Wikipedia:Reference desk/Archives/Science/2008 January 19

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January 19

Eyelid Vision

(Subject is for lack of a better name)

I have a strict policy of never making stuff up, so please believe this. A few months ago I had horrible insomnia, and in one fit of boredom while trying to fall asleep I stared at my hand and then closed my eyelids. Naturally the low light didn't make an after-image like a brighter light would. But then something strange happened: when I moved my hand I could see it, even though my eyes were closed. The image was black on black, but I could see the movement none the less. At first I thought I had gone daft, but after trying it many times I found that if I focused correctly I could see things through my eyelids (even when they are not moving). I just tested it out in complete darkness to make sure I wasn't just seeing the shadows (which would explain it), and it works better in pitch darkness than in very dim light - whereas in bright light I can only see the shadows without the clear outlines that I can see in darkness. Now I have come here to ask how the hell this is even possible. My theory is that my mind knows where my hands are and fakes an image, but that doesn't explain how I can see my window, along with anything I am holding. It only works when my eyes have adjusted to the darkness, before then I "see" nothing. Is there any explanation? Chris16447 (talk) 01:37, 19 January 2008 (UTC)

When your eyelids are closed, your irises opens up fully and your eyes therefore become even more sensitive to light passing through the thin skin of your eyelids. They may also be slightly sensitive to infra red which may be why you can see your hand.--TreeSmiler (talk) 04:46, 19 January 2008 (UTC)

Another explanation is simple brain activity. You don't really "see" what you see, your brain is building a model that is updated when new information comes in, and that integrates not just your vision, but also other inputs. A good example is movement - your brain compensates the effect of your felt movement, so that a still scene appears still although the physical image on your retina changes. That is e.g. why you are supposed to look at the horizon when you are sea-sick - if you can see your the motion against the motionless horizon, your brain copes, but if you feel the movement without getting visual confirmation (because everything you look at moves along with you), you get dizzy. In your case, your mental model includes both the visual and the tactile senses. If you move your hand, you know where you should see it, and so your brain creates the proper illusion. --Stephan Schulz (talk) 12:25, 19 January 2008 (UTC)

Incidentally, the term that describes your brain's awareness of where your body is in space is proprioception. TenOfAllTrades(talk) 15:23, 19 January 2008 (UTC)

Thank you very much for your informative answers. I wasn't aware this phenomenon went so far as to fabricate images! Chris16447 (talk) 18:25, 19 January 2008 (UTC)

3-D without glasses (parallax barrier?)

How did this work? Our one-sentence-long parallax barrier article is, um, lacking. Obviously the different eyes must somehow be receiving different signals, but how exactly? --24.147.69.31 (talk) 02:35, 19 January 2008 (UTC)

Click on the illustration halfway down this review. --169.230.94.28 (talk) 03:13, 19 January 2008 (UTC)

Stirling Engine formula

Is there a formula which relates Stirling Engine displacement cylinder height and diameter (or volume) with displacer height and diameter (or volume) and piston cylinder height and diameter (or volume) with piston stroke and displacer stroke? —Preceding unsigned comment added by 71.100.12.59 (talk) 03:54, 19 January 2008 (UTC)

how to improve in transcription?

—Preceding unsigned comment added by Vidyat (talk • contribs) 06:43, 19 January 2008 (UTC)

I've given proper format to your question, despite it being just a heading. --Taraborn (talk) 11:11, 19 January 2008 (UTC)

This still isn't really a question, since it's not clear which type of transcription the OP is referring to, or in what way he wants to improve it. Nevertheless, since this is the Science RD, I'm going to guess you're looking for ways to improve genetic transcription. Our article(s) may be a good place to start. You could also elaborate on your question; we'd be glad to help. (EhJJ) 15:49, 19 January 2008 (UTC)

Long Term Climate Change

A VERY long time ago (1970's maybe) I read about a theory of climate change that tried to explain long term effects in terms of the motion of the sun through the galaxy. I recall that two elements of the sun's motion in this theory were:
1. Fixed stars rotate around the galactic centre at a different rate to the spiral arms and thus periodically pass through the dust of the arms.
2. The sun has a bobbing motion similar to a floating buoy and cyclically passes through the galactic disc.
Have these ideas survived into the age of the internet anywhere? They don't seem to be mentioned in the Climate change article. Possibly they are discredited now, but I would like to know what happened to them. Please note that I am not talking about the well known ice age cycles, but something on a much longer time scale (100s of millions rather than 100s of thousands of years). SpinningSpark 11:26, 19 January 2008 (UTC)

Yes, those patterns of motion do exist. Yes, some people think they may influence long-term climate. However, there isn't a lot of information to support those theories. Most of the focus has been on the encounters with the galactic spiral arms (which is probably the larger effect), but the time between passages is poorly constrained with various researchers arguing for 140 Myr, 250 Myr, or even longer. As it turns out, it is very hard to measure the galactic pattern speed, since the pattern as whole evolves independently of the motions of individual stars in the same way that ripples run across the surface of a lake much faster then the motions of individual water molecules. On top of that there are a whole seperate set of problem associated with measuring climate over tens of millions of years when even continents don't stand still. So, yes, it is possible that the galactic motions of the solar system affect climate, but it is not yet a well-established conclusion.

However, if you are interested in recent work in this area, you can read: [1] Dragons flight (talk) 19:04, 19 January 2008 (UTC)

Do you think that this is a suitable subject to add to the paleoclimatology article, or is it too "on the edge"? SpinningSpark 04:37, 20 January 2008 (UTC)

Time travel and parrelle univerises

Assuming the existance of parrelle univerises, where every action which could (or couldn't) happen plays out differently in an infininate number of ways like forks in the road, would a disurption of the timeline (eg time travel resulting in the past being changed and thus changing the furture) in one universe affect all the other parrelle universes? Eg:

=============================================== =Parrelle Universe 6727 = = = =Past Present = = ------------------------ = = = =============================================== =Our Universe = = = =Past Present = = ------------------------- = = | <- Time travel = = -------------------- Event = = | = = ------------------------- = =Altered Past Alternate Present= = = =============================================== =Parrelle Universe 95579483472323457 = = = =Past Present = = ------------------------ = = = ===============================================

Weasly (talk) 14:26, 19 January 2008 (UTC)

Wouldn't time travel (whether you travelled or not) simply create more forks? Essentially, by going back in time, you'd make brand new forks and the ones that already "existed" would not be altered. (BTW, I'm not any kind of expert, but this is what I would expect as a result, given your first assumption). (EhJJ) 16:32, 19 January 2008 (UTC)

Almost forgot, Wikipedia has a lot of articles about Parallel universes, if you're interested. (EhJJ) 16:34, 19 January 2008 (UTC)

Standard deviation devide by N or by N-1

Maybe one of our statistiscians can explain this one to me. When calculating the standard deviation we can either devide by N-1 or N. Deviding by N-1 gives us a prediction, whilst deviding by N doesn't give this predictive value. What I'm wondering is what's the difference? How can one be predictive whilst the other is not? And why N-1, why not..N-2 or N-0.25. What's so magical about N-1?PvT (talk) 17:35, 19 January 2008 (UTC)

The Mathematics desk had an recent entry on this topic. I hope you find that discussion useful, as well as the articles on standard deviation and variance. In this latter it is explained why N-1 is used when estimating the variance of a population by means of a sample (using N instead in this case generates a biased estimation of variance). Pallida  Mors 17:48, 19 January 2008 (UTC)

Thank you for the information. The reason I asked the question btw. was that I'm doing an internship and had to calculate (for a rapport) the SD for duplicates. And to me this was confusing. Because N-1 would also take future measurements/samples into account. But that's idiotic. With such a small number of measurements taken the difference between N-1 and N is huge. My logic thus was that it was better to just show the true/known variance between the two samples taken. I showed that to some of the workers (one of which has a phd) and they are fine with it. It still bugs me though. I think the real answer to the problem is to do more tests!PvT (talk) 18:13, 19 January 2008 (UTC)

The basic question is: are the things you have measured the only things you care about? If they are, then you can sensibly calculate the standard deviation of the measured quantities (dividing by N). If they are just samples from a larger population (which you care about) then calculating the SD of the samples isn't helpful. Dividing by N-1 gives an unbiased estimate of the variance of the population, but with only two samples, this estimate is so laughably bad as to be pretty useless. If you care about the population, you certainly need more than two data points. Algebraist 18:26, 19 January 2008 (UTC)

That was my line of reasoning as well. I'm not taking samples from a larger population. The duplicates are the population. Thank you for you assistance.PvT (talk) 18:35, 19 January 2008 (UTC)

Formally speaking, if you are defining the mean based on your measurements, then you are reducing your effective sample size by 1 in the process. Hence N-1 rather than N is better estimate of the variance in your underlying population. If your mean is specified in some other way, for example because you already know the properties of the underlying distribution, then you don't reduce the effective sample size, and N is the correct estimate. Of course, ideally, you want to have enough data that difference between N and N-1 is not a big deal. Dragons flight (talk) 18:44, 19 January 2008 (UTC)

weather and climate

what is the importance or essence of studying weather and climate —Preceding unsigned comment added by 195.166.237.254 (talk) 18:01, 19 January 2008 (UTC)

Obviously for its predictive value: how could we give hurricane warnings or tsunami warnings, or more practically, how could we plan the party this weekend when we don't know if it will rain?18.96.7.61 (talk) 20:07, 19 January 2008 (UTC)

Coordination

Does being shortsighted or having astigmastism affect balance and performance in sports? If so, in what way? Thank you. Clover345 (talk) 20:38, 19 January 2008 (UTC)

Most people who are shortsighted or have astigmatism will use contact lenses during sports. This usually results in normal or better-then-normal vision. I don't know of any problems that are otherwise associated with myopia and sports (and I have -9/-8 vision!) (EhJJ) 18:41, 20 January 2008 (UTC)

Better-than-normal? Are you sure? --Taraborn (talk) 19:14, 20 January 2008 (UTC)

I am shortsighted and have a very severe astigmatism. I am a Judoka, and wearing contacts in Judo can lead to problems in the type of sport that Judo is. Since I don't wear any glasses/contacts, I *have* to rely on my sense of balance and other senses more than others do. This has come at a great advantage to me, and my balance and performance are heightened while I play. :-) I doubt it helps in Basketball or other precision, throw the ball into a hoop/glove/net type sports. --.ιΙ Inhuman14 Ιι. 19:40, 20 January 2008 (UTC)

I was just wondering because there are alot of shortsighted and astigmatic people who don't use lenses during sports and are still good. Surely, it would affect hand eye coordination in such sports as basketball, tennis, baseball, fencing which all require good hand eye coordination and balance. Clover345 (talk) 19:53, 20 January 2008 (UTC)

There an article on Vision training, small but with links etc. Julia Rossi (talk) 00:24, 23 January 2008 (UTC)

urea

ok, i have a lot of urea wat experiments can i make from it? perferably pyro. —Preceding unsigned comment added by 76.14.124.175 (talk) 21:45, 19 January 2008 (UTC)

Using bases you can decompose it into carbonate and ammonia. With heat you can convert it to biuret which has a nice blue reaction with copper sulphate. You can oxidise it with chlorine bleach. A reaction on the pyro front that I am not sure I believe is nickel powder heated with urea produces hydrazine and nickel carbonyl. With a mineral nutrient solution some bacteria can oxidise urea into nitrate. With formaldehyde you can make a urea formaldehyde polymer. Production of melamine or Cyanic acid could also be possible by heating urea. Graeme Bartlett (talk) 04:20, 20 January 2008 (UTC)

Please note that many of these reactions produce deadly poisons. Mixing urea with chlorine bleach produces nitrogen trichloride, which is nasty stuff, and nickel carbonyl is sometimes called liquid death, for good reason. So I would recommend against doing any of these, and if you want to do it anyway, use common sense and do it in a well-ventilated area. (And don't sue Wikipedia.) —Keenan Pepper 19:10, 20 January 2008 (UTC)

Here's a non-chemical experiment: testing the effectiveness of urea as a plant fertilizer. Try raising two identical plants without urea, two with it, and two with commercial fertilizers in otherwise-identical conditions. --Bowlhover (talk) 06:05, 20 January 2008 (UTC)