Wikipedia:Reference desk/Archives/Science/2009 October 16

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October 16

Wy does refraction occur?

What is the cause of refraction? I know about change in speed of light, but why does the speed of light change in different media? --shanuu (talk) 05:06, 16 October 2009 (UTC)

To quote our Speed of light article: "Light travels more slowly in a transparent material than it does through a vacuum due to interaction of the light with the electrons in the material." The Refractive index article explains it in terms of other intrinsic properties of the material. DMacks (talk) 05:12, 16 October 2009 (UTC)

In addition to what what DMacks said, the book QED: The Strange Theory of Light and Matter by Richard Feynman is an excellent (advanced) secondary-school-level treatment of the physics of optical phenomenon. I'd highly recommend it to everyone. -- 128.104.112.179 (talk) 14:30, 16 October 2009 (UTC)

That's sort of like asking, "I know that's it's harder to swim in maple syrup than in water, but why?" Perhaps it's difficult to conceive how light has to "act," so to speak, like a moving body, but the "viscosity" of the medium can have an effect on the light -- while this may not be the most precise way to describe it, it sure makes it more easy to understand for those whose name doesn't rhyme with "-eynman." :) DRosenbach ^{(Talk | Contribs)} 13:05, 23 October 2009 (UTC)

Special relativity.

I am currently reading a book on theoretical physics called, "The Elegant Universe, by Brian Greene". I've just finished reading the section on special relativity, and I had a question.

According to special relativity, every observer sees the speed of light at the same speed, regardless of their state of motion. This raised a question in my mind. I've read that light can be slowed down when it's in different media, eg rubidium gas. So, my question is this: If we always see light travelling at the same speed, how do we know that it changes speed in different media?Unique and proud of it (talk) 08:38, 16 October 2009 (UTC)

There is a constant called c, which is the speed of light travelling though a vacuum, as observed by all observers. If instead the light is travelling through a medium, all observers will instead observe the speed of the light to be less than c. Furthermore, if the light is travelling through a medium, the observed speed of the light depends on the velocity of the medium relative to the observer. It's only in a vacuum that the speed of light is the same for all observers. Red Act (talk) 09:09, 16 October 2009 (UTC)

Just as with the question right before this one, the speed of light and refractive index articles will provide a more thorough explanation. Red Act (talk) 09:17, 16 October 2009 (UTC)

I don't think it's correct to say "according to special relativity, every observer sees the speed of light at the same speed". The constancy of c is an empirical result (e.g. from the Michelson-Morley experiment), and Special Relativity examines the consequences. AndrewWTaylor (talk) 13:44, 16 October 2009 (UTC)

Right—Special Relativity comes out of assuming c is constant for observers. It does not prove it or even argue for it (and it's unclear how attentive Einstein was to experimental results like M-M, anyway). Of course, confirmations of SR's predictions further lend themselves towards believing the assumption is correct. An easy way to explain SR is that it is to say that it's the combination of two assumptions—that Galilean relativity is generalizable, and that c is constant. Everything else in SR just falls out of those two assumptions and what they imply about physical reality. --98.217.71.237 (talk) 14:24, 16 October 2009 (UTC)

No, it isn't entirely right. Special relativity gives rules for transforming space and time coordinates for an observer in motion. Those rules were formulated with the aim of making Maxwell's equations valid in moving reference frames. The constancy of the speed of light is a consequence of them. Looie496 (talk) 17:13, 16 October 2009 (UTC)

Yes,the Special theory of Relativity was inspired from Maxwell's equations, how they referred to a specific speed of light, c. Rkr1991 ^{(Wanna chat?)} 05:14, 17 October 2009 (UTC)

The "coordinate" approach to SR was not inherent to Einstein's original work, but is a later development by Minkowski (which Einstein originally was skeptical of, but later embraced totally). Einstein's original papers do not explain the constancy of the speed of light, but take it as a given. I am aware this is not how SR might be taught in a modern physics classroom, but as an assessment of its historical development, it is correct. As our article on the Annus Mirabilis papers puts it: "First, he applies the classic principle of relativity, which states that the laws of physics remain the same for any non-accelerating frame of reference (called an inertial reference frame), to the laws of electrodynamics and optics as well as mechanics. In the second postulate, Einstein proposes that the speed of light has the same value in all inertial frames of reference, independent of the state of motion of the emitting body." This is very clear from reading his 1905 paper on the subject. --Mr.98 (talk) 16:44, 17 October 2009 (UTC)

nuclear reactor model

i want to make a pressurised heavy water reactor for a science exibition to be held on 30th october. i need to complete my model till 26th october. i have find many a thing about its theory. but i'm unable to make the model of nuclear reactor of this type. i've the full explanation.

i'm in trouble in making its inside structure through which i can explain its working and its components. (full working,coolent used,its component) i'll also be including india's advanced heavy water reactor theoratically to enhance this project.

this project cannot be a working one but somehow we can use some goodoies to make it a attracting one.

can you please help me out in making this project i've very less days left with me. so pls help me. —Preceding unsigned comment added by Sunny kuchya (talk • contribs) 12:35, 16 October 2009 (UTC)

What materials are you using to make the model, and how large is it going to be? Our article on the CANDU reactor has a pretty nice schematic of one; this page gives a more clear explanation of the core area. Perhaps others will have concrete ideas as to what could be done to make something similar on a small scale without too much work? --Mr.98 (talk) 12:43, 16 October 2009 (UTC)

black hole

How do we know that a black hole is in fact a singularity and not simply an object where all dimensions are smaller then the event horizon, but not zero (say where all the mass is in a volume the size of a grape)? Googlemeister (talk) 16:24, 16 October 2009 (UTC)

We don't, as we have no way to retrieve information from beyond a black hole's event horizon. However, general relativity predicts the existence of a gravitational singularity, and since GR holds up well where we can observe and test it, it's reasonable to expect the math here to be a good model as well. It may also be worth noting that my understanding of the singularity is that it expresses the unavoidable eventuality that all matter within the event horizon will converge to a point. This statement does not preclude that, at the present time, the matter still has some non-zero volume (such as a grape). — Lomn 16:54, 16 October 2009 (UTC)

I'm afraid "at the present time" doesn't make much sense here. I don't believe there's a natural coordinate system in which you can talk about events inside the event horizon with the same time coordinate as events outside. In the usual coordinate system — I think they're called Schwarzchild coordinates or something like that — the time dilation means that, at the event horizon itself, time completely stops. --Trovatore (talk) 19:00, 16 October 2009 (UTC)

There are coordinate systems that cover the whole space except the singularity itself, but the timelike coordinate doesn't really coincide with what we would call "time" - the whole point of relativity is that there isn't really a global concept of "time". It makes sense to talk about the time things near you happen but if they happen somewhere else (and the other side of an event horizon definitely counts as "somewhere else") then it gets complicated. See Relativity of simultaneity. --Tango (talk) 09:06, 17 October 2009 (UTC)

Apparently the Penrose–Hawking singularity theorems prove it, but I don't understand that article. --Sean 17:01, 16 October 2009 (UTC)

Those are mathematical theorems, they just prove that the basic assumptions of GR imply singularities at the centres of black holes, they don't prove that those basic assumptions hold. I think most (if not all) scientists agree that those assumptions are perfect, since they don't explain quantum effects. --Tango (talk) 09:06, 17 October 2009 (UTC)

It isn't actually presumed that there physically exists a gravitational singularity at the center of a black hole. The singularity predicted by general relativity is commonly perceived as signaling the breakdown of the theory near that point. In particular, in some volume near that point, the extremely high density should result in quantum mechanical particle interactions that are not yet understood. To date it has not been possible to combine quantum and gravitational effects into a single theory. It is generally expected that a theory of quantum gravity will feature black holes without singularities. (Borrowing some from Black hole#Singularity.) Red Act (talk) 07:06, 17 October 2009 (UTC)

Perhaps for all we know the black hole, singularity or whatever else cease to exist inside the event horizon, but the effects still occur on the outside because the information about this non-existance cannot be transmitted to the outside without exceeding the speed of light?Trevor Loughlin (talk) 08:42, 17 October 2009 (UTC)

That sounds a little like the holographic principle. --Tango (talk) 11:30, 17 October 2009 (UTC)

Does "dark gravity" exist?

Scientist talk about "dark matter" and about "dark energy". They say that gravity "pulls the Universe together", while dark energy "pushes the Universe apart". But there are weird results from the formulas. So does "Dark Gravity" exist? Are any scientists investigating dark gravity? Lemonyjuicy (talk) 17:38, 16 October 2009 (UTC)

It's not clear what sort of concept "dark gravity" would be. Have you read our articles on dark energy and gravity? Our dark energy article in particular discusses its relationship with gravity and the rationale behind its proposal. — Lomn 17:55, 16 October 2009 (UTC)

We unfortunately don't yet have an article on dark gravity, not even a stub. Dark gravity just redirects to dark matter, which is incorrect.

It is not yet known whether dark gravity "exists" or not, i.e., whether a dark gravity theory will wind up matching experimental data better than a dark energy theory. Dark energy, in some form or another, is currently the most popular way to explain the acceleration of the universe's expansion. And general relativity is generally taken to be a very solid part of the foundations of modern physics, so any idea like dark gravity that involves tweaking general relativity faces an uphill battle.

Yes, scientists are investigating dark gravity, in that some theoretical physicists are playing around with trying to find an appealing dark gravity theory to fit the available data. Whether some complete, self-consistent dark gravity theory will be developed, which will hold up to theoretical scrutiny to the point where experimental physicists will start to get involved in testing the theory, is at this point a matter of speculation. Red Act (talk) 06:27, 17 October 2009 (UTC)

Really? I've never heard the phrase "dark gravity". There is lots of work going on trying to find new theories of gravity to explain various things, but do any of them use the term "dark gravity"? "Dark matter" and "dark energy" are just names, the word "dark" doesn't have a specific meaning, it just sounds good in applications for research funds. --Tango (talk) 09:08, 17 October 2009 (UTC)

Agreed. Some theories that try to explain the effects we ascribe to dark matter by modifying gravity are called Modified Newtonian Dynamics or MOND. They can explain some of the discrepancies in the rotation rate of galaxies, but not e.g. the Bullet Cluster. --Stephan Schulz (talk) 09:20, 17 October 2009 (UTC)

Yeah, MOND and some other older alternate gravity theories couldn't explain the gravitational lensing in the Bullet Cluster. But there are a couple newer alternate gravity theories, STVG and TeVeS, that supposedly can. Red Act (talk) 09:57, 17 October 2009 (UTC)

Yeah, I'd never actually heard of dark gravity either, before I started researching this question. But Google turns up tons of hits. It's basically modifications of the Einstein field equations in ways that would explain the acceleration of the universe's expansion, in ways that don't fit the form of the cosmological constant or an additional contribution to the stress-energy tensor. It's rather new; I think the oldest date I saw on anything was 2001 or 2003, with most stuff being since 2005 or 2006. I'm sure the "dark gravity" name arose as a contrast to "dark energy", which it's an attempt to avoid hypothesizing. Dark gravity basically differs from earlier alternative gravity theories like MOND in that it's addressing a different problem. Dark gravity addresses the expansion of the universe; MOND was developed to address the much smaller scale problem of galaxy rotation curves. However, very recently there are also alternate gravity theories like STVG and TeVeS, that address both cosmological and galactic gravitational problems. Red Act (talk) 09:51, 17 October 2009 (UTC)

I don't think the recent appearance of the term "dark gravity" in place of "modified gravity" means much; it's just become fashionable to put "dark" in front of everything in cosmology. I don't think the bullet cluster can be explained without dark matter. STVG and TeVeS introduce new vector and scalar fields, and their explanation of the bullet cluster, if it works at all, involves independent motion of those extra fields. Scalar and vector fields are also known as "particles", and particles introduced to explain gravitational anomalies are known as "dark matter". There are lots of other dark matter candidates, so it's hard to get excited about one more. MOND was interesting because it didn't have those extra degrees of freedom. -- BenRG (talk) 00:27, 18 October 2009 (UTC)

Chewing gum or no chewing gum on the pavement

Are these marks on the pavement of every city which look like chewing gum, really chewing gum? I read somewhere they were something else, like dirt getting clumped together with oil rests, but I am having trouble in finding the article. --Quest09 (talk) 17:57, 16 October 2009 (UTC)

If it looks like this picture, then it's chewing gum. If it looks like this on the other hand, it's still chewing gum, but after Ben Wilson has dealt with it. SpinningSpark 19:21, 16 October 2009 (UTC)

Could these be some kind of bird excrement or perhaps treesap blobs? What about insect residue like snail trails or worm matter? 66.102.199.179 (talk) 14:31, 17 October 2009 (UTC)

no dude, its gumm. smails and worms would have a trail not splotches and sap would have a shine to it, normaly. —Preceding unsigned comment added by DanielTrox (talk • contribs) 17:38, 19 October 2009 (UTC)

Water supply in developing countries

Why is it that in many developing countries, it is not safe to drink water from the mains supply but yet it is safe to use it for washing etc? Clover345 (talk) 18:49, 16 October 2009 (UTC)

Because it is far more easy for a pathogen to infect you if you directly consume it. You are far less likely to get infected from using infected water to wash your clothes as most of the nasties will die when the clothing dries, or from soap you use while washing. Of course, it would be better to use uninfected water for washing as well if it is available.

To put it another way, drinking the infected water would be like making out with someone who has the flu, vs washing would be like entering a room that someone with the flu exited 10 minutes ago. Which is more likely to give you the flu? Googlemeister (talk) 19:11, 16 October 2009 (UTC)

Similar to the reason you (well, children anyway) wipe your hands on your clothes before eating - the dirt and grime is a drag to get on your cheeseburger, but it won't do too much harm on your shirt sleeve. ~ Amory (u • t • c) 21:00, 16 October 2009 (UTC)

I understand that in many parts of India, for example, the water supply drawn from wells and acquifers (sp?) is naturaly contaminated with low levels of arsenic - not enough to kill you, buy enough to cause health problems. 78.151.108.233 (talk) 11:42, 18 October 2009 (UTC)

that said, volatile toxic contaminants are often more easily acquired by inhalation than by ingestion, meaning that you wouldn't want to shower in water that was too polluted to drink, either. [1] although most of the work, at least the well publicized work, done appears to be studying chlorinated compounds in the First World water supply. Gzuckier (talk) 20:13, 18 October 2009 (UTC)

Oxygen production in the dark?

Can photosynthetic organisms produced oxygen in the dark?

Aaadddaaammm (talk) 20:43, 16 October 2009 (UTC)

Yes. Photosynthesis is technically two reactions, and only the first requires light energy. The second requires the products of the light reaction, not light itself, so it is not directly dependent on sunlight. Often plants will perform this second reaction (called the "dark reaction") during the daytime, when the reactants made from the light reaction or readily available, although in climates where water is scarce they will often wait until after dark to prevent water loss. ~ Amory (u • t • c) 20:56, 16 October 2009 (UTC)

Thanks for the reply. But only the first reaction generates oxygen, doesn't it? The light reactions produce proton gradient (then ATP), NADPH and O2, and then the dark reactions use the ATP and reducing power to fix CO2. So in the very basic scheme, oxygen shouldn't be evolved in the dark, but are there certain circumstances or other processes known?? Aaadddaaammm (talk) 21:06, 16 October 2009 (UTC)

In general terms yes, most of the oxygen is likely to be released during the day. Some crassulacean acid metabolism plants uptake most of their CO₂ during the night but oxygen release is still during the day [2]. I'm not aware of any plant that releases significant amounts of oxygen in the dark Nil Einne (talk) 09:32, 17 October 2009 (UTC)