Wikipedia:Reference desk/Archives/Science/2012 April 26

= April 26 =

what is dark matter
what is matter which has only gravitational activity ,not electromagnetic not thermodynamic and so on,does the light pass through such matter ?I have problem with thermodynamic properties of dark matter.Akbar mohammadzade--78.38.28.3 (talk) 04:10, 26 April 2012 (UTC)
 * No one really knows what dark matter is, although there are numerous hypotheses; see the Dark matter article. Red Act (talk) 04:18, 26 April 2012 (UTC)
 * It is an invention, to explain the break down of gravitational theory on the large scale of galaxies and the universe. Gravity does not seem to function as the current threories predicts it to, so instead of modifying existing theory, cosmologists have invented dark matter. That is dark matter's sole purpose. Plasmic Physics (talk) 05:49, 26 April 2012 (UTC)
 * What Plasmic said. It doesn't "exist" except on paper, and even there some have challenged the rationale behind it. Evanh2008 (talk) (contribs) 05:51, 26 April 2012 (UTC)
 * Well now saying it like that makes it sound like physicists are lazy. Truth is there have been dozens if not hundreds of proposed alternative explanations for the observed gravitational anomaly, but the only one that is completely consistent with observations is some "missing mass": dark matter. And until we come up with better observations or a better theory we'll just have to go on not knowing what exactly that dark matter is. I don't like it, but it is what it is. And don't even get me started on dark energy.- Running On Brains (talk) 06:07, 26 April 2012 (UTC)
 * Ok then, what do you make of Einstein's self professed fudge: the gravitational constant, which now appears to be not so constant after all? Plasmic Physics (talk) 07:17, 26 April 2012 (UTC)
 * It's actually just a sign of how good his theory was. The GR equations popped out, they said "the universe is not static," Einstein said, "dang, that doesn't sound like what the astronomers are saying, does it?" and so he added the cosmological constant in there to make it match the observations. Later the astronomers figured out that they had been wrong and it was expanding, and if you remove the constant, GR works great as it ought to. So it's actually something in favor of GR, not against it. Einstein called it a "blunder" because he ought to have trusted his equations more and discovered something fundamental; instead he trusted the astronomers too much and fudged the equations. --Mr.98 (talk) 12:40, 26 April 2012 (UTC)
 * It is just as reasonable for a child to believe in the tooth fairy, because they can't explain why money magically appears when they leave a tooth under a pillow. Plasmic Physics (talk) 07:21, 26 April 2012 (UTC)
 * And your argument is just as bad as saying to that child, "You're wrong because your theory is silly." We've been waiting 80 years for someone to propose a theory of gravitation that doesn't require dark matter to explain galactic motion. Alternatives to the tooth fairy dark matter theory have been proposed but they all suck. You could certainly argue that the theory of dark matter also sucks since it never told us how to identify the invisible little buggers, but at least it didn't require a fundamental rewriting of well-tested physical laws. Someguy1221 (talk) 07:36, 26 April 2012 (UTC)
 * I wasn't saying that the theory is wrong, just that everyone of them tastes funny because they are all half-baked, they took their dark matter brownies out of the oven and served it too early. Plasmic Physics (talk) 07:44, 26 April 2012 (UTC)
 * Note that we now have fairly strong evidence for dark matter. MOND theories can, with some fudging, explain some features of galaxy rotation. But they are hard to apply to things like the Bullet Cluster, MACS J0025.4-1222, and Abell 520, where galaxy collisions have basically separated the interstellar gas (and hence most of the baryonic mass) from the stars, but the majority of the total mass of the galaxy has stayed with the stars. --Stephan Schulz (talk) 19:40, 26 April 2012 (UTC)
 * I would also note that the 'half-baked' part suggests a lack of understanding of how science and the scientific method works. You don't leave a theory in the oven (whatever that means) because it isn't cooked yet. Speaking in a very, very simplified manner, someone comes up with one or more hypothesis to explain some finding. (You don't generally wait until you have an extreme amount of evidence before publicly proposing something since it means both that someone may get there ahead of you, and that others can't consider your findings and attempt test them in various ways.) As evidence accumulates, other scientists come to accept your hypothesis/es as a valid scientific theory. If at some stage, evidence emerges which appears to contradict your theory you either refine it or (you or someone else) come/s up with a new one that better fits all the evidence. Scientific understanding is constantly changing, so it's difficult to say something is 'done' or fully baked, although obviously different theories can be at different stages of acceptance with different amounts of evidence in support of them. Nil Einne (talk) 20:55, 26 April 2012 (UTC)
 * Compare with Darwin's theory of evolution. Plasmic Physics (talk) 02:47, 27 April 2012 (UTC)
 * I guess what I'm getting at is: is the dark matter theory predicting observations, or are the observations predicting the theory? Plasmic Physics (talk) 02:54, 27 April 2012 (UTC)
 * Both, of course, as per the scientific method (observation->hpothesis->prediction->test). Darwin had finches and domestic animals, Einstein started with e.g. the Michelson–Morley experiment, Newton even had the benefit of Kepler's laws and his apple, and so on. Dark matter had a mismatch in galaxy rotation rate, and predicts things like the clusters I mentioned above. --Stephan Schulz (talk) 04:20, 27 April 2012 (UTC)
 * OK, I see. I then retract my critisism. Plasmic Physics (talk) 04:40, 27 April 2012 (UTC)
 * Returning to the original question, dark matter (hypothetically) does not interact with or "feel" the electromagnetic force, which is "carried" by photons - so, yes, light will pass right through it. It does not interact with the strong or weak forces either. The only one of the four fundamental forces that affects it is gravity. I don't think this means it has no thermodynamic properties - thermodynamics arises from statistical mechanics, which would still apply to dark matter. However, with only relatively weak gravitational interactions to distribute energy, it could take a very long time for a "cloud" of dark matter particlaes to reach thermodynamic equilibrium with itself or with ordinary matter. Gandalf61 (talk) 08:27, 26 April 2012 (UTC)
 * Dark matter can interact by the weak force. Neutrinos are dark matter (but there aren't enough neutrinos to account for all of it). Dark matter can even interact by the strong force—see SIMP. -- BenRG (talk) 05:30, 27 April 2012 (UTC)
 * Yes - my bad. Fixed above. Gandalf61 (talk) 08:27, 27 April 2012 (UTC)

thank all specially Gandalf61. for using "electrodynamic of moving objects" we ought to know that the matter is made of hadrons ,or dark matter(for full discussion) .we need to have its properties, as its state , mass and  center of mass ,internal and external reactions , radiating ,... the termodynamic relations in statistical physics strongly depends on matter properties and state ,gas liquid or solid ,and plasma mode .DARK MATTER is which one of such states ? — Preceding unsigned comment added by 78.38.28.3 (talk) 09:42, 26 April 2012 (UTC)
 * Since dark matter particles are not bound to one another by any electromagnetic forces, then dark matter must be a gas. On a small scale, where gravitational interactions are not significant, you could probably model it as a monatomic gas. On a larger scale I imagine you would use something like the virial theorem. Gandalf61 (talk) 09:49, 26 April 2012 (UTC)


 * Dark matter isn't in atomic form so it is not a gas, plasma, solid or plasma. Weakly interacting massive particles (a leading DM candidate) are more like slow fat neutrinos. Sky Machine   ( ++ ) 09:56, 26 April 2012 (UTC)


 * I don't think a gas has to be composed of atoms - see photon gas, Bose gas, electron gas. But my point was that dark matter can (on small scales) be modelled thermodynamically as an ideal gas, regardless of whether you think it is strictly a gas or not. Gandalf61 (talk) 10:34, 26 April 2012 (UTC)


 * It reminds me of the commercial for dawn dish soap where one drop of soap disperese a large amount of grease on the surface of the water. if you saw the grease moving away from that spot you would thing there was something pulling that grease in the direction you see it going. but actually it was the soap pushing it.165.212.189.187 (talk) 13:02, 26 April 2012 (UTC)


 * Something is pulling it - the surface tension of the water. Whoop whoop pull up Bitching Betty 19:13, 26 April 2012 (UTC)
 * There you go.165.212.189.187 (talk) 19:27, 26 April 2012 (UTC)


 * "There you go" I can't tell which side you're analogizing for. - Running On Brains (talk) 20:47, 26 April 2012 (UTC)


 * well, you often use a "flat" universe analogy to understand it so there's another one.68.83.98.40 (talk) 01:34, 27 April 2012 (UTC)

The most favored theory is that dark matter (DM) is the lightest supersymmetric particle. There is indirect evidence for this, e.g. from the fact that such a particle would interact with matter via the Weak interaction. This then more or less fixes the dark matter density of the universe. The smaller the annihillation cross section of DM particles, the sooner after the Big Bang it would have decoupled, leading to freeze-out at a higher temperature and in turn to a higher present day DM density. The present day DM density is consistent with DM particles interacting via the Weak interaction.

Also, the annihilation cross section is related to cross section with ordinary matter partcles, via crossing symmetry. E.g., if you take a Feynman diagram for DM-anti-DM articles annihilating into a positron and an electron, and you rotate it 90 degrees, you get a Feynnman diagram for the scattering of a DM particle off an electron. Count Iblis (talk) 19:26, 26 April 2012 (UTC)

the source of planets' atmospheres
How had planets been able to contain surrounding atmospheres?Akbar mohammadzade — Preceding unsigned comment added by 78.38.28.3 (talk) 05:54, 26 April 2012 (UTC)
 * Gravity. Evanh2008 (talk) (contribs) 05:57, 26 April 2012 (UTC)
 * Also internally generated planetary magnetic fields prevent the solar wind from steadily stripping away the atmosphere. - Running On Brains (talk) 06:12, 26 April 2012 (UTC)
 * You can also read the question on Europa's atmosphere found above. Plasmic Physics (talk) 07:09, 26 April 2012 (UTC)
 * Not really. ;) Evanh2008 (talk) (contribs) 07:39, 26 April 2012 (UTC)
 * "Not really"? Plasmic Physics (talk) 07:46, 26 April 2012 (UTC)
 * I think he was refering to the other Europa NSFW section above Sky Machine   ( ++ ) 07:58, 26 April 2012 (UTC)
 * Silly me, it's already been archieved. Plasmic Physics (talk) 08:07, 26 April 2012 (UTC)
 * The larger the mass of the planetary body, the easier it is for the planet to hold on to an atmosphere, thus gravitational attraction is the important factor here. Compare Jupiter with Mars or Earth for instance. Earth loses its lighter gases such as hydrogen and helium where Jupiter holds on to these gases. A cold temperature helps too, Pluto keeps its atmosphere because it keeps refreezing when it gets further out in its orbit. Sky Machine   ( ++ ) 07:48, 26 April 2012 (UTC)

for analyzing the combination of carbon dioxide for atmosphere of planet we can result one of this reasons : 3- 1) The gas has been absorbed from flattened primordial disk  3-  2) The gas has been absorbed from interstellar space 3- 3) The gas has been created from volcanic activities (as there are several high mountains in planet)  3- 4) Existence of  any growth process (for example process which increased oxygen                      on earth surface  for crust oxidation and its existence in our atmosphere     3- 5)Our scenario that the gas has been absorbed from solar wind (the solar wind                       carried particles are pulled to atmosphere, the light ones such as hydrogen  and helium cannot  remain there for their velocity :    Vrms=√3kT/m  (Boltzmann equation) source : Analytical mechanics calculations for finding main  reasons for  opposite rotation of Venus Author: Akbar Mohammadzade)  — Preceding unsigned comment added by 78.38.28.3 (talk) 09:50, 26 April 2012 (UTC)
 * Yes, that is true. Atmospheric gas come and leave, different types of gas leave more often than others. As gravity increases, they will leave less often, but come regularly. Statistically, they will never stop leaving.
 * Question for anyone: as solar wind travels long distances through space, does it cool down and radiate energy away as light, while slowing down enough to be captured by the outer planets? Plasmic Physics (talk) 10:15, 26 April 2012 (UTC)
 * It is not right to say that they cannot remain, it is not so black and white. You can say that statistically one molecule of hydrogen or helium is less likely to remain within the atmosphere for as long as a molecule of something heavier. Plasmic Physics (talk) 10:20, 26 April 2012 (UTC)

Plasmic Physics ! please see http://www.gsjournal.net/Science-Journals/Research%20Papers/View/4000 the solar wind particles generally remain in solar system and they are cooling by synchronious radiating. the archimedian spiral of particles movement model is shown in : [] — Preceding unsigned comment added by 78.38.28.3 (talk) 10:30, 26 April 2012 (UTC)

. — Preceding unsigned comment added by 78.38.28.3 (talk) 10:26, 26 April 2012 (UTC)
 * I cannot, I have no access to your computer. Plasmic Physics (talk) 11:00, 26 April 2012 (UTC)
 * This link should work. Evanh2008 (talk) (contribs) 21:21, 26 April 2012 (UTC)
 * Akbar Mohammadzade your hypothesis has atmospheres growing over time as they accumulate gas from solar wind, how is this consistant with Mars having lost considerable atmospheric density since its volcanism, magnetic dynamo (and magnetic field) came to a halt? Sky Machine   ( ++ ) 21:47, 26 April 2012 (UTC)
 * If I may, I believe that at the pressure of the solar wind is too high at that distance from the Sun, so rather than bing absorbed, it blows away the preexisting atmosphere. For planets further out from the Sun, the solar wind pressure is low enough to be overcome by gravity. As an example: let a mug be called a planet, and a water tap be called the Sun, and the water be called the solar wind. Then open the tap to maximum, you'll notice that the high pressure of the water blows any water clear out of the mug. However, if you close the tap to a gentle flow, the water will fill the mug. Plasmic Physics (talk) 00:53, 27 April 2012 (UTC)

Electric Field & Electric Force
Whatis the difference between electric field and electric force ? — Preceding unsigned comment added by Temesgen Mengesha (talk • contribs) 07:07, 26 April 2012 (UTC)
 * One is a field one is a force experienced by an electrically charged object moving in that field. Plasmic Physics (talk) 07:09, 26 April 2012 (UTC)
 * What Plasmic said. An electric field is something that occupies a three dimensional space, but it does not do anything as long as no charged particles are within the field. In contrast, an electric force is something that acts, and is located on a particle. In an electric field E, a particle with charge q experiences a force in the direction of the field equal to q*E. - Lindert (talk) 08:14, 26 April 2012 (UTC)

Influence of electric field on chemical equilibrium
How is expressed in formulae the influence of the electric field on chemical equilibrium in cases where this influence occurs like the dissociation of hydrogen molecule and hydrogen atom ionization? (to name a few processes pertaining to gas phase ion chemistry and also electrolytes namely effects like Wien effect and Debye-Falkenhagen effect)--188.25.241.99 (talk) 15:59, 26 April 2012 (UTC)
 * You (simply) factor in the change in energy of the products and reactants due to the electric field and perform the usual maneouvers.
 * (note) In some cases the electric field will cause separation of chemical species - this is effectively the same as the field 'doing work' to the system - in these cases you also need to factor this in (eg if it affects the starting species). Clearly if the field is doing work in separating species then this affects the positional entropy of the reaction products - something to bear in mind... In extreme cases it can change the reaction or reaction mechanism or species..
 * First order effects of electric field are just that of the Electric potential energy of a charged particle in a field... The situation gets more complex as you try to be more accurate. (eg dipoles, polarisable molecules etc etc)Oranjblud (talk) 01:21, 27 April 2012 (UTC)

for how long duration of time the sun was T-tauri star
first solar system emberyo of star formed in our global cloud, was T-tauri mode of it, the question is the age of such star which changed to be modern sun?Akbar mohammadzade.


 * T Tauri star says they join the main sequence (which is the type of star the Sun currently is) after about 100 million years. --Tango (talk) 13:15, 26 April 2012 (UTC)

Blue glow when pulling apart a Band-Aid wrapper?
For a couple of nights, I've been putting a Band-Aid on my finger after turning off my lights for the night. I've noticed that when I pull the wrapper apart in a dark room, the wrapper glows electric blue where the adhesive that's sealing in the Band-Aid is separating. Why does it glow blue? - Purplewowies (talk) 17:19, 26 April 2012 (UTC)


 * Your description suggests triboluminescence. Dragons flight (talk) 17:27, 26 April 2012 (UTC)


 * The article for which mentions Band-Aid wrappers specifically. -- Finlay McWalterჷTalk 17:28, 26 April 2012 (UTC)


 * Here's an interesting article from Nature about producing X-rays with scotch tape using the same effect: Sticky tape generates X-rays. - Running On Brains (talk) 21:01, 26 April 2012 (UTC)


 * Here is Natures video version of the same article. Producing an X-ray image of a finger by unwinding sellotape is just extraordinary.  Spinning  Spark ' 00:27, 27 April 2012 (UTC)


 * There is a whole heap of videos on YouTube showing tribolumiescence in duct and scotch tapes.  Spinning Spark  00:42, 27 April 2012 (UTC)

The triboluminescence article says some kind of asymmetry might be needed, such as impurities, but does not say what that is. Do they mean polarity and if so any ideas on how would that happen with the tape? --Agentundertables (talk) 00:59, 27 April 2012 (UTC)


 * "...triboluminescent activity is usually assumed to be a consequence of crystal asymmetry, ie only piezoelectric (non-centrosymmetric) crystals may be triboluminescent since only such crystals can develop opposite charges on the opposing faces of a developing crack" from .  Spinning Spark  02:24, 27 April 2012 (UTC)

How do they make large, curved sheets of glass?
Actually, I'm not even sure how they get normal windshield to the exact shape, but my question is inspired by those staircases you see at some Apple stores. They have pieces of glass that are quite a few square meters in a perfect cylinder section. I understand how they get glass really flat, by floating it on melted metal, but how do they get those perfect curves? I'd imagine they would bend it while still hot, but how do they then support it without causing dents or irregularities? 196.210.179.55 (talk) 20:56, 26 April 2012 (UTC)Eon


 * "How Do They Do It? How Do They Make Curved Glass?" video. Clarityfiend (talk) 21:23, 26 April 2012 (UTC)


 * An example of a manufacturing facility involving curved glass. Bus stop (talk) 22:28, 26 April 2012 (UTC)


 * Another video showing aspects of a process manufacturing curved glass. Bus stop (talk) 22:35, 26 April 2012 (UTC)


 * This explanation says that curved glass is a 'pane' to produce. Bus stop (talk) 16:39, 27 April 2012 (UTC)

Color change
Hello, is there any way to change the color of you're semen? I need to for an experiment Astro Philip (talk) 23:41, 26 April 2012 (UTC)
 * Add some food coloring to it. ←Baseball Bugs What's up, Doc? carrots→ 00:16, 27 April 2012 (UTC)
 * Lol, I'm guessing they mean "before" it comes out. You know, like eat lots of vitamin C to change the color of your pee. Otherwise it's trivial. I've heard you can change the smell, but not heard you can change the color by anything you eat... Vespine (talk) 00:23, 27 April 2012 (UTC)
 * There are also documented ways to change the taste. Anyway, I'm not aware of any research into the color. What are you trying to do? Make it more white, more clear, or something really weird, like turning it blue? If that's the case, would you mind if your entire body turned blue as well? Someguy1221 (talk) 01:30, 27 April 2012 (UTC)
 * Diet has no direct effect on semen color. Any effective method will be impractical, expensive, or hazardous. Drugs such as finasteride are known to cause green semen, but this is due to infection. If you want reddish semen, you could induce a mild case of hematospermia by undergoing a transrectal biopsy of the prostate, a procedure which usually causes bloody semen as a side effect. If you desire a more exotic color, you might persuade a urologist to inject some food coloring directly into your seminal vesicles. That would be benign, but I bet you'd have a hard time finding a doctor willing to do it. L ANTZY T ALK 19:04, 27 April 2012 (UTC)
 * Please note that hematospermia increases the already high propensity of seminal fluid to act as a vector of disease. AlexTiefling (talk) 21:51, 27 April 2012 (UTC)
 * At point does this wander into "medical advice" territory? ←Baseball Bugs What's up, Doc? carrots→ 23:04, 27 April 2012 (UTC)
 * My thoughts exactly, bugs. I'm aware of a former graduate student at MIT who managed to turn his entire body (and all bodily fluids) blue by injecting himself with "something". I don't feel right saying what per WP:BEANS. Then again, if anyone reading this has a habit of giving himself intravenous injections of random chemicals because someone on the internet said it was safe, he's probably hopeless. Someguy1221 (talk) 03:13, 28 April 2012 (UTC)
 * I assume you're talking about methylene blue. Note however that the decolorization of methylene blue is a routine test for semen quality.  though there are aspersions cast on its accuracy, and in any case I'm finding it hard to find a straight answer about how long it takes.  I suppose I'd have to see it to believe it.  Methylene blue was administered widely to troops in the second World War, being convenient for the purpose as it is a moderately effective antidepressant, with the added excuse of being a moderately effective antimalarial drug...  despite reading various ditties about how "Even at the loo, we see, we pee, navy blue" I didn't see anything about blue ejaculate.
 * My first thought was that you might put a fluorescent label on fructose, and as asinine as that sounds, there's actually a publication about such a thing! . But I have no idea if this substance would be absorbed and concentrated by the seminal vesicle, indeed, whether seminal vesicles even accept outside fructose that is un-fooled-around-with. Wnt (talk) 04:59, 28 April 2012 (UTC)