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November 25

light peak

i have some doubts in light peak topic which and what is the use of the micro controller used in it and how the data conversion takes place (electrical to optical) —Preceding unsigned comment added by 117.254.115.64 (talk) 00:30, 25 November 2010 (UTC)

Have you read our article on Light Peak and checked the official Intel Research Light Peak Overview website? Nimur (talk) 00:49, 25 November 2010 (UTC)

The microcontroller probably buffers, handshakes, and maybe adjusts output power. The optical signal comes from infrared lasers. Ginger Conspiracy (talk) 04:12, 27 November 2010 (UTC)

File:Brake shoe metal specimen.JPG

Does phosphorus (P) really look like that? --Chemicalinterest (talk) 00:58, 25 November 2010 (UTC)

The two standard allotropes of phosphorus are white phosphorus and red phosphorus; this could be something like "violet phosphorus" or "black phosphorus", both of which are more stable that white or red phosphorus, but require high temperatures and/or pressures to form. It could also be some form of phosphate. Phosphate minerals and Phosphorite are important sources of phosphorus, often for use in fertilizers. --Jayron32 01:07, 25 November 2010 (UTC)

The only time I recall encountering elemental phosphorus, it looked like white phosphorus. More often, I've encountered calcium phosphate, or various mineral forms like apatite. Nimur (talk) 02:06, 25 November 2010 (UTC)

My money's on that being a phosphate-containing mineral, rather than pure phosphorous. (Similarly, the samples for 'chromium' and 'magnesium' have the look of metal-containing mineral ores rather than the pure metals themselves — though I'm not qualified to say whether or not they're just very badly oxidized.) TenOfAllTrades(talk) 14:48, 25 November 2010 (UTC)

Brachistochrone problem

Can anyone explain the Brachistochrone problem in a more simple way than "The curve connecting two points displaced from each other laterally, along with which a body, acted on only upon by gravity, would fall in the shortest time."? Thanks in advance. Toolssmilezdfgsdffgrdsfrtdfh975243 (talk) 01:20, 25 November 2010 (UTC)

You have two fixed points. How do you connect a wire between them so that a bead moving under gravity gets from the higher bead to the lower bead in the least time? Assume the wire is frictionless. --140.180.14.145 (talk) 01:35, 25 November 2010 (UTC)

I think the word with is unnecessary, and makes the phrase confusing. 81.131.65.104 (talk) 14:17, 25 November 2010 (UTC)

I agree. Ginger Conspiracy (talk) 04:07, 27 November 2010 (UTC)

Corvus

Why were Corvuses so upseting to a ship's balance? Is it because it is a large, heavy mast sticking vertically in the air? If so, couldn't one sail with it lowered? And I can't imagine that it could weigh any more than the ship's mast, or at least not with the leverage the mast would have with such an enormous height. Thanks in advance for the answers. --T H F S W (T · C · E) 02:53, 25 November 2010 (UTC)

The illustration in the linked article seems to show it being much longer than a mast, and it had to be sturdy enough to support a column of rushing troops, so it would have been both heavy (they gave the estimate of 1 ton) and have had a high center of gravity. Lowering it along the deck of the ship, or better yet, into the hold, would certainly help, but it looks from the diagram like the only way it could be lowered was off the side of the ship, which would pull the ship over to that side (if it wasn't supported by another ship at the far end). StuRat (talk) 03:24, 25 November 2010 (UTC)

Well, I wouldn't really consider that diagram as an authority, and the beam supporting it is not a mast. And I think corvuses (or whatever the plural for corvus is) went off the front, rather then the side most of the time. And I think a mast would have to be stronger; I don't think a large group of soldiers would have nearly as much weight as a couple tons of sail. Plus, whereas soldiers rushing across are spread out and and are supported at both ends, a mast has to support the hole sail from the top. --T H F S W (T · C · E) 04:29, 25 November 2010 (UTC)

On a TV show (don't recall which one, but I think it was on the History Channel), they recreated a corvus and found it to be extremely clumsy and dangerous to its deployers. Clarityfiend (talk) 05:02, 25 November 2010 (UTC)

One other factor to consider is that wood is better under compression (like a vertical mast) than it is under a sheering or bending load (like a deployed corvus). Thus, much more wood is needed to support the 2nd case. Although, while the weight of the sails and the mast itself compresses the mast, the force on the sails, from the wind, is a sheering force, so the mast must be strong enough to withstand that, too (up until the point where they pull in the sails in high winds). StuRat (talk) 17:10, 25 November 2010 (UTC)

No. The mast is not taking up most of the sheering force of the sail. That is, instead, transferred via the rigging, in particular the stays, shrouds, and sheets. --Stephan Schulz (talk) 00:16, 26 November 2010 (UTC)

In that case, my point (about the corvus needing to be bulky to support sheering forces not experienced by the mast) is even stronger. StuRat (talk) 04:58, 26 November 2010 (UTC)

Yes. And the corvus was also intentionally made heavy. It was supposed to smash down any defenders, and to securely fix itself into the enemy ship with a spike. It's not very effective if your armored legionaries run over the boarding bridge while it slips off the other ship...swimming with 40 pounds of bronze armor may be borderline possible for a good swimmer, but it is definitely not a battle-winning strategy. --Stephan Schulz (talk) 10:12, 26 November 2010 (UTC)

The reason carrying a corvus in rough seas is problematic is that it raises the ship's center of mass. A stable ship will have a low center of mass. The higher a ship's center of mass, the more likely a slight tilt will cause it to capsize. WikiDao ☯ (talk) 15:29, 26 November 2010 (UTC)

I believe everyone here knows that, the real Q is why it raises the center of mass more than the mast does, which we seemed to have answered fairly well. There is also the point about the center of mass being off center, which has also been addressed. StuRat (talk) 17:58, 26 November 2010 (UTC)

Mast? I had thought the corvus was deployed on oar-ships. Any heavy weight above the water-line, whether in addition to a mast or not, would have made the ship top-heavy, I'm just trying to make that answer clear. WikiDao ☯ (talk) 20:00, 26 November 2010 (UTC)

I thought they had masts for sails and resorted to oars when there wasn't enough wind or it wasn't blowing the right way, but I could be wrong. StuRat (talk) 05:41, 27 November 2010 (UTC)

At the time in question, warships were primarily oared. They used sails to move around if the wind was favorable, but they would actually take down the masts and sails, and even store them on land if possible, before a battle. One detail of the Battle of Actium was that Mark Anthony's forces, contrary to prevailing practice, took their sails and rigging on board, which decreased maneuverability of the ships and may have been bad for morale. --Stephan Schulz (talk) 17:42, 30 November 2010 (UTC)

How does radiometric dating work?

I understand the principle behind radiocarbon dating, namely that the fraction of C-14 in the atmosphere is maintained at a constant level by the flux of cosmic rays activating nitrogen. Since organisms exchange carbon with their surroundings, the fraction of C-14 in an organism also remains constant during its lifetime. When the organism dies, C-14 is no longer in steady-state and its fraction begins to decrease, allowing its use for dating.

My question is about other forms of dating, like uranium-lead dating. As far as I understand, uranium concentrations are not being held in steady-state like C-14 is, so all uranium created from the supernova (or equivalent) that formed the solar system should decay at the same pace. If this is so, how can uranium be used to find the age of a rock, and what exactly does this age correspond to? The Wikipedia article on the subject goes in depth on the calculations involved, but not this background. —Preceding unsigned comment added by 68.40.57.1 (talk) 05:48, 25 November 2010 (UTC)

Quoting from the article you linked to:

Uranium-lead dating is usually performed on the mineral zircon (ZrSiO4), though it can be used on other minerals such as monazite, titanite, and baddeleyite. Zircon incorporates uranium and thorium atoms into its crystalline structure, but strongly rejects lead. Therefore we can assume that the entire lead content of the zircon is radiogenic. Where this is not the case, a correction must be applied. Uranium-lead dating techniques have also been applied to other minerals such as calcite/aragonite and other carbonate minerals. These minerals often produce lower precision ages than igneous and metamorphic minerals traditionally used for age dating, but are more common in the geologic record.

From later parts of the article, it can be more complicated then that as lead can sometimes leach but this seems to be the basics.

Nil Einne (talk) 06:59, 25 November 2010 (UTC)

Sunrise still becoming later after solstice

I wanted to find when sunrise would be in Cincinnati, so I went to sunrisesunset.com, selected Cincinnati at http://www.sunrisesunset.com/custom_srss_calendar.asp, and got a calendar for December. To my surprise, sunrise is latest at the end of the month: yes, it's only three minutes later than at the solstice, but it's still later. Why would sunrise continue to happen later after the solstice? 66.161.250.230 (talk) 12:29, 25 November 2010 (UTC)

I think we've discussed this before so you may find something in the archives. In any case [1] [2] should get you started Nil Einne (talk) 14:07, 25 November 2010 (UTC)

If you look at those data for Cincinnati, you will see that both sunrise and sunset are getting later in late December, and the daylight period is getting longer (as you would expect). The reason is that the moment that the Sun is highest in the sky is not exactly midday, even after accounting for time zones. You can get a better explanation of why in our article on the Equation of Time. Physchim62 (talk) 14:15, 25 November 2010 (UTC)

That happens because earth's orbit is not a perfect circle. It is slightly elongated. Because of that some days are actually slightly longer or slightly shorter than 24 hours. That causes a slight shift in the sunrise time that compounds the shift in sunrise time due to the change o seasons. The final result is that the latest run rise is slightly shifted from the solistice. 76.123.74.93 (talk) 14:13, 25 November 2010 (UTC)

People have known about this for a long time, even the ancients who, lacking the distractions of the modern entertainment industry, filled their time with astronomical observations and careful calculations of recurrant celestial events. The Analemma was devised as a means of calculating deviations in the actual day from the mean solar day. --Jayron32 14:58, 25 November 2010 (UTC)

The earliest sunset is in mid December, the latest sunrise is in early January. http://www.timeanddate.com/worldclock/sunrise.html The shortest day of the year is at the winter solstice. I do hope that the proposed bill coming before parliament in the UK is succesful and we switch to continental-european time so that the evenings are lighter and more enjoyable, particularly in the spring and autumn. 92.28.251.194 (talk) 18:34, 25 November 2010 (UTC)

If it does, you will find, not immediately but over time, that the nominal times of events start getting later and later, until in the end an equilibrium is reached not much different from if you had done nothing. Ultimately, the times people do things are determined by the Sun. Messing around with the clock, as with daylight saving time, works if you change back and forth every six months, because there isn't time for the adjustment to happen. But it's not going to work to make people get up in the dark on a long-term basis. --Trovatore (talk) 19:32, 27 November 2010 (UTC)

I infer that you do not work for a living? Because most people's routines are set by their 9 to 5 working hours. 92.15.11.45 (talk) 19:55, 27 November 2010 (UTC)

Those hours, in the long run, are adjusted to the convenience of the workers (including managers; managers are also workers, and also dislike getting up in the dark). So before too long it'll be 9:30 to 5:30, and then 10 to 6.

This is actually a good example of what I was talking about. When I was a student, doing a summer job for IBM in Tucson, my hours were somewhat flexible, but to match other people I was getting rides with, I went along with the most common schedule, 7 AM to 3:42 PM (the 42-min lunch hour has some historical significance at IBM). It took me a long time to realize why it was so early. Eventually I hit on it: Arizona doesn't observe daylight saving time. So in the summer, people made their own.

That will occur in society at large. --Trovatore (talk) 20:02, 27 November 2010 (UTC)

I've heard that people start work at an early hour in the US, but in the UK I do not believe that will happen. Being further from the equator, the time of sunrise and sunset is much more variable throughout the year than where you are. In mid-winter on a cloudy day it gets dark at 3.30pm for example, with dawn around 9am. In mid-summer it gets light around 3.30am and dark around 10pm or later. 92.15.14.132 (talk) 20:37, 28 November 2010 (UTC)

Fine, sure. I didn't mean that particular schedule. I meant that, in the long run, schedules will be determined by solar time, not by nominal time. If you artificially set your clocks an hour later, schedules will eventually be an hour later by nominal time. Not necessarily exactly — different organizations will respond differently — but that's the best first-order estimate. --Trovatore (talk) 01:14, 29 November 2010 (UTC)

I used to do office work on flexitime: some people arrived and left early, but that was due to wanting to avoid the rush hour crowds; others used to start and leave late. The statistics show that most people do fixed hours. The point of switching to European continental time is the longer-lit evenings allowing more and pleasanter outdoor recreation in the evenings, plus the syncronisation of business hours. 92.24.176.72 (talk) 13:45, 29 November 2010 (UTC)

But even "fixed" hours will change (possibly, to different fixed hours). You can't change people's preferences for when they get up, work, sleep, etc, just by putting a different number to it. The current equilibrium, ultimately, is driven by the hours of sunlight. Put a different number to the same hours, relative to sunrise and sunset, and the hours that people do those things will likewise change. --Trovatore (talk) 20:50, 29 November 2010 (UTC)

Are you not forgetting the effect of electric light on society? The proposed change is an optimisation of that. You should try getting a job in the UK - then you will rapidly drop these fantasy pastoral ideas. If you are an employee and not on Flexitime then you cannot choose. People in the UK are accustomed to getting up or starting work before dawn in winter. Perhaps in the backwoods of the US, where there is little contact with the rest of society, your ideas are true, but not in the typical urban culture. If what you are suggesting is true, then business hours would change with the seasons (particularly in the north where the UK is) but they do not. 92.29.120.7 (talk) 11:09, 30 November 2010 (UTC)

I explained why business hours don't change with the seasons. It's because seasons are too short. Let a season go on long enough, and they would indeed change.

The hour on a clock is just a number. In the long run, the equilibrium is not determined by that, but by sunlight. And no, I'm not forgetting about electric lights. --Trovatore (talk) 19:07, 30 November 2010 (UTC)

Lots of places I have worked in the UK have offered me flexibility of about half an hour, so I could work 8:30-4:30, or 9-5, or maybe even 9:30-5:30. Loads of desk jobs work like this, and times shift depending on many factors. Lots of offices use hours slightly offset from 9-5. If people wanted to start and finish such jobs earlier, they could. People are just as likely to start them later as they adjust. But these are desk jobs. Many, many people in the UK work shift jobs that do not run 9-5: they do not currently experience the light mornings/dark evenings that you and journalists complain of, nor will they win lighter evenings as a result of a change to continental time. Although we could then declare 1am (which would really be midnight) to be the beginning of a new day, solving the illogicallity another user was complaining about :) 86.161.109.130 (talk) 01:19, 29 November 2010 (UTC)

I raised this same question earlier this year and received some excellent answers. See HERE. Dolphin (t) 01:15, 26 November 2010 (UTC)

Special relativity

It's been said that special relativity raises the status of measurement - that the value of a quantity is intimately tied to how it can be measured. But where in special relativity does that become important. I learned SR by first having the Lorentz transformations derived, and (almost) all the results followed from these equations. So where does the elevation of measurement come into effect? 70.52.44.192 (talk) 13:02, 25 November 2010 (UTC)

Are you sure that the statement refers to special relativity, not quantum mechanics? --Wrongfilter (talk) 14:34, 25 November 2010 (UTC)

How did you derive the Lorentz equations? I've certainly seen them derived starting with a discussion about how time and space intervals are measured e.g assuming distances are measured by sending and receiving light signals. As I recall the term "operational definition of measurement" was thrown about. 129.234.53.175 (talk) 18:19, 25 November 2010 (UTC)

The Lorentz equations are so basic that the way you derive them depends a lot on which assumptions you want to use. It's kind of like proving 2+2=4.

I don't think I agree that special relativity changes the nature of measurement, versus Newtonian physics. What I do think, though, is that physicists and mathematicians overlooked special relativity for decades because they weren't thinking carefully about measurement. That was the only real contribution of Einstein's 1905 paper. the Lorentz transformations had been derived already (by Lorentz) but people were still stuck to Newton's idea of absolute mathematical time, which they believed was the thing being measured even if physical clocks were affected in such a way that they measured it wrong. Einstein finally threw that away, and people who read his paper finally realized why the Lorentz transformations made sense. It's interesting that the key to understanding quantum measurement turned out to be the same as the key to understanding special relativity: treating the measurement apparatus as a physical system that follows the same rules as the thing being measured. I have the fond hope that whatever is preventing us from understanding quantum gravity will turn out to be equally fundamental... -- BenRG (talk) 21:35, 25 November 2010 (UTC)

Relative velocity

I think it is more practical saying Lorentz assumed the Lorentz Transformation (LT) and Einstein is the first person trying to derive it. If you look at the time equation of LT, you will wonder how did Lorentz get it? But after you change the spatial equation to x=(x'/γ)+vt, and change the time equation to t=(t'/γ)+(vx/c^2) then put the right part of the equation into t of the previous equation, you will find out that the result is x = γ(x'+vt'). Yes, after you combined equations in LT you get the spatial equation of the inverse LT. Do you know how Lorentz get the time equation? It is so simple that Lorentz just assumed the "hypothesis of ruler contraction" to get x'= γ(x-vt), and the x= γ(x'+vt') for inverse LT, then replaced the x' in the later equation by γ(x-vt); and ha, there is the time equation in LT. Any two of the four equations in LT and inverse LT can derive the other two of them. That means, no matter how people derive LT, the equations of LT and inverse LT will always coexist. Logically speaking, we may assume S' is moving at velovity v and S is rest or we may assume S' is rest and S is moving at velocity -v but to assume both conditions to coexist we should be able to find something very bad in the LT. It is bad, in LT, the v is always zero.Jh17710 (talk) 04:29, 26 November 2010 (UTC)

The phrases "is at rest" and "is moving at velocity v" are meaningless, unless it's clear what movement is being measured relative to. If the x and x' axes point in the same direction, then the statements "S' is moving in the x direction relative to S at speed v" and "S is moving in the x' direction relative to S' at speed –v" mean the same thing. There is no inconsistency there, and no problem with the LT and the inverse LT coexisting in the same problem. Red Act (talk) 19:40, 26 November 2010 (UTC)

It is not easy to see the problem by looking at the four equations only. However, if you add two equations t'=t/γ and t=t'/γ that we could derive within LT and inverse LT, then you will see the problem very clearly. We will have γ=1 and so that v=0. How do we derive t'=t/γ and t=t'/γ within LT and inverse LT? Please refer to http://en.wikipedia.org/wiki/Wikipedia:Reference_desk/Archives/Science/2010_November_19 .Jh17710 (talk) 05:28, 27 November 2010 (UTC)

I've updated the page you linked to with explanations of your latest errors. Red Act (talk) 08:09, 27 November 2010 (UTC)

Platinum arsenide

Does anyone here know how to extract platinum metal from platinum arsenide (in my case it is sperrylite) and disposing the arsenic safely? I have search the internet and found no information on this.

Another thing I want to know that when we burn platinum arsenide,does it decompose? I gues it is because platinum is noble metal,and I have done and experiment,when I burn it,it turn to black and somewhat seem shrinked. If it does,it will decompose to platinum metal rather than its oxides?

I heard that platinum arsenide is attacked with oxidizing acid like nitric acid, when it attacked,does it form platinum nitrate and arsenic nitrate or just eat the arsenic away leaving the platinum metal?

Thank you very much! —Preceding unsigned comment added by 124.82.11.255 (talk) 14:08, 25 November 2010 (UTC)

Chemically extracting metals from their ores is actually usually a very dangerous process. Platinum extraction, as you note, produces nasty arsenic compounds. Gold extraction often involves the use of cyanides, see Gold cyanidation. Metal extraction is almost universally poisonous, environmentally destructive, and/or energetically expensive. See tailings and slag for examples of toxic wastes from various stages of metal extraction processes. --Jayron32 15:02, 25 November 2010 (UTC)

aqua regia will convert this to chloroplatinic acid H₂PtCl₆, and nitric acid to Pt(NO₃)₄. Burning the arsenide should remove the arsenic as an oxide vapour, but this could be polluting. Graeme Bartlett (talk) 21:17, 25 November 2010 (UTC)

Will the reaction with nitric acid produce arsenic nitrate?

ECG

Performing ECG on a patient having a metalic pin in his femurKhuloodm (talk) 16:12, 25 November 2010 (UTC)

Dose a metalic pin in a patient's bone affect ECG readingsKhuloodm (talk) 16:22, 25 November 2010 (UTC)

I have no idea, having a look at the Electrocardiography article, I can't deduce any plausible reason why a metal pin should affect ECG readings, but if that's a homework question, maybe it's one of those trick questions that only ECG technicians know the answer to. Vespine (talk) 21:44, 25 November 2010 (UTC)

No, a conductive metal pin would not influence a ECG any more than normal conductive saline fluids. MRI is a different story, but femoral traction pins should be safe at 1.5 Teslas: PMID 19897988; unlike other kinds of implants at higher magnetic strength: PMID 16835741. If this is an for an actual patient, please don't ask the reference desk. If it's for homework, you shouldn't really be asking it here either. Ginger Conspiracy (talk) 02:25, 27 November 2010 (UTC)

Human urine

What does human urine taste like ? Obviously I have no interest in trying it out and I'm not asking you to go do some original research I'm just morbidly curious. I'm more looking for scientific answers in the form: human urine contains these compounds, which are also found in these more commonly ingested substances, so it might taste like this. Thanks. 24.92.78.167 (talk) 17:29, 25 November 2010 (UTC)

Our article on urophagia discusses this a bit - it seems to vary quite a bit based on what the excreter has recently eaten and drunk. Because salts are concentrated in it (per our article), I assume it is usually salty unless something odd is affecting the flavour/odour. Matt Deres (talk) 18:33, 25 November 2010 (UTC)

It's definitely salty. It tastes pretty much how it smells: like urine. Urine is generally sterile, and safe enough to drink in small quantities; there's no reason not to try some (other than the grossness factor, which isn't insignificant). Buddy431 (talk) 00:03, 26 November 2010 (UTC)

Its salty (due to the salts) and has a bitter / ammonia-like taste due to the urea. Beyond that, it very much depends on what else the person has been eating drinking. Rockpocket 00:13, 26 November 2010 (UTC)

In the early days of medicine, physicians would taste the urine of any patient suspected of suffering diabetes. If the urine was slightly sweet due to the presence of sugar this confirmed the diabetes. See History. Dolphin (t) 01:05, 26 November 2010 (UTC)

You're clearly referring to diabetes mellitus, as distinguished from diabetes insipidus (the latter associated with dilute, not sweet, urine). Note that urine contains no sugar until the diabetes mellitus is sufficiently advanced for blood glucose to exceed the renal threshold (about 180 mg/dL or 10 mM), so the "taste test" would not rule out early diabetes mellitus (of course, diabetes mellitus generally isn't symptomatic until the renal threshold of glucose is exceeded, so it was a useful test). -- Scray (talk) 01:44, 26 November 2010 (UTC)

Mellitus means sweet in Greek... that's an intentional correlation. Apparently a number of cultures either tasted the urine themselves or used animals (ants or bees) to determine whether or not a particular individual's urine contained sugar, making it sweet. Shadowjams (talk) 12:25, 26 November 2010 (UTC)

Much of taste comes from smell. 66.108.223.179 (talk) 23:06, 27 November 2010 (UTC)

Actually, much of flavor comes from smell, taste is pretty much specific to the taste buds. Rockpocket 17:34, 28 November 2010 (UTC)

cloning plants

Were scientist cloning plants before Dolly the Sheep? If so, for how long? —Preceding unsigned comment added by 69.247.48.131 (talk) 20:47, 25 November 2010 (UTC)

You don't need to be a scientist to clone plants. Everytime you take a cutting of a house plant, you are cloning it! 86.162.106.18 (talk) 21:07, 25 November 2010 (UTC)

King's Holly Mac Davis (talk) 23:16, 25 November 2010 (UTC)

Banana's are famously propagated by cloning. Bananas have probably been cultivated for the past 7000 years in some areas [3], but it's unclear how long they have been cloned, rather than grown from seed. In any case, grafting has been practiced in the far east for the past 4000 years or so. Buddy431 (talk) 00:12, 26 November 2010 (UTC)

The Navel Orange is my favorite example, since all of the millions and millions consumed over the last 180+ years have been, essentially, the same orange! The Masked Booby (talk) 03:30, 26 November 2010 (UTC)

Along these lines, the same can be said for every commercial apple variety. There's only one granny smith, only one red delicious, etc. The point is that if a certain apple tree makes tasty apples, there is no guarantee that the offspring grown from seed (indicating sexual reproduction) will taste the same. The only way to ensure this is by grafting (cloning) one tree over and over. SemanticMantis (talk) 15:13, 26 November 2010 (UTC)

That's not strictly true: as the Red Delicious article points out, there have been a number of mutations that have occurred over the last hundred years, leading to multiple strands (genetically similar, but not identical), that may truthfully be called Red Delicious apples. In general, though, you are quite correct. Buddy431 (talk) 23:32, 26 November 2010 (UTC)

Thanks for the clarification Buddy431. Wouldn't we expect the same state of affairs for navel oranges and bananas then? Or do you think that the other fruits have not (for some reason) mutated over myriad cycles of cloning? SemanticMantis (talk) 23:13, 29 November 2010 (UTC)

For naval oranges, definitely. See this list of orange types. I count many dozens of varieties. Note that in most cases, the first tree where the mutation occured is identified, from which all trees of that variety were cloned. I'm not sure about the Cavendish banana; it's only been cultivated heavily for the last 60 years or so, leaving less time for mutations than red delicious apples or navel oranges. Still, we'd expect some mutations, but I can't find any list of varieties like you can for red delicious apples or navel oranges. Buddy431 (talk) 03:21, 30 November 2010 (UTC)

Research subject

Is there a way I could get paid to become a psychological/cognitive research subject? I have a lot of strange abilities. I lucid dream every night, I can depersonalize, derealize, and überrealize very easily and at will. Psuedohallucinations are common, and so are false memories, illusions of precognition, and many other little oddities. None of these things have ever bothered me in life. I enjoy them and learning how to manipulate them. I would like to learn to do more of these things, and help humanity's understanding of these mental effects. I am also very interested in being injected with psychoactive drugs during the dream state to see how sensations and perceptions are altered. My dream recall can go very deep, and is easily trained and untrained. I would really like to find a situation where I can spend some time delving into my own mind in waking and dream states while free from the responsibilities of civilization. Any ideas? -- Mac Davis (talk) 23:33, 25 November 2010 (UTC)

Its unlikely anyone is going to inject you with psychoactive drugs, but this site should give you some pointers of how to get involved in dream research. Rockpocket 00:18, 26 November 2010 (UTC)

Hey! That's a really good site. I read a lot of it. I also found a presentation from DEF CON with some scripts and schematics. I also already have somebody reliable to work with. —Preceding unsigned comment added by Mac Davis (talk • contribs) 04:12, 26 November 2010

You can look on the bulletin boards around your local college or university psychology department to see who needs research subjects; they often pay nominal fees for your time. But please don't specifically seek out experiments involving areas in which you think you may be a statistical outlier. To do so will skew the experiments' results. Ginger Conspiracy (talk) 01:37, 27 November 2010 (UTC)

Car turbochargers and torque

What causes turbos to produce so much torque (especially smaller twin turbos)? They smaller twin turbos are always capable of producing so much more torque than horsepower. Why is that? —Preceding unsigned comment added by 76.169.33.234 (talk) 23:38, 25 November 2010 (UTC)

Your statement that some turbos are capable of producing more torque than horsepower is meaningless. Torque and power are two different quantities. Torque is typically measured in Newton.metres and power is measured in Newton.metres.second^-1, Watts, kilowatts or horsepower.

Consider an engine producing a torque of 1000 N.m. If the speed of this engine is 1000 RPM it is producing power of 104.7 kW, but if the speed is 10,000 RPM it is producing 1047 kW.Dolphin (t) 04:58, 26 November 2010 (UTC)

Let me use a specific example to rephrase the question in a way that the OP actually means and would be helpful to follow-up RefDeskers: Why do turbo-charged engines such as that in the SEAT Leon Cupra (2L Turbo, 177kW @ 5,700-6,300rpm, 300Nm @ 2,200-5,500rpm) produce so much more torque so much lower down in the rev range than naturally aspirated engines of the same power e.g. the Honda S2000 (2L NA, 177kW @ 8,300rpm, 208Nm @ 7,500rpm)? The torque curves of turbo charged engines are disproportionately swelled at lower RPMs as compared to NA engines. What makes turbochargers so naturally effective at lower RPMs? Zunaid 10:05, 26 November 2010 (UTC)

Thanks for that clarification. It was perfectly clear to me what was being asked; I don't know if Dolphin really didn't understand or was just being difficult. StuRat (talk) 17:51, 26 November 2010 (UTC)

See:Turbocharger--Aspro (talk) 17:16, 26 November 2010 (UTC)

Does that article actually answer the question ? If so, I must have missed it. StuRat (talk) 17:55, 26 November 2010 (UTC)

Yes. More air drawn into the cylinder on the intake stroke.--Aspro (talk) 18:01, 26 November 2010 (UTC)

And how does that alter the HP to torque ratio ? StuRat (talk) 23:01, 26 November 2010 (UTC)

More oxygen availability causes a faster burn with the same amount of fuel, and possibly more importantly for the low revs case, allows a complete burn with more fuel. Ginger Conspiracy (talk) 02:36, 27 November 2010 (UTC)

Ginger Conspiracy has written More oxygen ... with the same amount of fuel ... If so, that would be a higher air-fuel ratio and I would disagree with that. Use of turbo-charging does not require, or warrant, a change in air-fuel ratio. The primary effect of supercharging and turbo-charging is to significantly increase the volumetric efficiency - more air and fuel per intake stroke, and therefore more heat released during each power stroke; so greater torque and greater power. An increase in volumetric efficiency can cause a 1 litre engine to have the output of a 2 litre engine - greater torque, greater power and greater fuel consumption at the same engine speed. Dolphin (t) 05:26, 27 November 2010 (UTC)

I don't think the above answers explain why one gets more torque at lower RPM than an equivalently powered NA engine. A turbocharged 1L producing the power of a 2L (say 110kW) will more than likely produce the torque of a 2.4L (say 240Nm). The torque boost is always disproportionately greater than the power boost when compared to equivalent NA engines, and always at lower RPM. It seems that turbos boost more effectively at lower RPM. Why? Zunaid 16:22, 27 November 2010 (UTC)

What makes turbochargers so naturally effective at lower RPMs?

That been answered. If you were to ask why turbo's don't show the same proportion of torque at the upper end as NA . The answer would be that if they were allowed to, the stresses imposed upon the components would shorten engine life. --Aspro (talk) 20:40, 27 November 2010 (UTC)

Turbocharged airplane engines, and possibly turbocharged car engines, are equipped with wastegates solely for the purpose of regulating the boost of inlet manifold pressure. The higher the engine speed the more exhaust gas is diverted away from the turbocharger to reduce the effectiveness of the turbocharging and limit the speed of the turbine. If the inlet pressure was not limited by the wastegate the turbine could overspeed itself to destruction, and there would be increasing potential for detonation at higher engine speeds. If turbocharged engines in cars also have wastegates then that explains why the effectiveness of the turbocharging is so significant at low engine speed, but less so at higher engine speeds. Also see Turbocharger#Wastegate. Dolphin (t) 00:31, 28 November 2010 (UTC)

November 26

Swallowing one's tongue

I read the following sentences on the Wikipedia: "Teammates Ivica Dragutinović and Andrés Palop immediately ran to his side as he lost consciousness. Moments later, club medical staff and other players followed suit, as Dragutinović stopped Puerta from swallowing his tongue." What is "swallowing ones tongue"? Can you give some explanations? Thank you! —Preceding unsigned comment added by 72.198.195.96 (talk) 05:43, 26 November 2010 (UTC)

When you lose consciousness your tongue can lose muscle tone and (especially if you're flat on your back) can fall back in your mouth occluding the airways. It's not that you actually 'swallow' it in the way that you swallow food, but merely that if can block the airway. That's why one of first steps in first aid with an unconscious patient is to turn them on their side (along with preventing choking on things like spontaneous regurgitation). --jjron (talk) 08:24, 26 November 2010 (UTC)

RFID

   Can RFID tagging (i.e. of products exposed for sale in a retail store) be used to defeat aluminium-lined bags used by shoplifters? In addition, what do RFID scanners which are to be placed at a store's entrance look like; are they conspicuous enough to deter would-be shoplifters from even trying? Alternatively, can RFID scanners placed at store entrances be made inconspicuous enough to aid in apprehending shoplifters? Rocketshiporion♫ 05:50, 26 November 2010 (UTC)

Aluminum foil will attenuate RFID signals in the same way it attenuates most security tags. The amount of foil required to prevent a detection will depend on the details of the system, but for many RFID uses the signal may be undetectable with only 1 to a few layers of foil. Dragons flight (talk) 07:30, 26 November 2010 (UTC)

Note that current security measures try to detect an object in the vicinity of the door, but a better approach is to continuously monitor the position of every item in the store, and alarm when any of them disappear, noting the site of the disappearance. This requires greater range, bandwidth, and computing power to track all those objects, but it is possible right now. However, the cost of the system and size of the tags makes it not yet practical for most items. Perhaps very expensive items, like jewelry, might be the first to get this treatment. StuRat (talk) 17:46, 26 November 2010 (UTC)

mining asteroids

Hi, I came across this page where they say an asteroid (3554 Amun) that is a mile wide contains 30 times as much metal as Humans have mined throughout history (ever). Surely this is wrong, or am I understanding it incorrectly? Sandman30s (talk) 06:45, 26 November 2010 (UTC)

I agree it is wrong, though perhaps not as wrong as one might guess. According to list of countries by iron production, global iron mining was 2.3 billion tons / year in 2009. As raw metal, that would have volume of 0.3 km³. 3554 Amun is only about 7 times that annual volume. So definitely not 30 times all metal ever, but still a large amount on the scale of iron mining. Also, iron stands out for its very large production volumes. Most other metals we mine are in much smaller quantities (e.g. copper and aluminum are only a few percent of the iron values), so a concentration of those metals would be comparatively more significant if one existed. Dragons flight (talk) 07:50, 26 November 2010 (UTC)

Thanks, I would never have imagined those figures... Sandman30s (talk) 09:48, 26 November 2010 (UTC)

Hang on a moment. I could not find a web page giving the volume of Amun (I did find multiple sources giving its "diameter", but since small asteroids are not spherical, this gives little idea of its volume). But Wikipedia's page shows its mass as 1.6e13 kg, which is 16 billion metric tons. This accords with Dragon's figure of 7 times the 2.3 billion tons of iron mined in 2009, if those are metric tons; if they're short tons, as one might expect from a US source, it would be nearer 8 times. But several other Internet sources give Amun's mass as 30 billion metric tons, which (if iron) would be equivalent to 13 or 14 years' terrestrial production rather than 7 or 8. Still nowhere near 30 times the world's all-time production; perhaps someone slipped a factor of 1,000 in their original calculation. --Anonymous, 02:02 UTC, November 26, 2010.

Hangh on another monent, list of countries by iron production actually gives iron ore mining statistics, not refined iron production. Thar page needs to be fixed. 75.41.110.200 (talk) 05:01, 27 November 2010 (UTC)

Good catch! I've put a "needs expert attention" flag on it. Still, judging by the some of the numbers mentioned in one of the linked references, the proportion of iron in iron ore is often pretty large, between say 25% and 40%, so this only introduces an error of a factor of 3 or so. Maybe something like 40 years' production, then. --Anonymous, 05:50 UTC, November 27, 2010.

Hmmm, that's 40 years CURRENT production. If production grows at 7% a year which is not unreasonable to assume, this equals to a doubling of production every decade. I don't know specifically about iron, but this is what has been happening to Oil for over a century. If production doubles every decade, it means that in the last ten years, you have actually produced more resource then ALL THE PRECEDING production combined. Think about it: If you mined one "unit" of iron in the 1st decade, and you mined 2 units in the next decade, in the decade after that you'll mine 4 units, this is more then 1 and 2 combined. The next decade you will mine 8 units, this is more then 1, 2 and 4 combined. And so on. So saying 40 years of current production could very easily be orders of magnitude more then what has ever been produced, if the rate of growth is just 7% annually. Scary but true. Hubbert curve, Hubbert peak theory, Peak oil. Vespine (talk) 23:30, 28 November 2010 (UTC)

Perfectly true, but we don't hear people talking about "peak iron", so I doubt there has been such a rate of increase. Data? I couldn't readily find anything by googling. --Anonymous, 03:17 UTC, November 29, 2010.

I wasn't trying to say anything about peak iron specifically. My main point is that even with growth rates of 7% a year which is not considered "aggressive" in our consumerist culture, 40 years worth of current production can easily be far more then has ever been produced in the past, regardless of how much time is "in the past" or the resource. Vespine (talk)

Right sorry, i think i misunderstood your question a little, this article has a LOT of data which I find a bit confusing because it talks about "exports" and all sorts of other "percentages" which i'm not sure how to interpret. One pertinent point near the beginning states : "World production of iron ore grew by 12% in 2006 to reach 1.5 billion mt (See Table 1). This was a fifth consecutive record high. " Vespine (talk) 04:28, 29 November 2010 (UTC)

How can a solvent be non-polar but be made of polar molecules?

My organic chemistry textbook makes the distinction between polar molecules and polar solvent. It says that polar molecules are identified by the high dipole (u) seperation of the molecule, while polar solvents are identified by a high dielectric constant.

It says that all polar solvents are made from polar molecules, but the opposite is not true and provides the example of formic acid vs acetic acid to demonstrate this.

Both formic acid and acetic acid are polar molecules by virtue of their dipole moment. However, formic acid, with a dielectric constant of 59, is also a polar solvent, while acetic acid, with a dielectric constant of 6.1, is not a polar solvent.

Why is this? Would both solvents be adequate for dissolving ionic compounds such as NaCl? Acceptable (talk) 08:05, 26 November 2010 (UTC)

Solubility of polar substances (and by extension, ionic) substances is dependant almost solely on dielectric constant. Your textbook is pretty much spot-on on this one. The solubility of something like NaCl is dependent on the ability of the solvent to solvate the ions; that is to make bonds to the ions which are stronger than the bonds the ions would make to each other (strictly speaking, it is defined thermodynamically; the substance is soluble if the free energy released in the formation of the solvent-ion bonds is greater than the free energy required to break the ion-ion and solvent-solvent bonds). Dielectric constant takes this ability into account, whereas dipole moment does not. --Jayron32 16:32, 26 November 2010 (UTC)

But why does formic acid have a higher dielectric constant than acetic acid when both have a carboxylic acid functional group? They differ only by the fact that one has a methyl group and the other has a hydrogen. Acceptable (talk) 21:24, 26 November 2010 (UTC)

The methyl group is MUCH more "electron donating" than the hydrogen atom is. You can think of this in two ways; either you can think of it as the methyl group donating electrons to the carbon part of the COOH dipole OR you can think of the methyl group as acting like a "positive charge sink", again to the same effect. The dipole moment is calculated for the bond, while the dielectric constant is calculated across the whole molecule; while the dipole moments will be similar (but not the same, due to the methyl's effect described above), the dielectric constant will be much lower on the acetic rather than the formic acid. You can see the effect even greater on very large molecules; take something like stearic acid; the dipole moment on the COOH will not be that much smaller than it will be on the acetic acid, indeed after propionic acid, longer chains of carbons do not markedly affect the dipole moment in that part of the molecule. However, the dielectric constant continues to go down to nearly nil. I am pretty sure that after 4 or 5 carbons, the bulk substance is considered essentially non-polar; despite the acid group. --Jayron32 21:33, 26 November 2010 (UTC)

Ah, ok, thanks a lot for the helpful explanation. Acceptable (talk) 02:08, 27 November 2010 (UTC)

Is it possible to vomit in your sleep and die by suffocation without being under the influence of anything?

In other words, could a perfectly normal, sober person who hasn't had anything to drink//taken any mind-altering substances vomit and thereby asphyxiate during slumber, or would the body's involuntary control measures trigger countermeasures like gagging, rolling over to one side, etc. before the person awoke?

Or perhaps it's not even possible to vomit in one's sleep? Sign me "curious" The Masked Booby (talk) 09:21, 26 November 2010 (UTC)

Did a few google searches on it, most of the results appear to be people asking about it on various forums - One answer on this forum referred to GERD - Although perhaps not vomiting in the sense that you mean, perhaps mildly along the same lines. All other mentions that I've come across thus far (the last few minutes), seem to point to factors enducing the vomiting while asleep: alcohol; obesity; underlying illness; directly related illness.. etc. Darigan (talk) 09:54, 26 November 2010 (UTC)

I think a better question is can you do that and stay asleep, and I doubt it. If I vomited I'd wake up for sure. Same goes x1000 for not being able to breath. Ariel. (talk) 11:15, 26 November 2010 (UTC)

I think the best anyone could say is it is unlikely. The human body is a fantastically unpredictable thing, so I would not be surprised to find, if I looked hard enough, isolated reports of one or two people who have died by choking on their own vomit, in their sleep, without any complicating factors. --Jayron32 15:22, 26 November 2010 (UTC)

One thing of course if we are thinking there is no illness or other factor as someof the above answers are discussing, vomitting itself is not common asleep or not. Nil Einne (talk) 16:50, 26 November 2010 (UTC)

I recall a report or two of people suffocating to death from their vomit while very drunk and thus probably unconscious. 92.24.178.149 (talk) 22:05, 26 November 2010 (UTC)

The suffocation reflex from carbon dioxide buildup in the lungs is very painful if you aren't used to it, and very powerful. Vomit is a fluid which is almost always easy for a conscious person to expel. A conscious person in shock may be effectively paralyzed, however, which is why CPR and related forms of first aid instruct the person administering the aid to check to see that the airway is clear, even when the victim is conscious. Ginger Conspiracy (talk) 01:03, 27 November 2010 (UTC)

Copper containing biomolecules

Why did copper containing biomolecules appear later in evolution than their iron based analogues that do the same job? —Preceding unsigned comment added by Blackmetalgrandad (talk • contribs) 10:57, 26 November 2010 (UTC)

I can't answer your question, but you should know that there is something like 100,000 times as much iron on earth as there is copper. Ariel. (talk) 11:22, 26 November 2010 (UTC)

Oops, I meant why did the copper based ones appear earlier!144.32.126.11 (talk) 11:33, 26 November 2010 (UTC)

It could be that hemocyanin would have been more efficient in the early seas which where oxygen poor, than iron based oxygen carriers. Also, it allows (or favours) simpler body organs. --Aspro (talk) 16:44, 26 November 2010 (UTC)

Iodine heptafluoride

What is its appearance? Colorless gas? (just a guess) --Chemicalinterest (talk) 15:02, 26 November 2010 (UTC)

Yes, good guess! It sublimes at 4.8 °C under atmospheric pressure. Physchim62 (talk) 15:12, 26 November 2010 (UTC)

Iodine from caliche

The article does not state how iodine is extracted from the iodide and iodate in caliche. How is it extracted? All of these iodine questions are because of this. --Chemicalinterest (talk) 15:25, 26 November 2010 (UTC)

It can be reduced with sodium bisulfite but there are more advanced multi-step processes which seem to be more popular these days.[4] Ginger Conspiracy (talk) 00:54, 27 November 2010 (UTC)

Why kilogram as a base unit rather than gram?

Why was the kilogram chosen as a base unit of the International System of Units rather than the gram? The kg is the only base unit with an SI prefix as part of its name. Our article on the Kilogram says, "Since trade and commerce typically involve items significantly more massive than one gram...[the standard became] one thousand times more massive than the gram—the kilogram." Although this seems common sense, wouldn't the desire for consistancy in the new system be more important? Doesn't having one base unit that already contains a prefix in its name cause confusion when multiplying it by a another prefix? It seems that the term "kilokilogram" would be necessary to describe a mass of 1000 kg, which is awkward. --Thomprod (talk) 15:26, 26 November 2010 (UTC)

(1) "Since trade and commerce typically involve items significantly more massive than one gram...[the standard became] one thousand times more massive than the gram—the kilogram."

(2) No.

(3) No.

--Shantavira|^{feed me} 15:45, 26 November 2010 (UTC)

The OP is making a common mistake; the confusion between the metric system and SI. The SI is a subset of the metric system chosen for convenience in the widest possible applications. The other thing about SI is that its "base" units are used to derive the so-called "derived units"; thus the Newton and the Joule and the Pascal and other units are always expressed as ratios or products of things like kilograms, meters, seconds, etc. There are other systems besides the SI which use different metric units, see cgs system, which uses units like the erg. In summation: The SI is not the metric system. It is a set of units speficially chosen from the metric system, chosen to be convenient for use in certain applications. --Jayron32 15:52, 26 November 2010 (UTC)

See grave (unit). Basically a grave (from gravity) was going to be the base unit; a gram was an alias for a milligrave; just as a ton is an alias for 1000 kg. However, this was just before the French Revolution, and Grave is also a French title; similar to the German Graf, or English Count. Grave as a title has the same etymology as Graff, which is different from Gravity. After the revolution it was felt that this would be contrary to égalité so grave as a unit was dropped. BTW, this came up a few months ago. CS Miller (talk) 16:00, 26 November 2010 (UTC)

Jayron: So the kilogram was chosen over the gram as the base unit of mass in the SI because the former was (and is) more "convenient" in most applications. I understand that. But didn't the kg stick out to the designers of the SI as the only base unit name to include a prefix? If I was designing a new system (based on mathematics and powers of ten and such) with base units and modifying prefixes (somewhat analogous to nouns and adjectives in grammar), why would I build in a point for possible later confusion by including a modifier in the name of a base unit? There is no color named "lightred", for example.

CS: I understand the politics of not using the word "grave". But, why didn't they just come up with a completely different word to represent 1000 grams, if that amount was thought to be more convenient than one gram? --Thomprod (talk) 17:00, 26 November 2010 (UTC)

Pass. Perhaps it was just an interim decision use gramme instead of milligrave, and no-one got around to making a new word for grave/killogramme. I made a slight mistake in my previous statement - originally the definition was the gramme - 1cc of water at 0°C, and the grave was the practical physical object to represent it. I don't know why the definition was moved to kg/grave - perhaps it was to make the definition and representation the same. CS Miller (talk) 17:28, 26 November 2010 (UTC)

Ok, now that makes sense. If the unit called "grave" was practical, but now politically incorrect, it would have made more sense to call it something brand new, rather than the hybrid "kilogram". --Thomprod (talk) 18:35, 26 November 2010 (UTC)

Don't expect SI to be this perfect, logical system. It's a system of units like any other. Use it when it's convenient; ignore it when its pronouncements are silly. In particular, ignore completely any recommendation not to use convenient units like the curie, or to avoid the name micron. --Trovatore (talk) 08:20, 27 November 2010 (UTC)

Force as Gradient of potential

We can define the quantity U such that dU is to equal -F·dr. But dU = ∇U⋅dr. So F⋅dr = -∇U⋅dr For this to be true in general, F = -∇U.

If a force can be written as the gradient of a scalar field, then this is taken as a definition that F is conservative. But where in the above derivation was F assumed to be conservative? 70.52.44.192 (talk) 20:54, 26 November 2010 (UTC)

Please do your own homework.

Welcome to the Wikipedia Reference Desk. Your question appears to be a homework question. I apologize if this is a misinterpretation, but it is our aim here not to do people's homework for them, but to merely aid them in doing it themselves. Letting someone else do your homework does not help you learn nearly as much as doing it yourself. Please attempt to solve the problem or answer the question yourself first. If you need help with a specific part of your homework, feel free to tell us where you are stuck and ask for help. If you need help grasping the concept of a problem, by all means let us know.

Having said that, you might benefit from reviewing force, gradient, and reading Newton's laws of motion#Variable-mass systems carefully. Ginger Conspiracy (talk) 23:52, 26 November 2010 (UTC)

No, it isn't a homework question, although looking back it does seem that way. The reason I spelled out the F = -∇U derivation was so that someone could point out more clearly where the assumption that F is conservative was made.

I looked at the articles you mentioned. I don't see what variable-mass systems have to do with conservative forces, and the gradient article doesn't mention potential energy. The force article starts by saying "a potential scalar field $U({\vec {r}})$ is defined as that field whose gradient is equal and opposite to the force produced at every point". But why can't such a potential be defined for any force? You might say, well U isn't a function of just r for all forces, just conservative ones. But if dU is defined as above, then it does seem, incorrectly, that all forces can be constructed to be the gradient of a scalar field. 70.52.44.192 (talk) 01:46, 27 November 2010 (UTC)

Oh, so by conservative you mean subject to conservation laws instead of covering only the particle case instead of the variable mass case? In that case, would the assumption of elastic force transfer be the reason, as opposed to force from an inelastic collision? Ginger Conspiracy (talk) 02:32, 27 November 2010 (UTC)

(ec) Such a U does always exist for a radial force whose magnitude is a function of only the radius, i.e., all such forces are conservative. But it doesn't always exist for more general force fields. For example, take ${\vec {F}}(x,y)=y{\hat {x}}-x{\hat {y}}$. -- BenRG (talk) 02:37, 27 November 2010 (UTC)

F is assumed to be conservative in the line "We can define the quantity U such that dU is to equal -F·dr." If F is not conservative, there is no field U that satisfies this definition. Looie496 (talk) 05:35, 27 November 2010 (UTC)

In BenRG's example, wouldn't dU = -ydx + xdy satisfy dU = -F⋅dr? 70.52.44.192 (talk) 07:23, 27 November 2010 (UTC)

Yes, but what U satisfies dU = -ydx + xdy? -- BenRG (talk) 08:47, 27 November 2010 (UTC)

A conservative force is a force for which the work done by the force on a particle that moves from A to B depends only on the start and end points A and B and not on the path taken between them. Equivalently, a conservative force is a force for which the work done by the force on a particle travelling round any closed path is always 0. As has been said, a force that is the gradient of a scalar field is conservative - you can see this by looking at the integral of F·dr around any closed path. Also, any conservative force can be expressed as the gradient of a scalar field U by picking a fixed point A and defining U at any point B to be the integral of F·dr along any path from A to B - the conservative property means that this definition gives a well defined value for U at each point. U depends only on the choice of the point A, which is the same as saying U is uniquely defined up to an arbitrary additive constant.Gandalf61 (talk) 09:32, 27 November 2010 (UTC)

Thanks 70.52.44.192 (talk) 16:06, 27 November 2010 (UTC)

November 27

Absolute rest point

According to the book "Four Symphonies of an Oriental Universe" the author, Zhi-Zhong Cai, said we can locate an absolute rest point with some imagination. Let a point-source of light shine once at a point E, then the out going wave front will make a growing but thin out ball of light. Do you agree that the center of that growing ball of light is a fixed point as in the absolute rest universe? The reason is clear, because the speed of light is independent of the speed of the source of light so that the growing ball of light is growing from a fixed point in the whole universe or whole space. The only difficulty is that, unless the source of light stays shining like once every two seconds at that same event point E while the ball of light is growing, otherwise, we have no way to point out where is E. Because our technology is unable to show us the actual growing ball yet. It is the same for every event with a point-source of ligh shines once so that the location of that event is an absolute rest point that is secured by the growing ball of light, if we can see it, then we can try to locate its center point. That means, if we shine four point-source of lights at each of (0,0,0),(1,0,0),(0,1,0), and (0,0,1) of a Cartesian coordinate system roughly at the same time then we have an absolute rest frame. We cannot deny their existence just because we cannot figure out some way to use those growing balls of light to locate their centers. Please comment, thanks.Jh17710 (talk) 06:13, 27 November 2010 (UTC)

I don't understand why that reference frame would be anymore 'absolute' than any other reference frame or why it would be considered an 'absolute rest point' or even how it would be determined to be 'at rest' unless you just wanted to define it that way in your coordinate system. This may help. Sean.hoyland - talk 07:23, 27 November 2010 (UTC)

Thanks for the reference. The reason why it is an absolute rest point to any absolute rest frame is based on the character of light. Since the way light moves away from the souce point is independent from the speed of the source. That means, if just shine once, the center point of that growing ball of light is guarded by that thinning out and growing ball of light as an absolute rest point in the whole space; at least at time while the ball is recognizable. The problem is that one single growing ball of light will thin out as 1/(r^2) of density and the speed of light is too fast that scientists have no way to watch that growing ball so that even the growing ball can show them an absolute rest point, its center point, they have no way to mark it yet. But, there are rest points if we can figure out some way to see those growing balls to locate their center points.Jh17710 (talk) 22:30, 27 November 2010 (UTC)

Actually, all reference frames will agree that the expanding ball of light is spherical, and that its center is at rest. So you can't use it to pick out a preferred reference frame — all the frames will think they're the preferred one. --Trovatore (talk) 07:25, 27 November 2010 (UTC)

Trovatore, thanks for yuor kindness to show me the right way to describe that expanding ball. Let's look at the issue of spherical shap first. Because of the farther length looks shorter, the only perfect circle shape of a ball is when the ball is infront of us or when we turn our eye sight directly to it. That means even within the same reference frame, the spherical shap is not automatically maintained. Secondly, not all inertial frames can think they are the rest one and it will be very easy to figure out if, well, the if part is very difficult; if we can locate the expanding ball, then, instantly locate the center point, then, instantly decide if that center point is at REST in their systems.Jh17710 (talk) 22:30, 27 November 2010 (UTC)

OK, first of all, we need to distinguish between measurement in a reference frame, and visual appearance in a frame. There are complications involved in the visual appearance of objects at relativistic speeds (we surely have an article on that, but I'm not sure where it is).

But those complications don't interest us here. What we're concerned about is measurement of time/space coordinates. For example, you might set up a spherical shell of photodectors, all with synchronized clocks. When they detect the light, they send out a signal, time-stamped by what their clocks say. If all of those time stamps are the same, you conclude that the shell of light is centered at the center of the photodetectors.

With me so far? Here's the surprising thing: No matter what reference frame you happen to be in, you can set up a concentric system of such shells, according to which you will conclude that the center of the shell of light is at rest. The why of that is a little involved to explain in detail, but it all works out. --Trovatore (talk) 22:45, 27 November 2010 (UTC)

Trovatore, I mean, it is really hard to figure out if a shape is spherical at far away, because our tools are very limited; it is even more difficult to measure from another inertial system, no tool at all. But you are right, that is not the main point. On the shell of photodetectors, we will need four wave peak counters at tips of the largerest internal triangular pyramid, let the center send out fixed frequency signals, then, if all four frequencies remained the same for a little while, then, yes we are lucky to witness an absolute rest point at that little while. I don't trust clock.Jh17710 (talk) 00:40, 28 November 2010 (UTC)

I made a wrong statement above. I will say it at the end of this paragraph. I found out it is not easy to synchronize clocks. If we think a little bit deeper, the four clocks on the spherical shell cannot have the same gravity so that their speeds will be different! Even if we can adjust their speeds, the mistake I made is that, all wave peak counters will count the same frequency within one minute of time, since we assumed that their distances from the source of light are all fixed. The first wave front may reach four clocks at different time, but, since all the following waves are all have the same differences in between four clocks so that the wave peaks count at each of four clocks will be the same in the same period of time. Sorry for the mistake.Jh17710 (talk) 15:43, 28 November 2010 (UTC)

The problem of detecting the wave front of that SINGLE expanding ball of light is our technology is way behind on two issues relative to measurement, one is to adjust the speed of four clocks so that they are the same against to gravity and temparature or other factors, and another one is to match the numbers showing on four clocks. Until we can control the synchronization of four clocks in this discussion, we have no way to find out if the SINGLE expanding ball of light reaches four clocks at exactly the same time.Jh17710 (talk) 15:43, 28 November 2010 (UTC)

Well, there is no "exactly the same time" (or "exactly" anything) in engineering. But you don't need it. You put the detectors in free space. They keep their relative positions constant, and their clocks synchronized, by sending laser messages to each other with timestamps, and making corrections with little hydrazine rockets when necessary. It's not going to be perfect, but it can narrow down the center of the expanding ball of light within some reasonably small margin of error.

And you can put several larger collections of these assemblies of photodetectors at larger distances, and decide whether the center of the ball of light is moving (again, within some margin of error) in your reference frame.

Then the surprising, counterintuitive thing is this: No matter what reference frame you are in, you will determine that the center of the ball of light is at rest, within your experimental error. --Trovatore (talk) 20:59, 28 November 2010 (UTC)

Trovatore, there are quite a few issues in your last comment. First of all, if you like to let four detectors keep constant relative positions do you think it may be better if we use one solid frame to install them? The second issue is laser messages with timestamps, since they are at four tips of a triangular pyramid, that can be a good idea to synchronize their clocks; but, the top one with different gravity potiential may adjust its speed a little bit to match other 3 clocks at about the same altitude. Then the third issue is if we have two sets of this kind of shell, one is rest and the other one is moving directly away from the rest set at any speed then back and forth at any other speed, we should find out, logically speaking, all of eight devices will count the same total amount of wave peaks within same period of time. Do you expect this result? The fourth issue is that above result does not mean both center points are absolute rest points. The key point is the SINGLE expanding ball of light, does it reach four devices at the same time? Or, we can arrange the experiment in one dimensional space to get more chance of being rest and to make things easier, just use a section of train. Logically speaking, if we fire two bullets at the same time at the center of that section of train to two ends, then, no matter how that train moves, bullets reach ends at the same time, but, not if we fire two rays of light. Is that right?Jh17710 (talk) 02:22, 29 November 2010 (UTC)

The problem boils down to relativity of simultaneity. The center of a growing light ball is the spatial center of a set of events which occur at the same time. But observers disagree about what "at the same time" means, which makes them disagree as to where the "center" of the growing light ball is. Red Act (talk) 08:21, 27 November 2010 (UTC)

Red Act, yes, the center of an expanding ball of light is the spatial center of a set of events, like when the radius of that ball is at 10', 20', 30',...but I don't see you point of "at the same time". Could you explain it a little bit? Thanks.Jh17710 (talk) 22:30, 27 November 2010 (UTC)

I assume you're familiar with conic sections. If you slice a cone along parallel planes at the correct angle, you get circles; the centers of those circles lie on a line through the cone. If you slice the cone along parallel planes that are inclined to those other planes by less than 45°, you get ellipses. The centers of the ellipses (for any particular set of parallel planes) also lie on a line, but it's a different line. The slope of that line is the velocity of the inertial reference frame whose planes of simultaneity are the parallel planes that you used. Thus, depending on which reference frame you use, you get a different "center". That center is always at rest with respect to the reference frame that you used. -- BenRG (talk) 01:14, 28 November 2010 (UTC)

BenRG, for events happen at points, lines, or even plans, we may have problem of "at the same time" or different time, but, this will not happen for an expanding BALL like this case. They will never happen at the same time from any observers. Am I correct?Jh17710 (talk) 03:25, 28 November 2010 (UTC)

Absolute time

According to the book "Four Symphonies of an Oriental Universe" the author, Zhi-Zhong Cai, said we can have a better clock than current atomic clock. If I have some device can count wave peaks then just select a source of light with fixed frequency which is moving at constant speed v relative to me along the line connecting that source and me, and we are ready to use that device to measure time. According to Doppler effect, no matter the moving party is the source or me and no matter what is the value of v, the frequency and the wave length of light may change but the speed of light will remain the same, so that my time can be calculated by (the total wave peaks I count by the device x the wave length)/(the speed of light). That means, the time period calculated at different relative speed will be the same. Could we use this idea to make a clock to measure time period? If yes, then, a moving clock will not slow down. If the device can be made very small, then we can put the device and the source of light in a small vacuum box and use it as a clock, that will be v=0, and since the speed of light is independent of the change of gravity, it will be a better clock than atomic clocks. Isn't it?Jh17710 (talk) 07:01, 27 November 2010 (UTC)

Yes, you can make a clock that measures a time coordinate independently of your motion if you use an external reference. A more familiar example is a cell/mobile phone that gets its clock setting from the cell tower. Another theoretical example would be a clock that measures the temperature of the cosmic microwave background, though that's cooling so slowly that it wouldn't give you a very precise time in practice. There are also inertial navigation systems that, in certain restricted cases, can keep time independent of your motion without any external reference. But this has nothing to do with the accuracy of atomic clocks. They do a very good job of what they are designed to do, which is count very nearly equal intervals of proper time. The clocks I described before are measuring some form of coordinate time, which can also be useful, but it's not the same thing.

I'm not familiar with the book you mention, but I would not use it as a way to learn physics. It sounds like the author is pretty clueless. A lot of people like that write "physics" books, for some reason. -- BenRG (talk) 09:43, 27 November 2010 (UTC)

Thanks. That book just published 9-1-10 at Beijing, it is in Chinese.Jh17710 (talk) 03:26, 28 November 2010 (UTC)

The article Comoving_distance#Comoving_coordinates covers comoving time in a bit of detail. --Jayron32 13:48, 27 November 2010 (UTC)

I think there are at least two issues. First, the speed of light does change in a gravitational field. Second, and more importantly, the method depends on the distance between the two objects remaining constant to an extremely high degree of precision. This means that any vibration, such as caused by sound waves, will cause a wobble in the distances. I don't see how it would be possible to solve that problem. Looie496 (talk) 17:05, 27 November 2010 (UTC)

Looie496, the frist issue is still unknown, we don't even know if the gravity will change the direction of light; the change might be caused by the different density of air around the sun. The second issue is very true. It is even worse for a constant velocity v in between two objects. However, the spirit in the idea is that, theoretically speaking, in front of a light source, we let a counting device move under a relative constant velocity v, then, if we use (total wave peaks x the wave length)/(the speed of light) to define time, that kind of time will be independent of v. What do you think?Jh17710 (talk) 23:13, 27 November 2010 (UTC)

The GR prediction of gravitational lensing has been experimentally confirmed to very high precision. Maybe 90 years ago you could have argued that it was refraction in the Sun's atmosphere, but not today. See Tests of general relativity.

The questions you ask in this thread about time in special relativity could be rephrased as questions about position in Newtonian physics. Can you build a device that determines its absolute position (relative to Earth's surface) by triangulating with cell towers? Yes. Can you put the cell towers and the receiver together in a box to get a device that knows its absolute position without an external reference? No. Can you use inertial navigation to keep track of your absolute position without an external reference? Yes (subject to certain caveats). In special relativity the same things are true of the time coordinate, for essentially the same reasons. -- BenRG (talk) 00:36, 28 November 2010 (UTC)

That way of counting time is just an idea. Like Looie496 said, it is almost impossible to make a device to count wave peaks due to the difficulty of keeping same distance. However, even if gravity can change the direction of light, the speed is the same so that the idea of that book still works, right? About a device to locate an absolute rest point, it is even more difficult as discussed in the thread above this one.Jh17710 (talk) 01:58, 28 November 2010 (UTC)

Pulsars are also better "clocks" than nuclear decay. ~AH1^(TCU) 03:27, 28 November 2010 (UTC)

If we have source and device

Even if you had such a light source, and an object capable of counting the light pulses, that still won't tell you how much time passed. You'd have to know the frequency of the light being emitted. You'd need another clock to figure out how fast that one goes, at which point you might as well use the other clock. — DanielLC 05:52, 28 November 2010 (UTC)

DanielLC, the key issue of a light source is having stable frequency. We have some known natural frequencies and radar will emit EM waves with almost fixed frequencies. So long as the souce and the device are keeping same distance or one of them is moving away directly from each other at constant speed (I will say this is impossible in real world, but, theoretically we should include the relative moving situation.) then the device will receive EM waves with fixed frequence. If we do have a known source, like red light, and a device, then we don't need any clock. That source and device make a perfect clock themselves so long as we can find some way to keep a constant distance between them. With the known frequency, we can calculate the time by the total count of wave peaks. Right?Jh17710 (talk) 17:22, 28 November 2010 (UTC)

Force at a distance without any connection

How does the force from gravity or magnetism communicate itself to distant objects, when nothing connects or passes between them and the attractor? And there's no aether either.

I'm aware of the teaching model that looks like ball-bearings on a rubber drum, but what is this curved surface and the third-object gravity embodied in in reality? 92.15.11.45 (talk) 14:18, 27 November 2010 (UTC)

There isn't any need for solid objects to be involved. In fact, what we see as solid objects ... aren't. The electromagnetic repulsion between electrons is what keeps the things we see as solid about an angstrom away from each other instead of passing right through each other. It's more like the whole universe is made of infinitely-small particles than of things that can actually touch each other. Paul (Stansifer) 15:02, 27 November 2010 (UTC)

I'm already aware that things are made of atoms and smaller things. That does not explain how for example a star pulls on the planets orbiting it, or how a magnet attracts things from a distance. 92.15.11.45 (talk) 15:28, 27 November 2010 (UTC)

Just about everything attracts or repels anything else at a distance. --Chemicalinterest (talk) 15:37, 27 November 2010 (UTC)

I'm asking how this happens. 92.15.11.45 (talk) 15:45, 27 November 2010 (UTC)

When I first read your question I found the article Action at a distance (physics). This didn't seem much help and I didn't understand it, so I didn't mention it, but on reflection, maybe it's helpful to you, so there you are. There seems to be some debate about how it happens and what it means. 81.131.46.126 (talk) 16:32, 27 November 2010 (UTC)

This is a good question, and has been answered on the desk many times. Action at a distance was one of the most plaguing problems of 20th century physics, but for the most part it has been resolved. There are a few critical mechanisms that apply to different kinds of fundamental interactions:

• Forces like electromagnetism and the strong nuclear force are mediated by a particle, the photon or gluon. These forces "propagate" at the speed of their carrier-particle, and in the case of the photon, that particle also conveys energy and momentum. The mathematics to ensure energy-conservation are explained by treating force-carriers as virtual particles until they undergo an interaction.
• The special case of gravity is explained by general relativity - there is currently no known force-carrying particle for gravity. Instead, Einstein's theory of gravity explains that all mass perturbs the geometry of space - so simply by existing, massive objects cause other objects to change their trajectories. Gravity therefore changes the shape of the inertial trajectory by warping the geodesic of space. General relativity mathematically describes the relationship between mass and the resulting warping of space-time geometry - in other words, allowing you to calculate the effective force. It also can describe how quickly gravitational information propagates - that is, when you move a massive object, how soon other objects in the "neighborhood" realize that something has changed.
• The last case, quantum entanglement, applies to certain quantum properties, but in general does not convey energy or information.

Hopefully this explains the various ways things can "act at a distance." Nimur (talk) 16:35, 27 November 2010 (UTC)

Note that (virtual) gravitons are hypothesized as the force carrier particles for gravity. And if you're having a hard time understanding how gravity can both be transferred by a particle and by curvature in space-time, you're not alone. Reconciling the two is one of the key stumbling blocks in constructing a theory of everything which combines quantum mechanics (forces-as-virtual-particles) and general relativity. -- 174.24.198.158 (talk) 19:10, 27 November 2010 (UTC)

you might be intested in this answer. 62.54.13.205 (talk) 23:04, 27 November 2010 (UTC)

Midnight sun question

Because of possible changes to the earth's tilt on its axis and to the earth's orbital path around the sun, is it possible for a place right around or near the Arctic Circle to barely experience the midnight sun on one year during the summer solstice night, but on another year, not experience the midnight sun let's say, for 10 minutes during the summer solstice night? Willminator (talk) 15:43, 27 November 2010 (UTC)

Well, the largest nutation is only some 20 sec of arc. On the Earth's surface that's about 2000ft of distance. The sun takes only 2 minutes to cover its own diameter. So on a very flat piece of land at approx 66½ degrees north (or south, six months later) you might just be able to catch a 'momentary' extension (or reduction) in night-time, as measured against the previous year. The sun would cover the 2000 ft in about a second or so I would think. This is a bit rough and ready, so I think it needs someone else to double check the assumptions I'm making.--Aspro (talk) 16:47, 27 November 2010 (UTC)

I don't see where you get the 2000 ft from (20 seconds of arc north/south on the Earth would be 1215 ft)(no, you were right about the distance), and while the Sun does indeed cover that apparent angle in less than a second, it's not what matters here, since the direction of the Sun's apparent motion is very close to east/west in the case of the midnight sun. What cannot happen is that the Sun never touches the horizon in one year but sets completely below the horizon in the next year (or the year after that), because the Sun's apparent size is roughly 30 minutes of arc. And if you consider not nutation but the fact that days and years don't align (i.e. if the exact time of solstice is at midnight for one location, it will be roughly at noon for that same location 2 years later, since 1 year is approximately 365 and a quarter days), then the difference will be only 3.3 seconds of arc (the formula is arccos(sin(axial tilt)*cos(angle that Earth travels in its orbit in half a day)) - arccos(sin(axial tilt))). Icek (talk) 23:43, 27 November 2010 (UTC)

Good questions. To do it in my head, I assumed a nautical mile is about 6080 ft and the illuminated parabola as a simple triangle with a 'lateral' base (therefore) of about 2000 ft west to east (not east-to-west as you put it because that is not the way the sun orbits), and about 2000 ft in the NS axis to keep things simple. I wanted to avoid discussions about twilight because it would be obvious to all that the effect would not be great enough to go into proper night. Likewise, whether 'day' starts at first rays or at the full disc becoming visible, is more of a academic question than what the OP is really asking. So I am assuming sun's rays. Perhaps the OP can comment on whether s/he meant this. The point about 'two years' is misleading pedantism in regards to the spirit behind the question. For we are talking about the apparent altitude extreme at the solstice, not a date on somebodies calendar. For it to be noticeable to the eye, the ground would need to be very flat, or you would need to assume the exact same position each year. Yet, if one used say a sextant, the difference in altitude -I expect - would be just discernible. Be on guard though. In such cold climes atmospheric 'tunnelling' may cause the sun to be visible when it is actually below the visible horizon. --Aspro (talk) 01:05, 28 November 2010 (UTC)

By midnight sun I mean the sun's disk being above the horizon at night in the summer solstice. A place that gets white nights is where the sun's rays are visible in the middle of the night, but the sun stays below the horizon around summer solstice. What I'm talking about is about a place where at least half of the sun's disk is above the horizon right in the summer solstice night, which happens around the Arctic Circle line. What I'm asking is if there a year around the Arctic Circle line where at least part of the sun's disk is exposed in the middle of the summer solstic night, but on another on another year in the summer solstice night it's not, or will the sun's disk be always partly exposed at that such location every year on the summer solstice night. I hope I've clarified what I tried to ask. Willminator (talk) 03:04, 28 November 2010 (UTC)

In that case the answer is no. The sun's angular diameter is about 1920 arcseconds and we are only talking about a change of about 20 arcseconds in altitude. As this would only represent the edge of the disc showing or not showing, it would be unlikely to cast a shadow bright enough to be seen above the twilight. However, with the aid of a post driven deep enough into the ground to prevent frost movements' so as to provide a fixed sight-line to the horizon, it should be able to indicate that in some years a few rays of midnight sun are still observable and in other years there is a brief moment of no disc to be seen at all. The effect though is very small and not something one would notice unless one took the trouble to observe it carefully. Even then, as I have already mentioned, the closer to the horizon that one observes events, the more the atmosphere can distort the true position of an object and make those observations unreliable. So the effect is of academic interest only.--Aspro (talk) 11:19, 28 November 2010 (UTC)

If you want to take a long-term perspective, the Earth's obliquity periodically changes from 22.1 to 24.5 degrees with a ~40,000 timescale due to interactions with other planets. So, if a hypothetical observer that could stay around 20000 years, then they would notice a quite substantial variation in the range of the midnight sun, even though these changes are pretty negligible on the scale of human lifetimes. Dragons flight (talk) 11:43, 28 November 2010 (UTC)

Aspro, which illuminated parabola are you referring to?
Why do you think that my 2 years have anything to do with the different human-made calendars? All I wanted to say is that the exact time of the solstice - the point in time when the angle between Earth's axis and a line connecting Earth and Sun reaches a minimum - may be at midnight for one particular location and one particular year. In the following year the exact time of solstice will be shifted by about 6 hours in local time of day, and in the year after that by about 12 hours. Obviously the distance in time to the next midnight cannot be more than 12 hours. So for a midnight 12 hours distant in time from the exact time of solstice the Sun's apparent position is 3.3 seconds of arc lower than for a midnight at the exact time of solstice (if there was no change in axial tilt). For 6 hours it would be 0.8 seconds of arc. Icek (talk) 11:55, 28 November 2010 (UTC)

The places you can see the midnight sun are those north of the polar circle. The arctic one is moving north by 14.2 metres per year. make of that what you like :-) EverGreg (talk) 20:48, 28 November 2010 (UTC)

What does unranked mean in wikipedia's Scientific classification ?

What does unranked mean in wikipedia's Scientific classification ? In the horticulture class I am taking they dicuss Plant Hierarchical Classification. Kingdom,Phylum,Class,Order,Family, Genus, Species. There is no "unranked". Does unranked refer to the plant or the subdivision of the classification. Is wikipedia using scientific classification or something else? —Preceding unsigned comment added by Zoztrog (talk • contribs) 17:12, 27 November 2010 (UTC)

It is impossible to cleanly fit the Linnaean categories to the evolutionary trees produced by modern evolutionary biology -- there aren't enough levels, and the Linnaean levels don't have any basic underlying biological reality anyhow. "Unranked" means a grouping of species that does not match up with any of Linnaeus's levels. Looie496 (talk) 18:13, 27 November 2010 (UTC)

As a footnote, my experience is that textbooks tend to run about 10 years behind the leading edge of science. I expect that the textbook presentations will gradually change over the coming decade. In current biology, the Linnaean scheme is definitely on its way out. We can expect the concepts of genus and species to stick around, but above that, pretty much every Linnaean grouping is useless. Looie496 (talk) 18:19, 27 November 2010 (UTC)

Agreed. While I think that the lagging of textbooks behind the leading edge of science has its upsides (some call it the "bleeding edge" for a reason), what disturbs me is the way in which clearly-dated dogma is still being taught in classrooms. The Linnean hierarchy is still being drilled by rote in many universities, rather than using it as a shared construct with a tangible (if flawed) basis in observation. I hope that university biology faculty take the leading edge into account when they teach, and avoid having students memorize things that will have no utility in a few years. It's hard to teach concepts, but it's rewarding. -- Scray (talk) 21:06, 27 November 2010 (UTC)

CLONE

THE PROCESS OF CLONING,IS SEXUAL OR ASEXUAL REPRODUCTION —Preceding unsigned comment added by 41.211.232.70 (talk) 17:19, 27 November 2010 (UTC)

Our cloning and asexual reproduction articles should clear this up for you. Is this homework by any chance? -- Scray (talk) 17:57, 27 November 2010 (UTC)

See Dolly (sheep). (and please don't write in ALLCAPS) ~AH1^(TCU) 03:22, 28 November 2010 (UTC)

Growth of trees

I've searched and maybe I just don't know how to use it well, but I'd like to know, If I make a notch/ scratch/ marking in a tree, will the marking rise as the tree grows? or will it stay the same distance from the ground? This is for research for a book I am writing. thank you!

46.117.94.124 (talk) 21:18, 27 November 2010 (UTC)

The only part of the tree trunk that is living or growing is the cambium. So the tree trunk just gets fatter with new rings, whilst only the new shoots grow upwards from the top. In other words, a blaze will stay at the hight that it was made. Does that help? --Aspro (talk) 21:33, 27 November 2010 (UTC)

A tree (and almost all plants) grows from the top, so a mark will stay at the same height. However grass is unusual - it grows from the bottom. Ariel. (talk) 23:09, 27 November 2010 (UTC)

You can also see this with signs, e.g. street signs, that are put on trees. The signs will remain at the same height above the ground. The tree may eventually even grow around the sign if it's there long enough. There is an example of this not far from the house I'm staying in now. The bark is slowly enveloping the signs. Dismas|^(talk) 00:50, 28 November 2010 (UTC)

Pics or it didn't happen. -- BenRG (talk) 02:30, 29 November 2010 (UTC)

I was actually going to ask the same question! I recently moved into a house that has a data cable which is secured to a tree between the street and my house. The anchor is barely half way up the tree and I was wondering if I would have to have it moved at some time in the future. I did suspect the answers above but glad to have it confirmed. Vespine (talk) 23:15, 28 November 2010 (UTC)

November 28

Sleep and lifespan

A friend of mine said he wants to "experience a longer life", so he's going to cut out an hour of sleep from now on. I thought this was pretty bizarre logic. It got me thinking though, if you were to cut out an hour of sleep from now on, would that shorten your lifespan? Would the extra hours you stayed awake equal the additional hours you would have been alive for? ScienceApe (talk) 02:34, 28 November 2010 (UTC)

Sleep deprivation can lead to immune system and other health problems. However, if your friend is cutting from say 8 to 7 hours of sleep a day as an adult, this could potentially increase lifespan. Simply not sleeping, and yet still expecting one to live much longer is not possible. ~AH1^(TCU) 03:18, 28 November 2010 (UTC)

You may be interested in scheduled napping to achieve more time awake. — DanielLC 05:41, 28 November 2010 (UTC)

I believe there have been studies in mice which show that limiting sleep has a positive effect on lifespan. Here is one article I found, i'm not sure how reputable the source is, Link. Calorie restriction has also been found to increase lifespan in mice but it's thought to not have a huge effect in humans, but there is an effect Link. But you have to ask yourself, if you're going to go through life tired and hungry, is it worth the few additional years at the end of your life? Vespine (talk) 23:13, 28 November 2010 (UTC)

And, of course, it bears stating that what applies to mice doesn't necessarily apply to humans: we are fairly different, particularly in this case. After all, sleeping and dreaming clearly has some important effects on our brains, and our brains are very different to mouse brains. And a lot of popularly-reported studies on how much sleep people need seem to give an average, which is reported as a universal optimum, when of course there is a lot of individual variation. I suspect this is more a problem of the reporting than the studies. Caveas. 86.161.109.130 (talk) 00:50, 29 November 2010 (UTC)

Looking for name of strange wart-like cutaneous growth on human foot with tentacles

Hi. First off, a few notes, I am NOT looking for medical diagnosis or opinion ... except that "diagnosis" might be a sliding rule here. I do not have this problem, no one I know has this problem, and I am not going to lie and say this is for a "book I'm writing". I have been looking every few months for this gross thing, and I can't find it, so I'm turning to this as my last resort. I'm merely curious what it is.

20+ years ago, I caught something on my foot, possibly from a public swimming area (I swam a lot). I went to the doctor and had it treated and it's never returned. But in my adult years, I have been curious as to what exactly it was, since I no longer have medical records of this thing. So please: I hope someone can tell me what it was.

It is not a plantar wart. But it was like a very large wart on the heel of the foot, and it had thick, white tentacular cilia growing from the center. It was really gross, I admit. To cure the thing, I had to pluck the tentacles every night and then apply a strong liquid.

One could ask why on earth I would want to remember such a thing, but I'm simply curious. Again, if I had this problem in real life, at the moment, I would immediately go to the doctor's. I just want to know what it was that I had. Anyway, if this question still breaks some rules, feel free to remove it. Thank you, Reflectionsinglass (talk) 09:25, 28 November 2010 (UTC)

Have you been through our slightly gruesome article List of cutaneous conditions? 87.81.230.195 (talk) 10:16, 28 November 2010 (UTC)

I have! But not thoroughly; it's such a long list, and it's one of the pages I'd been perusing. If it's listed there, it would take me quite a while to find it. I just took another gander, but no luck. Lots of other interesting things that prove to me I was not born to be a doctor :) Reflectionsinglass (talk) 10:53, 28 November 2010 (UTC)

To discount a parasitic infestation: What geographical location were you in when you first noticed it and for the few weeks before? Was blood or other liquid observable when tentacles were removed. Was it painful or just uncomfortable during the day and during tentacle removal. Did you think of the medical tincture applied as being 'strong' because it had a pungent acidic smell, or of alcohol or did it smart, etc.? Were the tentacles smooth all round or did they have texture? Did they end as a point or flattish or bluntly. Where they elastic or not when pulled ( i.e. did they become longer and thinner before coming out). --Aspro (talk) 11:37, 28 November 2010 (UTC)

The "tentacles" could be hyphae, suggesting something like Phaeohyphomycosis. DuncanHill (talk) 14:39, 28 November 2010 (UTC)

If there is a scientific answer to this, it would be a diagnosis that might still be relevant to the OP's health. To arrive at that diagnosis, we would need a back-and-forth exchange of information about symptoms, signs, other conditions. It seems clear to me that this is a request for medical advice, and I suggest that this should end here. -- Scray (talk) 17:03, 28 November 2010 (UTC)

The original poster wrote: "I went to the doctor and had it treated". If that doctor's still around, I'd suggest writing to them to ask what the thing was. You never know, they might still have records. --Anonymous, 03:22 UTC, November 29, 2010.

Why shopuldn't you fill a hot water bottle from the domestic hot water supply?

I brought a hot water bottle that says in the instructions"Do not fill using water from a domestic hot water system as this can considerably shorten the life of the hot water bottle". Since it also warns against using boiling water the alternative is to part-heat a kettle. How is partly heating water in a kettle different from heating it in a domestic hot water system? The only difference I can think of is that the water from the hot water tap may have been kept warm for some time, but I can't see how this would make a difference. -- Q Chris (talk) 12:17, 28 November 2010 (UTC)

My loose guess is that it might have to do with impurities in the water — see, e.g. water softening. --Mr.98 (talk) 14:47, 28 November 2010 (UTC)

It's a wonder chemicalinterest has not answered this yet, is he asleep still :) It's the copper that's the problem with rubber. Case studies hot water bottle premature failure Obviously our article on hot water bottles requires this addition by a thermophilic editor who does not like to wake up in a wet bed.--Aspro (talk) 15:07, 28 November 2010 (UTC)

The original poster cited a British URL for the bottle. Many houses in Britain have water systems where only one (I hear) cold-water tap is fed directly from the mains, the hot-water system and other taps being fed from the tank. I can't find a Wikipedia article directly about this, but Water heating#Gravity system addresses it. Anyway, with such a system the hot water is in it longer and has more time to collect impurities. But I can't say if that actually is what they have in mind. --Anonymous, 03:33 UTC, November 29, 2010.

Bose-Einstein condensate of light

Many years ago I imagined an idea in a fantasy game, in which people would lay out spheres of "one-way mirror" in the sun until the light would "crystallize" into "flasheesh", a basic magical reagent better known for its recreational abuse by magic users who would shimmer and eventually spontaneously combust from their addiction. It seemed to me just about the most absurd thing I could come up with. Until that is today, when I read someone had done something about like this, and that the Bose-Einstein condensate of photons was being considered as a viable commercial process for solar cells and lasers!^[5]

The above story in Nature News is written to be very accessible, but even so it leaves me with a host of unanswered questions. Why can only a certain number of photons enter the cavity before thermal equilibrium is no longer possible? Where do the photons go, and are they "bound" to the dye molecules? Can you physically shove such a condensate of photons to change their wavelength? When they talk about a "super-photon", are they merely waxing poetic?

As you can tell, I haven't accessed the paper, but this looks like a big story, and I hope that the people who understand it will make a new article to explain it on Wikipedia. Is it time to call the DEA? ;) Wnt (talk) 13:56, 28 November 2010 (UTC)

The photons are trapped in a resonant optical cavity (basically imagine a box with mirrors on all sides, though this box is microscopic and two dimensional). Quantum mechanics selects for wavelengths that are integer fractions of the dimensions of the box. For example, if you had a 1 m box, then you could have 1 m waves, and 1/2 m waves, and 1/3 m waves, etc. The actual box selects for light in the visible spectrum. So that's one selection constraint. The other constraint is that the temperature of the gas should be in thermal equilibrium with the box. (The dye they used is basically there to promote the creation of thermal equilibrium quickly enough that photons can't escape from the box or be absorbed on the walls before equilibrium is achieved.) The energy per photon increases as the wavelength decreases. The result is that certain photon states are forbidden. For example, even a single photon in a high enough state would require more energy than the entire collection of photons at thermal equilibrium is supposed to have. Similarly, the resonant nature of the box forbids very low energy photons from being formed because they would require wavelengths much longer than the box. As you push more and more photons into the box they will try to reach equilibrium, but because certain energy states are forbidden you end up with an excess being forced into the lowest available state. It is this non-thermal excess in the ground state that characterizes the Bose-Einstein condensate.

As for what the result is like, the resulting ground state population is basically a laser. You get a coherent population of photons at a particular wavelength. The trick here is that the wavelength is determined by the physical properties of the box and not the atomic properties of the substances involved. This means you could tune the wavelength by adjusting the size of the box, which could have a variety of practical applications and allow for lasers at wavelengths which we don't have an effective means of generating now. It is also interesting that this system essentially transforms arbitrary light of sufficient intensity into a laser beam (with some unknown efficiency). That could have a variety of applications with respect to solar cells and other processes that require light concentration. Dragons flight (talk) 21:29, 28 November 2010 (UTC)

By "super-photon" they presumably mean many photons in the same quantum state. Lasers do that too, so "super-photons" have been around for a while. This looks like an interesting experiment, but popular science magazines have a habit of reporting everything as though it's going to change the world. Chances are this won't. -- BenRG (talk) 02:41, 29 November 2010 (UTC)

Water not freezing below the bridge

I was walking around in park (the temperature was 0 C or below) and noticed that the water below the bridge and in nearby area for some reason did not freeze (and the birds were swimming there). Why is that? —Preceding unsigned comment added by 89.77.158.172 (talk) 14:31, 28 November 2010 (UTC)

Currents. Kittybrewster ☎ 15:01, 28 November 2010 (UTC)

The bridge may provide shelter from the cold (e.g wind chill factor) and it may also be emitting heat absorbed over the summer / from usage which does just enough to prevent the water there freezing. ny156uk (talk) 15:11, 28 November 2010 (UTC)

The simpler explanation is that the open water has been able to radiate its heat into space but under and around the bridge it has been reflected back. This is why ice can form over-night under a clear desert sky, even though the upper air temperature is above freezing.--Aspro (talk) 15:43, 28 November 2010 (UTC)

Has it been snowing or frosting where you are in Poland recently? If so, it seems likely that they would have been out there laying down salt on the bridge, which is then running off or blowing off the bridge onto the ice below, melting it. Especially if the bridge is a road bridge and not a footbridge, and has a lot of traffic over it. WikiDao ☯ (talk) 19:32, 28 November 2010 (UTC)

As this is a common phenomena under structures which have not seen gritted, I take that explanation with a pinch of salt ;-) No one has mentioned yet, that the air temperature reported is only around the freezing point. Therefore, I was expecting someone to also say, that perhaps the water has not had time to stratify (remember science lessons at school about the density of water and what parts of a lake are the first to freeze and which parts are the last). The water may be a lot deeper around the bridge which would be the reason why that spot will be the last to ice. Here is a link Understanding Pond Stratification. We need the OP to come back with some more observations.--Aspro (talk) 21:04, 28 November 2010 (UTC)

Beneath what other structures does this commonly occur? WikiDao ☯ (talk) 21:22, 28 November 2010 (UTC)

Roofed over boat moorings on pleasure lakes is one example. Common in the Victorian period before people could stay in all night and watch I love Lucy. --Aspro (talk) 21:40, 28 November 2010 (UTC)

An alternate explanation is that they have installed some sort of heaters around the bridge to stop the water from freezing. Having ice directly freezing against the bridge supports could have a detrimental effect on the life of the bridge. They may be deliberately stopping the water from freezing... --Jayron32 21:02, 28 November 2010 (UTC)

Is a common feature of these cases that water is moving near some fixed object (pier, etc.) and/or something is moving in the water (birds paddling around, etc.)? I've often heard folklore (yeah yeah /) that turbulence makes water freeze more slowly, so water lapping at a piling would freeze after a still pond or a smooth gently-flowing stream. Sea ice#Formation of sea ice suggests this isn't a complete crock either. DMacks (talk) 22:07, 28 November 2010 (UTC)

Folk can be right too. Turbulence warms water and Mechanical equivalent of heat is your citation. Cuddlyable3 (talk) 22:47, 28 November 2010 (UTC)

Growing bacteria, beef bullion

I'm trying to grow bacteria with unflavored gelatin in petri dishes. I've heard to add beef bullion to the boiling water, then let it dissolve. Beef bullion is high in sodium, and sodium was used as a preservative in meats. Would sodium inhibit the growth of bacteria? Also, how would you identify the bacteria that grows, and what surfaces would bear the most bacteria? Albacore (talk) 17:00, 28 November 2010 (UTC)

Different bacteria require different Growth mediums. Once you know what you what to grow, you can choose a suitable source to provide the nutrients. Some of these bacteria can be very dangerous in such large quantities that end up in culture dishes -so beware. --Aspro (talk) 17:09, 28 November 2010 (UTC)

I'm really concerned, now that I've thought about it, that you are going to try doing this in the house – don't! Here are two links you might find useful: Safety Guidelines & MICRO-ORGANISMS FOR EDUCATION. The salts (sodium) in the beef bullion doesn't matter. After all, its best diluted down to half the strength normally used for making stock. At that level the salts will have little inhibitive effect. --Aspro (talk) 18:37, 28 November 2010 (UTC)

Geologic Time Scale

Found in the discussion of geologic time is a Geologic Time Scale. Where can a full size copy of this document be acquired?

Rolf Olson Salem, OR —Preceding unsigned comment added by 24.22.3.126 (talk) 18:44, 28 November 2010 (UTC)

Other than Geologic time scale, what exactly are you looking for? BTW: End the Holocene now, ask me how. Hcobb (talk) 18:54, 28 November 2010 (UTC)

How? WikiDao ☯ (talk) 19:38, 28 November 2010 (UTC)

Oh! Don't you read all of Wikipedia from A to Z? You don't need to do nothing, as it finishes in two years time.--Aspro (talk) 21:15, 28 November 2010 (UTC)

There are several diagrams on the discussion page. Do you mean this one:[6]. If so you can use the right-hand download tool. --Aspro (talk) 19:02, 28 November 2010 (UTC)

Skinny vs baggy jeans in freezing weather

Being a student up here in Oswego, I know a lot about walking around all day in freezing wind/death snow. But this question has been seemingly unanswerable by my friends and I. What would be, according to science (if it is science that applies here), warmer, skinny jeans or loose, normal fitting jeans? Skinny jeans are closer to the skin, and seem to not let any of the cold air in, but because they are so close to the skin, the cold air gets closer to the skin, too. Looser jeans seem to have more room for the cold air to circulate around your skin. I've clearly tried to think about this for a long time! The jeans are both made of the same fabric, so that shouldn't be an issue. ?EVAUNIT^{神になった人間} 19:00, 28 November 2010 (UTC)

Yes, you need some trapped air. A pair of these underneath would also make all the difference. After all, if they're good enough for Clint Eastwood, they should be good enough for you. --Aspro (talk) 19:09, 28 November 2010 (UTC)

Looser will be better on the basis of allowing sweat to escape, and on the same note will allow hairs to stand out more. Also, what heat is emitted by your legs will travel more freely up the wider pants, creating a chimney-effect (if rather negligible). The baggier jeans will also be made of more fabric. In addition to this, any consideration of moisture favours baggier jeans: Skintights that grow wet will immediately cool down parts of your legs, while baggier jeans can keep this moisture at a range. 88.90.16.188 (talk) 19:11, 28 November 2010 (UTC)

Not answering the question, but there exist flannel lined jeans, such as these jeans from L.L.Bean (not spamming for them, but that's where I got my pair). They are quite comfortable in colder weather, though if they do get wet, they take forever to dry. Back on topic, tighter jeans may restrict circulation in extreme cases, which would be undesirable in colder whether. Buddy431 (talk) 19:34, 28 November 2010 (UTC)

Definitely looser. The amount or air that comes in because the opening is larger is nothing compared to the insulation the air gap gives you. Ariel. (talk) 20:55, 28 November 2010 (UTC)

Best to wear baggy jeans with layers underneath, such as jogging trousers, long underwear, leggings, or all three. Insulation is provided by trapped air, so loose layers are best. 92.15.14.132 (talk) 22:49, 28 November 2010 (UTC)

Sleep, tiredness, exercise

1) Why does taking a lot of exercise in the day make you sleep well that night? 2) Why does not getting enough sleep make you feel tired? You havnt been exercising all night, so why should you feel tired? 3) Why is the tiredness you get from not sleeping enough the same as the tiredness you get from exercising a lot? Do they have any differences? Thanks 92.15.14.132 (talk) 23:10, 28 November 2010 (UTC)

Circular logic

I had to do an experiment designed to show the "equivalence of mechanical work/energy and heat". This was done by heating a piece of copper with friction. The amount of work was calculated, and change in temperature of the copper was predicted using the heat capacity of copper. This was compared to the actual change in temperature of the copper.

But if the heat capacity of copper is determined experimentally by how much energy it takes to raise a mixed mass of copper one degree, then isn't the whole experiment circular? 70.52.44.192 (talk) 23:16, 28 November 2010 (UTC)

But the heat capacity of copper could be determined using chemical energy, like gas. Vespine (talk) 00:23, 29 November 2010 (UTC)

Or by Resistive heating, where the amount of energy put in can be measured quite accurately by maintaining a certain (known) current over a certain (known) voltage drop for a certain (known) amount of time. Buddy431 (talk) 01:50, 29 November 2010 (UTC)