Wikipedia:Reference desk/Archives/Science/2009 July 2

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July 2

Acceptability of wave function

How I mathematicaly prove e^ -|x| is acceptable function .plese give a example .Supriyochowdhury (talk) 01:22, 2 July 2009 (UTC)

First you need to define "acceptable" - this term does not have a specific meaning in mathematics. Do you mean "finite-valued," "bijective," or some other description of a function? Acceptable redirects to Proper, which does have several mathematical meanings - but out of context, I think it's too vague to pick which meaning is intended. You mentioned wave-functions, so do you intend to show that the integral of e^ -|x| is integrable and that the probability function integrates to 1? If that is your intent, you should perform the integral and check whether it is suitable for your problem. Nimur (talk) 03:34, 2 July 2009 (UTC)

I am assuming the domain is Quantum Mechanics...I don't think it needs to Integrate out to 1, if it doesn't, you can always normalize it..The important thing is, it must be finite. So, I think being continuous and well defined in the whole region from minus infinity to infinity would do... So now you can see it is satisfies both conditions... I'll leave it for the experts to explain more... Rkr1991 (talk) 05:27, 2 July 2009 (UTC)

Yes. Simply prove that the integral of abs(exp(-abs(x)))^2 over all x (from -infinity through + infinity) is finite —Preceding unsigned comment added by 81.11.170.162 (talk) 07:14, 2 July 2009 (UTC)

Solution to biting insects?

Having been bitten (by midges, I suspect) no less than 10 times in the past two days, I've declared a state of emergency and am searching for a way to combat this continued assault. I would like something cost-effective and long-term and low-maintenance.

• I've heard that moving-air is a deterent.
• I've been told that barbed-wire wont help.
• I've read that cheap consumer units for electric-based destruction are ineffective (insects not attracted to blue ("UV") light) - I have first-hand experience of this, from a couple of years ago
• I'm considering deet but it may dissolve my mosquito net (which I suspect is made out of some plastic polymer) and needs to be replaced every day.

I'm a student, living in a single room, so I don't have a large area to cover but I don't think my windows are amenable to nets due to their manner of opening, besides which insects can invade via housemates's rooms.

I'm living in NL. ----Seans Potato Business 01:40, 2 July 2009 (UTC)

Where exactly is NL? Nimur (talk) 03:36, 2 July 2009 (UTC)

That refers to .nl surely. 208.70.31.206 (talk) 07:45, 2 July 2009 (UTC)

• I've had problems with flies in the past. Sticky fly paper is the way to go - just hang up a bunch of them. It works, and you can see it work. Also make sure you don't leave wet trash out - that's where flies breed. Don't trust in ultrasonic deterrents. You can also look at some of the heavy duty systems used in horse stables - you can read more for example here. Actually fly zappers do work, but this can depend on the particular type of fly. Dcoetzee 01:48, 2 July 2009 (UTC)

I've had some success with insect repellent candles, particularly if you are sat out on a balcony in the evening, or something. --Tango (talk) 01:56, 2 July 2009 (UTC)

I've heard that Citronella and lemon grass are plants which can act as a deterant to many biting insects. --Jayron32.talk.contribs 03:44, 2 July 2009 (UTC)

Citronella and lemon-grass plants are an interesting idea! Citronella oil is the active ingredient in insect repellent candles. DMacks (talk) 05:56, 2 July 2009 (UTC)

The bug patch is the latest thing and has been recommended to me. You stick it on your skin. Get it on the net or in camping stores.--Shantavira|^{feed me} 07:35, 2 July 2009 (UTC)

If the bugs are only bothering you at night, try sleeping under a mosquito net. 208.70.31.206 (talk) 07:45, 2 July 2009 (UTC)

You might be able to find Neem oil at a local health food store, herbalist or flea-market nl:Azadirachta indica. Try a bit on a paper towel first, some people find the smell hard to bear, others hardly notice. (OR To me it sort of smells like cold vegetable soup.)71.236.26.74 (talk) 09:14, 2 July 2009 (UTC)

It's claimed that biting insects are attracted to carbon dioxide (that's how they find living, breathing things) - there are several CO2 generators out there that claim to be able to rid you of mosquitoes "naturally". Sadly though, this may not help our OP - you wouldn't want to use one of those indoors for obvious reasons. SteveBaker (talk) 11:19, 2 July 2009 (UTC)

Try any of the artemsia plants, especially southern wood. Unfortunately the fly family, Tabanidae (horseflies & deerflies) find you using visual cues. 67.193.179.241 (talk) 14:39, 2 July 2009 (UTC) Rana sylvatica.

I removed the leading space from your answer 67.193, so that it didn't extend further that the width of the page. Hope you don't mind! --Kateshortforbob 15:46, 2 July 2009 (UTC)

The best insecticide/repellent I have ever used is a pyrethrin-based aerosol. Gets rid of all flies, mosquitoes, midges, cockroaches... Have a google search for pyrethrin-based products available in your part of the world. Gwinva (talk) 04:19, 3 July 2009 (UTC)

Who has a "blood-brain barrier"?

Obviously, humans do. Animal experiments are mentioned, so perhaps all mammals have the blood-brain barrier; is this true? What about other animals which have brains; do they all have a blood-brain barrier, or ??? Thank you. - Hordaland (talk) 05:06, 2 July 2009 (UTC)

(Whoever has the answer, please improve the cited article. Good catch, Hordaland.) Tempshill (talk) 06:30, 2 July 2009 (UTC)

This is an excellent question indeed. I know mammals have a BBB as adults, but usually do not have a functional BBB when very young. I also know that birds (or at least some birds) have a BBB. As for other vertebrates - I don't know, and I could not find anything useful in Google Scholar. I know frogs have a functional choroid plexus, though, so they may have a BBB as well. I'll do a more thorough search tomorrow. I don't know if arthropods have anything homologous to a BBB, either. Of course arthropods can not have a proper BBB, as they do not possess a closed circulatory system to begin with; however, they may still have some sort of an epithelial barrier preventing at least some parts of their nervous system from the direct contact with the haemolymph. Anyway, great question. I'll fix the article, but I need to do some reading first :) --Dr Dima (talk) 07:15, 2 July 2009 (UTC)

All vertebrates have a BBB that consists of epithelial cells. Some invertebrates have an analogous system (insects for example), that consists of glial cells that surround blood vessels. Some sources taken out of the german wikipedia article (which is much more scientific and complete than the english one): [1], [2], [3] --TheMaster17 (talk) 09:31, 2 July 2009 (UTC)

Crayfish and fruit flies, even. The one paper speaks of "CNS barrier systems", plural. Thanks, all (so far)! - Hordaland (talk) 16:26, 2 July 2009 (UTC)

Ether Physics

Due to the limit of speed for the propagation of waves being based on the characteristics of the medium through which the wave travels, what would be the characteristics of the medium (Ether) through which light and gravity travel in the absence of transparent materials like glass such as the vacuum of space?

7 August 06

Summary: The forces of magnetism, electricity, and gravity are simply different types of perturbations in a single field that permeates the universe and comprises the fabric of existence. “Everything is flux” – Heraclitus -- Taxa (talk) 05:46, 2 July 2009 (UTC)

Joking, of course. To the OP: Have you read our speed of light article? Tempshill (talk) 06:36, 2 July 2009 (UTC)

I do not recall a reason being given in the article why the speed of light has a limit in a vacuum. -- Taxa (talk) 09:15, 2 July 2009 (UTC)

It doesn't have a limit, it has a fixed speed. --Tango (talk) 17:33, 2 July 2009 (UTC)

Does not light travel slower through glass than through a vacumn? -- Taxa (talk) 04:32, 3 July 2009 (UTC)

See our article on luminiferous aether. Gandalf61 (talk) 10:12, 2 July 2009 (UTC)

Our electromagnetic wave equation article may be of use to you. The equation, derived from the Maxwell equations describes the propagation of electromagnetic fields through a medium. It turs out the speed of propagation is dependent on the electric permittivity and magnetic permeability properties of the medium. For a vacuum, these are known universal constants (see vacuum permittivity and vacuum permeability). OP, did this answer your question? I couldn't tell if you were actually asking about the obsolete luminiferous aether theory, annd the last bit of your final question was worded oddly to me. —Akrabbim^talk 10:32, 2 July 2009 (UTC)

Our OP seems a little confused - so let me try to spell it out more simply.

When we talk about the speed that electromagnetic and gravitational waves are moving - we have to be extremely careful. Light does "slow down" when it moves through air or glass or something (that's why lenses and prisms work - and why diamonds are so sparkly). But then we're talking then of the "group velocity" of the waves (the speed at which information carried by the wave travels) - not the "phase velocity" (the speed that the actual waves move)...it gets a bit technical/mathematical and it's hard to explain in words without getting into deep waters! This image may help though:

• The red dot moves at the "phase velocity" - a speed which represents 'c' - "the speed of light in a vacuum" - the thing that is the cosmic speed limit. This speed never changes - no matter what the medium is.
• The green dots are moving at the "group velocity" - the speed at which a change in the size or frequency of the waves can be propagated. This speed is slower than the phase velocity in air, water, etc - but happens to be the same as the phase velocity in a vacuum. When you hear of scientists "slowing down" or even "stopping" light - they are talking about the group velocity.

In a true vacuum, the two speeds are exactly the same. Unfortunately, when we use the term "the speed of light" - we tend to get slightly fuzzy about which speed we're talking about. For all of the exciting relativity kinds of thing - we're talking about the phase velocity - and we should strictly be saying: "the speed of light in a vacuum" or "the phase velocity of light" - and not just "the speed of light" because that is confusing. For measuring the speed of light in (for example) air and glass so that we can calculate the refractive index and make the right kind of lenses for your spectacles - we're talking about the group velocity - and we should say so - but we usually don't.

'Ether' - in this context - is correctly spelled 'aether' (and often expanded to luminiferous aether) because there is a chemical called 'ether' that is an entirely different thing! The idea that there is an 'aether' through which the waves travel is a seriously outmoded concept - the utter failure of the many efforts to detect the presence of the aether is the reason that Einstein discovered relativity in the first place.

As for "reasons"...these are always difficult for physics. We mostly deal in the "what" - not the "why" of nature. We can measure what it does - but not why it does that. So I don't think we know of a reason why there is a cosmic speed limit. If we had to say anything, it would probably be that he nature of relativity is that there is a cosmic speed limit built into the laws of nature - you can see it clearly in the equations. Light and gravity travel at that speed. We call it "the speed of light" mostly for historical reasons. In the case of light, photons (having zero rest-mass) can't help but travel that fast - if they were (hypothetically) ever to travel any slower then even an infinitesimal force would impart an infinite accelleration (because F=ma - so a=F/m - and if 'm' approaches zero and 'F' is finite - then 'a' approaches infinity). That infinite acceleration would instantly push the photon them up to the cosmic speed limit. Now - as to why there is a cosmic speed limit...that's something we don't know the answer to.

SteveBaker (talk) 11:06, 2 July 2009 (UTC)

Wow Steve, that was incredibly informative. My knowledge of the subject seems to me much less definitive than I had thought. Your response leaves me wondering, though, are photons always traveling at the phase velocity? Or are they tied into the group velocity? —Akrabbim^talk 12:11, 2 July 2009 (UTC)

Steve, if a change in size or frequency of light can only pass through air, water etc. at less than 'c' how can one prove that any higher velocity exists in the medium ? Cuddlyable3 (talk) 12:28, 2 July 2009 (UTC)

See Cherenkov radiation. --Tardis (talk) 16:22, 2 July 2009 (UTC)

Steve, your response was pretty inaccurate. The phase velocity of light is not generally equal to c (the vacuum speed of light). It can be smaller or larger. It's effectively infinite for a standing wave, even in vacuum. For light in glass it's roughly the same as the group velocity (that animated image shows a gravity wave, not light, and as an illustration of group velocity it's kind of dubious anyway since there's no real wave packet). It's the phase velocity that's defined to be c/n in a refractive medium and that figures into Snell's law and lensing (see the animated image at the top of refractive index). The group velocity can also exceed c and is not the speed at which information is communicated in general. This Java applet shows how group velocity can be larger than c or negative (that is, opposite the direction of information propagation). The relationship between phase velocity, group velocity and "actual information propagation velocity" is subtle, and I think it may even be an open question. See the last part of this page, starting from "How about using tachyons to transmit information faster than the speed of light..." -- BenRG (talk) 00:52, 3 July 2009 (UTC)

The OP's post is confusing because of the lengthy indented quotation and the mysterious date "7 August 06" in the middle. Cuddlyable3 (talk) 12:12, 2 July 2009 (UTC)

Apologies: I had added the indent when I responded. Because of the date in the middle, I mistakenly thought Taxa was responding to an unsigned OP. Indent removed. Tempshill (talk) 19:27, 2 July 2009 (UTC)

A minor nitpick to Steve's answer. The phase velocity does not have to exactly equal c (the universal constant). There are plenty of materials (optical prism?) where both phase velocity and group velocity are non-constant or vary with frequency. Nimur (talk) 16:33, 2 July 2009 (UTC)

I too am wowed by Steve's answer, whether it is right or wrong. Steve's answer is still extremely informative as to how one might go about answering the question. Most important is his introduction of the relationship between force, mass and acceleration and ultimately energy, which should properly be responsible for a speed limit. The problem I have with group versus phase velocity is that if you can clip the intensity and in effect use it to propagate information then group velocity would always equal phase velocity if such a clipping could be done. Another response that comes to mind is that the limit may be simply be the limit that a magnetic field can generate an electrostatic field and vice versa as the means of propagation. This process must be very efficient and use up very little energy which may or may not support the idea of the existence of a luminiferous aether]]. -- Taxa (talk) 05:02, 3 July 2009 (UTC)

As a side note, see this. It's an issue of a webcomic -- in this case atypical in comic content for this comic, but typical for interesting commentary, in this case on Maxwell's equations. -203.129.49.222 (talk) 11:32, 3 July 2009 (UTC)

Oh yeah. That fully explains it. I suppose that at some point in the transition of magnetic to electric field light would take on characteristics of a particle while in the opposite phase the characteristics of a wave? -- Taxa (talk) 13:43, 3 July 2009 (UTC)

Second Generation Growth

First generation crystal right, second generation left

The owner of this crystal tells me that the first generation crystal (right) formed first. Then the second generation solution came in and formed the crystal on the left. Does wikipedia have an article on second generation crystal growth, or can anyone find a reference? This would be be a perfect illustration if I could find the article. Noodle snacks (talk) 09:38, 2 July 2009 (UTC)

I notice the OP uploaded the image of calcite (?) today and it is not used in any article yet. Cuddlyable3 (talk) 11:50, 2 July 2009 (UTC)

The OP did, but the OP is looking for what is described above to add the image there too. Noodle snacks (talk) 11:54, 2 July 2009 (UTC)

Hydrogen atom

defination of Degeneracy.Supriyochowdhury (talk) 11:12, 2 July 2009 (UTC)

Your quesstion seems to be a homework question, as do the ones below. This desk is not answering such questions per policy. Some answers might be found at Hydrogen,degeneracy, Ion, potential as well as other related Wikipedia articles. As aditional advice: The utilization of socks is frowned upon. Try to aviod making the same spelling mistake all over your questions. Its a deat giveaway. --91.6.12.225 (talk) 11:50, 2 July 2009 (UTC)

Running around acting wild and paying no heed to the expectations and norms of society. Like a semiconductor, a human can be doped into total degeneracy. Edison (talk) 23:55, 2 July 2009 (UTC)

Haha, Edison -- good one! 76.21.37.87 (talk) 04:04, 4 July 2009 (UTC)

quantum mechanics

defination of simple harmonic motion.Supriyochowdhury (talk) 11:14, 2 July 2009 (UTC)

Simple harmonic motion. No quantum mechanics is involved in the definition (correctly spelled). Cuddlyable3 (talk) 11:55, 2 July 2009 (UTC)

Note that a quantum-mechanical treatment of a simple harmonic oscillator is not the same as a classical physics treatment. This example is commonly the first example worked in a quantum physics text-book. Nimur (talk) 16:35, 2 July 2009 (UTC)

Quantum harmonic oscillator. Algebraist 16:36, 2 July 2009 (UTC)

conductivity of electrolites

define ionic mobility.Rikichowdhury (talk) 11:24, 2 July 2009 (UTC)

We have a relevant page: Conductivity (electrolytic). --Scray (talk) 12:14, 2 July 2009 (UTC)

thermodynamics

defination of chemical potential.Rikichowdhury (talk) 11:28, 2 July 2009 (UTC)

We have a page for this, too! Chemical potential --Scray (talk) 12:15, 2 July 2009 (UTC)

conductance

defination of molar conductance.Rikichowdhury (talk) 11:31, 2 July 2009 (UTC)

I think this is described here as well: Conductivity (electrolytic) Before asking for things that could easily be found by searching, please search Wikipedia and other sources a bit, as suggested at the top of this page. If you need to ask, please keep in mind that we are all human beings, here to enjoy interaction, not automatons waiting for your command. --Scray (talk) 12:21, 2 July 2009 (UTC)

That latter statement is untrue. I am waiting for his command, as are others like me. Tempshill (talk) 19:23, 2 July 2009 (UTC)

fingers and toes

Is there a scientific reason why humans normally have 5 fingers and five toes on each limb? Why not 4 or 6? Why is 5 the magic number? 65.121.141.34 (talk) 16:29, 2 July 2009 (UTC)

Depends on what you mean by "reason." The simplest answer is that we have five digits because our ancestors had five digits (pentadactyly). Someone else might be aware of research showing why 5 is more adaptive than 4 or 6, but I wouldn't be surprised if there's no such research. Many traits are not adaptive. Adaptive or not, an expert in ontogeny could probably give you another kind of "reason," that is, exactly how our genes encode for various proteins that lead a fetus to develop five digits.

This probably doesn't answer your question, but hopefully it's something to think about pending a more complete answer from someone with more expertise. --Allen (talk) 16:57, 2 July 2009 (UTC)

Increasing or decreasing the number of digits presumably would make the overall body function less effectively. Bus stop (talk) 17:12, 2 July 2009 (UTC)

Not necessarily. We can presume that it wouldn't make the body function more effectively, but that is all. --Tango (talk) 17:18, 2 July 2009 (UTC)

Why one presumption over the other? To me they both seem equally likely presumptions. Bus stop (talk) 17:25, 2 July 2009 (UTC)

They can't be equally likely since one includes the other (unless it is impossible for 5 and 6 fingers to be equally effective and there is no reason to assume that). If 5 fingers was worse than 6 fingers then we would expect natural selection to select people with 6 fingers and by now we would all have 6 fingers, but if 5 and 6 fingers are equally effective then there is no selection pressure and it just gets chosen at random. So we can presume that 5 is either better than 6 or as good as 6, but we can't tell which without more research. --Tango (talk) 17:31, 2 July 2009 (UTC)

I think we are comparing 5 to any other number. Isn't that what we are really comparing? The questioner mentions both four (a lesser number) and six (a greater number). Bus stop (talk) 17:33, 2 July 2009 (UTC)

Tango, you say, "If 5 fingers was worse than 6 fingers then we would expect natural selection to select people with 6 fingers and by now we would all have 6 fingers". I don't think that's true. It might be that 6 fingers confer a very slight advantage, and that there has not been sufficient environmental pressure to make that advantage play much role in determining reproductive success. -GTBacchus^(talk) 17:44, 2 July 2009 (UTC)

I am pretty sure Tango misread my statement. This is getting hideously confused. Bus stop (talk) 18:04, 2 July 2009 (UTC)

If only I had a couple of extra digits, I could probably sort it out, and then get on with my reproductive goals... -GTBacchus^(talk) 18:19, 2 July 2009 (UTC)

lol Bus stop (talk) 18:27, 2 July 2009 (UTC)

Our ancestors have had 5 fingers for a long time, all primates have 5 fingers/toes so that means it happened at least 10s of millions of years ago. Polydactyly is fairly common (and can be inherited and, as far as I know, isn't associated with any other negative conditions), so there has been plenty of opportunity for us to evolve to have a different number of fingers. It wouldn't need to be a very large advantage to it to have happened. --Tango (talk) 21:55, 2 July 2009 (UTC)

Ok, so we're saying the same thing, for different values of the phrase "very large". :-) -GTBacchus^(talk) 01:15, 3 July 2009 (UTC)

I was using 6 as an example, replace 6 with any other number at it is still true. --Tango (talk) 21:55, 2 July 2009 (UTC)

I don't have time to do a search, so I can just try to piece together fragments from an old memory... In genetic research, the gene group that is responsible for creation of the digits (fingers and toes) are also responsible for creation of the reproductive organs. So, if there is a genetic abnormality that causes more or less digits, it is highly likely that reproduction will be hindered. Therefore, the ability to stray from five digits is limited by the ability to reproduce. What comes to mind is something like HAX or HEX or HOX. I don't actually work in genetic research. I read this while researching genetic information on the four-toed hedgehog. Does this nonsense ring a bell for anyone who actually knows about genetics? -- kainaw™ 18:29, 2 July 2009 (UTC)

I just googled for "genetics h.x" and got a lot of hits for Hox genes. -- kainaw™ 18:30, 2 July 2009 (UTC)

HOX is short for homeobox and they determine all sorts of things about the arrangement of the body (eg. that your face is on the front, not the back). It wouldn't surprise me if HOX genes were responsible for the number of fingers, however I've never heard of people with Polydactyly being infertile. --Tango (talk) 22:01, 2 July 2009 (UTC)

Not the scientific answer you're looking for, I know, but I like to attack the questioner's frame of mind, not the question itself:

If we had 4 or 6, you would just as soon ask, "why not 5?". There's 5 because evidently, that's what works best. Trying to break down that 'why' into some bio-mechanical or socio-evolutionary explanation would be so lengthy and obscure that it wouldn't be worth the effort – in my opinion. Vranak (talk) 19:09, 2 July 2009 (UTC)

Fingers started as stiffeners for fish fins. The practicality of mittens shows that the number of human fingers doesn't matter much. The evolutionary step that did matter was when a primate gained an opposable thumb. Hands would no more be just for walking on, now they could be used to fruitpicking, hitchhiking and typing the spaces in Wikipedia. Cuddlyable3 (talk) 22:34, 2 July 2009 (UTC)

The number of digits is (partly) mediated via CRABP-II. Axl ¤ [Talk] 20:02, 3 July 2009 (UTC)

Could you give a few more examples of what else CRABP-II mediates besides the number of digits? I'm not a geneticist, but I'd like to know if a modified CRABP-II gene is likely to be maladaptive. 76.21.37.87 (talk) 04:12, 4 July 2009 (UTC)

In Mutants (ISBN 0006531644), Armand Marie Leroi points out that until recently all known tetrapods (i.e. land vertebrates) had no more than five digits, and those with fewer than five always had ancestors with five digits: the normal conclusion was that the common tetrapod ancestor was pentadactyl, and all subsequent tetrapods had inherited this trait, though in some cases subsequently lost digits. But the discovery of Acanthostega, Turlepreton and Ichthyostega has changed this, and it now looks as though there was considerable variation in the dactyly of early tetrapods before pentadactyly became universal.

Of course you could use this fact to argue for an adaptive advantage in no more than five digits, but I think such an argument would be weak ;) --ColinFine (talk) 16:18, 4 July 2009 (UTC)

Try this article (although it's a little old). CRABP-II is involved in neural crest development. "CRABP-II mRNA have been shown to increase in tissues and cell lines of various origins, including F9 teratocarcinoma cells, P19 embryonal carcinoma cells, adult human skin, skin fibroblasts, and lesional hyperplastic psoriatic skin following treatment with retinoic acid." Axl ¤ [Talk] 19:57, 4 July 2009 (UTC)

This article does a pretty good job of answering your question. Rockpocket 01:10, 5 July 2009 (UTC)

Nice. I added that to the See Also of Dactyly. --Allen (talk) 06:20, 5 July 2009 (UTC)

Any particular reason the OP has an interest in six-fingered men? 75.157.28.248 (talk) 08:21, 6 July 2009 (UTC)

I would not want to be the six-fingered man. 208.70.31.206 (talk) 04:21, 7 July 2009 (UTC)

Light speed clarification

To me I'm still confused. I haven't got the chance to study about special and general relativity in the university. I tried to read some books and visit such web pages as Relativity of simultaneity to understand some more. Now I need to distinguish the following: 1- Light speed is absolute: What can prove this other than Maxwell's equations? 2- Assuming 1 holds true; what makes us assume that the observer in moving frame of reference will find that light in that frame passes both (front and back) directions and reaches equal distances at the same time (So far we shouldn't conclude that time is relative till both 1 and 2 postulations are verified)?--Email4mobile (talk) 17:55, 2 July 2009 (UTC)

That the Speed of light as an absolute and unchanging quantity, independent of the speed of the observer has actually been proven experimentally, and not just via deduction. Consider the seminal work of people like the 17th century Ole Rømer. Of course, the definitive works is the Michelson–Morley experiment, which essentially disproved the idea of 'aether', and the Ives–Stilwell experiment, which showed that redshift could be accounted for only if the observed speed of light is identical looking forwards and backwards. Also see History of special relativity which explains the development of all of these ideas and more. --Jayron32.talk.contribs 18:10, 2 July 2009 (UTC)

I've read about Ole Rømer and Michelson–Morley experiment but non of them explained that light is absolute nor is simultaneity observer's dependent. I'd like to know more about experiments done to realize or prove the 2 points in question before Albert Einstein or his followers formulated the time dilation equation.--Email4mobile (talk) 08:58, 3 July 2009 (UTC)

Did you read the other articles, or folow any links in the see-also sections of any of those articles? The information is here for you to find; the History of special relativity has lots of details and lots of links; also the Ives–Stilwell experiment which also explains your direct question regarding perceptions of the speed of light in a moving observer. --Jayron32.talk.contribs 20:32, 3 July 2009 (UTC)

Solar Panels

If I have a solar panel operating at a certain voltage and current, and I cover part of it, what will decrease, the voltage, the current, or both? Not homework, just curious. 149.169.104.64 (talk) 18:02, 2 July 2009 (UTC)

Assuming it is a photovoltaic panel, I would think that it would directly reduce the voltage output. Depending on the load network that the panel is attached to (I have no idea how they are integrated into power grids), the current would probably be reduced as well. —Akrabbim^talk 19:19, 2 July 2009 (UTC)

It's going to depend on what else is on the circuit. I don't know a whole lot about electronics, but if we assume a simple case where whatever you're powering has constant resistance R no matter what, then the voltage and current are going to stay in the ratio V/I = R. So cutting the power by half, and using P = IV, the voltage and current would both be reduced by sqrt(2). Maybe someone who knows more about this can comment on how likely that assumption is. Rckrone (talk) 23:36, 2 July 2009 (UTC)

Actually looking at Electromotive_force#Solar_cell it looks like it's a bit more complicated than I thought. The current and voltage will stay proportional for a fixed resistance, but the power isn't determined only by how much light is hitting the solar panels like I was assuming, it depends on the current, so instead of using the equation P = IV you would have to use the one listed there to figure out exactly by what factor the current and voltage would both drop. Rckrone (talk) 00:04, 3 July 2009 (UTC)

Voltage and current both reduce (and the covered part of the panel gets hotter). Polypipe Wrangler (talk) 14:41, 4 July 2009 (UTC)

Green stars?

Most stars are colored red, or yellow or white because of their temperature. Are there any known green colored stars? Say one that has a lot of chlorine or copper in it? Is such a star theoretically possible, even if we don't know of a current example? 65.121.141.34 (talk) 18:16, 2 July 2009 (UTC)

I wish I could back this up better, but the gist of it is this: stars output their light in a curve, so to speak. Bright, blue stars output most of their light in the blue wavelengths, whereas red stars output most of their light in redder wavelengths. Now, a star with a peak in the green won't appear green for reasons I don't quite remember, but it had something to do with the other (bluer, IIRC) wavelengths drowning it out. So yes, there *are* green stars, but the human eye won't ever perceive them as green. Actually, here is a reference, and here's another one. I left my old text since I think it's mostly correct. ;) -- Aeluwas (talk) 19:10, 2 July 2009 (UTC)

That's a pity. The green stars I see in Freelancer are particularly nice-looking. —Akrabbim^talk 19:21, 2 July 2009 (UTC)

Human color perception is a strange thing in many ways. Consider that there are actual two different "yellow" colors, which are VERY different, but which our minds cannot distinguish between. One is a single wavelength color consiting of pure yellow light, and the other is a mixture of two wavelengths of light, one red and one green. The human mind only sees these as the same color, though one is clearly a primary color, and one is a secondary color. There are LOTS of colors that work this way. See Color vision for more on how we perceive color. --Jayron32.talk.contribs 20:52, 2 July 2009 (UTC)

How do you define the colour of a star apart from what colour it appears to humans? Those are the only real definitions of colours we have (except for monochromatic colours that can be defined by their wavelengths). --Tango (talk) 21:46, 2 July 2009 (UTC)

Except for absorption lines whose effect is relatively minor, stars emit blackbody radiation, which is spread over a quite broad range of wavelengths. For cooler stars, most of their output is in the infrared region, so we can only see the top of their spectrum which is red; for hottes stars, it's mostly ultraviolet, so we can only see the blue part. Stars with intermediate temperature emit over the whole visible spectrum, so they are white. Black bodies can only have colors on the Planckian locus; so there can be no green or purple star. --A. di M. (formerly Army1987) — Deeds, not words. 21:42, 2 July 2009 (UTC)

Interesting fact: our Sun is a "green" star because its spectrum peaks in the green, or at least very close to it. Not surprisingly, the human eye's sensitivity peaks around the same wavelength, but why leaves reflect green light most strongly is a bit harder to explain. --Bowlhover (talk) 23:31, 2 July 2009 (UTC)

Insect identification

This morning I found these on my front gate; I've never seen anything like them before, and was wondering if anyone could identify? I'm assuming they're moths, and they're mating (?), but beyond that I'm lost. I'm in Belfast, Northern Ireland where I found these guys about 8am. Unfortunately I wasn't able to line anything up for size comparison, but the body length was approx 1.5 inches and 2 inches from wingtip to wingtip. Are they mating? They don't exactly look like the same species - are males and females often significantly different in appearance? Thanks --Kateshortforbob 22:14, 2 July 2009 (UTC)

I think the species is Poplar hawk-moth, Laothoe populi. So, yes, they are moths; and yes, they are mating. Regarding the appearance - I need to look it up. I've seen both grey and brown form in the photographs, but I do not remember if it is sexual dimorphism or just different color forms. --Dr Dima (talk) 22:24, 2 July 2009 (UTC)

Oops, I gave our article less credit than is due. It explains the colors. --Dr Dima (talk) 22:26, 2 July 2009 (UTC)

You should replace the current 'mating' photo with this one and explain the color difference in the caption. It's a prettier photo than the current one. Tempshill (talk) 23:15, 2 July 2009 (UTC)

That might be a good pic for the main Mating article too... --Kurt Shaped Box (talk) 00:16, 3 July 2009 (UTC)

Cool! Moth-porn. SteveBaker (talk) 14:27, 3 July 2009 (UTC)

Disgusting! Will no one think of the larva? Dragons flight (talk) 19:55, 3 July 2009 (UTC)

I boldly added this excellent pic to both articles. --Jayron32.talk.contribs 00:45, 4 July 2009 (UTC)

Thanks Dr Dima, for the identification - I will update the description page accordingly. Thanks also, Jayron32, for adding the picture to those articles - I suspect I would have committed some embarrassing captioning error if I did it myself! --Kateshortforbob 10:28, 4 July 2009 (UTC)