Wikipedia:Reference desk/Archives/Science/2006 August 18

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six degrees of separation

^Moved...

Virtual Particle

Can a photon be called a virtual particle,as it only acquires mass during interaction,but not during transit?

The photons that make up light are called "real" or "on-shell" particles; they obey the energy-momentum relation for having zero mass, in that their energy and momentum are equal. "Virtual" or "off-shell" photons are Fourier components of the electromagnetic field with differing energy and momentum; they don't really propagate, but they do cause the Coulomb interaction... at least in the formalism of quantum field theory. I'm not sure what you're asking, but the short answer is that a photon can be either real or virtual. Melchoir 21:54, 18 August 2006 (UTC)

THanks for shedding some light on that Melchoir 8-)--Light current 00:02, 20 August 2006 (UTC)

Process Instrumentation and Analyzer maintenance

Dear Japanese brothers and sisters,

Keeping process instruments, particularly in oil refineries, calibrated and online presents numerous obstacles in our country. Does this problem exist in Japan, as well. Thank you for your reply.--202.4.4.23 09:46, 18 August 2006 (UTC)

Dear anonymous user, in case you are not clear on what this page is, it is part of the English-language Wikipedia, a general online encyclopedia. The people who answer questions on this page are English speakers from all over the world who volunteer to answer questions because of an interest, and usually some expertise, in general scientific topics. The odds of one of them being a petroleum engineer from Japan is probably not all that high, however, so your chances of getting an opinion from such a person by asking on here are not good.

I've tried to dig up some possible contacts who might be able to put you in touch with people who are able to help you. One relevant web link I was able to find through google was JEMIMA, the Japanese association for electric measuring instrument manufacturers. Also, you might try contacting people connected with petroleum engineering in Japan, for instance, the Petroleum Engineering Laboratory at the University of Tokyo.

Good luck.--Robert Merkel 12:11, 18 August 2006 (UTC)

Thanks for your kind advice. You are probably right, no reply from our Japanese colleagues. I had actually thought they might have the most sophisticated experience in Asia as we are based in the Philippines. --202.4.4.23 06:26, 24 August 2006 (UTC)

negative energy and paul davies

I was reading a book called "how to make your own time travel machine" by paul davies who is a respected physicist[ i read on the book cover anyway i have never heard of him" but in the book he claims that we have the capacity to create an anti-gravity force from the quantum vacuum and an exapmple of this is the casimir effect, apparently it produces negative energy so therfore anti-gravity. what puzzles me is that if gravity is an attractive force and anti-gravity is a repulsive force, then how come the casimir effect produces an attractive force between the two plates? is it something to do with the setup? bernard haicsh and paul davies claim that the casimir effect can be engineered to be repulsive and there could be other effects that are casimir like that produce negative energy and there for repulsivve antigravity, paul davies even cites one which is a single vibrating mirror. is this true? Robinresearch

• well, have you tried Casimir effect, Vacuum energy, Negative energy, or anti-gravity yet?...actually, the last one isn't the best source, but the first 2 should work out--71.247.125.144 12:29, 18 August 2006 (UTC)

Well have been on those no mention of why the casimir effect produces atractive force from what should be an anti-gravitational reaction because it produces negative energy. Looked up paul davies and he is not a controversial physicist, and have gone on NASA website and read that indeed negative energy can produce propulsive force. So it stands to reason that it is possible that if negative energy can be produced by the quantum vacuum there could be an effect that produces more negative energy and therefore more anti-gravity correct? Apparently NASA are looking into this.

However still no mention of why the casimir effect produces attractive rather than repulsive force. Teh casimir effect page says that it is possible for the casimir effect to be repulsive in theory, so that question is answered.

So in summery, negative energy can be produced by the quantum vacuum [zero-point energy], negative energy creates repulsive anti-gravity, the casimir effect can produce repulsive and attractive forces and the casimir effect only produces a small amount of negative energy but theoritically only for now but other reactions that could produce more negative energy from the vacuum and therefore more pronounced anti-gravitational effects are possible. This is not however a free energy source as the activation of enough negative matter would take at least a small bit of energy to cause the reaction that would effect macro-objects. Is all of this correct?

But i still need an answer as to why the casimir effect produces attractive force if it produces negative energy that means it should produce repulsive force. Is it something to do with the mirrors themselves?

Robin research

DVD ram camcorder video movie creation problem

When I try to transmit video from my hitachi dvdcam camcorder(model dzmv550a) from a dvd ram disc to my pc using movie album se software It wont transmit and a flashing hourglass and arrow cursor come up on the screen. What can I do to fix this problem? Thanks Erich

Rubix cube

If you had to design a liquid rubic cube, that would be at least 1x10³ orders of magnitude more difficult than a solid state rubix cube, how would you do it? Keep in mind, the only restrictions on this exercise are that it must be liquid, and must also resemble a normal rubix cube in either form or function--71.247.125.144 13:05, 18 August 2006 (UTC)

First explain what a "Rubix Cube" is and then, what class is this homework for? --Kainaw ^(talk) 13:49, 18 August 2006 (UTC)

Do you mean a Rubik's Cube? It is not possible to make anything that has much shape using only liquid.--Shantavira 14:13, 18 August 2006 (UTC)

No, the X was intentional, after all, there's no such thing as a liquid Rubik's Cube, therefore such a question wouldn't really make any sense--71.247.125.144 14:44, 18 August 2006 (UTC)

Is there any limit on how viscous the liquid needs to be? --Dweller 15:20, 18 August 2006 (UTC)

I'm going to say less than 10²⁰, otherwise you could just make the thing out of glass, and that wouldn't be any fun, now would it, so probably anything more viscous than a syrup just wouldn't be sporting--71.247.125.144 15:24, 18 August 2006 (UTC)

Bah. You spoiled my fun. I think a glass one would be quite attractive. --Dweller 15:29, 18 August 2006 (UTC)

Didn't I read on this very desk just a couple days ago that all that talk about glass being viscous was nothing but a myth?  freshofftheufoΓΛĿЌ  17:13, 18 August 2006 (UTC)

As I understand it, glass comes in two varieties, glass crystal (like leaded glass crystal) is actually a solid, and doesn't flow, while regular amorphous glass is a supercooled fluid, and does flow (although so slowly that it can be ignored). Glass isn't the only thing with that characteristic, however. Metals are similar. Only crystals are really solid. StuRat 17:29, 18 August 2006 (UTC)

I was taugh that glass was a liquid, but the glass article says that is a common misconception. --Kainaw ^(talk) 20:17, 18 August 2006 (UTC)

Actually, the article's source says it could be considered a solid or a liquid, although it also says there is no evidence for any measurable flow: [1] StuRat 03:24, 20 August 2006 (UTC)

glass strangely enough, is in fact a glass (aka an amorphous solid). Xcomradex 23:20, 18 August 2006 (UTC)

Common glass is, yes, but glass crystal, is, as the name would imply, a crystal. StuRat 23:44, 18 August 2006 (UTC)

true, and the line continues it slow journey to the right hand side of the screen... Xcomradex 00:37, 19 August 2006 (UTC)

You just need more glass for a wider screen (or even one of those cheap ones with plastic instead of a glass covering). --Kainaw ^(talk) 00:42, 19 August 2006 (UTC)

That's easy. Just suspend several colors of sand in a bottle of water. The challenge would be to get the sand to settle in recognizably stratified layers. Solving the puzzle would involve shaking the bottle a lot and hoping for the best, which is remarkably similar to how I solve a regular Rubik's Cube, with the one exception that, as specified, it would be astronomically less successful. Not that I've ever succeeded that way on a normal cube. Black Carrot 02:32, 19 August 2006 (UTC)

Oh, and if you want to get pissy about the "must be liquid" rule, you can go ahead and change the sand bit to something else. I'm sure we could find something that has the requisite characteristics. Black Carrot 02:32, 19 August 2006 (UTC)

You could have a bunch of hollow glass cubes or bubbles connected to each other by valves. In each bubble you could have a different colored liquid which is not miscible with any of the other liquids, and the challenge could be to move the liquids to fit some pattern or something. I think you'd be limited by the number of mutually non-miscible liquids you could find, though. --Bmk 15:29, 19 August 2006 (UTC)

Is that your final answer?--71.249.31.135 13:18, 25 August 2006 (UTC)

time travel

What experiments could I do about time travel? -- 66.41.55.93 14:36, 18 August 2006 (UTC)

• You could sit in front of your computer for the next 60 seconds, you will have traveled exactly 1 minute into the future--71.247.125.144 14:48, 18 August 2006 (UTC)

Oh boy, I didn't know that the reference desk would be entertainment :)--Kungfu Adam ^(talk) 14:50, 18 August 2006 (UTC)

I seem to recall that in Alice in Wonderland, the faster you ran, the longer it took to get somewhere. Try timing yourself running faster and faster over, say, 1500 metres. <grins> --Dweller 15:31, 18 August 2006 (UTC)

Run a block as fast as you can and run back. Compare yourself to your identical twin. He will have aged more than you by a very tiny bit. Edison 15:36, 18 August 2006 (UTC)

SERIOUSLY you could get two atomic clocks synchonized together. One is left on the ground and one is put on an aircraft which travels around the world. When the clocks are compared after the flight there is a discrepancy. One has travelled in time relative to the other. It was done in 1971 [2] .Also the perehelion shift of the planet mercury is proof that Einsteins theory of relativity works. So I guess you have the last laugh and HAVE asked a relevent question!!

I forgot who the scientist was, but I remember an article in my newspaper about how it may be possible to take an extremly massive object, an put it in a circular path at high velocity and the space inside the circle time will travel slower, so, in say 50 years in the circle 500 years passed. Not sure on reason69.29.78.229 18:06, 18 August 2006 (UTC)

I wonder if it is possible for a kid to get an airline to allow him to leave a rather accurate clock on a plane for a year. Of course, a kid can't afford an atomic clock, but having a few rather accurate ones is fine. Then, have a few rather accurate ones in his room. In the end, compare the microseconds. That leads to the question that I don't know... how many years of average air travel does it take to lose a single microsecond? --Kainaw ^(talk) 20:24, 18 August 2006 (UTC)

I don't know, but it felt like I lost a couple of hours flying from Edmonton to Toronto. - Cybergoth 21:46, 18 August 2006 (UTC)

MIT student, Amal Dorai, came up with a very elegant way of proving time travel will be possible. He organised the first (and possibly only) Time Traveler Convention. Sadly, as of yet, no traveller has found a slot in their busy schedule to attend. Rockpocket 22:04, 18 August 2006 (UTC)

You could listen to late night radio, I tuned in at 2:30 a.m. once, and it was "Time Traveler Night." Most of the people who called in said that time travel involed reaching 88 m.p.h. or getting struck by lightning while drinking large quantities of beer.

I also have a time machine that can travel 8-10 hours into the future once a night. Sadly, the backward travel function is broken.--67.172.248.207 03:03, 19 August 2006 (UTC)

There is a web site available now on the internet that, once you are logged into it, propels you quite quickly into the future. Again it doesnt work the other way round. Its called wikipedia or something.--Light current 06:35, 19 August 2006 (UTC)

If anyones really interested in this stuff, I recommend the book 'Black Holes and Time Warps' by Kip S. Thorne (a close friend of Stephen Hawking)--Light current 07:53, 19 August 2006 (UTC)

If i recall correctly, the actual experiment was done of flying an atomic clock on a jetliner, and showing that it "aged" less than the identical synchronized atomic clock who remained on the groung. Edison 03:22, 20 August 2006 (UTC)

You need a computer and a programming language that allows you to dump the state of the whole running program to a file and continue execution. Then, experiment with loading older images after the program has continued. Now that's time travel. If you even find a way for the program to reinvoke the older image but pass an object back to the older image, that's moving objects back in time. A particularly fine variation on this is the call/cc function which saves the call stack (which is called a continuation) but not the object space (the content of existing objects). Call/cc can go not only forward in time but backwards, when you save the exit procedure until when the call/cc call has already returned. Call/cc has a very interesting application when you are programming in a side-effectless manner so call/cc saves almost every aspect of the program, except that you can pass an object back from the future as the return value of call/cc: this is McCarthy's ambiguous operator.[3] (Thanks for David Madore who gave me the idea by describing call/cc as being able to travel backwards in time.[4]) – b_jonas 20:53, 21 August 2006 (UTC)

Standard error

No-one seems to be around on the Mathematics desk and I'm sure you scientists are equally capable of helping me with the question I recently posted there about standard error. Sorry for (almost) cross-posting, but there's an element of time pressure on me. Help gratefully received. --Dweller 14:59, 18 August 2006 (UTC)

error function (erf) Its related to a gaussian curve isnt it?

--Light current 05:40, 19 August 2006 (UTC)

bioluminescent pigs

Taken from (August 12^th): No, but seriously, bioluminescent pigs, how cool is that?--71.247.125.144 15:29, 18 August 2006 (UTC)

Will I be able to find the bacon when the refirgerator light has burned out?Edison 15:37, 18 August 2006 (UTC)

Pork: the other light meat. --Fastfission 16:26, 18 August 2006 (UTC)

These are pretty cool too. Rockpocket 21:56, 18 August 2006 (UTC)

No you'll need to look at the pig under a UV light. David D. (Talk) 22:27, 18 August 2006 (UTC)

The fish and pigs use the same biotechnology. GFP and its varients can been seen in certain tissues without the need for a UV source. Its much more dramatic under UV, of course. Rockpocket 23:59, 18 August 2006 (UTC)

Remember daylight does have UV light. GFP needs to be excited to emit. David D. (Talk) 05:27, 19 August 2006 (UTC)

Of course. Sorry, rereading your answer i now realise your "no" was in response to the refrigerator question. I thought you were commenting on the fact that the the pigs were different from the fish. Rockpocket 05:35, 19 August 2006 (UTC)

Better link: fluorescent green pig. Henry^Flower 22:07, 18 August 2006 (UTC)

question on evolution

could any other animals other than humans evolve to be as smart as humans. Would it happen, as in, could chimps get smarter and eventually be as smart. Also, could humans stimulate animals like birds through enviroment modification to spur evolution in that area?69.29.78.229 18:49, 18 August 2006 (UTC)

It could happen, yes, but humans most likely prevented this from happening by outcompeting other intelligent animals, like Neanderthals. Think of "intelligent animal" as one ecological niche, which we humans have totally taken over, leaving no room for any competitors. StuRat 18:56, 18 August 2006 (UTC)

An interesting science fiction series that discusses this topic is the Uplift Universe series by David Brin. --Ginkgo100 ^{talk · e@} 20:20, 18 August 2006 (UTC)

And, of course, the less interesting Planet of the Apes. StuRat 20:33, 18 August 2006 (UTC)

Although, in the Uplift Universe, every intergalactic species (except for humans) was artificially bred for intelligence. The rest of the intelligent species in the universe look down on humans for being the only species which had ever done it on their own. Humans are in the process of uplifting chimps and dolphins in the books. Very good series, by the way. User:Zoe|(talk) 23:29, 18 August 2006 (UTC)

Intelligence is subjective. If I put you in the airport and asked you to tell me which bags had cocaine in them without opening them, could you do it? Humans have selectively bred dogs that have the olfactory sense to do it and the intelligence to use the sense in an intelligent manner. --Kainaw ^(talk) 20:26, 18 August 2006 (UTC)

Yeah but if I had a nose as big as a dog, I might stand a better chance. I wouldnt say that was intellingence. THe intelligence comes in knowing that dogs are better at sniffing that almost any other animal.--Light current 17:07, 19 August 2006 (UTC)

It took more than 200 million years from the emergence of mammals to the genus Homo. So even if all humans were to disappear overnight, it might take some time before some other species picks up on editing Wikipedia again. As to using modified environments, what do you think: Could we stimulate spiders to develop flight through environmental modification? It is not at all clear how we could even start approaching it. And we understand the mechanisms of and pathways to flight a lot better than intelligence. --Lambiam^Talk 20:44, 18 August 2006 (UTC)

I would say we already have effected the evolution of other species in many ways:

1) Several species have evolved to live with people. Cats, for example, may have evolved purring as a way to seem cuter than non-purring cats, and thus get more food and shelter from humans.

2) Other species, which seemed like a threat to people, were driven to extinction, such as saber-toothed tigers.

3) Other species, which were easy prey for humans, such as the dodo bird, were also driven to extinction.

4) Human caused global warming, has, and will continue to, favor some species at the cost of others.

5) Pollution favors some species over others.

6) Humans have introduced non-native species to many areas, allowing some species to flourish and move into new ecological niches, at the cost of the native species.

StuRat 21:11, 18 August 2006 (UTC)

I'd be interested to know how you explain purring pumas. ;) Henry^Flower 22:12, 18 August 2006 (UTC)

I imagine some version of purring existed in many cat species prior to humans, and probably served a social purpose (making the purrers seem less threatening to other pumas, etc.). However, once humans showed up, purring became a very important skill to feline survival, so rapidly evolved from barely audible to quite loud. StuRat 22:24, 18 August 2006 (UTC)

Cats purr to each other, audibly and probably have done so for a long time. They purr when intereacting with other cat "friends" and most commony when nursing or being nursed on. Since cats likely see humans as a mother or friend substitute, their purring makes complete sense in that context and is probably not affected by humans or an example of evolution. The whole "cats only purr to humans" is a common misconception floating around. Also, evolution is notoriously slow, and the small time domestic cats have been with humans is not enough to make that change. Besides, other small cat species purr. The large cats are however, physically incapable. pschemp | talk 02:40, 19 August 2006 (UTC)

pschemp, please don't modify what I write, that's inappropriate Wikipedia behavior. You can add your comments, but you shouldn't change mine. I've undone your edits to what I wrote. StuRat 15:50, 20 August 2006 (UTC)

I've read that cats only "Meow" to humans, never to other cats. That noise may sound to a cat like human speech. just as humans think that cows say "MOO!."Edison 03:26, 20 August 2006 (UTC)

The "meow" cats use on humans to get attention is very similar to the "mew" used by kittens to their mother — in fact, it's probably the exact same sound, except for the trivial pitch change caused by the cat growing in size. Adult cats do meow to each other for various reasons, but those are generally different sounds; the word "meow" can cover a very broad range of feline vocalizations. —Ilmari Karonen (talk) 14:57, 27 August 2006 (UTC)

"Would it happen, as in, could chimps get smarter and eventually be as smart." If chimpanzees were specifically bred for intelligence—either by humans or by the needs of nature—then maybe. But it would take many, many, many generations for a chimpanzee to become anywhere near as intelligent as a human being (i.e. it would take a loooong time and you'd need to breed a lot of chimpanzees). It's also highly likely that we'd do it in an inefficient manner—i.e. work really hard to breed chimpanzees who were good at playing chess and then realize that being able to just play chess is not exactly what we meant by intelligence (but perhaps I allude too much to the history of artificial intelligence?).

"Also, could humans stimulate animals like birds through enviroment modification to spur evolution in that area." Well in intelligence, maybe but the odds are that there are quicker ways for birds to evolve to fit into an ecological niche created by humans, i.e. the way that the common rock pigeon has become such a well-adapted being for living in urban areas in comparison to, say, a hawk. With rapid change to environments on the scale done by humans, though, you'd probably just get a lot of extinction and only a little bit of adaptation, and I'm not sure that adaptation would favor general intelligence in particular. General intelligence is not, as should be clear by the more-or-less lack of it in the animal kingdom, always the best adaptation. If you already know how to fly, the species will probably just become better at that. It takes a lot of specific jumps for general intelligence to be a really powerful adaptation (i.e. the ability to manipulate tools is no doubt up there as a very useful prerequisite—it is something which more general intelligence can be directly applied to with devastatingly powerful results). I'm not sure that sparrow which is only incrementally smarter than other sparrows is unlikely to be able to translate that brainpower into a real reproductive difference. --Fastfission 23:53, 18 August 2006 (UTC)

Mad scientists have already inserted human brain tissue into rats. In time, they will likely do the same with chimps. Edison 03:24, 20 August 2006 (UTC)

I'd love to see your source for that statement... Rockpocket 07:58, 20 August 2006 (UTC)

Who is to define intelligence? Us? Then the question becomes if other animals would be able to become like humans. Sounds unlikely. Then again, there's the phenomenon that very different animals can evolve to be very similar if the conditions are similar enough (like dolphins looking rather like fish) (I forgot the term for this). But such animals would never really have our inteligence, and for us we are the ones who define the meaning of 'intelligence'. By their standards, we would probably look silly, having some intelligence, but not quite enough. Imagine what a cow thinks of us when we say 'moo' to it. Or a cat when we meow to it. If it tries to imitate our speech and we respond by saying 'meow' back, then it probably thinks it got it right and thinks it's pretty smart. Or maybe it is trying to communicate something and we reply with some weird answer. It would probably think us fairly intelligent (can talk) but somehow lacking (talks nonsense). DirkvdM 08:11, 21 August 2006 (UTC)

Gold and Silver

I need to find a table or chart that shows the optical density and emissivity of Gold and Silver. Thanks for any help!

matweb will give you the emissivity, at least. Apparently the term optical density is ambiguous. You ought to be able to estimate the absorbance from the electrical resistivity given in matweb. That's left as an excercise for the reader. -- The Photon 02:57, 19 August 2006 (UTC)

Nystagmus upon waking

Is it common to have nystagmus upon waking from a vivid dream? It happened to me the other night when I woke suddenly from a nightmare. It had a semicircular pattern with the fast phase to the left. It only lasted a few seconds. In the dream there were a lot of video screens with images flashing rapidly up and down. --Joelmills 22:02, 18 August 2006 (UTC)

The article states that it is relatively common, although I've never read anything about association with dreams or waking up before. — [Mac Davis] (talk)

Changing DNA on the fly

In Star Trek, every other episode you get a case of characters devolving into monkeys or growing giant brains or turning into spiders because they've had their DNA changed. Presumably, the idea behind this is that if you change a individual's DNA (genotype, I suppose) you can almost immediately affect their phenotype, their outside characteristics. So to use a simple example, if I could somehow change the allele for brown eyes to blue eyes in a human's DNA, at some point (instantly? in days? weeks? years?) the human's eyes would turn blue. I assume the science is that as the iris cells die and are replaced, the new DNA is used to code for the iris colour, resulting in a different colour.

This has always seemed rather dodgy science to me, but it has a spark of believability about it. So the question is: if I change a person's DNA, will it ever change their appearance, etc.? Sum0 22:18, 18 August 2006 (UTC)

• (^{edit conflict})Of course it will, it just won't turn you into another organism, transcription errors happen all the time, but rather than changing a phenotype in any noticable way, they usually just wind up killing a cell, or even worse, leading to uncontrolled cell growth, aka, tumor formation ^{(David D. beat me to it)}--71.247.125.144 22:28, 18 August 2006 (UTC)

If you could change the gene in all cells then you could change the appearance. Cancer cells are an example of mutations that change the genotype and phenotype. Except in this case only a single cell is involved. In star trek i think they usually invoke a virus whivch presumably infects (and changes) all cells. Or maybe only the brain cells in your " giant brains " example. David D. (Talk) 22:25, 18 August 2006 (UTC)

For some changes, yes, for others, no. Some changes that take place in our bodies are not reversible. Your height, for example, can't decrease by much (it can a little, due to osteoporosis), since there is no biological process in place to remove large bones and replace them with smaller bones. Some changes, however, can take place "on the fly", and don't even require DNA changes, such as changes caused by testosterone injections. StuRat 22:38, 18 August 2006 (UTC)

Aside from your brain, your a totally new you ever 7 years due to cells dying and being replaced. Some cells, like red blood cells, are replaced faster tahn, say, bone.69.29.78.229 23:11, 18 August 2006 (UTC)

• and if you use five ~~~~~ it just gives the date--71.247.125.144 23:13, 18 August 2006 (UTC)

If you change DNA on the fly, you get... The Fly! --Lambiam^Talk 01:37, 19 August 2006 (UTC)

It's important to always take into consideration how DNA works when thinking about stuff like this. Your DNA becomes RNA which becomes a protein (more or less—there are more complications of course). These proteins have big phenotypic effects, some of which are fast (formation of new cells with specific characteristics) and some of which are not. Some things only happen at specific times in your life (i.e. puberty) and will not happen again. It seems unlikely to me that changing the DNA of an already fairly well-developed organism (even if you had a way of changing all of the DNA in all of the cells at the same time) would ever make it able to grow into a super-sized organism. You might be able to, however, change a gene which encourages a never-ending appetite and make them very fat. But even that would be pure speculation that a simple full-organism knock-out would have phenotypic effects which would alter those which had already gone through a long period of development to get to where they were. --Fastfission 04:32, 19 August 2006 (UTC)

Eyes rolling to the back of the head

Why is it that people's eyes sometime roll to the back of their eyes? Also, why do some people's eyes roll back when they die and some don't? 63.23.82.53 22:46, 18 August 2006 (UTC)

No-ones eyes do, it would detach the nerve, plus the muscle arrangement means nothing over 90 degree rotation in any direction from straight forward is possible. Philc _T^E_C^I 22:56, 18 August 2006 (UTC)

Yeah, its just something you see on television. — [Mac Davis] (talk)

I think they just mean rapid up and down, or side to side, movements, not literally 'behind' their head--71.247.125.144 23:07, 18 August 2006 (UTC)

when you are in non-rem sleep, your eyes will roll slightly so that the apperance of your pupils are in your skull, which, for most people, doesn't happen just some of the pupil is, there are anomolies out there, though, but nobody's eyes are even somewhat capable of even looking towards "the back of the eyes"69.29.78.229 23:16, 18 August 2006 (UTC)

Source? — [Mac Davis] (talk)

When you are dead, you become cross eyed. (The optician told me)--Light current 05:36, 19 August 2006 (UTC)

Do it your self X-ray

I've heard stories of people who, while witnessing atomic testing, have held their hand in front of x-ray film, and gotten a clear picture. It sounds sketchy to me, but I suppose it could be true. Is it?--67.172.248.207 23:56, 18 August 2006 (UTC)

Wouldn't the film be overexposed? or even completely washed out?--71.247.125.144 23:59, 18 August 2006 (UTC)

not over x-ray film, but over their eyes. common veterans story here from the days of nuclear tests. Xcomradex 00:40, 19 August 2006 (UTC)

I don't know what an x-ray film is, but if you took an x-ray of somebody's hand, no matter how much nuclear testing they witnessed, it would not be clear picture, that's not how it works. An x-ray works, because bones absorb more of the xray region of the electromagnetic spectrum than surrounding skin, fat, muscle, etc. See the radiation article. — [Mac Davis] (talk)

It's the "hand over the eyes and see your bones" tale that is usually given. A large part of a nuclear explosion does come in the form of X-rays (which is what drives the compression in a hydrogen bomb), but I have no idea about how true the story is, except that a lot of people have told it, though that hardly makes it true. It strikes me as a little fishy as I think about it, since the human eye can't see X-rays unaided (i.e. without film or a fluorescing screen). A lot of different things are released during a nuclear explosion though so something else could be going on there as well. --Fastfission 04:41, 19 August 2006 (UTC)

obviously not x-rays, i'd say the enormous amount of visible light released does the trick. Xcomradex 05:06, 19 August 2006 (UTC)

Yeah, flesh is translucent to visible light, just not very. Hold your hand over the end of a torch (or, better, a white LED) to see. (Of course, some of that's just subsurface scattering.) EdC 23:45, 19 August 2006 (UTC)

Hand over the eyes story is absolute bullshit, since the eyes are not especially sensitive to x-rays, and the lens and cornea are not remotely capable of focussing xrays on the retina. If you were close enough to a nuclear explosion to absorb about 100 millirems, and you held your hand over an x-ray film or other photographic film in a lightproof paper wrapper oriented so the hand was normal to the vector between the film and the explosion, you could hardly help but take a fine x-ray.Edison 03:31, 20 August 2006 (UTC)