Wikipedia:Reference desk/Archives/Science/2009 March 26

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March 26

Gravitomagnetism

Why can't the gravitomagnetic equations be quantized as an approximation to GR if they are so similar to Maxwell's equations? —Preceding unsigned comment added by 76.67.79.89 (talk) 01:52, 26 March 2009 (UTC)

You seem to be misunderstanding the gravitomagnetic effect; the gravitomagnetic effect is a (small) effect of general relativity which is governed by equations very similar to Maxwell's for electromagnetism. The gravitomagnetic effect can't be quantised as an approximation to general relativity as it is a direct consequence of relativity. The similarity of gravitomagnetism to electromagnetism is like the similarity of Newton's law of universal gravitation to Coulomb's law, the two forces use similar equations but you can't infer more about one by using the other... - Zephyris _Talk 09:09, 26 March 2009 (UTC)

You're talking about the gravitational analogue of magnetism (which is what I think of too when I hear the word "gravitomagnetism"), but our article gravitomagnetism appears to be about a weak-field approximation to GR that looks like Maxwell's equations. I don't know whether that specifically can be quantized, but weak-field gravity can be—see gr-qc/9512024. -- BenRG (talk) 12:56, 26 March 2009 (UTC)

Is human food killing the seagulls?

Is it true that the seagulls living in urban areas that feed on discarded human junk food are starting to drop dead from heart disease or develop diabetes? I was told this today by a taxi driver and I don't know whether he was winding me up or not. It sounds slightly plausible to me, considering that some of these gulls seem to exist on chips, pizza, burgers, fried chicken and kebabs. --84.66.64.241 (talk) 01:57, 26 March 2009 (UTC)

I rather doubt they live long enough for those diseases to be a problem. But let's see what our resident expert has to say. (Are they even subject to diabetes?) Clarityfiend (talk) 05:42, 26 March 2009 (UTC)

Birds, like mammals, have a pancreas that produces insulin and glucagon. I suppose that anything with pancreas may develop diabetes under certain environmental and/or genetic conditions; I can't see why not. OTOH, I've never seen avian diabetes studied or even mentioned. Feeding sugar or HFCS to a seagull is an exceedingly bad idea, at any rate. They don't normally put sugar on their fish or crab :) . Fried foods, as you can imagine, are also not a part of their natural diet. Heart problems stemming from overeating and lack of exercise are expected, too. Finally, plastic and foil wrappers are potentially a serious problem. AFAIR, seagulls can dispose of the inadvertently swallowed pieces of mollusc or crab shells; but a swallowed piece of a nylon bag may well prove fatal. --Dr Dima (talk) 06:53, 26 March 2009 (UTC)

As with a lot of human diseases, animals tend not to suffer from them because they are so short-lived. It takes years of a terrible diet to develop these conditions - and seagulls simply don't live that long. SteveBaker (talk) 19:44, 26 March 2009 (UTC)

No to be contrary, but at least 1 gull has lived to the age of 49... http://web1.audubon.org/waterbirds/species.php?speciesCode=hergul&tab=natHistory (talk) 20:01, 26 March 2009 (UTC)

FWIW, the larger gull species tend to be up there amongst the most long-lived of birds. 25-plus-y.o. Herring/Lesser BB Gulls are not uncommon, as I understand it. They don't even start breeding until they're at least four. Here's a couple of slightly-related links I just found (see here and here) - they doesn't specifically answer the original question but seem to suggest that a diet high in fat and sugar is indeed having an effect of some kind on the gulls. --Kurt Shaped Box (talk) 22:17, 26 March 2009 (UTC)

Adverse drug reaction: Rabeprazole

Is their any evedience that long term use of rabeprazole like Proton pump inhibitor is associated with incease risk of gasric carcinoma or gynecomastia??? —Preceding unsigned comment added by Samir doc (talk • contribs) 08:35, 26 March 2009 (UTC)

I found no evidence of these on a literature search. This study noted a number of side-effects, but not gynaecomastia or gastric cancer. Axl ¤ [Talk] 11:29, 26 March 2009 (UTC)

Identify this fish!

I took this photo last summer of small (~10cm long) fish trapped in a rockpool on Holy Island in North Wales. Does anyone have any idea which species they are? - Zephyris _Talk 08:58, 26 March 2009 (UTC)

I think they are lesser sand eels. Axl ¤ [Talk] 11:14, 26 March 2009 (UTC)

Electric arcs: Possible terahertz sources?

It seems that lightning and other arcs are shown, sometimes unexpectedly, to produce electromagnetic radiation in virtually every part of the spectrum where detection attempts have been made: Radio and microwave ^[1] , infrared ^[2], visible (hence visibility of lightning and sparks), ultraviolet ^[3] , X-rays ^[4], and even gamma rays ^[5]. So why not terahertz? Since commonly discussed THz sources, even incoherent ones, are extremely expensive and high-tech it seems like something as obscenely low-tech and low-cost as source of high voltage electric arcs deserves some attention. Wouldn't it be easy to try shooting high voltage arcs through random gases at random pressures and observing in the THz region of the spectrum, just to see what happens? Wouldn't a THz arc-lamp/discharge-lamp be far cheaper than other sources?

69.140.12.180 (talk) 15:29, 26 March 2009 (UTC)Nightvid

I'm no expert - but isn't the problem to get enough power into those THz ranges to be useful? What you do by producing (essentially) Radio-spectrum white noise is to put power into the spectrum in roughly the inverse of the frequency (or maybe the inverse of the square of the frequency...I forget). At any rate, that means you've got to put an insane amount of energy into your arc to get enough THz stuff to be useful. Lightning can do it because it discharges an ungodly amount of energy in a very short space of time...you can't sustain that kind of power for very long. SteveBaker (talk) 19:42, 26 March 2009 (UTC)

If that were so then lightning and other arcs wouldn't be effective in radiating visible light. I emphasize that as far as I know there is no part of the electromagnetic spectrum that arcs and lightning are terribly bad or inefficient at radiating in, and it would be very strange if unlike all other parts of the spectrum one got so little in the THz region for a reasonable input power.

69.140.12.180 (talk) 19:57, 26 March 2009 (UTC)Nightvid

I think you're missing what I think is at least part of Steve's point: if they are indeed radiating in all parts of the spectrum, they can't also be highly efficient in radiating in an any/every arbitrarily-chosen narrow range. With a fixed amount of energy, you can either radiate all of it at one frequency or spread it out thinly. So if you have a huge amount of energy over a broad spectrum, you get a decent amount in your band of interest, but that's not efficient because so much of the energy is in other bands. DMacks (talk) 20:18, 26 March 2009 (UTC)

This has gone slightly in the wrong direction. The distrubtion of energy with respect to frequency is important, as nothing will emit radiation equally in all frequencies - the result would be infinite power radiation. That's why the notion of lightning radiating in "all other parts of the spectrum" is inherently flawed. This isn't even like blackbody radiation with a smooth curve on the power vs frequency distribution graph - the distribution of lightning's radiation is going to have peaks and valleys, corresponding to the different mechanisms that produce that radiation during the strike. For example, the visible light just under the 1,000 THz range is due to photons with an energy on the order of a few electron-volts, being generated by molecular-level reactions from the oxygen and nitrogen in the atmosphere being strongly ionized by the strike. This does not neccessarily imply that THz radiation will also be emitted strongly. There just happen to be various peaks associated with their respective generation criteria: just under 1,000 THz (or PHz) radiation, i.e. visible light, corresponds to electrons hopping around in orbits (flames, sparks, neon signs, etc) as well as blackbody radiation around a couple thousand kelvins (incandescent lightbulbs) - Ultraviolet, at a few PHz, typically comes from higher energy electron hopping (black light phosphors) and blackbody radiation of around 9000 kelvins (electric arcs) - X-rays up in the hundreds to thousands of PHz (10e5 to 10e6 THz or 10e18 Hz) typically come from high-energy electrons knocking into heavy atoms, either knocking out inner valence electrons (causing outer electrons to undergo a huge drop to fill the hole) or Bremsstrahlung from nearly hitting the nucleus - Gamma rays up in the 10e20 Hz range typically come from state changes of million of electron-volts, typically found in nuclear reactions - Infrared in the ten to hundred THz range is abundant, with the power decreasing with temperature, from simple blackbody radiation at various sane temperatures (like human body heat, a low power emitter of 30 THz radiation) - lower frequencies go from microwaves generated by ballistic electron motion within a small but macroscopic cavity, all the way down to radio waves that are easily generated with discrete electronics equipment. At around 10-20 kelvins, you can generate blackbody radiation with a peak in the THz range (interstellar dust does exactly this), but the power of this radiation is too low to be useful for anything. It's just a fact of life that there aren't any common mechanisms in nature that generate photons with the right wavlength (around a millimeter). That's why THz radiation is hard to generate. DeFaultRyan 23:09, 26 March 2009 (UTC)

What you describe as "different mechanisms" of generation of radiation are really just phenomena which occur on different timescales - for instance bremsstrahlung processes generally emit radiation at frequencies comparable to the inverse of the time it takes the electron to stop or be deflected - if it passes close to the nucleus and is going really fast this time may only be around 10^-18 seconds, corresponding to an X-ray period. If it is farther from the nucleus and not so fast it will be a longer time such as 2*10^-15 sec. and emit lower frequencies such as visible light. (And because by definition a plasma has free electrons this emission must also include free-free radiation, not just transitions between quantized bound states or "hopping") And the free electron motion over yet longer timescales emits radio waves and microwaves, as the chaotic nature of the process means current in the discharge flows erratically, electron motion changes on the scale of 1 ns would produce 1 GHz radiation. So although there are in a sense different mechanisms involved, the issue really amounts to the timescales of the motion changes and irregularities of the electrons in the discharge. To say that little THz radiation is emitted is to say that there are not significant features of the motion of electrons on the timescale of 10^-12 seconds. This seems to be questionable to me because the discharge is chaotic and analogous to turbulence. Turbulence in a fluid produces sound waves (that is why jet airplanes are so loud is this happening in the air jet's turbulence) and analogously, the electrons constituting a discharge current are in "electromagnetic turbulence" / "electromagnetic turbulent flow" and radiate electromagnetically. But this would imply that the electron motion is highly irregular and spans many orders of magnitude in timescale, from those corresponding to interaction of electrons with atoms, with molecules, with groups of molecules, with micron-scale thermal fluctuations, with small filaments, with large filaments, and with macroscopic irregularities in the structure of the arc. But lightning and other sparks in some sense appear "fractal", meaning it has spatial structure at different scales, so it would be natural to expect the same of electron motion at different timescales, including 10^-12 seconds. If in air at 1 atmosphere it just so happens that there isn't much in the way of electron motion features at that timescale, then surely that could be changed by using a different gas, pressure, arc current density, electric field, and/or arc length. When you said "It's just a fact of life that there aren't any common mechanisms in nature that generate photons with the right wavelength (around a millimeter).", did you mean THz emission from arcs has already been sought but not found? We don't know until we try, because many radiations which have been discovered were described by the scientists as "unexpected" including X-rays from arcs in air at 1 atmosphere in the laboratory. In light of all this (no pun intended), how could one not justify an experiment wherein electric arcs are made in different gases at different pressures (say, from 10^-3 to 1, and to 10² atmospheres), with THz detectors watching? (or has this been done already?) 69.140.12.180 (talk) 15:30, 27 March 2009 (UTC)Nightvid

A quantum mechanical "proof" that any positive real number is zero

About a week ago, while browsing Wikipedia, I stumbled onto a (fallacious) proof that any positive real number is zero. It was quantum mechanical, basically "proved" that the Planck constant equals zero. The resolution had something to do with bra-ket notation not working on a sphere or hiding a functional analysis fact from the plain sight. It ended in words like: "Thus $h$, an arbitrary positive real number, must be zero." The exact wording must have been different, as googling doesn't help. I really cannot remember anything else. Could anyone please point me to the Wiki article? — Pt _(T) 22:59, 26 March 2009 (UTC)