Wikipedia:Reference desk/Archives/Science/2008 January 9

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January 9

Population genetics problem

I'm trying to figure this out, and I can't: "A/A and A/a individuals are equally fertile. If 0.1 percent of the population is a/a, what selection pressure exists against a/a if the A → a mutation rate is 10^−5?" I have found online that the answer is 0.01, but I would like to know why. Any help is much appreciated. Thanks! —anon. —Preceding unsigned comment added by 141.155.31.61 (talk) 01:41, 9 January 2008 (UTC)

The frequency of the alleles can be determined using the equations described in the article Hardy-Weinberg equilibrium. The relationship between selection pressure, mutation rate, and allelle frequency is described in the article mutation-selection balance. William Avery (talk) 08:24, 9 January 2008 (UTC)

That double-crossing cerebellum!

Are there any theories as to the evolutionary benefit of the many neural decussations, either individually or together? Tuckerekcut (talk) 02:11, 9 January 2008 (UTC)

A predator or enemy whacks you on the right side of the head, and your left cerebral hemisphere directs your right arm to whack 'em back. Edison (talk) 07:04, 9 January 2008 (UTC)

here is an (unpublished) paper by Shinbrot and Young of Rutgers University, demonstrating "that as the number of wiring connections grows [in 3D networks], decussated arrangements become overwhelmingly more robust against wiring errors than seemingly simpler same-sided wiring schemes." I'm not sure I buy it, but its certainly a theory. Rockpocket 07:41, 9 January 2008 (UTC)

Google Book Search yields the following, from "Mind As Machine: A History of Cognitive Science", By Margaret A. Boden, p. 1289: "He [Braitenberg] argued that decussation hasn't evolved by chance as a selective advantage. Rather, it's a natural consequence of a mathematical fact: if each computational unit is connected to all the others then, given bilateral symmetry, there will be more connections crossing the median plane than staying on one side of it." Arguably, then, the question isn't why there are so many decussations, but why there are so few. Here's the ref: Braitenberg V (1965). "Taxis, kinesis and decussation". Prog. Brain Res. 17: 210–22. PMID 5893240. --Arcadian (talk) 03:54, 10 January 2008 (UTC)

Earth's mass

How do we estimate earth's mass? From what I heard scientists don't even know exactly the composition of the earth's interior. 199.76.154.127 (talk) 05:08, 9 January 2008 (UTC)

Measure the acceleration of a falling body or the force the Earth exerts on a mass and use Newton's Law of Universal Gravitation. The tricky part is determining the gravitational constant, but that can be done in the lab by making precise measurements of the force between two masses. Bubba73 (talk), 05:16, 9 January 2008 (UTC)

Henry Cavendish very famously "weighed the Earth" with a torsion pendulum. Someguy1221 (talk) 19:51, 10 January 2008 (UTC)

Hypersensitivity of the senses

Do we have a list of conditions which cause people to be hypersensitive to light, sounds, odors, tastes, and touch ? The linked article in the title seems to be more about immune system over-reactions than what I have in mind. For example, someone suffering from a hangover may react badly to bright lights, loud noises, and strong smells and tastes. StuRat (talk) 16:55, 9 January 2008 (UTC)

I know photophobia is hypersensitivity to light. That led me to guess at the name of others, such as hyperacusis. However, there appears to be no such thing as hypergeusia and hypersmia (as opposed to hypogeusia and hyposmia. Perhaps I need study Latin more. -- kainaw™ 18:24, 9 January 2008 (UTC)

You missed the o in hyperosmia. Rockpocket 19:12, 9 January 2008 (UTC)

So there isn't any term for the condition of being hypersensitive across all the senses ? StuRat (talk) 19:10, 10 January 2008 (UTC)

Sensory defensiveness is probably closest to what you are looking for, but this is primarily a form of anomalous perception rather than anomalous sensation. See also here. Such hyper-perception can be a long term condition. There are certain "conditions" that can result in temporary general sensory hypersensitivity, such as pregnancy (though not usually to the extreme of sensory defensiveness). It is thought that this change could be at the level of sensation, rather than or in addition to the level of perception. Rockpocket 20:31, 10 January 2008 (UTC)

That sounds like what I'm looking for, although our article didn't mention hangovers specifically as a cause (it did mention drug and alcohol abuse, though). StuRat (talk) 06:50, 11 January 2008 (UTC)

btw, Roderick Usher in the 'Fall of the House of' suffered this. In one of his novels, I think William Gibson calls the process of tuning the sensorium for high sensitivity 'to Usher'. Adambrowne666 (talk) 10:24, 14 January 2008 (UTC)

Is that why Ushers in movie theaters are always telling me to be quiet and turn off my flashlight ? :-) StuRat (talk) 14:08, 14 January 2008 (UTC)

there is a name for the general condition: hypereasthesia

Advantage &disadvantages of Industry

D/Sir/Madam, For my high school project, I want some article on the above subject. Can you please help me out ?. Regards ! D.J.Sen —Preceding unsigned comment added by 220.226.37.124 (talk) 16:57, 9 January 2008 (UTC)

The article industrialization may be of use to you. I don't see explicit sections on advantages and disadvantages, but it's a start. Friday (talk) 16:58, 9 January 2008 (UTC)

Check out Bhopal disaster and Minamata for examples of extreme disadvantages. Matt Deres (talk) 17:24, 9 January 2008 (UTC)

Check out chlorination and Green revolution for some examples of advantages. Delmlsfan (talk) 23:46, 9 January 2008 (UTC)

History of quantum mechanics

I'm reading up on the history of quantum mechanics and I must confess I'm a little confused. I have a strong background in physics, having graduated with a degree in mechanical engineering from the University of Minnesota Institute of Technology, but one particular aspect of the quantum theory is bothering me.

It would seem that the precipitating factor in the development of quantum theory is that atomic emissions appear to be discrete (in terms of energy or frequency). In other words, it was observed that light is emitted from excited atoms (or rather atoms exiting an excited state) in specific frequencies, and thus the concept of energy levels was born.

However, it seems a logical leap to say that because electrons exist in an atom at discrete energy levels (and therefore the photons emitted by those electrons occur at discrete frequencies) that light itself must be quantized. Light could very well be continuous, and it's just the structure of the atom that is discrete.

What experiment or theory substantiates this leap? How do we go from the idea that atomic structure is quantized to the idea that all energy must be quantized. It's like saying because bottled water from the grocery store only comes in discrete quantities, that water itself must be organized into discrete packets. In a macro-sense, water is continuous, it is only the container that is discrete.

Thanks! FusionKnight (talk) 17:14, 9 January 2008 (UTC)

You may already have looked here, but does photon help? There's a historical section at least. I notice this sentence: "In 1905, Einstein was the first to propose that energy quantization was a property of electromagnetic radiation itself." Sounds like this is on the right track. There's a reference for that statement too, it may help. Friday (talk) 17:18, 9 January 2008 (UTC)

There are many reasons to assume the quantization of light. Look at photoelectric effect (Einstein actually got the Nobel award for its explanation, not for relativity) and Planck's law, which derives the black body spectrum using the assumption of discrete oscillators which emit discrete quanta. --Stephan Schulz (talk) 17:36, 9 January 2008 (UTC)

Thank you for the responses! I appreciate it. However, I think you've just restated my original problem. The reason scientists believe energy to be quantized is because of observations of the black body problem and the photoelectric effect. However, the observed quanta are a result of atomic structure only allowing electrons to exist in specified energy levels or orbitals (and therefore only oscillating between those specific levels). I'm still missing the part where energy levels imply the quantization of energy rather than just the quantum nature of atomic structure. FusionKnight (talk) 18:41, 9 January 2008 (UTC)

(Pedantry alert!) "Discreet" means subtle/prudent/unobtrusive, while "discrete" means quantized/noncontinuous/individual. Atomic emissions can be both, I suppose. :) --Sean 18:52, 9 January 2008 (UTC)

I think you may be misunderstanding what quantization of energy means. It is refering to the fact that energy is transfered in discrete packets. The easiest examples to understand involve photons. The amount of energy in each packet can be arbitrary (which is equivalent to saying that a photon can have an arbitrary choice of wavelength), but you can only absorb or emit an integer number of photons. As you appear to understand, that's related to the fact that atoms absorb and emit energy 1 photon at a time. The discreteness of energy, as understood in the early 20th century, was exactly a consequence of the fact that physical systems were made of atoms that could only transfer energy in discrete amounts. Dragons flight (talk) 19:09, 9 January 2008 (UTC)

No, Dragons flight, the OP makes a quite valid observation that the hypothesis that light itself comes in discrete energy packages ("photons") does not follow from separate observation that the energy levels of atoms are discrete. The crucial early experiments that supported the photon hypothesis were the spectrum of black body radiation, and the photoelectric effect. Planck explained the black body spectrum by hypothesizing that light could only be emitted by matter in discrete packets of energy hf, where h is Planck's constant and f the frequency of the light. (Note that the "matter" here is typically a solid, which unlike isolated atoms do not have discrete energy levels.) In the photoelectric effect, an electron is ejected from a material which is illuminated by (typically ultraviolet) light. It is observed that the kinetic energy of the emitted electron equals hf minus an offset that is fixed for each material. Einstein explained this observation by hypothesizing that the incoming light itself came in packets of energy hf, of which a fixed amount was used to free the electron, and the rest was available as kinetic energy of the electron. In that experiment, the energy hf does not correspond to any inherent energy level difference in the material; it is purely a property of the light. Further strong support for the photon model came from Compton scattering, in which a photon looses some amount E of energy to an electron it passes (sending the electron flying off to the side), and in the process decreases in frequency by precisely the amount deltaf=E/h predicted by the photon hypothesis. --mglg_(talk) 20:37, 9 January 2008 (UTC)

I think we are talking past each other. Of course I know all that. Look at the post I replied to, he is suggesting that blackbody radiation and photoelectric effect don't imply quantization. Obviously this is false, since the photoelectric effect is essentially impossible to understand without quantization. I am assuming that the original poster's comment is thus coming from a conceptual misunderstanding about what "quantization" actually means. Specifically, we know that the "quantization of energy" refers to the transfer of an integer number of energy packets (e.g. photons, phonons, etc.). The transfer of energy, in this way, is a property of both matter and light. What it does not mean is that energy only forms in multiples of some standard packet size. His comments suggested to me that he was suffering from this latter form of confusion, i.e. that energy was like electic charge and came in discrete multiples of some fundemental unit. I was trying (perhaps unsuccessfully) to explain that the historically relevant "quantization of energy" was meant to explain that energy was exchanged in discrete chunks (e.g. photons) and not that all energy existed as a discrete multiple. Dragons flight (talk) 21:40, 9 January 2008 (UTC)

Make sure you have a look at Wave_particle_duality. In short, any matter or energy exhibits at all times both wave an packet (or quantized) behavior, except only one of those behaviors can be observed at a a time. Don't forget that it wasn't a simple or quick step from one idea to the next here but the result of a few centuries of theory, experiments, nobel prizes, and arguments which continue to today. Furmanj (talk) 21:01, 9 January 2008 (UTC)

Without wanting to belabor a point, the introduction to photon seems to address this directly in regards to what are called the semiclassical models, whereby light is described according to Maxwell's equations (continuous) but interacts with matter in a quantized way. This was apparently an objection some had and a way some tried to "save" the classical theory, and was only later shown to be incompatible with experiments. If you are asking why Einstein would have assumed it was quantized, well, Einstein was, in those days anyway, a self-styled revolutionary and a strong Machian at that. The former quality would make him not shudder away from making as strong a statement as he felt possible, and the latter would cause him to regard any speculation about the nature of light other than how it interacted in a measurable way to be pointless metaphysical speculation. Anyway, footnote 31 in the article seems to be a reference relating to the semiclassical models, you might turn there. This isn't an aspect of the the history of physics I'm acquainted with myself, though I have a passable knowledge of the history of quantum theory. --24.147.86.187 (talk) 03:10, 10 January 2008 (UTC)

I think the relevant timeline (see history of quantum mechanics) is as follows:

1905 - Einstein proposes that the photoelectric effect can be explained if electromagentic energy is quantised in packets which he called "light quanta", and which we now call "photons". Einstein also predicts that the energy of emitted photoelectrons will increase linearly with the frequency of incident light. Note that this hypothesis says nothing about the internal structure of the atom.

1909 - Einstein shows that his "light quanta" model can also explain the empirically-derived Planck's law of black body radiation. Planck had regarded his "action quanta" as a purely formal device, and initially rejected the physical reality of Einstein's "light quanta".

1913 - Bohr proposes the Bohr model of the atom, in which electrons can only orbit the nucleus with discrete energy levels. Bohr uses this model to provide a theoretical explanation for the empirically-derived Rydberg formula for the wavelength of spectral lines. No need to assume that light is quantised in the Bohr model.

1915 - Milikan completes an experimental program intended to show that Einstein's explanation of the photoelectric effect was incorrect, but which instead confirms Einstein's predictions in every detail.

1916 - Einstein extends Bohr's explanation of spectral lines, and introduces Einstein coefficients.

1923 - Compton discovers Compton scattering, which establishes the physical reality of photons.

In short, the discovery of the photon and the discovery of atomic orbitals proceeded along independent paths, although the use of Planck's constant in both models was a good indication that there was an underlying connection. I don't think these paths finally came together until Schrödinger's sythesis of wave mechanics and the Schrödinger equation in the 1920s. Gandalf61 (talk) 11:46, 10 January 2008 (UTC)

So, let me see if I'm understanding correctly. The photoelectric effect showed that light must be absorbed by electrons discretely because the intensity of light had no effect on the threshold but frequency did. This meant that photons are particle-like; a single photon, if cold, won't eject an electron, but a hot one will.

If the wave theory of light had been correct, the threshold energy could be overcome either by increasing amplitude or frequency since the power of a (mechanical) wave is dictated by both its amplitude and frequency. Since the photoelectric effect was seen to be non-additive (i.e. a more intense wave or a less intense wave over time did not have the same effect as a wave of higher frequency), that meant that the "amplitude" of a light wave didn't carry energy in the same way as a sound wave or other mechanical waves.

And that was the leap. There was no luminiferous aether in which the wave propagated, because there was not "true amplitude" in a mechanics sense. So Einstein said "amplitude" for light must be something else, like... number of photons per second! Then the photoelectric effect makes sense.

So, as a previous poster said, energy quantization doesn't say that a photon's energy has to be an integer multiple of some constant, but rather that energy (photons) don't behave entirely like ordinary waves, and in fact seems to act like particles on occasion. Does that sound right? FusionKnight (talk) 15:17, 10 January 2008 (UTC)

PROVIDING LINK FOR FRESH USERS FOR SYSTEM DYNAMICS

SIR/MADAM I HAVE STARTED TO RESEARCH ON SYSTEM DYNAMICS, BUT I AM STILL LOOKING FOR SOME USEFUL LINKS WHICH COULD HELP FRESHERS IN THIS FEILD —Preceding unsigned comment added by 202.70.201.120 (talk) 18:28, 9 January 2008 (UTC)

Please do not post in all UPPER CASE, it is the equivalent of shouting. Please do not post the same question multiple times or on multiple reference desks. Please do be patient and realize that everyone here is a volunteer, and there may very well not be somebody around who can answer your specific query. --LarryMac | Talk 19:10, 9 January 2008 (UTC)

Why do most penguins have black feathers?

I recently saw a picture of a penguin with an all-white covering, and it made me think of the question. I would guess that in the evolution of penguins, white feathers would be favored, or at least being not totally black, so as to camouflage in the snowy/icy environment. Or does the black covering help in camouflage under water? But still, I'm amazed at how much black feathers dominate in the diverse penguin population, as opposed to lighter feathers. 128.163.224.198 (talk) 18:35, 9 January 2008 (UTC)

Two thoughts: 1) black is good for absorbing heat, and 2) many aquatic animals are camouflaged with light-on-bottom and dark-on-top coloration so that the shark below you doesn't see you against the light sky, and the polar bear above you doesn't see you against the dark depths. See Countershading. --Sean 18:56, 9 January 2008 (UTC)

Penguins seldom need to fear terrestrial predators (except man) and thus have no necessity to be camouflaged on land. Thus their need is to be inconspicuous in the water (as Toto says above). Polar bears are in the far north where penguins are never found.--Eriastrum (talk) 19:05, 9 January 2008 (UTC)

Camouflage is not only to hide from predators. I believe the March of the Penguins movie (or perhaps some other documentary at the time) showed penguins diving deep below their own prey and then coming up at them from beneath. By being black against the black depths of the sea, they can sneak up on their prey better. -- kainaw™ 23:53, 9 January 2008 (UTC)

FYI - Except in zoos, penguins live in the southern hemisphere while polar bears live in the northern hemisphere. So, in the wild, penguins don't have to worry about polar bears. Besides avoiding predators, they also need to avoid being spotted by their prey, so countershading helps with that too. -- HiEv 01:58, 10 January 2008 (UTC)

Why don't polar bears eat penguins? Because they can't get the wrappers off of course. Lanfear's Bane | t 12:40, 10 January 2008 (UTC)

How ironic, for a penguin to eat a polar bear, it would have to eat the wrapper: [1]. StuRat (talk) 19:50, 10 January 2008 (UTC)

Heh that's pretty neat. 217.43.59.220 (talk) 21:46, 10 January 2008 (UTC)

incorrect 3d structure

I was looking at the sucrose_molecule_3d_model.png/800 and I am pretty sure it is incorrectly drawn. It shows a six membered ring connected to a six membered ring. The structure directly above it is a six membered ring connected to a five membered ring. The correct number of carbon, hydrogen and oxygen are attached. However the carbon attached to the oxygen holding the rings together should have no hydrogen attached to it.

This will alter the structure correctly. —Preceding unsigned comment added by 141.154.107.189 (talk) 20:03, 9 January 2008 (UTC)

• You are referring to the two images at the top of Sucrose. I think both images are drawn correctly. The bottom (3D) image does show a 5-member ring connected to a six-member ring. However, it is flipped horizontally compared to the top (2D) image. In the 2D image, the six-member ring is on the left; in the 3D image, the six-member ring is on the right. Johntex\^talk 20:35, 9 January 2008 (UTC)
  • That is pretty confusing though…one of my pet peeves is when diagrams of something that is "same" is drawn "differently", such that people that already understand it don't notice a problem but ones who are trying to learn get caught up in irrelevant details. The 3D image is actually rotated 180° in 3D space, not simply flipped in the Photoshop image-transformation sense. Will ask creator to fix it. Maybe this previous version of the image is clearer? DMacks (talk) 20:12, 10 January 2008 (UTC)

Bears in the south

Are there any native bears in the southern hemisphere? 202.168.50.40 (talk) 23:13, 9 January 2008 (UTC)

• The Sun Bear lives in Indonesia. Johntex\^talk 23:48, 9 January 2008 (UTC)
• The Spectacled Bear lives in South America, including Peru. Johntex\^talk 23:50, 9 January 2008 (UTC)
• The Drop bear lives in Australia. --M@rēino 17:10, 10 January 2008 (UTC)

Very funny, but please don't put misleading information in an answer like that. Matt Deres (talk) 17:14, 11 January 2008 (UTC)