From Wikipedia, the free encyclopedia

Science desk
< January 11	<< Dec | January | Feb >>	January 13 >

Welcome to the Wikipedia Science Reference Desk Archives
The page you are currently viewing is an archive page. While you can leave answers for any questions shown below, please ask new questions on one of the current reference desk pages.

Contents 1 January 12 1.1 Car crash 1.2 Farting between sexes 1.3 How does light slow down in glass? 1.4 Light speed and time 1.5 Tell me about the physical substance of memory and personality 1.6 Quantum Physics 1.7 Tungsten 1.8 Thermal Conductivity?

[edit] January 12

[edit] Car crash

There is a 2000 Kg car going 14 m/s north, and it crashs into a 4500 Kg truck that is going 22 m/s east. what is the resultant magnitude and its direction.24.215.147.206 00:10, 12 January 2007 (UTC)

You can calculate the momentum of both bodies (m × v) and add the vectors using the top-and-tail method. (You find the magnitude by using Pythagoras' theorem to find the momentum of both bodies combined, then dividing the figure you get by the sum of masses of both bodies (i.e., 6500 kg) to give you the velocity; then work out the angle.) You need to assume that the bodies stick together. BenC7 00:28, 12 January 2007 (UTC)

I am assuming that at this time of year that that was not a homework question. BenC7 09:25, 12 January 2007 (UTC)

Where are you located that doesn't have schooling at every level this time of year? --66.195.232.121 19:09, 12 January 2007 (UTC)

Dear anon, might I suggest that ben is located somewhere in the Southern Hemisphere? --Robert Merkel 02:10, 13 January 2007 (UTC)

Whoops. BenC7 03:10, 13 January 2007 (UTC)

[edit] Farting between sexes

Is it true that women actually fart as much gas as men but pretend thay dont and try to hide it by coughing and blaming the smell on the dog etc?

Yes. Poor dogs. --jpgordon^∇∆∇∆ 03:20, 12 January 2007 (UTC)

I don't believe there are any scientific studies on the subject. X [Mac Davis] (DESK|How's my driving?) 12:13, 12 January 2007 (UTC)

But there's no real reason to assume that females have less or more flatulence than males. Though I could imagine that if male and female diets differed by much, which is plausible, that there could be different rates. But no physiological reason. --24.147.86.187 15:26, 12 January 2007 (UTC)

Quite the opposite. The male and female bodies exhibit numerous physical differences (e.g. anatomy, average height, hormonal levels, physiological parameters, ...); there is no reason to assume that the incidence of flatulence is exactly the same in both sexes. Believing otherwise in the absence of evidence is to base a conclusion on ignorance. --71.175.23.226 18:33, 12 January 2007 (UTC)

Farting is a function of a few things: 1, amount of swallowed gas/carbonated beverage; 2, amounts of certain carbohydrates in the diet; 3, type of gut flora; 4, amount of gut flora; 5, gut motility. I would think that the only likely significant differences would be in 2 and 4 (because women are generally smaller and thus would both eat less and have shorter intestines). Note that 2 and 3 are linked. Different people have different kinds of bacteria in their guts, which use different kinds of carbon. Almost everyone has hydrogen sulfide-producing bacteria, only about 1 out of 3 people has methane-producing bacteria. Tuckerekcut 22:03, 12 January 2007 (UTC)

Didn't Mythbusters did that? ...here you go youtube link

[edit] How does light slow down in glass?

Why is light slower in glass than in vacuum? Do the photons actually slow down, or do they bump into atoms, or do they curve around the atoms? Why is red light faster than blue light? Thanks, AxelBoldt 03:45, 12 January 2007 (UTC)

See Permittivity --Tbeatty 03:47, 12 January 2007 (UTC)

Couldn't find the word "photon" in that article, nor the dependency of the speed of light on the wavelength. AxelBoldt 04:00, 12 January 2007 (UTC)

Light is a form of electromagnetic radiation, so all the discussion of electric fields is relevant. Regarding wavelength dependence, see the "Complex permittivity" section. DMacks 04:07, 12 January 2007 (UTC)

Sorry, I just read the article again, but couldn't find information as to whether photons slow down in glass, nor could I extract from the complex permittivity section an explanation of why red light is faster than blue. Maybe it's in there, I just couldn't get it out. AxelBoldt 22:48, 12 January 2007 (UTC)

Blue light doesn't travel faster than red. All wavelengths of electromagnetic radiation move at the same speed in the same medium. --Cody.Pope 04:51, 12 January 2007 (UTC)

I don't believe this is true. Nonlinear dielectrics can have permittivity dependant on frequency. Also, the speed of light in a dielectric does not have same "speed limit" as light in free space does. Namely other particles can travel faster than light in that medium (see Cherenkov radiation. Tbeatty 05:06, 12 January 2007 (UTC)

In vacuum all wavelengths have the same speed, but in other media the speeds typically differ. A prism separates the colors because of their different speeds.AxelBoldt 22:59, 12 January 2007 (UTC)

Also see refractive index. Tbeatty 05:06, 12 January 2007 (UTC)

Also see Dispersion (optics). The material will have a different permittivity for different wavelengths of light. The Photon 05:25, 12 January 2007 (UTC)

The actual photons must travel at c individually, as much as any one speed and energy can be assigned to objects that may exist for so short a time over so short a space. The usual interpretation is that the waves of polarization and magnetization in the material (which are responsible for regenerating the wave, albeit at a lower speed, causing transparency) result from absorbing the incident batch of photons and that a new batch, somewhat behind where the originals would be if they had been unimpeded, is created shortly thereafter (although of course it's not in coherent bunches but rather on an atom-by-atom basis). --Tardis 06:40, 12 January 2007 (UTC)

Yes, that makes sense, thanks. Does this model also somehow explain why red light is faster than blue light? AxelBoldt 22:48, 12 January 2007 (UTC)

Different colors of light have different frequencies and so the polarization and magnetization of the particles in the medium proceed somehwat differently. (In an ideal linear medium the differences are nil.) See dispersion. --Tardis 23:57, 13 January 2007 (UTC)

[edit] Light speed and time

It is my understanding that one appears to slow down (to an outside observer) the faster one moves.

If I was travelling close enough to the speed of light so that my motion looked to another observer like it had ceased, but headed for the other side of the room, what would the observer get when he reached over and touched me? A light "echo"? Would his hand just go through? 05:06, 12 January 2007 (UTC) —Preceding unsigned comment added by 164.11.204.52 (talk • contribs)

The room distance also contract so it works out. See Lorentz contraction and Relativity of simultaneity --Tbeatty 05:11, 12 January 2007 (UTC)

The below answer is probably closer to the target, but your response reminded me of a wonderful physics problem I came across in high school: Suppose a pole with a rest length of twenty feet is traveling fast enough that it is relativistically shortened to 15 feet. Now imagine that it enters a 15-foot-deep garage. It will, of course, fit (briefly) in the garage. The problem, of course, is to explain how this is consistent with what happens in the frame of reference of the pole, in which the pole is 20 feet long, and the garage is only 11.25 feet long. Dfeuer 07:46, 12 January 2007 (UTC)

IIRC, that problem is addressed in some article over here at Wikipedia. The article mentions a fly inside hole when something

Yeah, I decided to look that up too. Here it is: Wikipedia:Reference_desk/Archives/Science/2006_October_19. The page linked has a another page on the specific pole example at [1]. — Kieff 03:32, 13 January 2007 (UTC)

Actually, Kieff, the page you pointed to contains errors, and misses the interesting point about what happens if you do close both garage doors while the pole is inside. (The answer to that question has to do with the finite speed with which the information that the front has hit a door can propagate back along the pole to tell the rear end to stop: the front end of the pole by necessity scrunches up (for real, not by Lorentz contraction!) while the rear end keeps going, and in the pole's own frame this compression is enough to allow the rear end to pass through the front door before it closes.) I edited the Wikibooks page about this [2] to explain it better. --mglg_(talk) 22:40, 12 January 2007 (UTC)

I wouldn't know, I never really checked it (I just had it on my "to check later" bookmark folder along with some other 50 thousand pages I'll probably never look at.) That Wikibooks page is pretty good, though. — Kieff 03:07, 13 January 2007 (UTC)

I think what you're saying is that you expect your friend to see you stationary, "frozen". While it's true that he will see you as experiencing not much time (your watch will rarely tick, for instance), you will still be seen as moving at the proper speed: there's no confusion about where you are, merely how much time you experience. --Tardis 06:35, 12 January 2007 (UTC)

Tardis is absolutely right. Although time in your frame of reference, as measured by a stationary observer, appears to have slowed down that does not mean that "my motion looked to another observer like it had ceased". You still appear to be mvoing very quickly in the observer's frame of refernce - but your watch appears to be running slowly. Relativistic particles do not stand still - they just last longer before they decay. See time dilation for more details. Gandalf61 11:36, 12 January 2007 (UTC)

For a refreshingly different (and stimulating) view of light, space and time, try Alice in Quantumland by Peter Russell. --Seejyb 16:30, 12 January 2007 (UTC)

[edit] Tell me about the physical substance of memory and personality

I'm sorry if my question is unclear but I don't know how else to articulate it. Are there any accessible books on the subject?

Thanks.

66.91.225.188 10:08, 12 January 2007 (UTC)

You might try to articulate what you mean by "physical substance" — it is a rather vague term. One approach to articulating it might be to try and articulate what sort of answer you are looking for — neurological? genetic? psychological? physiological? etc. If you are talking about the neuroscience of memory, our article on memory does a relatively good and understandable introduction to it. Personality is a much more ambiguous term, referring to a number of different types of mental activity, but our article on that has some links as well. --24.147.86.187 15:24, 12 January 2007 (UTC)

You seem to be looking for something much more than encyclopedia content. I assume you mean "hard coding", i.e. where exactly is memory (or personality) stored, and what are the structures or molecules involved? You are asking for books, not articles, so you are likely to get a more satisfactory response at a sites designed to point you to suitable books. For memory you would get quite a few reviews by searching for "memory and learning" literature, e.g. this very helpful thread. At sites such as BrainConnection you can use the search box to find articles and related books. --Seejyb 16:19, 12 January 2007 (UTC)

You might find this online textbook to be useful. The book "Memory: From mind to molecules" by Squire and Kandel is fairly easy to read. --JWSchmidt 18:08, 12 January 2007 (UTC)

The Scientifically Correct expression to use here is physical substrate, as in this Google search query.  --Lambiam^Talk 20:17, 12 January 2007 (UTC)

[edit] Quantum Physics

Can two observers detect one photon and by extension,would an object emitting only one million particles exist for two million observers?208.101.159.59 14:17, 12 January 2007 (UTC)Giles

My instinct would be that the answer should be "no" — "detection" of a photon should mean that another "observer" should not be able to detect it again in strict terms (in semi-practical terms, it would be the equivalent of saying that once a photon has triggered a detection in a photomultiplier tube it cannot trigger any further detection in another tube; at least, not in a way which would associate it with the object in question). But I am not a physicist, and this could be totally wrong. --24.147.86.187 16:35, 12 January 2007 (UTC)

Certainly - here's a thought experiment proof-of-concept: if a single photon is detected by a photomultiplier tube, anyone standing around the output readout will notice the single photon; i.e. multiple observers have detected one photon. I actually think your question is not well defined; what do you mean by two observers detecting one photon? Do you mean simultaneously? I believe that a single particle can only interact with one other particle at a time (Feynman's idea, I think), but I don't know much about quantum field theory. -bmk

What you've done there is taken the detection of the one photon and used a machine to generate more photons to describe it. I'm pretty sure that's not what is being asked about — it is not a quantum physics problem in any case if that is all it is. I'm pretty sure the question is whether or not more than one observer (in these examples, the tube, which uses the photoelectric effect to determine whether a photon has hit it or not) can detect the same photon. Transforming a single photon into a bit of information and then having other people see that piece of information is not the same thing as having multiple observers detecting the same photon—detecting here should have a pretty well-defined meaning. --140.247.249.100 18:13, 12 January 2007 (UTC)

Through quantum entanglement two observers can obtain information about the same photon in independently conducted measurements. Some source must have emitted two photons for that, though.  --Lambiam^Talk 20:24, 12 January 2007 (UTC)

I still maintain that the question is ill-defined. Is the question, "can two observers simultaneously observe a single photon", or is it "can one observer observe a photon, then another observer observe the same photon?". I'm not really sure what the answer would be to the first question, but the second question is somewhat trivial; photons have no identities. They are just quanta of the electromagnetic field. They are identical bosons, and are fundamentally indistinguishable. --bmk

... and does "observe" only mean "interact with the photon directly" or can it mean "observe effects of the photon" or "observe a record of an interatcion" ? And if "observing" the photon changes its energy/frequency/wavelength, is it still the same photon after the interaction, thus allowing it to be "observed" multiple times ? Agree that question as it stands is not well defined. Gandalf61 11:12, 13 January 2007 (UTC)

[edit] Tungsten

What are some of the chemical properties of tunsten? The article isn't much help... 209.81.119.178 15:01, 12 January 2007 (UTC)

Try WebElements, following the links in the menus. --Seejyb 16:59, 12 January 2007 (UTC)

I believe their high melting point makes them excellent for use in incandescent light bulbs (unless this is just a filament of my imagination). StuRat 05:35, 15 January 2007 (UTC)

[edit] Thermal Conductivity?

What materials have high thermal conductivity, but are not metals? (other than diamonds) —The preceding unsigned comment was added by 216.182.149.254 (talk)

Probably not what you were thinking of (single materials), but heat pipes.

Atlant 23:46, 12 January 2007 (UTC)