Wikipedia:Reference desk/Archives/Science/2007 July 25

= July 25 =

Technetium
Having read the article, I see that Tc does not occur naturally on the earth, but it never says why. Of all the lower elements, what makes Tc so unspecial that it doesn't naturally form? H YENASTE 01:34, 25 July 2007 (UTC)
 * Technetium is radioactive in all its isotopes, the halflife is short enough that any original has all decayed away. No isotope of technetium has a half-life longer than 4.2 million years. And also note that on Earth, technetium occurs in trace but measurable quantities as a product of spontaneous fission in uranium ore or by neutron capture in molybdenum ores. (lifted from that article) GB 01:39, 25 July 2007 (UTC)


 * Technetium says "Since technetium is unstable, only minute traces occur naturally in the Earth's crust as a spontaneous fission product of uranium." So natural uranium fissions spontaneously, forming natural technetium.  But it's unstable, and has in general a short half life (see Isotopes of technetium), so that technetium in turn spontaneously fissions into something else. -- Finlay McWalter | Talk 01:41, 25 July 2007 (UTC)


 * I suppose I should rephrase thusly: why does Technetium, an element with considerably fewer subatomic particles than other radioactive elements, decay so quickly? H YENASTE 01:49, 25 July 2007 (UTC)


 * The article has a "Stability of technetium isotopes" section, which explains. -- Finlay McWalter | Talk 01:54, 25 July 2007 (UTC)


 * Thank you, Finlay. I can't believe I missed that the first time though. H YENASTE 02:04, 25 July 2007 (UTC)

Constant laws of physics?
Also based on Tc: What if Tc does form naturally somewhere in the universe? Is it possible, then, that certain truths in our part of the universe don't hold elsewhere? Could it be that beyond our realm of the universe, our accepted laws of physics do not apply? H YENASTE 01:34, 25 July 2007 (UTC)


 * Well, it would depend. If the reason there's no Technetium on Earth is (in effect) a coincidence, then if it appears somewhere else, there's no problem.  Or if we know that the conditions for it to "form" were never found on Earth or in our Solar System, but they do elsewhere, then still no problem. Or if the reason there's none is that it's all decayed away by now, as Finlay McWalter just explained, then still no problem.


 * As far as we know, the Laws of Physics (as we know them in our Universe) apply everywhere. As far as we know.  Obviously we can't prove that they apply beyond the realm of our knowledge and experience.  (Except maybe by Occam's Razor.  Which is actually a pretty good argument.) —Steve Summit (talk) 01:43, 25 July 2007 (UTC) [edited 01:46, 25 July 2007 (UTC)]


 * It does form naturally. It forms on Earth, as mentioned above, and in red giant stars. All of this is in the Technetium article. -- Finlay McWalter | Talk 01:45, 25 July 2007 (UTC)


 * Yes, stars that we have observed. I'm imaging a distant area where Technetium is as common as iron (figuratively). H YENASTE 01:46, 25 July 2007 (UTC)


 * You're not asking a science question, then, but postulating some figurative imaginary land where different laws of nature apply. -- Finlay McWalter | Talk 01:52, 25 July 2007 (UTC)


 * As in the Isaac Asimov novel The Gods Themselves. But that's, y'know, science fiction. --Anonymous, July 25, 07:17 (UTC).


 * Technetium has been detected in other parts of the universe. However it can be explained in terms of physical laws also used on earth. This is part of science to find out what is the same on earth as the rest of the universe. Some things are different, and then the laws have to be generalised - eg air pressure, day of 24 hours. GB 01:49, 25 July 2007 (UTC)

Rephrase
I believe my example overshadowed my question. To rephrase:

In the observable universe, all laws of nature do apply consistently. Could it be possible, in part of the universe unobserved, billions of light years away, that certain inalienable laws here do not apply? A fair example is: what if the gravitational constant G was not constant, but depended on the objects' distances from the center of the universe if the universe has a center. H YENASTE 02:12, 25 July 2007 (UTC)


 * Well, yes, it's possible, in the sense that just about anything is possible. The general trend of science, though, is to say "there should be one set of laws governing everything we can observe" and then try to find that set of laws, or a reasonable approximation to it. When scientists observe something that doesn't follow the current theory, they look for a more expansive theory that explains both the previous observations and the new one (a good example is how we went from Newtonian gravitation to General Relativity). That said, I think I've read somewhere about the possibility that if there are sections of space expanding away from each other faster than light, so that they can never communicate, then certain aspects (I can't remember if they talked about physical laws, or constants, or something else) could be significantly different. Confusing Manifestation 02:34, 25 July 2007 (UTC)


 * There is some discussion of whether such physical parameters do vary within the observable universe (e.g. Fine-structure constant). Beyond the observable universe, we have few theoretical constraints, so it is not really science in the usual meaning; however, there are some theories that would predict that physical parameters vary on scales much larger than the current observable universe. So yes, it is possible. Is it necessary? No. Right now there is no conclusive evidence that would require the parameters of physics to be anything other than constant. Dragons flight 02:45, 25 July 2007 (UTC)


 * Alot of science is actually based on the assumption that certain laws and characteristics of the universe are the same everywhere.
 * For example, cosmology is based on the "Cosmological Principle." That article says:
 * The Cosmological Principle is not a principle, but rather an assumption or axiom that, when applied, severely restricts the large variety of possible cosmological theories. It follows from the observation of the Universe on a large scale, and states that:


 * On large spatial scales, the Universe is homogeneous and isotropic. 


 * In laymen's terms, (I quote ISBN 0471265187) At any given time, the distribution of matter is the same everywhere in space, and the universe looks the same in all directions. So in the field of cosmology, scientists tend to ignore notions like "what if the laws of physics are completely different over there?" It's justifiable to ignore ideas like that (as we can't study things we can't see), and those "what if"s aren't strictly scientific. So, to answer your question: yes, it is possible for certain physical laws to be different in unobservable reaches of space, but there's no evidence to support that claim, and cosmologists tend to quickly rule out ideas like that. Jolb 05:25, 25 July 2007 (UTC)

Yes it's possible - in the same way that anything is possible until proved or shown otherwise. It is however usual to assume that behaviour in one place is the same in any other place unless there is a reason to believe otherwise (it's simpler that way).87.102.10.79 12:39, 25 July 2007 (UTC)

Of course if our 'laws of physics' don't apply it would be fair to describe such a place as a different (new) universe.87.102.10.79 12:41, 25 July 2007 (UTC)


 * We really only know what we can observe - so we have two hypotheses: One says that the universe has the same laws and constants everywhere (evidence for which is that for every experiment we have ever done - and every observation no matter how distant a galaxy we look at - those numbers are still correct). The other hypothesis is that these numbers change - but so gradually that we can't observe it with any existing equipment or theories.  Since we have at least some evidence for the first theory - and no reason to suspect the second, all we can do is employ Occams razor and say that in all likelyhood, the simplest explanation is the right one.  The simplest is that everything is the same everywhere.  So that's what we go with...but knowing, in the backs of our minds, that MAYBE we're wrong about that. There have been lots of scientific papers written about this kind of thing and it's definitely taken as a serious possibility.  The Universal Gravitational Constant and the Fine Structure Constant are the two that are most often questioned.  We don't (for example) often ask whether the charge on the electron might vary - although I have no clue why not!  Obviously there are mathematical constants like 'pi' that we know for sure can't vary.  Another possibility (much more likely IMHO) is that these 'constants' are only constant under a certain set of circumstances - maybe they are actually variable depending on the curvature of space or the intensity of an electromagnetic field or something. This kind of thing was initially suggest as a 'get out clause' to avoid the need to hypothesise black holes. SteveBaker 15:09, 25 July 2007 (UTC)


 * A third possibility is that things are the same everywhere but our understanding is only local. The world is measurably flat on very small scales, for example. --140.247.238.102 15:22, 26 July 2007 (UTC)

A rephrase seems to be, is it possible our scientific model is wrong? and yes would be the answer, as all the previous ones were. Philc 19:28, 25 July 2007 (UTC)
 * It's a good idea to note, however, that many previous models were largely untested before their acceptance, while the existing base laws of physics have been well tested since their creation starting a century ago. The well tested models that were thrown out (but still used today! Newtonian mechanics, and Maxwell's theory) failed testing only once sufficiently accurate analytical devices were created, non-existent when said theories were invented.  But then this just brings us back to the one of the original, and most basic answers, which is that our current model of the laws of the universe are almost certainly accurate, to within our ability to test them.  151.152.101.44 21:10, 25 July 2007 (UTC)
 * Well, the philosophy of science generally accepts that its a bit more complicated than simple test/verification/falsification in most cases, but in any case, I think the most safe way to pose the positive statement is as far as we know, our current scientific model is correct, and we do not have compelling evidence that it is exceptionally wrong, though that doesn't apply to some fields (e.g. we know that something must be exceptionally wrong/incomplete in our understandings of General Relativity and/or Quantum Mechanics, because they aren't compatible). --140.247.238.102 15:22, 26 July 2007 (UTC)

Climate Change
I'm confused about why scientists are so concerned about climate change. If the Earth is warming, why can't humans just cool it down to solve or at least mitigate damage done by greenhouse gasses? We've all heard of nuclear winter, another kind of man-made climate change. Why don't scientists look into using aerosolized dusts (like in a nuclear winter) to reduce the amount of solar radiation reaching Earth? I'm not suggesting nuclear winter to prevent global warming, but, for example, couldn't we spray aerosolized sulfur dioxide or aluminum oxide into the upper atmosphere to reflect solar radiation to counteract greenhouse gasses? Jolb 05:02, 25 July 2007 (UTC)


 * And have you considered the consequence to your solutions? Do you want to smell like Rotorua get acid rain or get lung cancer from breathing in microscopic pieces of aluminium oxide? It is simply easier to prevent climate change than to let it happen and undo its damage. --antilivedT 05:18, 25 July 2007 (UTC)


 * Antilived, I don't feel like the examples I gave are necessarily sound ecological solutions. Instead, I merely use those examples to illustrate the point that humans are capable of REVERSE climate change, not necessarily in the same way or with those exact chemicals. Also, preventing climate change has eluded us thus far, so I don't think it's as easy as you seem to think. I also don't suggest that we "let it happen." Jolb 05:29, 25 July 2007 (UTC)


 * Climate change is very small over a long period of time. It would be very difficult to respond over such a long period of time with the minute changes necessary to counteract it.  Secondly, the law of unintended consequences would most likely impact the earth in an unexpected fashion.  Third, the mechanism of global warming is not completely understood nor is the man-made contribution to global warming fully understood.  --Tbeatty 05:53, 25 July 2007 (UTC)


 * The short answer is that yes, there are people who are paying serious attention to geoengineering approaches to combating climate change. As Antilived suggests, so far these proposals are either extremely impractical/costly or carry significant negative effects on their own. As a result, most people feel that focusing on preventing/limiting climate change is probably the better opinion of those that have been presented. Dragons flight 05:58, 25 July 2007 (UTC)


 * There is a proposal in the scientific community right now too possibly use a shield of dust in orbit abound earth the shade us as a last ditch solution. (The article is from a recent New Scientist. I’m not sure which issue.) However, the proposed method is apparently only an absolute last resort, since it would probably cause an ice age or other unintended disaster. --S.dedalus 06:00, 25 July 2007 (UTC)


 * The question whether global warming could be reduced by technical means is, of course, a valid one, and hence actively researched since quite a while. So far, however, all ideas seem to be quite dangerous, in that they may pose risk for even bigger disaster. Even the simple idea of capturing and storing the carbon dioxide has its problems: the storage might malfunction and release giant amounts of CO2 in one go. Other scientists test whether one could increase the growth of algae in the oceans by iron fertilization. When the algae die, they sink down and take with them the carbon that they took from the athmosphere while growing. But will they stay down there? That's far from clear given the little we know about deep ocean currents. And recently, Paul Crutzen sparked a major debate with his idea of artificially introducing sulphur dioxide into Earth's upper athmosphere to cause a cooling effect similar to that following major volcano outbreaks. Most scientists, however, consider this way too dangerous. The problem with all this geoengineering is always that it is hard to judge effects and side effects from small-scale experiments, and large-scale experiments may cause irreperable damage. Simon A. 07:39, 25 July 2007 (UTC)


 * This is the kind of optimistic can-do thinking that made the former Soviet Union into the veritable paradise it is today, both economically and ecologically (note to origial poster, please don't take this as condescencing or insulting, it's not intended as such). Whereas our government is largely formed of lawyers who couldn't get lawyer jobs, with all the problems that brings, lawyers didn't have the same clout in the USSR (defense lawyers vs. the USSR; yeah, right), so the majority of the lawmakers were engineers. And the attitude was, we'll make it work; and if something breaks we'll patch it, and if that breaks something we'll patch that, etc. and like i said, that led to economic and ecological disaster. At the current state of human competence, best we not screw with things too much, and always with a clear pathway for rolling things back to The Way They Were In The First Place rather than patches on patches on patches.Gzuckier 14:36, 25 July 2007 (UTC)
 * Your 'our government' sounds a lot like my 'our government'. :) DirkvdM 06:32, 26 July 2007 (UTC)


 * The problem with these kinds of intervention is that they all too often do more harm than good. We didn't understand  the atmosphere well enough to avoid getting into this mess in the first place - it's unlikely we could safely take such drastic action to reverse it.  For example (and I have no clue whether this is a real issue or not) - the CO2 in the atmosphere predominantly blocks re-radiation of infrared light back out into space.  Regular visible light is not much affected.  If we put up some kind of cloud to reflect sunlight away, maybe it would reflect visible light away as well as infrared.  Lack of sunlight in the green region of the spectrum would cause plants to do less photosynthesis - which would cause them to absorb less CO2 - which would result in global warming getting worse - not better (and incidentally probably cause global crop failure and starvation).  I'm not saying that's definitely what would happen - but you have to see that it's plausible.  If I can imagine a problem that serious with just 10 seconds thought (and I'm no climatologist), just imagine how many other nasty possibilities there might be after 10 years of experts looking into it!  Think about the number of cases where we've tried to "fix" ecological problems and made things worse in the past.  We introduce some kind of animal or plant into a non-native habitat - it goes nuts and takes over from the native species - so we find the natural prey for that thing and introduce that - only to find that it's extra food for some native species that then undergoes a population spike that causes some other native species to go bust.  If there is anything we've learned over the past 40 years it's that intervention of any kind is dangerous.  By far the safest thing to do at this point is to drop the outflow of CO2 into the atmosphere as fast as is reasonably possible - and to let nature do the best it can to clean up and restore balance to the system.  SteveBaker 14:55, 25 July 2007 (UTC)


 * It's interesting that you should ask this question today. Wired News just posted this article up. In it, scientists suggest that pumping 20 to 25 litres of aerosols into the stratosphere per second would be enough to reach the temperatures of 1900 in five years. Unfortunately, this pumping of aerosols would have to remain constant. If it was stopped, temperatures would dramatically increase to the level governed by carbon dioxide levels. --80.229.152.246 22:02, 25 July 2007 (UTC)


 * Steve, your problem would be solved if the sunlight were not 'caught' before it reaches the Earth but when it reaches the Earth. We might do that with solar panels and create electricity at the same time. Two for the price of one!
 * Of course, those shouldn't be placed on rooftops in cold climates, because that would take away the natural heating of the house, increasing the necessity for burning fuel. But that's just another side-effect I happen to think of. Damn side-effects. DirkvdM 06:32, 26 July 2007 (UTC)
 * No - that doesn't work. If you absorb the sunlight, turn it into electricity, then use the electricity, the sunlight turns back into heat (eventually) and ends up warming the earth.  You have to reflect away the sunlight so that its energy is shed back out into space.  This is one reason why the melting of the North Pole is so serious.  Snow and ice are white and reflect sunlight back out into space.  Open ocean is dark and absorbs sunlight - making the world warmer.  As the polar ice retreats due to greenhouse effects - it actually accellerates global warming.  So you can't absorb the sunlight and claim to be helping.  Of course replacing fossil fuels with solar electricity may not be such a terrible thing - so long as your solar panels don't absorb more sunlight than the ground beneath them did.  Putting a huge array of solar panels on snowy ground (for example) is likely to be counter-productive. SteveBaker 11:58, 26 July 2007 (UTC)


 * Ah, but on sea it would help then. Of course the poles are not very effective places to put them (unless one day we start producing surpluses of solar panels). And the cooling could cause ice formation, which would cover them and reduce their effectiveness even more. But then what about pouring polystyrene bubbles out in the arctic ocean? That might cause some other ecological problems, but it could reverse the effect of ice with its high albedo being replaced by ocean with its low albedo, which aggravates the global warming. I actually once proposed that once before here on the ref desk, I believe. DirkvdM 05:08, 28 July 2007 (UTC)

Friction Ridges / Finger Prints
I was told that finger prints are formed by pressure in the womb, yet the Wikipedia article on skin suggests that it is genetically determined. Which one is right, or both?
 * AFAIK it's genetics, but I'm not an expert. Gzuckier 14:43, 25 July 2007 (UTC)


 * If it were pressure in the womb, wouldn't we have fingerprints all over? Naegeli syndrome is a genetic disease that results in people being born without fingerprints - which also says it can't be due to pressure in the womb because that couldn't depend on the genetic makeup of the baby.  Nope - I don't buy the 'womb pressure' theory at all.  Who told you this? SteveBaker 14:43, 25 July 2007 (UTC)


 * The fact that fingerprints fall into discrete patterns (spiral, whorl, etc.) implies a genetic cause, but identical twins have distinct fingerprints, so that implies an environmental cause. These links give that same (non-)answer: it's both nature and nurture. ,  --TotoBaggins 19:59, 25 July 2007 (UTC)
 * The article says that patterns are partly inherited but ridges are not. --Kjoonlee 20:47, 26 July 2007 (UTC)

which is more durable:metal or non metal?
which is more durable?metal or non metal?


 * it depends on the context. please elaborate. --Russoc4 13:46, 25 July 2007 (UTC)


 * Yep - more detail please! It depends on whether you are talking about resistance to abrasion or tensile strength or ductility or compressive strength, tendancy to crack formation and propagation.  At what temperature range?  Pure metals versus pure non-metals - there are some pretty durable compounds that contain both metals and non-metals.  We certainly need more information about the context.  Ceramic heat tiles on the space shuttle are much more durable than aircraft-grade aluminium under circumstances of re-entry into the earths atmosphere, but a bone-china (ceramic) teacup is no match for a stainless steel coffee mug if you drop them both from 6 feet onto a concrete floor!  Diamond is much harder than steel - but at high enough temperature the diamond will catch fire long before the steel will melt.  The answer can only be: "it depends". SteveBaker 14:36, 25 July 2007 (UTC)

Diuretics and potassium
Why is it with diuretics that potassium is the only ion that is lost? Why don't diuretics result in a loss of sodium, calcium, magnesium and so forth? I read the diuretic article but couldn't make much sense of it, so I was hoping that someone here might be able to provide with information that is easier to understand. Thanks in advance, Jack Daw 13:50, 25 July 2007 (UTC)
 * See Loop of Henle: "K+ is passively transported along its concentration gradient through a K+ channel in the basolateral aspect of the cells, back into the lumen of the ascending limb. This K+ "leak" generates a positive electrochemical potential difference in the lumen. The electrical gradient drives more reabsorption of Na+, as well as other cations such as magnesium (Mg2+) and importantly calcium Ca2+." Thus potassium is "sacrificed" to allow for more reabsroption of the other ions.-- Flyguy649 talk contribs 17:20, 25 July 2007 (UTC)


 * It actually depends on the diuretic used. Loop diuretics affect the Na-K-2Cl symporter, and can indeed cause hyponatremia (low sodium) and hypomagnesemia (low magnesium) as well as hypokalemia (low potassium).


 * On the other hand, potassium-sparing diuretics operate on other ion transport channels. For example, amiloride and triamterene inhibit the epithelial sodium channels, preventing the resorption of sodium from urine.  TenOfAllTrades(talk) 17:28, 25 July 2007 (UTC)

Eating excess sugar?
What are some health problems that can result from eating too much sugar?
 * You can see the appropriate article on sugar, specifically the sugar and health section. While that entry seems to be sadly lacking in necessary citations, it should be able to give you a start to looking into it in more depth.  ◄   Zahakiel   ►  18:04, 25 July 2007 (UTC)




 * There are a variety, but it basically boils down to this --->
 * --TotoBaggins 20:09, 25 July 2007 (UTC)


 * You mean sugar mutates the ob gene? alteripse 02:26, 26 July 2007 (UTC)




 * Or this >
 * --DuncanHill 22:59, 25 July 2007 (UTC)


 * Type II diabetes. Aaadddaaammm 01:27, 26 July 2007 (UTC)

Air speed and PSI
How fast does air have to be traveling to create pressures of up too 10 psi [pound per square inch], such as would be in a shockwave etc ?

Thank you - —Preceding unsigned comment added by 80.6.36.200 (talk • contribs)
 * I got a figure of 294mph from The Atomic Archive using the following google query: overpressure mph "10 psi" - But you'll probably have to show your work. - C HAIRBOY (☎) 20:49, 25 July 2007 (UTC)
 * Ha! Flyguy649 talk contribs 21:11, 25 July 2007 (UTC)

Name for a filtering problem formulation
Say I'm building some active noise control system, for example a noise-canceling microphone system in which one microphone faces the source of the sound, and a second microphone faces away from the source to pick up the ambient noise. So I have two different input streams: a noisy signal, and another stream that's correlated with the noise, and I want to clean up the first stream by subtracting out some estimate of the noise derived from the second stream. The problem is, I can't just subtract out the second stream as it is, because it's facing a different direction, and it might be a different kind of microphone with a different frequency response or whatever, so I have to filter it first. How do I design a filter that is optimal in the sense that the signal resulting from the noise subtraction process has the minimum possible power, i.e. it is most effective at removing noise?

I don't want a detailed answer (it would be too long and there are probably many different ways to attack the problem); I just want some keywords I can use to search for this problem. The book I have here (Optimal Filtering by Anderson and Moore) mentions three problems: filtering, smoothing and prediction, but they all use only one input stream, so they're not what I'm looking for. —Keenan Pepper 22:07, 25 July 2007 (UTC)


 * Presumably you have looked here? or here --SpectrumAnalyser 22:28, 25 July 2007 (UTC)


 * Um, yes. And don't use external links to link to Wikipedia articles. For example, instead of, use Noise-cancelling headphone . —Keenan Pepper 23:43, 25 July 2007 (UTC)


 * Oh sorry. So what your problem? You dont need a filter, you just need to capture the noise in the same phase as it would appear at the main microphone then subtract it from th e main signal. The trick is to get it in the right phase, but you would n use a filter to do that. You dont say what type of noise you are trying to eliminate. If its higher frequency stuff, the phase errors will stuff you.--SpectrumAnalyser 00:26, 26 July 2007 (UTC)


 * I beg to differ. You do need a filter. How else would you "get it in the right phase"? Ever heard of an all-pass filter? Also, as you say, active noise control is less effective for high frequencies, so if you didn't use a filter, you would end up increasing the power at high frequencies rather than reducing it. The optimal filter would automatically incorporate a low-pass filter.
 * None of which answers my question, which was: what is the name of this problem formulation? —Keenan Pepper 05:06, 26 July 2007 (UTC)


 * I dont think there is a specific name for this sort of system. You will have to treat it as any other system with two inputs. The main input will be S+N and the other input will be f(N). You have to create the inverse function (with your 'filter' as you call it) before you can subtract the noise (N) from S+N. Since you are likely to have little control over f(N) you must create a versatile system to obtain its inverse. If you new the exact nature of the noise (say a single sine wave) then of course it would fairly easier. But with broad band noise, I dont think you have a chance unless you were to utilize spectral subtraction methods. These methods may be too slow to perform in real time for all but the lowest frequencies of interference. None of which answers your question sorry.-- I dont think there is an answer.--SpectrumAnalyser 12:40, 26 July 2007 (UTC)

Related question

 * OK - so I have a related(ish) question. Why do this electronically at all?  Why not have a microphone that's just a hollow tube with a diaphragm blocking the tube halfway down it's length.  Sound that's coming from all directions would enter both ends of the tube and the pressure changes would cancel out - someone singing into one end of the tube would create a pressure differential on that side that would vibrate the diaphragm.  Noise cancelling without the electronics!  Clearly this doesn't work in practice or that's the way everyone would be doing it - but why would it be worse than two microphones pointing in opposite directions with electronics subtracting one signal from the other?  Noise cancelling headphones are a different matter of course...not at all the same thing! SteveBaker 01:47, 26 July 2007 (UTC)


 * Ding ding ding! Correct answer! Highly-directional microphones are the way everyone does it, using techniques such as "shotgun" microphones and parabolic reflectors plus microphones. Our microphone article discusses this.


 * Atlant 15:54, 26 July 2007 (UTC)


 * Why do you assume the noise on either side of the diaphragm would be perfectly out of phase? It could just as easily be perfectly in phase (doubling the amplitude and therefore increasing the power by a factor of four), or, more likely, incoherent (doubling the power). Electronics simply "subtracting one signal from the other" would suffer the same problem. You have to filter the noise to make the phase correlate before you can achieve destructive interference. At high frequencies it's hopeless and the best you can do it avoid increasing the noise by adding to it incoherently. —Keenan Pepper 05:06, 26 July 2007 (UTC)


 * I notice that expensive cell phones do a much better job of blocking ambient noise than cheap cell phones. I assume it's due to some intelligent acoustic engineering. Gzuckier 19:19, 26 July 2007 (UTC)

Medical Illness
When I was younger, I contracted a virus which gave me these weird red dots all over my skin that turned purple when pressed. It also affect my joints, making them so swollen I could barely move. I've heard that it could also cause kidney problems/failure. The only thing is, I don't know the name. I suppose I could ask my doctor, but I was wondering if this was a somewhat common infection that someone else knew. According to my mother, it was named after someone. Thanks! Delta 23:15, 25 July 2007 (UTC)


 * Rubella?

Henoch-Schonlein purpura? alteripse 02:25, 26 July 2007 (UTC)

Medical disclaimer? Nil Einne 16:23, 28 July 2007 (UTC)

metal
is there any metal on earth that can conduct lightning and not get super heated, with out it being grounded? —The preceding unsigned comment was added by Jeff1133 (talk • contribs) 23:38, 25 July 2007 (UTC)


 * I dont think the metal body of an aircraft gets super heated when struck. And thats not grounded (when flying of course)--SpectrumAnalyser 00:31, 26 July 2007 (UTC)


 * Yikes! That's not true! This 707 wingtip  (for example) shows a nice scorch mark on the wing where it took a mid-air lightning strike.  The electrical discharge takes the (electrically) shortest path.  It's easier for the electricity to pass through the body of the plane and out the other side than it is to go through the air nearby.  Immense amounts of current flows through the metal - and current flow heats metal...so yeah - it'll zap the plane pretty efficiently.
 * To answer the question, all metal conducts electricity to some extent - but all of them offer some resistance to the flow. Resistance means heat.  Some (eg silver) conduct better than others (eg aluminium).  The degree to which the metal gets heated depends on it's total resistance.  The larger the chunk of metal, the lower the resistance - the better the conductivity - the lower the resistance.  The most conductive metal known is silver - and a really large ingot of the stuff ought to conduct lightning without getting too hot.  As to whether grounding it helps - you have that backwards.  You're interested in what the potential difference is between the ends of the metal - grounding the metal helps to ensure it presents the easiest path to ground (that's why we carefully ground lightning rods) - and therefore that it'll experience a bigger potential difference than if it were suspended in the air.      So a grounded metal rod would heat more than one sitting out in the air.  But both get heated! SteveBaker 01:32, 26 July 2007 (UTC)


 * Generally superheating only occurs in liquids. I think that lightning would probably disturb the liquid enough (through air-shockwaves) for this to not happen. Out of interest, could it happen in a perfect crystal? Should this be added to the article? Capuchin 06:50, 26 July 2007 (UTC)


 * The questioner didn't say "superheated", but "super heated". Slang. very; extremely or excessively: super classy; a super large portion of food. —Keenan Pepper 10:19, 26 July 2007 (UTC)


 * You just need a thick enough piece of metal to conduct all the current safely. The current could be about 1000 amps, so a one inch diameter copper cable should be able to do the job.  However where the lightning strike goes from air to copper it will be plasma temperture and will cause excessive heating at that spot. GB 21:56, 26 July 2007 (UTC)


 * Yeah, it's mostly just a matter of having enough metal in a lump. Big buildings get struck by lightning all the time without damage, as you see here and here and here.  If the metal main structure got super-hot, it would set something on fire.  --Anonymous, July 27, 00:34 (UTC).


 * Superconductors have no resistance, and thus the lightning wouldn't heat them at all, but superconductors have to be so cold that the air would heat it a lot. If you tried to insulate it, the insulation would heat up from the lightning and then heat up the superconductor. — Daniel 00:35, 28 July 2007 (UTC)