Wikipedia:Reference desk/Archives/Science/2009 March 3

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March 3

do indoor roses need a night phase?

I notice my indoor miniature rose has a tendency to bloom during my sleep (while the lights are dimmer) and sprout new leaves yet remain static while I'm awake. (Maybe this is a watching the kettle boil thing.) It occurred to me that I don't know what type of photosynthesis a rose would carry out. I suspect C3 (why it needs so much water) -- but if not, does it need a minimum amount of dark per day in order to optimise its growth? I'm not sure if it would schedule some growing phases during the dark.

I know indoor roses often suffer from lack of light. But provided adequate water and fresh, cool air, is it possible to give rose too much light (I'm using purely fluorescent light). The new leaves are remaining yellower longer than expected. I'm growing this in a pot in my dorm until it warms up -- I've basically it elevated the plant until the topmost leaves are barely an inch away from the in-built desk fluorescent light, and I've got another study lamp (fluorescent) with a flexible stem shining on it at the side just centimetres away from the leaves. John Riemann Soong (talk) 03:24, 3 March 2009 (UTC)

Sorry, I don't know a lot about plants and next to nothing about roses. Fluorescent lamps come in many different types and generate various spectra. Plants usually need special grow lamp bulbs for optimum growth. Putting your pot near a window might get you better results. 76.97.245.5 (talk) 09:48, 3 March 2009 (UTC)

Three thoughts: [1] could the yellow leaves be a sign of overwatering? [2] indoor roses have a tendency to not do very well after the initial blooms, unless you let them go through a period of dormancy somewhere cool (like a garage). Are you doing this? [3] Roses require 5 hours of direct sunlight per day. I don't know how many hours of grow-light that translates into. - Nunh-huh 10:13, 3 March 2009 (UTC)

How bad are shrooms for you?

Really...how bad are they for you? I have tried doing research, but I've discovered NOTHING. I can't find anything! Everywhere I go they just talk about the immediate, post-consumption effects of shrooms. Nothing on effects on cognitive ability, memory, recognition of simple shapes/colors..etc. I haven't been able to find a source that describes any sort of long term health issues with chronic mushroom use. This has given me the impression that they aren't bad for you. Am I right? --71.117.36.86 (talk) 03:35, 3 March 2009 (UTC)

Thanks for posting your question. Your question seems to be a request for medical advice. It is against our guidelines to provide medical advice. You might like to re-phrase your question. You may also find it helpful to read the article: Psilocybin, and form your own opinion from the information there. Specific advice below pertaining to the consumption of hallucinogens has been removed. Note to editors: Any response containing prescriptive information or medical advice will be removed. Please ensure your responses are constructed so as not to be construed as medical advice! If you remove a response, please paste it and your reason for removal on the Reference Desk Discussion page If you feel a response you have given has been removed in error, please discuss it at the Reference Desk Discussion page before restoring it.

. Mattopaedia ^{Have a yarn} 06:06, 3 March 2009 (UTC)

Psilocybin#Toxicity is one relevant link. Friday (talk) 03:37, 3 March 2009 (UTC)

No, if you find long-term studies concluding it's safe, then it's safe. If you can't find any such studies because none have been done, then the best answer would be 'We don't know'. Given the track record of the vast majority of hallucinogens I sure wouldn't default towards 'safe'. --Pykk (talk) 03:53, 3 March 2009 (UTC)

There's no clear physiological damage of the kind that you might see with alcohol or cocaine, or with any kind of smoking. On the other hand, changes in cognitive abilities might be difficult to recognize and detect - I don't think anyone's done a controlled study. It seems that it would be difficult to organize one. -GTBacchus^(talk) 04:18, 3 March 2009 (UTC)

Response removed. See discussion page

Please note that this is dangerously close to a medical question, if not over the border. arimareiji (talk) 05:03, 3 March 2009 (UTC)

There are case reports in the scientific literature of Hallucinogen persisting perception disorder as a consequence of psilocybin abuse. HPPD is listed in the Diagnostic and Statistical Manual of Mental Disorders. See for example, PMID 15963699. Rockpocket 07:03, 3 March 2009 (UTC)

Erowid is a good souce for this kind of thing. This is their main mushroom page. Pfly (talk) 09:06, 3 March 2009 (UTC)

That is one big template! --Tango (talk) 13:51, 3 March 2009 (UTC)

To our OP: Absence of information most certainly DOES NOT indicate that there are no problems. There is probably no good online information on the consequences of hitting yourself on the head with the pointy and of an icepick either...but it's not generally recommended. SteveBaker (talk) 15:46, 3 March 2009 (UTC)

Thank you guys for your responses so far, I've got some interesting reading to do. Firstly, I was most certainly not requesting medical advice. I have never experimented with shrooms, but rather I inquiring about studies or general information regarding and demonstrating the long term effects of mushroom use. And yes, you guys who said that just because there is no definitive information about the negative effects of something doesn't mean it isn't harmful are absolutely right...I just found it unusual that I could not find much info on how horrible they are for you, considering how (esp. the government) publishes and propagandizes studies showing how terrible illicit drugs are for the body. I just figured if they didn't go on a huge crusade about the evils of mushrooms, then they might not actually be THAT bad for you. Certainly there is the more obvious reason of no credible or viable studies being undertaken..and @Steve--Just because there is an absence of information (and the lack of any information regarding negative long term effects) does not indicate that there are problems. Obviously, one can see how hitting oneself in the head with a heavy, metal object may be detrimental to one's health. Drugs, on the other hand, are not so easily categorized as some are beneficial and have no negative side effects in moderate doses. --71.117.38.74 (talk) 01:27, 4 March 2009 (UTC)

Yeah - well, everyone who I ever knew who messed with drugs in the 1970's and onwards either ended up in a very bad way for one reason or another (mostly because they escalated to worse and worse things) - or they got out of it eventually - but still come over as slow, dull intellects who have lost that sparkle they had when I knew them beforehand in college. When I go to college reunions - you can instantly tell who was in which crowd. The people who didn't touch the stuff are much more interesting to work with and talk to - and 99% of the time, they are earning more money for less work and are VASTLY happier in later life. That's anecdotal evidence - but it's over a fairly wide sample from the tail end of the hippie era when that stuff was pretty rampant. It's pretty clear that the effects these things have on your brain are the biochemical equivalent of that ice-pick. Every time you do it you get more stupid and you notice it less and less - it's amazing to me that every time a pro-drug-legalization person comes on TV or the radio, they come over as amazingly slow and stupid...but they don't seem to realize it. But do it over enough years - you'll wind up dull and stupid too - so perhaps you won't care anymore. My brain matters to me - it's where I spend most of my time! I intend to look after it and not hit it with random concoctions and just hope that because I couldn't find any mention of problems, that there are no problems. Modern medicine can do a lot to fix up abuse of your body - but to date there is hardly anything they can do if you screw with your brain. SteveBaker (talk) 04:22, 4 March 2009 (UTC)

Steve, you misunderstood the point of my question. It was in no way intended to push a pro-drug agenda or try to rationalize the use of mushrooms by myself. I don't plan on getting mixed up with illicit substances or abusing them. I was simply curious, however odd that may be, and was playing devil's advocate earlier (in regards to the fact that there is no evidence that it is very harmful over long term use). I don't doubt for a second though that any form of drug abuse will not have adverse effects if used long enough or in high enough doses. I appreciate the impassioned censure of abusing illegal drugs...largely in part because it's common knowledge that 99% of those substances will mess up your brain and its neurochemistry! Which leads me to why I originally asked the question in the first place...cocaine, marijuana, ecstasy, heroin, meth, etc., all have terrible long term side effects that are quite noticeable and dangerous, while mushrooms, on the other hand, apparently have no known long term use side effects which I thought was unusual. Hence my curiosity. So please do not misinterpret this as I was saying earlier as a rationalization for drug abuse- it's not. All it is, is simple curiosity. --71.117.38.74 (talk) 05:58, 4 March 2009 (UTC)

Steve, although I have much respect for you, I think your anecdotes aren't necessary here. Many people turn "slow and dull" after college. Correlation-causation problems also arise in that people who take recreational drugs may be predisposed to, and may also be predisposed to being slow and dull people (risk-seeking people are quite biochemically different to risk-averse people). Also, in order to get hold of recreational drugs, people will often start to hang around with those they can get them from (dealers). In order to befriend a dealer (usually someone lacking success in life and of a low intellect), they must start to behave differently, to be accepted in that group (peer pressure). Eventually people adapt to that lifestyle, a sort of "being dumb is cool" attitude, and the self-fulfilling prophecy comes true (acting dumb turns into being dumb). As for 5-HT_2A agonists being "biochemical icepicks", well, I don't quite understand what you are trying to say. They cause lesions? Or they just destroy one's intellect? I have found a review study (free, full text) which should be of much interest: "Role of the Serotonin 5-HT_2A Receptor in Learning". [1] To quote the abstract:

Agonists at the 5-HT2A receptor including LSD (d-lysergic acid diethylamide) enhanced associative learning at doses that produce cognitive effects in humans.

I'm not saying that we should take psychedelics whilst reading through a ton of papers, of course they produce many other effects. What I am saying is that there is no solid evidence that consuming a psychedelic drug will cause you to be "dumb". Unless Albert Hofmann, Alexander Shulgin and probably thousands of other scientists are dumb as well. --Mark PEA (talk) 13:34, 4 March 2009 (UTC)

I'm not sure I would characterize Carl Sagan as being stupid or dull-witted. Matt Deres (talk) 14:48, 4 March 2009 (UTC)

Another of the reasons I suspect there aren't very many studies of long-term shroom use is that, in my limited experience, it's not a drug that is regularly used over an extended period. While there are certainly some people that are sufficiently enamoured with it that they do it regularly, everyone I know that has taken shrooms has done it as a very occasional thing: they seem to be used far more for occasional fun trips rather than as a daily/weekly habit. It may well be that for most people the effect, while generally pleasant, is not something you actually want to do that often - I've heard the same about LSD. This is in contrast to drugs like marijuana, where studies of the long-term effects are most important as it is often used regularly (in some cases multiple times daily) over a long time. ~ mazca ^t|c 08:54, 4 March 2009 (UTC)

To OP, the original question is quite ambiguous. Asking if water is safe can be ambiguous (hyponatraemia). Due to ethical and legal reasons, psychedelic drugs are very poorly researched in humans (and you can't get solid evidence without placebo-controlled studies). As far as I'm aware, there are no "psychedelic models" in animals either. We can see if their 5-HT2a receptors are up/downregulated, but that is about it. As already mentioned, HPPD is a reported problem in some users. There is also some anecdotal evidence of "flashbacks", and a "bad trip" may lead to some form of PTSD. As for the government, don't they normally say that psilocybes are dangerous because they can change your behaviour and make you jump out of a window? (as for ketamine, you might walk into a road, not that anyone under the influence of ethanol has ever done that). --Mark PEA (talk) 13:34, 4 March 2009 (UTC)

Yeah, that dihydrogen monoxide is dangerous stuff. Inhaling a small quantity can kill. Axl ¤ [Talk] 18:06, 4 March 2009 (UTC)

LOL. Thank you for your comments everyone, especially Mark. --71.98.15.188 (talk) 00:54, 5 March 2009 (UTC)

I don't think chronic consumption of magic shroom is widespread and that would make another reason (beyond ethical and legal reasons) why you can't find much of a study out there regarding long term health effects. Besides, health issues related to psychedelic drugs being largely psychological in nature, a study about drugs with similar effects such as LSD or mescaline would likely be just as relevant. Equendil Talk 17:25, 5 March 2009 (UTC)

Are chard and beets the same plant? "beta vulgaris"

If so, is there a link to the USDA or a Dept. of Agriculture website that states this so I can be sure?Troyster87 (talk) 05:00, 3 March 2009 (UTC)

I think this site provides what you want - the family tree of chard, showing it's a member of the beet family. Our page on Chard essentially states this, but provides no references (eww!). --Scray (talk) 06:02, 3 March 2009 (UTC)

This link from our beet article provides very thorough scientific/taxonomic names and multi-lingual common names. It is from The University of Melbourne (article), may also be reliable for academic/citation purposes. Note that beets are classified into a large number of subspecies. "Swiss chard" is a type of beet, but it may be a different breed or cultivar, depending on your classification scheme. As this topic comes up often, classifying things as the "same species" is entirely a matter of your taxonomy preference. Nimur (talk) 07:08, 3 March 2009 (UTC)

When it comes to plant relatedness, agriculture has often created widely different cultivars or breeds of plants which are essentially genetically the same "species", but have been bred to force certain traits to express themselves in an "unnatural" way, i.e. in ways that they could never do in the wild. There are wildly different members of the cabbage family, for example Kale. Brussel Sprouts, and Broccoli which are all essentially descend from the same wild "proto-cabbage". See also the relationship between Teosinte and Maize. --Jayron32.talk.contribs 13:07, 3 March 2009 (UTC)

To make a analogy to something people may be a little more familiar with, a chihuahua, a poodle, a St. Bernard, a German Shepherd(Alsatian), and a wolf are all the same animal/species (a "dog"), but are vary different in their characteristics and their use due to selective breeding. -- 128.104.112.117 (talk) 19:06, 3 March 2009 (UTC)

okay so my question is a little more specific than perceived, i would like to know if beats and chard can be harvested from the same plant, i.e. i plant some beta vulgaris seeds, could i eat the root (beet) and the leaves (chard) and if so would they be the same i find in the supermarket or would they be the leaves that grow on beets that are not commonly eaten and the chard root that is not commonly eaten?

In a word, no. The plants grown for chard and the plants grown for beetroots, while the same species, are different varieties (see Beet#Taxonomy). That said, you can probably use the tops of garden beets as a leafy vegetable. Doing an internet search on "beet greens" reveals a lot of hits. (Although I can't be sure if they're all talking about the leaves of garden beets, or might instead be referring to the spinach beet, which is grown especially for its leaves.) -- 128.104.112.117 (talk) 00:37, 5 March 2009 (UTC)

You can eat both the root and leaves of the turnip. It might have a different name in American English - I'm not really sure what a "beet" is, I assume it is not a beetroot. 78.149.172.237 (talk) 12:20, 6 March 2009 (UTC)

sub : Quantum mechanics

The wave funtion corresponding to np hydrogenic orbital are imaginary the howI conceive of np orbitals in reality.Supriyochowdhury (talk) 06:06, 3 March 2009 (UTC)
I have edited this into a format that I believe was intended Mattopaedia ^{Have a yarn} 06:21, 3 March 2009 (UTC)

It's not clear exactly what your asking here, but perhaps you could first look at our article on quantum mechanics then direct any further questions here. Mattopaedia ^{Have a yarn} 06:21, 3 March 2009 (UTC)

If I understand S's question, the square of the wave function gives the probability of an electron "being" at a specified position at a specified time. You should try reading that article; it also has some nifty orbital diagrams. Clarityfiend (talk) 07:48, 3 March 2009 (UTC)

While the wave equations may be imaginary, certain sums and differences of wave equations can also valid solutions (a general property of solutions to differential equations). The imaginary terms can be made to cancel out, allowing three completely "real" p orbitals to exist on any energy level. Someguy1221 (talk) 08:36, 3 March 2009 (UTC)

More specifically, the square of an imaginary number is always real, which is why Ψ² is the relevent function here, not Ψ. --Jayron32.talk.contribs 12:59, 3 March 2009 (UTC)

It's not the square, it's the square of the absolute value. This is always a positive real number, as expected of probabilities. Algebraist 13:04, 3 March 2009 (UTC)

Is Ψ imaginary? I thought it was complex (ie. can contain both real and imaginary parts). While the square of a strictly imaginary number is always real (a negative real number, in fact), the square of a complex number can be absolutely anything. That's why, as Algebraist says, you take the the square of the absolute value, or equivalently the product of Ψ and its complex conjugate. --Tango (talk) 13:49, 3 March 2009 (UTC)

Tango is correct. Ψ is properly treated as a complex value, though for some special circumstances it only has a real or an imaginary component. For probability calculations, we use the square of the absolute value: |ψ|². Conveniently, one can calculate this value by multiplying ψ by its complex conjugate, often denoted ψ^*. So |ψ|² = ψ^*ψ. TenOfAllTrades(talk) 14:30, 3 March 2009 (UTC)

I do not understand Faster-Than-Light( physics, relativity) article

B and E fields travel faster than light but energy travels at the speed of light? How is it possible? How we know that magnetic and electric fields travel faster? By Maxwell's equations? Is the photon speed constant and it is 3*10^8 m/s? —Preceding unsigned comment added by Logicman112 (talk • contribs) 12:26, 3 March 2009 (UTC)

B and E fields propagate at the speed of light. What made you think they go faster? Dragons flight (talk) 12:28, 3 March 2009 (UTC)

Is there some confusion involving group velocity and phase velocity? --Tango (talk) 13:40, 3 March 2009 (UTC)

What article are you reading? I can't find B and E fields mentioned in Faster-than-light, although it does cover phase and group velocities which, as Tango suggests, might be the root of your problem. SpinningSpark 17:57, 3 March 2009 (UTC)

Could it be that OP fell into the "in a vacuum" trap? Light propagates through different media at different speeds. It is possible to exceed the speed of light in a given medium. (See Čerenkov radiation.) It's one of my pet peeves that last bit of the saying usually gets left out. 76.97.245.5 (talk) 21:26, 3 March 2009 (UTC)

This part of physics is really a horrible mess because of the choices people have made for the names of things.

• The speed of light IN A VACUUM ('c') is unchanging...and it's the cosmic speed limit beyond which energy, mass and information cannot travel.
• Photons always move at 'c' - even when they are travelling through (for example) water.
• The speed of light in a material like water (which is about 75% of 'c') is not the speed that the photons travel - for they HAVE to travel at 'c', no matter what. But if you take a flashlight and a large tank of water with a mirror at the other end - turn on the flashlight and start your stopwatch, then when you see the light bouncing back at you through the water - you'll notice that it took longer to do it than if the light was moving at 'c'.
• This ought to be obvious because from the perspective of a photon, most of what's in a glass of water is vacuum...the spaces between the molecules and between electron clouds and nucleus is all big open space as far as the photon is concerned.

The reason that the photons can be moving at 'c' but your light beam is only moving at 75% of 'c' is because the photons aren't able to make a simple straight-line trip through the water. They are being bounced around by the water molecules - absorbed and re-emitted and all sorts of other exciting things - which means it takes them longer to get from A to B - even through they are going at 'c' all the time. Personally - I thing physicist should stick with 'c' as "The Speed of Light" and explain that the water merely makes it travel further some factor due to all of these interactions. Talking about "The Speed of Light in Water" is a stupid and meaningless confusion.

So: that said - if the B and E fields carry energy, mass or information - they can't go faster than 'c' - but they may possibly be able to take a shorter route through water than a photon of visible light can - and thereby get there first. That's not the same thing as "Travelling faster than 'c'" - it's only "taking a short cut" - which is much less exciting.

SteveBaker (talk) 23:40, 3 March 2009 (UTC)

SteveBaker, the reason physicists don't stick with 'c' as "The Speed of Light" and then explain things the way you explained is because your explanation is wrong. If it was true that the bouncing of photons off other particles inside the material were slowing them down, you would get light scattering as well as dispertion (even for photons with identical frequencies). In fact that happens (see Rayleigh scattering), but that is another matter and is not the cause of the photon slowing down. Dauto (talk) 01:40, 4 March 2009 (UTC)

You aren't telling me that the photon slows down are you? Relativity applies equally in water and in vacuum - and the 'c' it uses is the speed of light in a vacuum - not the speed of light in water. Photons can't literally slow down because in order to have non-infinite mass when travelling at lightspeed in a vacuum - they would have to have zero mass when they slowed down in water...and that doesn't make sense. So while the photon is in the water - it has to be travelling (over short distances) at 'c' - and to be interfered with in some manner in order for it to take longer to travel over a given distance.

Our article: Čerenkov radiation says: "It is important to note, however, that the speed at which the photons travel is always the same. That is, the speed of light, commonly designated as c, does not change. The light appears to travel more slowly while traversing a medium due to the frequent interactions of the photons with matter. This is similar to a train that, while moving, travels at a constant velocity. If such a train were to travel on a set of tracks with many stops it would appear to be moving more slowly overall; i.e., have a lower average velocity, despite having a constant higher velocity while moving.".

In Speed of light we have: "In passing through materials, the observed speed of light differs from c. When light enters materials its energy is absorbed. In the case of transparent materials (dielectrics) this energy is quickly re-radiated. However, this absorption and re-radiation introduces a delay. As light propagates through dielectric material it undergoes continuous absorption and re-radiation. Therefore when the speed of light in a medium is said to be less than c, this should be read as the speed of energy propagation at the macroscopic level. At the microscopic level electromagnetic waves always travel at c."

Which is what I endeavored to convey...perhaps badly! SteveBaker (talk) 04:06, 4 March 2009 (UTC)

I find the second quotation to be misleading the point of being partially wrong. The delayed speed of light can be understood entirely classically (one doesn't need to introduce photons at all). The propagation of an electromagnetic wave into a dielectric excites a polarization wave in the medium of opposite sign (i.e. locally charges move to partially cancel the external excitation). In the continuum approximation of matter (useful since EM waves are useful much larger than atoms), the superposition of the excitation wave and polarization wave is a new wavefront of the same frequency but slower velocity. The net result is that the electromagnetic wave, expressed as changes in the E and B field, is honestly propagating at less than the speed of light and one doesn't need to invoke any silliness about macro vs. micro waves to get that. At a quantum level the wave is generated by (usually) a very large number of photons. The excitation of the polarization wave is caused by photons being absorbed by the medium. As the polarization wave collapses, new photons are coherently re-emitted in the same direction of travel, after some effective retardation in time. So, yes, the net flow of photons is delayed by interacting with the medium, but the physics of wave propagation in dielectric medium and the consequent delay is better understood by the reaction that retards the purely classical evolution of the wavefront. Dragons flight (talk) 09:46, 4 March 2009 (UTC)

SteveBaker, both your quotations from the Čerenkov radiation and Speed of light articles are rubbish. If I were in a particularly generous mood, I would say that the first one isn't entirely wrong, but needs clarification. I don't think I'm feeling that generous though. The second one really is wrong. Dragons flight already explained what's really going on. In short: Light in vacuum consists of oscillations on the electromagnetic field. Light in a medium consists of oscillations of the electromagnetic field plus oscillations of the charged constituents of the medium (mainly electrons). A different kind of animal entirely. Dauto (talk) 14:25, 4 March 2009 (UTC)

For what my opinion is worth, I don't think SteveBaker (or his quotations) said anything incorrect. It is worth bearing in mind that although you can consider wave propogation in a dielectric as a classical phenomenon, we understand that the world acutally behaves in a quantum manner. Any such classical model must be acknowledged as simply a model which is ony valid if it correctly describes the limiting quantum behaviour. So it's not silly to look at the interactions on a micro- level just because one can find a model which gives the same answers as the quantum case. I agree that the slowing of electromagnetic waves can be 'explained' by the classical continuum model but really we should try to understand this at a quantum level where all quanta of the electromagnetic field necessarily travel at c.

Dragons flight, It seems to me that (ignoring any classical model) your description actually agrees with the second quotation. You say that there are a large number of photons entering the material, this causes some kind of excitation of the material due to photon absorption which in turn leads to re-emitted photons after some delay. Could you elucidate your disagreement? Vespertine1215 (talk) 10:45, 5 March 2009 (UTC)

Vespertine, The interpretation of the slowing down of electromagnetic radiation as due to light being absorbed and re-radiated or following a zigzag path due to bouncing off other particles is not correct even within quantum mechanics. Dauto (talk) 14:52, 6 March 2009 (UTC)

Dauto, can you expand on that then? What do you claim is happening on the quantum level? As I said above, I don't see the difference between what the speed of light article says and Dragons flight's explanation which you seemed to agree with. Surely the only way an incident field can interact with matter is by absorbing/scattering photons. What process is occuring for it to interact in another way? Vespertine1215 (talk) 16:12, 6 March 2009 (UTC)

The problem with the idea that light slows down inside a medium due to constant bouncing off of electrons and being absorbed and re-emitted is the fact that it gives the wrong idea that light follows a zigzag line (it doesn't) or that the photon disappears part of the time and reappears latter. That would be a more apt description for what happens inside the sun, for instance. Photons produced at sun's core are constantly absorbed and re-emitted. Those photons take tens of thousands of years to find their way out of the sun (That's some serious slowing down). They follow a zigzag path and thermalize on the way out. photons inside a (perfectly) transparent medium don't thermalize. So why are photons inside a medium slower then c? Those photons are more complex then free photons. The charged particles inside the medium also oscillate along with the electromagnetic field forming what sometimes is called a quasi-particle but which in my opinion is better described as a dressed photon. In short, the charged particle oscillation is an integral part of the photon itself. Dauto (talk) 02:07, 7 March 2009 (UTC)

Ok, I think I see your point. We can't really think of our system (which is composed of coupled fields representing the matter excitations (e.g phonons) and the electromagnetic field) as simply some photons moving between interaction events inside the matter. A proper treatment of the excitations of this system leads us to introduce some group excitations (the dressed photons) which don't have the same properties as the free photons, specifically the possibility of non-zero mass. (If this is still faintly on topic for the OP, there is a little more about this in Photons#Photons_in_matter and Polariton). Thanks for your explanation. Vespertine1215 (talk) 15:47, 7 March 2009 (UTC)

Unpleasant screeching noises

Why is it that humans find it difficult to stand certain screeching noises like the one produced on scratching one's nails against a granite board, or steal, or even rubbing two metals against each other. What is the scientific and evolutionary reason for humans (like me) find such noises so unpleasant and anxiety producing? Thanks. --ReluctantPhilosopher (talk) 14:08, 3 March 2009 (UTC)

I'd say it's because the sounds resemble screams people (and before them, other animals) gave out as a warning of danger, causing us to flee immediately. Those who ran immediately when hearing such sounds are more likely to have survived and passed on their genes. StuRat (talk) 14:47, 3 March 2009 (UTC)

Equal loudness curve, with "perceived loudness" plotted vs. frequency. Nimur (talk) 15:33, 3 March 2009 (UTC)

Also, you may find phon interesting. A human's sensitivity to loudness is not "flat" - certain tones which contain the same total sound energy (or same sound pressure level) seem louder or quieter depending on their frequency. (This should be no surprise to any engineer who's worked with complex frequency-responses - we see poles and nulls of frequency response in all kinds of mechanical and electronic systems). Anyway, it's possible that your ear/brain audio perception pathway is "amplifying" the specific frequency of the screeching noise to a much higher level than equivalent non-screech-sounds. Nimur (talk) 15:31, 3 March 2009 (UTC)

The question you are asking has been researched rather extensively. The research culminated in the 2006 IgNobel Prize being awarded to people researching Sound of fingernails scraping chalkboard. The latter article lists a few ideas as to why the said sound is so disagreeable. --Dr Dima (talk) 18:44, 3 March 2009 (UTC)

You could try an experiment - play Chris Crocker vids at extremely high volume and ask people at the limit of hearing range to describe what they think it sounds like. If you get a lot of "Sounds like an animal being eaten alive," or "Sounds like the screech of a predator attacking," etc, you have an answer. Personally, I'd go with StuRat's answer for its simplicity, with Nimur's answer providing an excellent backdrop for a perception mechanism to amplify it. arimareiji (talk) 15:06, 6 March 2009 (UTC)

dual exhaust and fuel efficiency

My buddy and I are debating the merits of having dual exhaust. I have a 2000 F-150 with a single exhaust. Would having a dual exhaust improve the mileage? If so, what are the physics behind this? —Preceding unsigned comment added by 216.154.22.180 (talk) 14:40, 3 March 2009 (UTC)

I'd say probably not on mileage. Dual exhaust is normally used to make a car faster, not to improve mileage. By reducing back pressure, dual exhaust can allow for slightly more horsepower, but the added weight of the dual exhaust system will also reduce the mileage slightly. StuRat (talk) 14:43, 3 March 2009 (UTC)

No, actually, a less-restrictive exhaust system tends to improve engine efficiency by dint of reduced pumping losses. Depending on how restrictive the baseline (single) system was, the gain may be small or large. If the driver operates the vehicle so as to produce the same performance level as before, then fuel economy will improve on account of the increased efficiency. If the driver routinely takes advantage of the increased efficiency to accelerate faster, then fuel economy will not improve. Unless the original system is extremely restrictive — generally not the case — the efficiency gains are rather trivial at low, normal engine speeds. On anything but a vehicle specifically designed for minimal weight, and especially on a very heavy vehicle like the pickup truck under discussion, the weight of a dual vs. single exhaust system is insignificant with respect to the vehicle's overall weight and will therefore not materially affect fuel economy. It can be difficult and costly to retrofit a dual exhaust system while remaining in compliance with applicable emissions regulations, unless the vehicle was originally offered by the manufacturer with dual exhaust, in which case it is merely costly — the expense of doing so must be figured into the reckoning of whether it makes financial sense to proceed. Unfortunately, our exhaust system article is presently of low quality and in need of much work. —Scheinwerfermann ^T·_C15:07, 3 March 2009 (UTC)

Thanks. So, technically: we both win . . . and we both lose! Being a gentleman, I'll buy the beer as a gesture of good sportsmanship! —Preceding unsigned comment added by 216.154.22.180 (talk) 15:32, 3 March 2009 (UTC)

Maybe. The amount of 'back pressure' exerted on the engine is a rather important tuning parameter - and changing it has implications for engine power - which may or may not require it to burn more fuel. On a turbocharged car - where the exhaust is fed through the turbocharger's turbine to spin it up and allow better air compression, adding a more freely flowing exhaust will generally give you a few more horsepower because it allows more pressure to build up between engine and turbo. This improved efficiency might cut gas consumption - but if you actually DEMAND that extra horsepower, you'll burn more fuel doing it. On a normally aspirated engine (no turbo) you might not improve things at all - and you could make matters worse. But going from a single to a dual exhaust on most cars is nothing more than a decorative feature - since most of them feed all of the exhaust gasses through a single catalytic converter and silencer (aka muffler) before splitting the exhaust in two. In such cases, the effect is most likely to be zero - no matter what technology is used in the engine - because the limiting factor on flow rate is the catalytic converter and muffler. Doubling up on the catalytic converter and muffer gets around that - but it does add weight - and that might adversely affect your fuel consumption if the original single pipe was already perfectly adequate. SteveBaker (talk) 15:41, 3 March 2009 (UTC)

Angular momentum

http://img183.imageshack.us/img183/887/habitat.png
Say you constructed a Stanford torus, spun it, and climbed up to the exact center. You're just spinning in place, so if you let go your angular momentum is conserved and you continue spinning about your navel and stay perfectly still relative to the supports you were just holding. The article says you're probably spinning about one revolution per minute. But say you were down on the habitat level, facing forward, and you fell through a trap door out into space. Yes you fly off tangentially but do you additionally spin once per minute head over heels? Or do you spin a fraction of that depending on how far you were from the center? Or do you not spin at all? If the last case, where does angular momentum cease to be conserved? Is it when your axis of rotation is no longer inside your body and becomes kind of an axis of revolution?

http://img410.imageshack.us/img410/113/earth.png
Also, say you had a device, gyroscopically stabilized against any change in rotation from outside wind forces. It's on the north pole and it's lifted up and dropped. During the short free-fall the Earth didn't rotate below it because it spun at the same speed. But what if the device is on the equator, tipped over horizontally? It is spinning one rotation per day relative to the stars, same as on the north pole, as you make it around the globe, but is this really the type of rotation that's conserved or just a side effect of the centripetal force? Will the cylindrical device shown over the equator land on a different number than was pointing down when it was picked up? .froth. (talk) 19:23, 3 March 2009 (UTC)

When talking about angular momentum, you have to specify the point you are taking as the centre. In this case, that's the centre of the torus. When you move off tangentially, your angle to that centre will still be changing, since the point isn't on your line of travel. That means you still have angular momentum and, if there aren't any external influences (like the Earth's gravity) that angular momentum will remain constant. --Tango (talk) 19:52, 3 March 2009 (UTC)

Interesting.. but that angle-changing isn't constant. The rate of change of the angle is ${\frac {30\Pi }{t^{2}+900}}$ radians/second at t seconds, given the figures in the article. It starts out at the point of release to be equal to the rate of angle change of the wheel, but then rapidly decreases. In fact, the angle relative to the point of release will never be greater than 90 degrees. So how can you say that you still have the same angular momentum, when before it was whipping around at a steady π/30 radians per second? .froth. (talk) 20:32, 3 March 2009 (UTC)

With respect to your original question, the angular speed $\omega$ will indeed be the same, no matter where you are inside the space torus. With respect to your follow up question, The angular momentum $L\,$ will be conserved and from the formula $L=I\omega \,$ you can se that if your moment of inertial with respect to the center of coordinates $I=mr^{2}\,$ varies, so will your angular velocity around that center $\omega ={\frac {d\theta }{dt}}$, $r\,$ is the distance to the center of coordinates. As you pointed out, once you "fall" off the torus, $r\,$ will increase over time and $d\theta ={\frac {L}{I}}dt={\frac {L}{mr^{2}}}dt$ will decrease over time (for fixed $dt\,$). Do not confuse the angular velocity around the center (which is in fact decreasing over time) with the angular velocity that the poor fellow will have around himself (rotation). That last one will be constant. Dauto (talk) 21:47, 3 March 2009 (UTC)

But the angular velocity he will have about himself (rotation).. if he falls out of the bottom, will he have the same angular velocity about himself as the fellow standing at the hub of the wheel? I think you're saying No but it's hard to decipher .froth. (talk) 21:54, 3 March 2009 (UTC)

He may be spinning around his own centre of gravity, he may not, it's irrelevant. It will depend on exacting how he leaves the station - if he pushes off with just one arm, that would provide a torque that sets him spinning. That won't affect his angular momentum around the centre of the station, though. --Tango (talk) 22:15, 3 March 2009 (UTC)

Tango is right that any torque at the moment he leaves the station will afect his angular velocity around himself. But assuming that there was no torque, it is safe to say that his angular velocity around himself will be the same as the fellow standing at the hub. Dauto (talk) 22:22, 3 March 2009 (UTC)

I don't think that's true. The angular velocity of someone at the hub around their centre of gravity is the same as that around the centre of the station, since that is where their centre of gravity is. Assuming there is no ground underneath them and they just fall away from the centre unhindered then their angular momentum around that centre will remain constant, their angular momentum around their centre of gravity will not since their centre of gravity is moving (obviously, I'm working in an inertial, non-rotating reference frame, if you do it in terms of the person's inherent reference frame you will get different results - you'll have centrifugal force and things to contend with). I can't see anything that would set the person spinning. --Tango (talk) 22:46, 3 March 2009 (UTC)

His angular momentum relative to the hub will be unchanged from what it was just before he left the station. It will be much larger than that of the dude at the hub, because he was further from the hub to begin with. He may or may not be rotating about his navel depending on how cleanly he left the station, but that's irrelevant when it comes to computing his angular momentum relative to the hub.

A curious thing about angular momentum is that you can take an object undergoing linear motion, select a point, and compute the angular momentum of the object relative to the point. And as the object moves along, the angular momentum will remain constant. --Carnildo (talk) 01:52, 4 March 2009 (UTC)

For the first part of your question: If the floor of the space station beneath your feet simply vanishes - you'll proceed in a straight line at a tangent to your former circular motion - which will seem to you like you fell through the floor. You'll continue to rotate at 1 rpm about your center of mass. If you were to rotate about any other point, that would be a net acceleration - and there is no longer a force present to provide that. Rotational and translational inertia are both preserved and all is well with the universe...until your air supply runs out.

If that's hard to imagine - separate out the rotational motion from the translational. Firstly, while you're inside the space station, you are translating around in a circle, secondly you are spinning about your center of gravity at exactly the right rate to keep your feet on the floor and your head up by the ceiling - which is 1rpm. When you take away the floor - you take away the force that's making you translate in a circle - but your 1rpm rotation remains.

For the second part: This is really the same thing - the object is rotating about it's center of gravity at one rotation per day - and travelling around in a circle at one rotation per day. When your "device" (it's really a kind of dice) is tossed upwards (let's do the experiment someplace where there is no atmosphere), it'll continue to rotate at one revolution per day - so when it comes back down again, it'll land on the exact same number. The result is the same no matter where you are on the earth because you are rotating about an axis that's parallel to the axis of the earth,

SteveBaker (talk) 23:23, 3 March 2009 (UTC)

Tango, nothing sets the person spinning. He is already spinning to begin with, along with everything else inside the station. Dauto (talk) 01:14, 4 March 2009 (UTC)

Yes, I missed the fact that his feet always point away from the centre, which requires a rotation around his centre of mass in addition to the rotation around the hub. --Tango (talk) 12:31, 4 March 2009 (UTC)