't fly with any apparent intent to avoid obstacles. Easy to move objects near them in my experience. DMacks (talk) 08:30, 9 December 2012 (UTC)[reply]
They don't pose many problem to humans. Sure, they are a little annoying, but they will not eat your food or furniture, spread disease, or suck your blood. Most of them will quietly die in a corner, and you just clean them up when you sweep. They are mildly attracted to lights, and if they bother you, you can usually easily capture them in a jar, and dispose at your convenience. Because they are stink bugs, they can emit foul odors, but in my experience, they usually don't if you catch them slowly. SemanticMantis (talk) 04:38, 9 December 2012 (UTC)[reply]
Thank you! The girlfriend was right then; she said stink bug, and I said it couldn't be that, because they don't stink. I'll continue my strategy of ignoring them, throwing them out the window when I see one, and encouraging the cat to eat them. -FisherQueen (talk · contribs) 12:42, 9 December 2012 (UTC)[reply]
I love the critters. They are cute and docile and quite fascinating, really. They usually fly to my reading lamp, and then just walk around a bit on my end table, until I put them back outside (or the smarter ones go hide and escape my banishment). Feeding them to a cat however would likely be equally unpleasant for both. μηδείς (talk) 17:25, 9 December 2012 (UTC)[reply]
Why are they called "stink bugs" if they don't stink? Alansplodge (talk) 22:28, 10 December 2012 (UTC)[reply]
Some varieties of Pentatomidae can exude a foul smelling chemical when threatened. And, of course, squashing them releases all of it at once. — The Hand That Feeds You:Bite 23:42, 12 December 2012 (UTC)[reply]
Drugs not working the first time
Why some drugs, like marihuana, don't work for some people the first time they use them? OsmanRF34 (talk) 19:47, 8 December 2012 (UTC)[reply]
I don't think the answer is really known. It might be that the drugs do work but that on the first exposure people don't realize it because they don't know what changes to look for; or it might be that the first exposure induces some changes in brain chemistry that potentiate the effects of later exposure. Looie496 (talk) 20:40, 8 December 2012 (UTC)[reply]
Marihuana is very fat soluble and requires a little bit of a build-up in the liver before it will be effective--but everyone I know who's tried it has gotten high the next day on their second joint. I am sure we have some sort of source on drug half-life and so forth that will be relevant to this. μηδείς (talk) 01:26, 9 December 2012 (UTC)[reply]
Genetics are also a factor -- for example, people who are homozygous for the short version of the D4DR gene appear to be less affected by marijuana and other drugs. 24.23.196.85 (talk) 02:36, 9 December 2012 (UTC)[reply]
Genetics might be a factor for a drug not working at all, but does genetics make the drug useless the first time we use it but not subsequent times? OsmanRF34 (talk) 18:53, 10 December 2012 (UTC)[reply]
As pointed out earlier, it's fat soluble. Which means continued use builds up more of the chemical in the body over time. "Less affected" doesn't mean "unaffected," after all. — The Hand That Feeds You:Bite 23:46, 12 December 2012 (UTC)[reply]
If some person grasps and briefly hangs on high-power cable line, what role would sweat or other moisture droplets on his/her hands play? Is their absence crucial to body's electric resistance? --93.174.25.12 (talk) 22:23, 8 December 2012 (UTC)[reply]
Sweat increases the skin conductance. But if you grasp a high voltage line, it really won't make any difference. Looie496 (talk) 23:28, 8 December 2012 (UTC)[reply]
But if you hang from the cable (not touching the ground) your body isn't part of an electrical circuit and the resistance won't matter. Sjö (talk) 00:00, 9 December 2012 (UTC)[reply]
So even if you grasp the line with wet hands and not touching the ground, you wouldn't be affected in any way? 93.174.25.12 (talk) 00:19, 9 December 2012 (UTC)[reply]
While you should generally avoid grasping high-voltage transmission lines under any circumstances unless you know what you're doing, generally speaking...yes, you can safely grasp a high tension line without harm as long as you're not in contact with (or near) anything else at a different potential (voltage and phase). In situations where power workers need to inspect or maintain live high-voltage transmission lines, they can bond themselves appropriately to the line so that their tools and bodies are at the same potential as the line and no current flows through them. (See live-line working and Faraday suit.) This is also why birds can land on high-tension lines without getting fried. TenOfAllTrades(talk) 00:49, 9 December 2012 (UTC)[reply]
Similarly, squirrels can cross such lines safely - unless they make the mistake of "grounding" themselves when they reach the next pole. Then you get fried squirrel, which may sound gross to us, but is a prized delicacy to your average crow. ←Baseball BugsWhat's up, Doc?carrots→ 12:48, 9 December 2012 (UTC)[reply]
Fried squirrel is a normal way of preparing it. Or stew, barbecued or ground in to sausage.[2]75.41.109.190 (talk) 18:14, 9 December 2012 (UTC)[reply]
Them's good eatin'. Coincidentally, I'm currently translating a scientific paper on Glis glis, the Edible Dormouse, which was, and still is to a limited extent, eaten in Europe. It was much prized by the Ancient Romans. By the way, it's off the menu here in Poland, where it is considered a protected species. Dominus Vobisdu (talk) 04:46, 10 December 2012 (UTC)[reply]
While we're here, and discussing squirrels and 'good eatin', a question for USAians (e.g. Bugs). What is the difference between 'critters' and 'varmints' (if any), and where do squirrels fall within the categories? Dictionaries seem to suggest that 'varmints' = vermin, and as such not 'good eatin' accordingly, but given the evident delight taken in shooting the tufty-tailed skyrats and man-cooking them (i.e. grilling, frying, or roasting with an oxyacetylene torch) taken your side of the pond, are they somehow what anthropologists tend to describe as 'liminal' - neither one thing nor another? If the local butcher (assuming such still exists) had "Fresh Squirrels, $1.40 each" advertised for sale, how would the customers react? AndyTheGrump (talk) 05:31, 10 December 2012 (UTC)[reply]
Yes, I think varmints are to critters as weeds are to plants. That is, they are unwanted critters, just as weeds are unwanted plants. And only rednecks eat squirrels in the US. Why do rednecks eat roadkill ? ... Cuz the tire tread pattern makes a great irrigation system for the gravy. StuRat (talk) 06:01, 10 December 2012 (UTC) [reply]
The word "critters" is derived from "creatures" and "varmint" from "vermin" so the "plants versus weeds" analogy is correct. Roger (talk) 06:06, 10 December 2012 (UTC)[reply]
I thought "varmint" was synonymous to "throw up" ;) - ¡Ouch! (hurt me / more pain) 11:15, 11 December 2012 (UTC)[reply]
"Excuse me, I have to go and varmint". Nah, that'll never float. -- Jack of Oz[Talk] 11:51, 11 December 2012 (UTC) [reply]
I'm in eastern Kentucky, where this is not uncommon. (And we prefer "hillbillies," thankyouverymuch. ;) ) Squirrels have so little meat that it's typically just consumed by the hunter & their family. There's no real business for squirrel meat. You might find someone selling some burgoo at a festival or such, but that's about as close as you'd get to "commercial" squirrel. — The Hand That Feeds You:Bite 23:55, 12 December 2012 (UTC)[reply]
It won't make much difference with a high voltage line, but moist skin plays a large role in electrocution at lower voltages. --Srleffler (talk) 19:31, 11 December 2012 (UTC)[reply]
head shave vs bald
Hi,
I wonder why when someone shaves his head completely bald with a razor, his scalp becomes gray. I mean the color of the hair isn't gray, so what do we see?
I read that bald people have hair at the bald parts, but it is too short to see, so why we don't see gray either at those parts? — Preceding unsigned comment added by 77.126.90.48 (talk) 23:32, 8 December 2012 (UTC)[reply]
It's because of hair follicles, whose multitude gives a shaved head its dark shade (similar to any shaved area of body), but shaved blonde scalps may have a lighter shade than those of brunettes. Permanent hair removal removes all such traces. Brandmeistertalk 00:13, 9 December 2012 (UTC)[reply]
If you shave your head with a razor, you're not removing the hairs, you're just cutting them very very short. My sort of baldness, on the other hand, is due to the follicles shrinking (in response to DHT), and what remains is either nothing or tiny tiny little soft, colorless hairs. I don't actually know if those tiny hairs grow, fall out, and are replaced. --jpgordon::==( o ) 01:13, 9 December 2012 (UTC)[reply]
December 9
Is the refrigeration technique advanced enough to preserve ice in July in Hittin during the 12th century?
In the article Battle of Hattin there is a sentence:"The exhausted captives were brought to Saladin's tent, where Guy was given a goblet of iced water as a sign of Saladin's generosity." I checked Tiberias#Geography and climate where was near the location of battle, and found that only in extreme occasions did the temperature there reach zero. Those articles about refrigeration such as Refrigeration, Timeline of low-temperature technology are alomst devoid of information about that time (There are some remnants of earlier Ice house in China and Rome, but I am not sure if it helps to this question). So was ice ever possible at that time and that location?--Inspector (talk) 08:01, 9 December 2012 (UTC)[reply]
Our article on Evaporative cooler doesn't mention the production of ice, but I believe it's possible at night in a dry atmosphere. In my student days when I couldn't afford a refrigerator, I kept milk cool by this method, but the humidity was too high to make ice. In some areas of desert, it is possible to produce ice at night purely by radiative cooling, using an ice pit insulated from the surrounding warm sand with straw, and the technique was developed in ancient times, with the resulting ice being stored in ice houses. Dbfirs 08:36, 9 December 2012 (UTC)[reply]
I don't know if radiative cooling is really a feasible way of producing ice. The linked article has some doubts about it. OsmanRF34 (talk) 13:51, 9 December 2012 (UTC)[reply]
I think it's possible, but only in certain conditions, especially including a clear dry atmosphere above the ice pit. The discussion seems to conclude that it doesn't violate any laws. Dbfirs 20:10, 9 December 2012 (UTC)[reply]
A couple of sources[3][4] state he had ice brought from the mountains. Clarityfiend (talk) 08:40, 9 December 2012 (UTC)[reply]
Ice can be both be brought frozen over the mountains from pits dug for the purpose or brought from the mountains. μηδείς (talk) 04:26, 10 December 2012 (UTC)[reply]
Subconscious phrases in mind
Sometimes, but not often, I experience short phrases that pop in while I rest or sleep. It's not like I hear them, they just flare up in mind spontaneously, without my intent. The phrases are like "Want to take a photo?", "No, thanks" etc. They appear before I fall asleep or so when I'm not dreaming. I think it's related to brain rest, but not sure. Is there a name for such phenomenon?--93.174.25.12 (talk) 10:42, 9 December 2012 (UTC)[reply]
In the Sleep article, it mentions a "non-REM stage 1", which might be what you're experiencing. When you're not quite awake and not quite asleep, things can seem "real" which aren't real. I've had this happen from time to time... as have many others, as this is presented as an explanation for some folks who think their bedroom is "haunted", as they feel as if they're being stifled by some spook... but that only happens when they're in that in-between stage, and it feels "real". Seems to me there's a more specific term than "non-REM stage 1", but it's not popping right now. P.S. If this is bothersome to you, as opposed to merely interesting, you should consider seeing a professional. (If nothing else, they could probably give you better terminology.) ←Baseball BugsWhat's up, Doc?carrots→ 12:43, 9 December 2012 (UTC)[reply]
No, this is known as "hypnagogic imagery" -- see Hypnagogia#Sounds. "Hypnagogia" is the formal term for the process of falling asleep. Looie496 (talk) 15:54, 9 December 2012 (UTC)[reply]
Very good. I don't know if either terms are what I saw on TV, but sleep paralysis explains the spooky stuff, and hypnagogic imagery explains my experience perfectly... and it sounds like it would explain the OP's as well. ←Baseball BugsWhat's up, Doc?carrots→ 17:13, 9 December 2012 (UTC)[reply]
I believe I saw that as well, many years ago, on one of those "unsolved mysteries"-type shows. I don't recall the details of the show, but it led me to a book called The Terror That Comes in the Night, which was quite interesting, though it looked at the phenomenon through the paradigm of folklore study. By coincidence, a few years later my young daughter began experiencing night terrors, which is a closely related phenomenon; she never saw any hags though. Matt Deres (talk) 20:55, 9 December 2012 (UTC)[reply]
As someone whose workplace was destroyed on 9/11, and who has been treated for night terrors, with good reason, I can assure you that mere vocalizations as one falls asleep do not count as night terrors. 04:24, 10 December 2012 (UTC)
It's not a contest; night terrors often have no trigger in the way that nightmares (for example) often do; the experience itself is the terror as the half-asleep brain attempts to interpret the paralysis it suddenly "finds" itself in. Night terrors are not necessarily worse or easier to live with than nightmares; they're something else entirely. Matt Deres (talk) 01:42, 11 December 2012 (UTC)[reply]
Humidifiers and temperature
The manual of my (ultrasonic) humidifier states that increasing the humidity with it will increase the felt air temperature and effectively reduce heating costs. Two questions: 1) Does higher humidity always mean higher felt air temperature or does it depend on temperature as well? At least to me, for sub-zero (Celsius) temperatures, a dry climate is much more pleasant than a wet one. 2) All costs included, can a humidifier really lower total expenses? bamse (talk) 19:53, 9 December 2012 (UTC)[reply]
Higher humidity will slow evaporation from your skin; and it increases the heat capacity of the air, which can make you feel warmer in hot temperatures and cooler in cold temperatures, also depending on the air flow, and other factors. See heat index for the combined effect of heat and humidity. μηδείς (talk) 20:05, 9 December 2012 (UTC)[reply]
Thanks. Any ideas on the second question? bamse (talk) 19:14, 10 December 2012 (UTC)[reply]
There are many factors that go into the second calculation:
1) What type of humidifier is it ?
a) You said yours is ultrasonic. Now, the energy usage for that should be low, however, you need to use distilled water, since it will otherwise vaporize all the minerals in the water and coat the area with dust. So, the question comes up as to how much you pay for your distilled water. If you buy it, that will get expensive fast. If you distill it yourself, then the energy to do that must be considered. If you just use tap water and ignore the dust, the cost will be lowest.
b) An electric evaporation humidifier will cost a lot, too, because electricity is expensive. Tap water can be used, but will create mineral deposits in the humidifier (which is much better than on your walls).
c) A natural gas humidifier will be less expensive. I essentially do this by having a huge stock pot on the stove with a tiny flame always under it. This has the advantage that moisture is also an exhaust product from combustion, so you get moisture from two sources. As above, tap water mineral deposits go into the pot, so don't plan on using it for food again. Use a huge pot so you can leave it on overnight or while at work without danger of it running dry (mine lasts about 2 days). Another approach is always having a big pot of stew of the stove in winter.
d) An integrated humidifier which works with the furnace, using air already heated there, is probably best of all. This also has the advantage of distributing it throughout the house. (With any single source of humidity, you will see higher humidity in that room, possibly with condensation at the windows leading to mold, and lower humidity elsewhere.)
2) What type of heat does your home have ?
a) Natural gas heating is the least expensive (unless you count heat pumps, in the temperature range where they work). However, the water vapor generated by combustion goes up the chimney, along with some of the heat.
b) Electrical is the most expensive, except that some forms can be set up for zone heating, so you only heat the areas you want.
3) How well insulated is your home for heat and humidity ? A poorly insulated house will lose heat and humidity through the walls, and water will also condense on the windows, and maybe walls, causing mold.
So, it's probably impossible for us to judge which is cheaper, in your home. However, humidity should be kept up for other reasons, like avoiding dry skin, callouses, chapped lips, cracked wooden furniture, etc. StuRat (talk) 19:40, 10 December 2012 (UTC)[reply]
entropy in information science and thermodynamics
what, if anything, is the relationship between the word "entropy" used in information science (e.g. 2 bits of entropy per english letter, etc) and entropy in thermodynamics? if nothing, why is it the same word - historical coincidence? or is there nevertheless something in common. thanks. 178.48.114.143 (talk) 20:45, 9 December 2012 (UTC)[reply]
Can you be a little more vague or general, please? I understand that your answer is very precise but I am neither a mathematician or physicist. I understand the latter because it's "obvious" to me or intuitive, but not the thermodynamic version. Why is it not the same formula? Could you 'dumb it down a tad' for me? more importantly, can you explain the relationship between the concepts. thanks. 178.48.114.143 (talk) 21:24, 9 December 2012 (UTC)[reply]
The difference between the formulae is just cosmetic. Both formulae define entropy as the negative of a sum each term of which is given by a probability times its logarithm. Dauto (talk) 22:59, 9 December 2012 (UTC)[reply]
This doesn't help me at all. Are the units the same? Is the concept the same? What are we talking about and what does heat have to do with winzip (for example ; i.e. higher-entropy files will be larger when zipped). I get the latter concept and would like to know what, if anything, this has to do with thermodynamics. Thanks. 178.48.114.143 (talk) 23:15, 9 December 2012 (UTC)[reply]
It is not a coincidence. Thermodynamic entropy came first, but the same concept was later applied to information. See Entropy in thermodynamics and information theory, which handles them both and describes the ways they do or don't overlap. One early and interesting example of the interrelation of thermodynamic and information principles is Leo Szilard's informational interpretation of Maxwell's demon and its implications for the second law of thermodynamics. --Mr.98 (talk) 23:43, 9 December 2012 (UTC)[reply]
Guys, you're asking me to read these articles (I'd found the one you just linked already) and I'm saying it's too hard. Can you help? Thanks. 178.48.114.143 (talk) 23:52, 9 December 2012 (UTC)[reply]
(I didn't mean to annoy you by posting a relevant link — we lack the ability to know what you have and have not read, if you do not tell us, and figured that if you had looked at the article, even uncomprehendingly, you'd have understood that the shared language was not one of historical coincidence — the very fact that there is an article seems to indicate that, I thought.) --Mr.98 (talk) 01:46, 10 December 2012 (UTC)[reply]
In both cases, entropy is a precise way to compute something that corresponds to the "randomness" of the system. Randomness is sort of vague and poorly defined. In information-theory contexts, we might hand-wavingly say that random information content is less structured, or has less redundant/repeated information. In statistical physics or thermodynamics, we might say that random arrangements of molecules have more atoms flying around in more different directions. All of these hand-wavey imprecise statements leave a lot to be desired, because we can't compute values for such vague concepts. So, we define entropy, which is a convenient and precise way to express this sort of concept. And, as it happens, our equation definition is useful because we can relate it to other computed quantites. We have equations to relate entropy and temperature, in the case of thermodynamics. In information theory, we have equations to relate entropy and algorithm efficiency, or entropy to expected data loss rate, and so forth. Nimur (talk) 00:01, 10 December 2012 (UTC)[reply]
This is very helpful. Are we saying that the entropy is LITERALLY the same concept, or an analogous concept? By literally the same concept, I mean, "air molecules on one side vacuum on the other take less bits to describe than in a highly mixed state" just like a file that is five million ones and then five million zeros takes very few bits to describe - it has little entropy. To me, entropy isn't "counter-intuitive" at all because I always equate it with the size if you try to compress or describe it (e.g. winzip). So is it literally the same concept - if you had a "replicator" and sent across a description of the system to be replicated perfectly, then you don't need very much information (it "zips" well) if it's all air molecules on one side and vacuum on the other (e.g. low-temperature) but if you wanted to replicate it perfectly at a higher temperature or better-mixed then you need more information? Is this LITERALLY the same concept (not just an analogy)? Or am I now going too far? 178.48.114.143 (talk) 00:45, 10 December 2012 (UTC)[reply]
Ok, you asked for it -- here's a very hand-wavy discussion that is not rigorous or precise, but I think captures the general similarities. The Shannon notion of information (Information_theory#Entropy) equates information with randomness. The idea is that there is more "information" in a specific random string, than a specific string with a lot of structure. For instance, Let X be the string with 50 ones and 50 zeros. This has low information, because it can be described much more succinctly, and we could transmit or store that shorter expression, rather than the whole string. On the other hand, let Y be a "random" string of 100 ones and zeros, say Y=10101110000101000011110100101010100101001...1
Y carries more information than X because we'd basically have to record every digit to faithfully transmit that string. Though I'm speaking qualitatively, we could work out specific quantities of entropy for X and Y by computing the formulae that are (tersely) quoted above. This discussion also matches up with the "entropy is a measurement of disorder" analogy in physics. In that case, X is similar to a box with air on one side and vacuum on the other, and Y is similar to a box of air at uniform pressure. So, the mathematical form of "structure vs. order" in physics is also useful for describing "high vs. low information". Counter-intuitively, the "structured" case, X, corresponds to the low information example. Another application of this same idea is to measuring biodiversity, see Shannon index.
In short, we use the same word because they are mathematically the same thing. The differences are in the way that we interpret the math, in hopes of that interpretation being useful for solving a certain problem or discussing a certain field of science. SemanticMantis (talk) 00:02, 10 December 2012 (UTC)[reply]
This is very helpful. See my question imm. above at same indent level as this. 178.48.114.143 (talk) 00:45, 10 December 2012 (UTC)[reply]
Let me give you a very explicit answer this time. Yes, the two concepts are LITERALLY the same concept, but applied to different situations, and that's why the mathematical formulae are identical. Dauto (talk) 14:29, 10 December 2012 (UTC)[reply]
Is my replicator analogy also 100.00% correct? Imagine you have a supernatural replicator that can make anything as described (that comes over its communications link). Is it the case that to perfectly replicate a hot drink, you need more bits (more coming over the line) than to perfectly replicate a cold drink? I mean, imagine that you describe an x by x by x cold vacuum, and it replicates that for you. That takes almost no bits, it's like zipping an empty file. Then as you are replicating more and more complicated things, it takes larger and larger space to get all that to the replciator, even compressed. Now here is the question: IS THE THERMODYNAMIC ENTROPY IN AN OBJECT LITERALLY EQUIVALENT TO HOW MUCH ENTROPY ITS DESCRIPTION TO THE REPLICATOR CONTAIN? Please bear in mind that the questino this time is hether this description is NOT hand-wavey but 100% perfectly rigorous. Is what I've just said, technically, perfectly, rigorously true in a mathematical and physical sense? Thank you. 178.48.114.143 (talk) 19:09, 10 December 2012 (UTC)[reply]
note also that by introducing the supernatural replicator I have stopped having to talk about "concepts" and reduced the question to a yes-or-no truth value about how many bits must get to the replicator given a certain object given optimal compression, and whether this is literally the same "number" as its thermodynamic entropy. There is no longer any question of "concepts" but instead, teh truth-value in a thought experiment. THank you. 178.48.114.143 (talk) 19:12, 10 December 2012 (UTC)[reply]
Yes, yes. The two concepts of entropy really are identical. The only difference is choice of units. Information entropy is usually measured in bits or nats while the thermodynamic entropy is measured in Joules/Kelvin. That's why the formula for the thermodynamic entropy is multiplied by Boltzmann constant which is nothing more than a conversion factor between those choices of units. Dauto (talk) 20:45, 10 December 2012 (UTC)[reply]
Achievement unlocked! Thank you, Dauto, and others, for your answers. In fact I now feel (if it really is this simple :) ) that I have understood it very well. Forgive me if I seemed incredulous - the relationship seemed a lot more complicatd than before I asked this question :). Thanks for all your help! 178.48.114.143 (talk) 21:30, 10 December 2012 (UTC)[reply]
Chloromethane on mars
The SAM instrument on Curiosity is a pyrolysis GC-MS with a relative slow heating ramp of less than 50°C / minute. They found chlorinated methane [5] in a sand dune on mars (they choose it because it would be the point with only minimal organics). (Viking found the same stuff in a sand dune in the 1970s) It is a very small amount. The other results make it relative obvious that perchlorate is present. Perchlorate decomposes to oxygen and chlorine. The source of carbon might be earth organics traveling with the rover to mars, mars organics, or inorganic carbon from carbonates.
Is it possible to get chloromethane from carbonates and perchlorate? Carbonates decompose and give off carbon dioxide which decomposes at temperatures above 800°C forming carbon and carbon monoxide Boudouard reaction. I can come up with no good rection forming that compounds from carbonates. --Stone (talk) 23:34, 9 December 2012 (UTC)[reply]
Neither can I, and I'm very familiar with these types of reactions. 24.23.196.85 (talk) 00:05, 10 December 2012 (UTC)[reply]
I can't answer the question directly but, chloromethane is known to be produced by several species of phytoplankton here on Earth. It's also seemingly becoming clear that liquid water once existed on mars, and as per [6], it's looking more certain than ever that life once existed on Mars, it's possible the chloromethane they found could be organic in origin. douts (talk) 22:14, 11 December 2012 (UTC)[reply]
Does Quantum entanglement apply to the entire visible universe?
Are we in a state of Quantum entanglement with those distant proto-galaxies at the edge of Hubble's fuzzy vision? Did we have time to mingle into a single cosmic scale wave function before Inflation (cosmology) went off like a bang? Just how far out of the box is the reach of the undead cat? Hcobb (talk) 00:46, 10 December 2012 (UTC)[reply]
No. Nonclassical entanglement isn't a normal state of affairs; it's very easily destroyed by environmental interactions. That's one reason building quantum computers is so hard. -- BenRG (talk) 02:54, 10 December 2012 (UTC)[reply]
But isn't the entire universe ruled by a single wave function and decoherence simply an mirage caused by our inability to fill in all the blanks? Coherence is said to be lost once one of the particles interacts with some "random element" from the outside, but that simply means that whatever is being interacted with is itself part of a larger coherence? Hcobb (talk) 03:23, 10 December 2012 (UTC)[reply]
Well, operationally speaking, there isn't any nonclassical result like Bell's theorem that applies to different galaxies. Philosophically speaking, in the many-worlds/relative-state picture (which is what you're probably thinking of), measurements entangle the measuring device with the system being measured, and this entanglement at the scale of the whole universe is responsible for the fact that the world appears as it does. I'm not sure it's right to call this entanglement, though. It's just plain old classical correlation, which is another name for entanglement that doesn't violate Bell's theorem. -- BenRG (talk) 05:21, 10 December 2012 (UTC)[reply]
What's to keep a distant galaxy from sending single photons at us followed by their observations to play Bell's theorem across the breadth of the Universe? Also wouldn't we use Entanglement-assisted classical capacity to chat with interstellar probes simply because of power considerations? There is no range limit on the wave function. Hcobb (talk) 16:55, 10 December 2012 (UTC)[reply]
Yes, in principle you can send one half of some Bell pairs toward another galaxy, wait until they've traveled far enough that they can be treated as part of that galaxy instead of this one, and then there's a small amount of nonclassical entanglement between the galaxies. This is an artificial situation maintained by careful isolation of the entangled particles. You could use those Bell pairs for superdense coding, but the amount of nonclassical entanglement would decrease as the Bell pairs were consumed. It's very hard for me to believe that this could ever be the most power-efficient option. For starters, the Bell pairs would presumably need active containment for the entire trip. -- BenRG (talk) 17:25, 11 December 2012 (UTC)[reply]
But why the stored glycogen is broken down? And how can a dead cell's mitochondria be active? --PlanetEditor (talk) 11:31, 10 December 2012 (UTC)[reply]
There may be trace amounts of glycogen or glucose in the meat, but a very small amount. Labeling laws allows rounding (usually to the nearest whole number, sometimes to the nearest 5, depending), so small amounts get rounded down to zero. --Jayron32 13:01, 10 December 2012 (UTC)[reply]
The mitochondria consume the glucose before they die - cell death is not an instantaneous process. Roger (talk) 15:25, 10 December 2012 (UTC)[reply]
Curie temperature of welding steel
I understand that stray drops of molten steel from a welding operation will not be deflected by a magnet because the steel is above its Curie temperature 700 deg C. However the drop cools as it falls and will become magnetic. Does its magnetism recover instantly or does it increase gradually over a number of degrees aabout the Curie point? Will the falling drop aquire a cooler magnetic shell around a paramagnetic interior? Is there a formula for the cooling of a molten drop falling in air? SkylonS (talk) 09:32, 10 December 2012 (UTC)[reply]
See Curie's Law. The long and short of it: Below the Curie temperature, magnetization is inversely proportional to the temperature, whereas above it, the steel does not magnetize and its magnetic susceptibility is inversely proportional to the difference between the steel's actual temperature and its Curie temperature. The steel also exhibits critical behavior at its Curie temperature -- which means that it changes abruptly from paramagnetic to ferromagnetic as it cools down past its Curie point. 24.23.196.85 (talk) 00:57, 11 December 2012 (UTC)[reply]
The drops will also be affected by the current induced by moving past a magnet, the effect would be to repel the drops. Dmcq (talk) 19:03, 11 December 2012 (UTC)[reply]
When toilets flush
What determines whether a toilet's flush will be more... forceful? Stronger? Basically, what makes it so that some toilets barely trickle the water and waste out while others make you think that the whole of reality will be sucked down the pipes? Is it a difference in altitude over where the sewer/septic system is? Or a combination of other factors? Dismas|(talk) 11:58, 10 December 2012 (UTC)[reply]
It may depend on local water pressure in pipes and on toilet's construction, as well as on the amount of water in toilet cistern. Brandmeistertalk 14:39, 10 December 2012 (UTC)[reply]
Yes, the amount of water discharged from the cistern, the rate at which it is discharged, the height of the cistern above the toilet, and the dynamics of the flow, are all significant, but the pressure in the pipes is a factor only in the time it takes to refill the cistern. Dbfirs 16:24, 10 December 2012 (UTC)[reply]
Height above the sewer line is not a factor, except in the rare cases where the sewer backs up all the way to the toilet. The size of the discharge hole in the bottom is also important, and any clog will reduce that. The infamous low flow toilets, which may require several flushes instead of one, are one cause of poor flushing. StuRat (talk) 18:30, 10 December 2012 (UTC)[reply]
Why is solid carbon at room temperature much denser than minus 300 degree oxygen solid?
Carbon atoms are lighter and larger. 96.246.63.155 (talk) 13:42, 10 December 2012 (UTC)[reply]
Only answer I can come up with without actually doing the work of researching it: carbon forms four bonds, allowing for packed crystal lattices, while oxygen almost exclusively forms O2 molecules, which do not pack well at all. i kan reed (talk) 21:59, 10 December 2012 (UTC)[reply]
But the OP asked about it at "minus 300 degree oygen solid", which does not make any sense temperature wise or in the english language, but presumably is meant to mean "at 300 K below room temperature, where oxygen is solid". Which it will be. Oxygen is not O2 molecules when solid, it is a continuous crystal structure. The OP is trying to compare room temperature carbon (a solid) to near zero K oxygen, which is a crystaline solid. Darned if I know why. Perhaps he meant the diamond form but forgot to say so. The OP needs to clarify what he wants. Wickwack 120.145.0.81 (talk) 00:39, 11 December 2012 (UTC)[reply]
He probably means -300°C. Yes, absolute zero is at −273.15°C, but they were just rounding imprecisely. StuRat (talk) 00:51, 11 December 2012 (UTC)[reply]
(ec) That would be below absolute zero, so in the everyday sense, there is no such temperature. There is a sense in which such a temperature could exist (see negative temperature) but it's "hotter than infinite temperature", so it doesn't fit with solid O2.
I would guess instead that the intent is −300 degrees Fahrenheit, but then it's incorrect, as O2 does not solidify until you get down to −362 F, if I remember correctly what I just looked up. --Trovatore (talk) 00:57, 11 December 2012 (UTC)[reply]
Solid oxygen is a crystal lattice of O2 molecules. "Molecular" and "lattice" are not mutually exclusive terms. It's simply bound O2 molecules connected in a lattice by van der Waals forces.[7]Someguy1221 (talk) 00:50, 11 December 2012 (UTC)[reply]
Only at very high pressures, I think. Wickwack 60.228.248.5 (talk) 01:56, 11 December 2012 (UTC)[reply]
No, it is true down to vacuum. At very high pressures, you can get O8 molecules in a van der Waals lattice, but you never get a covalent lattice as with carbon. Someguy1221 (talk) 02:01, 11 December 2012 (UTC)[reply]
One would think people would take the time to make sure their questions make sense, that way we could spend our time debating the best answer instead of debating the meaning of the question. Just saying it... Dauto (talk) 15:46, 11 December 2012 (UTC)[reply]
AIDS
I just got done watching a documentary about AIDS in Africa, and it got me thinking, do black people have some genetic predisposition towards getting HIV? Their continent has AIDS as an epidemic and while some people may blame lack of education, There are many other poor, uneducated areas of the world where HIV is not epidemic, although it is still high, such as South America, and the more rural areas of South East Asia.--Wrk678 (talk) 13:59, 10 December 2012 (UTC)[reply]
AIDS is only an epidemic in southern Africa. I don't know are they too poor to afford (enough) condoms? Then that's the reason. You can't really end humping now can you? Sagittarian Milky Way (talk) 14:20, 10 December 2012 (UTC)[reply]
Didn't it spread worldwide really quickly the first time a world traveler got it though? I believe Congo is the source but that isn't the place the hardest hit. Maybe places like Botswana with the ridiculous amount of AIDS are poor even by third world standards. It's hard to get poorer than sub-Saharan Africa (South Africa and a few others like maybe the Ghana with the hydroelectricity excepted). Sagittarian Milky Way (talk) 15:17, 10 December 2012 (UTC)[reply]
Actually Botswana is one of the wealthiest countries in Africa on a per capita basis, however with a high Gini coefficient the wealth distribution is very skewed. South Africa has the highest Gini coefficient in the whole world. Roger (talk) 15:32, 10 December 2012 (UTC)[reply]
Come on, people, we keep a copy of the world's largest free encyclopedia right behind the Ref Desk. Use it!
Getting back to the original poster's question, it is true that some populations have a greater innate resistance to HIV infection. Carriers of one copy of the CCR5-Δ32 mutant form of the CCR5 gene have some resistance to HIV infection; individuals with two copies of the CCR5-Δ32 mutation are strongly resistant to HIV. It is estimated that roughly 10% of northern Europeans carry at least one copy of CCR5-Δ32, and perhaps 1% have two copies. CCR5-Δ32 is essentially unheard-of anywhere else in the world. By itself, however, this improved resistance in a small fraction of the population has a negligible effect on HIV infection rates in Europe (or elsewhere) versus those in Africa; the social, political, and economic factors contributing to the high levels of HIV infection and AIDS in sub-Saharan Africa are far more important. TenOfAllTrades(talk) 15:46, 10 December 2012 (UTC)[reply]
Right, ignore anything I said, those articles are very good. Basically, it's complicated. Wow, that is one crappy place to live. Sagittarian Milky Way (talk) 15:53, 10 December 2012 (UTC)[reply]
No small part of the African AIDS epidemic has been due to religion. The Catholic Church has consistently made empirically false claims about the effectiveness of condoms, and generally opposed contraception at every step. Since contraception is by far cheaper than education or treatment, this has greatly hindered attempts to tackle the epidemic. See Catholic Church and AIDS.
Also, I challenge the OP's comparison between sub-Saharan Africa and South America or Southeast Asia. Look at the 2011 UN Human Development Index. Almost every country in South America has a decent quality of life; Chile and Argentina have a Western-level HDI. Southeast Asia is indeed poor, but almost all of it has a "medium" HDI, whereas almost all of sub-Saharan Africa is "low". There's just no comparison between the poverty and loss of social order in this part of Africa and that in any other part of the world. There's a reason that most charity TV ads you see are about Africa rather than Haiti, China, the Philippines, or Papua New Guinea--because it really is much worse. --140.180.249.194 (talk) 18:19, 10 December 2012 (UTC)[reply]
I have to agree with IP 140. You'd be surprised how many negroes Benedict has had buttsex with. (Whoops, no refs? Who cares! They are Catholics!) μηδείς (talk) 21:41, 10 December 2012 (UTC)[reply]
Was that supposed to be a sourced and helpful contribution, or are you just PMSing like you always do? --140.180.249.194 (talk) 22:09, 10 December 2012 (UTC)[reply]
Nice one Ten --Lgriot (talk) 08:25, 11 December 2012 (UTC)[reply]
John Scalzi is very wise... --Jayron32 13:02, 11 December 2012 (UTC)[reply]
The factors involving the HIV epidemics in Africa are complicated and, frankly, not yet fully understood even by experts. There are many things going on there, and any single-variable, pat answer is going to be inadequate. (I don't claim to be an expert on this, but I've been surprised as at how much question there still is on some of the basic questions, like to what degree needles versus sexual activity are responsible for the cases.) That being said, I don't think anybody who has studied the issue thinks that genetic predisposition has anything significant to do with the rate of exposure there. "Dire poverty" is obviously the context for the answer(s) but it's not the answer itself. --Mr.98 (talk) 02:38, 11 December 2012 (UTC)[reply]
Dilated eyes
Last time I had my dilated eye exam I drove home after dark and all bright lights had a starburst effect around them. What causes the starburst? (I thought maybe it had to do with the pupil not being a perfect circle but I don't know if that would explain it.) RJFJR (talk) 17:15, 10 December 2012 (UTC)[reply]
Star-bursts or halos? Have you had any laser work done? Do you have astigmatism – if so at which angles (for both peepers)?--Aspro (talk) 17:50, 10 December 2012 (UTC)[reply]
Was it in your eyes or in the windshield - moisture, soap residue, construction of the glass... 88.112.41.6 (talk) 18:19, 10 December 2012 (UTC)[reply]
'star-bursts' (rays around bright lights). I didn't think it was the windshield but I suppose it could have been the windshield and I noticed the effect much more due to the dilated. RJFJR (talk) 22:31, 10 December 2012 (UTC)[reply]
I too have noticed the star burst effect after dilated eye exams, bad enough to make using a PC difficult, but I'm sure it didn't happen when I was young. Perhaps it is due to imperfections in the cornea and/or lens, and isn't noticed normally because the aperture is small enough to avoid most of the imperfections. Wickwack 120.145.0.81 (talk) 00:48, 11 December 2012 (UTC)[reply]
This is unscientific, but I suspect it has more to do with the fact that your iris dilator muscle as well as your ciliary muscle both are affected by the norepinephrine in the drops they give you, and that makes it more difficult to focus, leading to that effect. However, that's mostly just an educated guess. I did find one medical text that referred to halos after certain eye surgeries apparently due to an optical phenomenon with larger pupils, which may be of some interest, but I don't have the technical background to know if that answers your question. ISBN 0865777594, page 358. Shadowjams (talk) 17:17, 11 December 2012 (UTC)[reply]
Spontaneous generation vs. abiogenesis
What is the difference between spontaneous generation and abiogenesis? I think spontaneous generation can be summed up by "maggots come from flesh", where as abiogenesis can be summed up by "first population of cells comes from a population of organic molecules in the primordial soup in the early earth. It seems to me that both are trying to suggest that life comes from non-life. 75.185.79.52 (talk) 19:27, 10 December 2012 (UTC)[reply]
Yes, both of them are life coming from non-life, though the two ideas are very different they do have that fact in common. Dauto (talk) 20:34, 10 December 2012 (UTC)[reply]
The difference is that one says it happens all the time, and the other that it's an extremely rare event that only occurs once over billions of years and, perhaps, billions of planets, or even an infinite number of universes. Also, spontaneous generation is supposed to produce far more complex organisms, initially. StuRat (talk) 20:37, 10 December 2012 (UTC)[reply]
The (long-ago-dismissed) idea of spontaneous generation really applied to large organisms. If you left a bunch of cheese lying around the house, soon afterwards you have an infestation of mice...so mice are made from mouldy cheese(!)...that kind of thing. It was a stupid idea that was widely dismissed pretty much as soon as it had been suggested. We knew how mice reproduce(!) and cheese isn't involved.
Abiogenesis is a much more subtle idea - which is by far the most widely accepted scientific explanation for the beginning of life. It suggests that life arose as just one tiny step - from molecules that cannot not make copies of themselves to molecules that can - and once a single self-reproducing molecule had formed and copied itself, evolution would kick in and everything else would follow from that. But abiogenesis isn't about large scale organisms...it talks about things like individual RNA molecules. You wouldn't exactly say that an RNA molecule was "alive" but the steps from that to the most primitive bacterial cell are never especially large ones. Evolution drives those steps once a basic self-reproducing molecule had come into being more or less by chance. The threshold at which we'd consider something to be "alive" is a fuzzy one - not everyone agrees on a single definition. So the point at which "non-life" turns into "life" becomes a matter of semantics rather than science. I would define abiogenesis as that point where the primordial soup became susceptible to evolution. It's a cleaner definition than "non-life" to "life". SteveBaker (talk) 20:07, 11 December 2012 (UTC)[reply]
Transportation gunpowder ingredients world war I era
Research for historical novel. Need to know how munitions raw materials such as camphor, pyrite and saltpeter were packaged for bulk transportation in 1914. I've tried many web searches to find information from this period, with no success so far. 98.125.189.95 (talk) 19:40, 10 December 2012 (UTC)[reply]
By the time of WWI, gunpowder (often referred to as "black powder") was rather obsolete as an explosive. Modern high explosives, mostly TNT, were in use for the bursting charges of artillery shells, aircraft bombs and those huge mines that they set off in tunnels under the enemy trenches. Low explosives, such as cordite, were used as propellants, to fire shells and bullets out of guns and mortars. I believe that gunpowder was still in use for artillery fuzes. There is some information about British munition plants in Filling Factories in the United Kingdom which says that "The Filling Factory's raw materials, such as TNT, RDX, or propellants, such as cordite, were manufactured in National Explosives Factories (World War I) or Explosive ROFs (World War II) and transported, by railway trains, to the Filling Factories for filling into munitions, produced at other plants." Not much to go on, but it might give you some useful search terms. Alansplodge (talk) 22:04, 10 December 2012 (UTC)[reply]
I wonder whether you'd get some useful information by reading about some of the failures of such bulk storage. The resulting explosions were large and prominent events - so lots has been written about them - and I'd imagine that you'd find information about storage and transportation in those accounts. Check out List_of_the_largest_artificial_non-nuclear_explosions#World_War_I_era and start following the references. SteveBaker (talk) 20:35, 11 December 2012 (UTC)[reply]
Gays and noses
Women have more sensitive noses, they can smell more things then men. (so I heard).
If its true, does gays (men) have a similar ability? does they have a better noses then straight men?
Maybe some scientist already did experiments about it? Yes, this sounds like a really dumb question to investigate, but I'm sure I'm not the first one in the world who think about it :)
UPD: Sorry if my question sounds arrogant or insulting, its because bad wording (I'm not a native English speaker). Also, you can make fun from my bad English (if you think its funny) --Ewigekrieg (talk) 21:03, 10 December 2012 (UTC)[reply]
You might be the second. I also did no know that gays are all men or that there were none left. I guess you learn something new...165.212.189.187 (talk) 21:08, 10 December 2012 (UTC)[reply]
No offence. I didn't say gays are women or something. One of my friends is a gay and he is a definitely a man (and a good guy) --Ewigekrieg (talk) 21:16, 10 December 2012 (UTC)[reply]
The issue is that some gay people are women, gay refers to a person of either sex. You were also using the passed tense: "did gays have a similar ability" which implies that homosexuals have vanished. Obviously you didn't mean any of this and intended to refer to gay men in the present tense, but this is a reference desk on the internet so a degree of pedantry and perhaps snark is to be expected. --Daniel(talk) 21:25, 10 December 2012 (UTC)[reply]
Speaking of which, if you're going to correct others, you have to be squeakily correct yourself. The term is past tense, not "passed tense". Also, he made it clear he was asking about male gays; he wasn't saying that all gays are male. -- Jack of Oz[Talk] 11:49, 11 December 2012 (UTC)[reply]
There's a reference desk guideline that would seemingly frown on a snarky response to an obvious verb conjugation mistake in a non-native English speaker's question. Red Act (talk) 04:13, 11 December 2012 (UTC)[reply]
He added the "(men)" after, Jack. And The question came across extremely ignorant and snarky in its own right, regardless of improper verb conjugation. is there a ref desk guideline for that, Red?165.212.189.187 (talk) 13:49, 11 December 2012 (UTC)[reply]
Gay men do have a greater olfactory sensitivity to some pheromones than do straight men[8], and the activation of neural circuitry in response to pheromones is different than in straight men; see Neuroscience and sexual orientation#Response to pheromones. But I'm having a harder time finding information about olfactory sensitivity in gay men in general. Red Act (talk) 21:45, 10 December 2012 (UTC)[reply]
"Gays" do not form a well-defined biological group. But I smell great. μηδείς (talk) 21:38, 10 December 2012 (UTC)[reply]
You seem to smell like anyone else on the internet Medeis. OsmanRF34 (talk) 22:17, 10 December 2012 (UTC)[reply]
And next time you tickle my chocha with your nariz, I'll admit that fact, Os. μηδείς (talk) 22:33, 10 December 2012 (UTC)[reply]
The olfactory has given way to the information age. Bus stop (talk) 11:59, 11 December 2012 (UTC)[reply]
Some of us can read Portuguese. Dauto (talk) 15:39, 11 December 2012 (UTC)[reply]
And most everybody else can read inuendoese.165.212.189.187 (talk) 19:45, 11 December 2012 (UTC)[reply]
Leaving the Solar System
Let's say I build a spaceship to travel to the stars. To accelerate the ship to the highest possible velocity, I aim it at multiple planets and make use of gravity assist. Assuming I don't really care in which direction my ship leaves the Solar System, what is the maximum velocity I can achieve? Can I also use the sun as a slingshot? Hemoroid Agastordoff (talk) 21:47, 10 December 2012 (UTC)[reply]
I think the question leaves way too many open variables to even hazard a guess. What is power source, your fuel supply, starting position(earth surface, leo, the moon), the frame of reference from which you are comparing your velocity? With "near future" sci-fi like p-b11 fusion ion drives, starting in L3, using the SUN, and unlimited resources to design your rocket, you'd get multiple orders of magnitude more momentum than a fairly typical liquid-solid multi-stage rocket launched from cape Canaveral. To make it out of the solar system, you'd need a (sun's frame of reference) velocity of 10 km/s. We've only ever launched two rockets with the intent of passing the edge of the solar system: Voyager probes 1 & 2. Voyager 1's current relative velocity to the Sun is 17,043 m/s. Voyager 2's is closer to 40 km/s. Sort of answer your question? i kan reed (talk) 22:09, 10 December 2012 (UTC)[reply]
I think the total speed you could reach is limited only by the drag experienced by encountering particles in the solar system (if this limit didn't exist, you'd eventually run into the speed of light limit). You could use the Sun, too, but the solar wind would decrease your speed as you approached (and then increase your speed as you leave, but to a lesser degree, since you are going faster then, thus exposed to it for less time, and also because your velocity relative to the solar wind is less, then). StuRat (talk) 22:18, 10 December 2012 (UTC)[reply]
You should get it all taken back though, and would require a bunch of time too (multiple inner planet assists, or maybe going all the way out to Jupiter just to get close to the Sun). Well you do go faster for awhile, don't know if it's quicker to just start outwards in the first place. Unless you have a reallly long timeframe and are talking about altering the probe's galactic orbit. Sagittarian Milky Way (talk) 22:32, 10 December 2012 (UTC)[reply]
Our article says, with each pass, the initial velocity V (relative to the sun) can be increased to V + twice the planet's velocity (relative to the Sun). However, if we're talking about speeds relative to the galaxy, then V (relative to the galaxy) can be increased to V plus twice the Sun's velocity (relative to the galaxy). StuRat (talk) 02:53, 11 December 2012 (UTC)[reply]
The sun cannot be used as an assist because it is the dominating body. However, its gravity can prove useful to approach a planet which can be used.
Once you get past the escape velocity of the solar system, you are not coming back, so then, you would not make many other passes. There can be slight exceptions if Mercury pushes you above the limit, and you happen to pass Jupiter on the way out, but that approach comes with its own limits. One, it's not possible to get back to planets "behind" you (Mercury in that example) once ypou're well beyond the planetary (Jupiter's) escape velocity, because you won't be close enough for long enough time to make the turn. At least not turns like the "simplified" one shown in the Gravity assist article.
Basically, gravity assist is a great method if you're limited to chemical propellants and/or low thrust ratings, because the kick you get is virtually "free", compared to the cost of more delta v. Once you have a continuous-beam plasma thruster with a cheap power source, you can just "floor it" without caring too much about assists.
Things would change again if you had a binary Black Hole to work with; these do crazy things to mass, light, and spacetime itself. You could go from near-zero to 42% c in one pass (or so I've read). - ¡Ouch! (hurt me / more pain) 11:38, 11 December 2012 (UTC)[reply]
Is it possible to do MS (surgery) or MD without MBBS? With MCAT score and without MBBS is it possible to do MD or MS (surgery)? — Preceding unsigned comment added by 199.224.149.10 (talk) 08:21, 11 December 2012 (UTC)[reply]
Is a masters in pediatrics course without MBBS considered a specialization in that field? after completing masters in diagnostics radiology without MBBS, is it possible to be considered as a professional radiologist? — Preceding unsigned comment added by 199.224.149.10 (talk) 08:22, 11 December 2012 (UTC)[reply]
In which country or countries? --Dweller (talk) 11:37, 11 December 2012 (UTC)[reply]
The OP's IP geolocates to Hyderabad, in case that helps. Marnanel (talk) 13:52, 11 December 2012 (UTC)[reply]
I should be able to find the formula for this...but I can't seem to find the right one...argh!
I have a small vessel that's pressurized to around 20psi with nitrogen gas from a large cylinder with a regulator valve. The gas is flowing out to atmosphere through a 5mm diameter circular hole. Everything is at room temperature. I'm trying to find out roughly how long an 80 cu.ft cylinder will be able to maintain that pressure before I run out of gas.
how much power does humanity consume with respect to the amount of solar power it receives?
I read from Dyson sphere that the Sun would provide only an order of a trillion times more energy than the power consumption of humanity. This makes humanity's power consumption seem quite large, as I would have expected it to be on the order of 10^15 or more. This means that humanity doesn't seem to have a lot of space to expand even within the solar system, energy-wise. 71.207.151.227 (talk) 23:02, 11 December 2012 (UTC)[reply]
Incoming solar power is 174PW [9]. World average power use is 15TW [10]. 15TW/174PW = 0.0086%. Sun's total output is 380YW[11], which is 2.5*10^13 times mankind's current energy usage. I doubt anyone can even make educated guesses on what the energy consumption of a civilization capable of building a dyson sphere would be like. Dncsky (talk) 23:35, 11 December 2012 (UTC)[reply]
Astronaut Lifestyle
Are (male) astronauts allowed to masturbate while in zero gravity? If not, due to, well, basically the mess it could create, would the prohibition have an impact upon their mental health? 86.13.97.144 (talk) 23:54, 11 December 2012 (UTC)[reply]
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![[5]](http://www.nasa.gov/images/content/710697main_Mahaffy-4-pia16576-43_full.jpg){kind=link}
