Wikipedia:Reference desk/Archives/Science/2009 March 27

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

March 27

Simple Motor -- Won't Work -- Please Help

I'm trying to make a simple motor to help my younger sister better understand circuits. It is composed of copper/metal wire, a "D" battery, and several strong magnets, which was supposed to create an electrical currents strong enough to flip a copper wire hoop suspended between 2 paper clips. It's supposed to look something like this Video

The problem is my demo isn't working. Any help here? Is my battery too small? Not enough magnetic power? Please respond by 5:00am EST/-5 GMT. Thanks! Zidel333 (talk) 02:20, 27 March 2009 (UTC)

Is your wire insulated? Did you half-strip the wire as instructed? When you spin the copper loop manually, can you feel the tug of the magnets? Not sure people here can help you without more details of what you've done and what you've observed (especially with your tight schedule, which makes this sound so much like homework). --Scray (talk) 02:31, 27 March 2009 (UTC)

Not homework, I'm 20; it's more like we had a deadline by Friday because we have spent the past 6 days trying to figure out the *cursing* problem. I'll try your suggestions. Zidel333 (talk) 02:34, 27 March 2009 (UTC)

Hmmm - that's a kinda crappy way to make an electric motor - but I agree that the biggest source of error is that bit about 'half-stripping' one end of the wire. This makes an extremely crude 'commutator'. The deal is that if you just apply current through the wire, it'll spin through maybe a half turn and then stop because the magnetic force that pulled you around (say) clockwise - now wants to push you back anticlockwise. What you have to do is to cut the current (or better still, reverse it) for the second half of the rotation. This movie covers things a little better: http://www.youtube.com/watch?v=PSNgsluUfhc - more turns of the wire will be a big help, I don't think you need more battery power. SteveBaker (talk) 02:50, 27 March 2009 (UTC)

Steve, that video you linked was very helpful. I'm going to try to construct his version instead. I'll keep you updated if it worked or not. :) Zidel333 (talk) 02:56, 27 March 2009 (UTC)

How the insulation is stripped is critical. One end may be stripped in its entirety. The other end shuld be stripped such that when center axis of the coil (rotor) is perpendicular to the magnet (stator) as shown in the first video diagram, the circuit is closed, and when it is rotated 180 degrees it is open. For a bottom magnet design, you should hold the coil off the edge of the table so that it is vertical (not flat) when you remove the isulation. If you lay it flat and sand, the system will be 90 degrees out of phase and will not work. -- Tcncv (talk) 03:10, 27 March 2009 (UTC)

• • UPDATE** I did a version based on Steve's video-- it's working, but won't rotate fully, it kinda wobbles for a while. Any suggestions? Zidel333 (talk) 03:24, 27 March 2009 (UTC)

It should be balanced as best you can, the magnet should be strong and as close as possible, if you have magnets above and below that would help. If you use 2 magnets, they should have the N pole of one facing the S pole of the other above and below the rotating coil. Be especialy careful to follow Tcncv's instruction about which half of the insulation is stripped relative to the coil orientation. I have made these and seen them spin. Edison (talk) 03:52, 27 March 2009 (UTC)

You do have to start it spinning by hand - it needs a certain amount of momentum to keep it spinning when the insulated half of the half-stripped wire is in contact with the frame. But I can't over-emphasise the importance of following that "half-stripping" advice in the video to the letter! Your description of it just 'wobbling' just screams "You messed up the half-stripped-wire part"!! The electricity has to be disconnected from the coil for half of it's rotation...and the correct half with respect to the orientation of the coil - or what you'll get is...well...exactly what you are getting! This approach to making a commutator is really kinda flakey...I've seen better designs for home-built motors. SteveBaker (talk) 04:19, 27 March 2009 (UTC)

Radio source SHGb02+14a

I'm a little confused by this New Scientist article, which states that the 8-37 Hz/s drift of this radio source corresponds to a speed much greater than that of Earth's rotation. 8-37 Hz/s of drift in a 1420 MHz signal should correspond to a speed of (8/1420*10^6)*300 000 000=1.69 m/s, correct? Perhaps Hz/s should really be KHz/s? There's frustratingly little information on the Internet about this signal, considering it's the best candidate for an extraterrestrial transmission SETI@Home has produced.

Two more questions: (1) Is it possible for a terrestrial planet that rotates 40 times faster than Earth to form? (2) For how long did the signal last each time it was observed? I can't believe New Scientist doesn't give this information. --Bowlhover (talk) 03:40, 27 March 2009 (UTC)

I'm somewhat doubtful that the signal is still of great interest given that the article was published in 2004 and it's now 2009. Also see Radio source SHGb02+14a. In terms of the New Scientist, while I read it regularly and find it of interest, they are sometimes guilty of being a bit sloppy in the articles and also of being sensationalistic or publishing minority viewpoints or ideas without making it clear they are such. Nil Einne (talk) 05:55, 27 March 2009 (UTC)

Check the units. (8Hz/s)/(1420*10^6Hz)*(300 000 000m/s)=1.7m/s^2. That's an acceleration, not a speed. That's two orders of magnitude higher then earths centripetal accelerations at equator. Dauto (talk) 06:46, 27 March 2009 (UTC)

Oops, that was a pretty silly mistake. This raises a new question, though: would radio transmissions from Earth really drift at 1.5 Hz/s as the article claims? That would imply an acceleration of 0.32 m/s^2, still much greater than the actual equatorial centripetal acceleration of 0.034 m/s^2.

Edison: You're right that the signal may not be of great interest among the general public, but I've been an amateur astronomer and SETI@Home participant for years, so I'd like to find more information on this signal candidate. Also, thanks for the tip about New Scientist. I'll remember to check other sources if it makes far-fetched claims. --Bowlhover (talk) 07:04, 27 March 2009 (UTC)

ecg explanation

Possible medical advice question

my question is - what does accelerated AV conduction with pre-excitation mean, in a way that I can understand as i am not in the medical field and is this condition serious —Preceding unsigned comment added by 124.181.75.11 (talk) 09:37, 27 March 2009 (UTC)

This question appears to be a request for medical advice, and this issue is being discussed here. It is best to discuss medical conditions with a trained and licensed professional. --Scray (talk) 11:39, 27 March 2009 (UTC)

You may find the these articles provide helpful background information to discuss the matter with your doctor: pre-excitation syndrome, electrical conduction system of the heart. – 74  16:21, 27 March 2009 (UTC)

Historical composition of shaving creams?

The rather short Wiki article on shaving cream only mentions one composition from ancient Sumer. I was watching the smoldering Seth Bullock lather up on Deadwood last night, set in and around the 1880's, and I got to wondering what *exactly* he was lathering up with... figuring that it was unlikely he was even using whatever the period standard material was, given the remoteness of Deadwood and the corresponding high cost for city luxuries...

So, shaving cream through the ages... what say you? —Preceding unsigned comment added by 61.189.63.137 (talk) 09:42, 27 March 2009 (UTC)

Soap? --TammyMoet (talk) 09:50, 27 March 2009 (UTC)

Specifically, shaving soap, as mentioned in shaving scuttle and shaving brush, and available at places like this. jeffjon (talk) 13:07, 27 March 2009 (UTC)

Modern shaving cream is essentially aerated liquid soap. You can create roughly the same substance by rubbing your favorite bar soap in your hands with some water. In fact, before canned shaving cream, this was exactly how it was done for thousands of years... --Jayron32.talk.contribs 13:28, 27 March 2009 (UTC)

Speed of light and time

As I understand it, if I got in a spaceship and travelled at the speed of light for a year, then turned around and came back, while two years would have passed for me, a different amount of time would have passed for everyone else on Earth. How much time would have passed?

Similarly, while I know that light takes 8.33 minutes to get from the Sun to the Earth, how long does it take from the light's POV? I mean, if I was to slap on my welding goggles and stare at the sun, at exactly the same time a sun-gnome popped out of the Sun's surface and sped towards me, I would wait 8.3 minutes, but what would the time be on the sun-gnome's stopwatch when he careened out of the sky and smacked me in the face?

Thanks

FreeMorpheme (talk) 12:58, 27 March 2009 (UTC)

Well, the answer is a bit weird, but... At the speed of light, there is no time. Time stops. The sun-gnome's watch wouldn't have moved a nanosecond. I'm not sure about the first question ("Captain, relativity gives me a headache!"). Not that it's possible to travel at the speed of light unless you're a massless particle. —Preceding unsigned comment added by 83.253.252.234 (talk) 13:17, 27 March 2009 (UTC)

Indeed, travel AT the speed of light is impossible for any particle with mass and volume. When you work out the mathematics, all sorts of weird stuff happens:

• You arrive anywhere instantaneously (on your clock), while everyone else sees you moving at the speed of light (on theirs)
• You become infinitely heavy. Not just really heavy, but your mass becomes infinite.
• You become infinitely thin in the direction of travel, and infinitely wide in the orthogonal directions. Not just kinda thin in one direction, and kinda fat in the others. In otherwords, you become as wide as the universe, but you become two-dimensional.

Based mostly on 2 and 3, it becomes readily apparent that some silly stuff is going on at the speed of light. It's not just "we can't travel that fast because we don't yet have the technology"; it's that "we can't travel that fast because of the fundemental way the universe is put together". --Jayron32.talk.contribs 13:26, 27 March 2009 (UTC)

Shorter answer: you can't travel at the speed of light, nor can a sun-gnome with a stopwatch. The principle of relativity ensures that motion at one speed is equivalent to motion at another if the speeds are less than c, but motion at c follows different rules. The physical laws that make your existence possible don't operate at light speed.

"You become infinitely heavy" is a reference to relativistic mass, which is not usually taught any more because it's not a very useful concept. It would be more useful if fast-moving objects behaved somewhat like stationary objects with an increased mass, but they don't. You can't plug the relativistic mass into F=ma, for example. Nor does a fast-moving object collapse into a black hole, even though its increased "mass" ought to put it inside its own Schwarzschild radius. It's pretty hard to think of situations where the relativistic mass does make sense. The only one I can think of is as the m in E=mc².

"You become infinitely thin" is a reference to Lorentz contraction. "Infinitely wide" is incorrect, though. Lorentz transformations preserve the volume of your world-tube in spacetime, but not the volume of spatial slices through it. Think of slicing a dowel. If you cut it at right angles you get a circular cross section. If you cut it diagonally you get an elliptical cross section that's wider in one direction by ${\sqrt {1+{\tfrac {\mathrm {rise} ^{2}}{\mathrm {run} ^{2}}}}}$ but not narrower in the other direction. That's pretty much what Lorentz contraction is, except that the plus inside the square root becomes a minus. -- BenRG (talk) 14:15, 27 March 2009 (UTC)

Your first question has it backwards. Two years pass on Earth. The time that passes for you is two years times ${\sqrt {1-{\tfrac {v^{2}}{c^{2}}}}}$, where v is your speed. In the second question, beware of saying that things happen "at exactly the same time" in different places. That's a reference-frame-dependent concept; it's no different from saying that two things happen at exactly the same x-coordinate. -- BenRG (talk) 14:15, 27 March 2009 (UTC)

Blimey. OK, so what if I were to travel at 99% of the speed of light, is that possible? Would there be time-jiggering effects then? And are you saying that it is impossible to say that two things happen at the same time in two different places in the universe? FreeMorpheme (talk) 16:32, 27 March 2009 (UTC)

Yes, you can (theoretically) achieve any speed below c, wether it is 99% or 99.999% (although at that speeds other real-world effects may become annoying). Unless my math is off, if you fly around at 99%c, the γ-factor is close to 7, i.e. your 2 years are seen by an outside observer as a little bit more than 14 years. And yes, it is impossible to say that two things in different places happen at the same absolute time - the order of events depends on the movement of the observer. --Stephan Schulz (talk) 17:00, 27 March 2009 (UTC)

Thanks. So am I correct in thinking that if my sun-gnome were travelling at 99% of c, then he would arrive at approx (8.33/7) = 1.19 minutes relative to himself, but I would wait 8.33 minutes for him? And if he were a massless object travelling at the speed of light then all bets are off, as he would arrive instantly from his POV (and possibly at the same width as the universe)?

Does this mean that the difference in speed between 99.99999% of c and c itself is infinite? FreeMorpheme (talk) 17:24, 27 March 2009 (UTC)

Yes, 1.19 minutes sounds about right (he sees the distance from the Sun to the Earth as having been contracted, so if he measures his speed it is still less than c). The difference in speed between 99.99999% of c and c is what you would expect it to be, 0.000001% of c. The difference in energy, however, is infinite. --Tango (talk) 18:45, 27 March 2009 (UTC)

Indeed, this is the problem - the difference in our answer for (say) 99.<twelve nines>% and 99.<thirteen nines>% is enormous - so just knowing it's "a bit less than c" doesn't help one bit in getting an answer. Since you can't travel at literally the speed of light - there is no good answer to the OP's question. SteveBaker (talk) 21:01, 27 March 2009 (UTC)

Question: Is it sensible to talk about two things in different places happening at the same time if those two places are at rest relative to each other (like the two ends of a space craft) and not accelerating? Zain Ebrahim (talk) 19:05, 27 March 2009 (UTC)

You can always define a standard of simultaneity if you want to. UTC defines a reference frame, for example, and you can take two events to be simultaneous if they happen at the same time UTC, regardless of the motion of any people or objects that are involved. On the other hand, distant simultaneity doesn't figure into the laws of physics. All influences propagate at a finite speed, so it just doesn't matter what's happening "right now" over there, it only matters what will be happening over there a bit later. It's kind of like the US before the transcontinental railroad, when every town had its own time standard and it didn't much matter because getting from one town to another took so long. -- BenRG (talk) 21:31, 27 March 2009 (UTC)

For a given frame of reference, such as for a rider on the spacecraft, the concept of simultaneous events at each end of the that spacecraft is well defined. You can measure the distance between yourself and each end of the spacecraft, adjust for any light-speed propagation delays in the observed events, and be confident in your results. That said, someone in a different frame of reference, such as the planet you are approaching, may just as confidently observe that the two events occurred at different times. This is a hard concept to comprehend. It is not just an illusion that observers in different reference frames reach different conclusions, the Lorentz transformation causes the actual geometry of space and time to be different in the two reference frames, so both observers are quite correct, even though they reached different conclusions. I went years without really understanding what was going on, until one day I watched episode 42 of The Mechanical Universe... And Beyond and the illustrations used finally allowed me to put the puzzle together. -- Tcncv (talk) 02:44, 28 March 2009 (UTC)

I try to avoid this conflation of observers (i.e. people making measurements) with reference frames (i.e. systems of coordinates). Einstein did not conflate the two in his original paper; that was done by later writers. There's a little more about this at observer (special relativity), particularly the "history" section. It is not true that people in different states of motion disagree about their measurements. People who choose different systems of coordinates disagree about their measurements, but this is no more profound than a disagreement between Celsius and Fahrenheit. Associating every person with a reference frame is... well, I can't call it wrong, since you can solve problems that way, but it's unnecessarily constraining and it isn't motivated by any property of the real world. Consider the analogous proposal that every person has their own Cartesian coordinate system with the positive z axis pointing forward, the positive x axis pointing to the right and the positive y axis pointing up. Then different "observers", i.e. people, disagree on fundamental properties such as "width", which I'll define as the difference between the minimal and maximal x coordinate of an object, and "elevation", which is the value of the y coordinate. Someone who's looking upward, relative to you, will say that certain things have different elevations that to you have the same elevation. That's exactly as profound as the relativity of simultaneity. If you think the relativity of simultaneity is more profound than that, you don't understand it! That's not to say that Einstein's 1905 paper wasn't profound. But what was new about special relativity was that it got rid of the idea of time as a universal parameter. It didn't add a bunch of incompatible observer-dependent universal time parameters. That makes as much sense as having a plane-of-constant-y associated with every person. You can do it, but the universe is indifferent to planes of constant y and it's equally indifferent to planes of constant t. -- BenRG (talk) 18:33, 28 March 2009 (UTC)

As others have pointed out, to simply say that an object is moving close to the speed of light isn't very informative. See rapidity for a more insightfull way to describe how fast something is moving. With respect to the question about wheather it's sensible to describe two events as being simultaneous, the answer is yes, but different observers will have different understandings (both correct) about what is simultaneous and what isn't. Dauto (talk) 05:36, 28 March 2009 (UTC)

Muscle Tearing from 1st Time Strength Training

Take someone who is out-of-shape and hasn't exercised in a very, long while. Then, that person decides to join a fitness club. The personal trainer makes this person do numerous strength training exercises the first day there which results in muscle pain the next day. Isn't muscle tearing adverse to the health of the muscles? Would it not be healthier to gradually build up these muscles instead of tearing them the first day? When I say tearing, I don't mean the kind that ends it in the hospital. I meant the kind miniscule muscle tearing that happens when you do strength training exercises. --Emyn ned (talk) 13:12, 27 March 2009 (UTC)

See Microtrauma. 76.97.245.5 (talk) 13:23, 27 March 2009 (UTC)

Ice cores

Can an icecore tell you the average temperature in 1900? If so, how does it work and how accurate is it? 65.121.141.34 (talk) 13:35, 27 March 2009 (UTC)

According to de:Eisbohrkern and Oxygen Isotopes (de) yes it can. Since O18 and O16 have slightly different evaporation rates one can measure the average temperature of a given time by measuring the respective contents in a given probe. Therefore accuracy depends on the amount of ice one has at his disposal for testing. The oldest found ice core is according to the articles approximately 900.000 years old. The respective english articles are Oxygen and Ice core--91.6.18.23 (talk) 13:56, 27 March 2009 (UTC)I changed your link to the German page, it didn't work the way you had it. Hope you won't mind.

There's also Ice core although that only hints at it. Follow the links from there for more information.76.97.245.5 (talk) 14:20, 27 March 2009 (UTC)

No, ice cores can not be used to determine the temperature in 1900 with any degree of reliability. Ice cores are used in paleoclimate to determine temperatures using the O18/O16 ratio in trapped air bubbles. But the air in ice is not sealed in instantly. Instead, as the ice is compressed more and more by new layers above, it becomes denser and denser, and air exchange with the environment becomes less and less. IIRC, the seal is near complete after about 30-50 years, depending on exact circumstances. So any air bubble will have a mixture of air from about 30-50 years (more from the early years, less from the later years). This allows a temporal resolution on about the same time scale - you can make statements about climate, because 30 years is about the shortest time frame we consider significant for climate (as opposed to weather). But you cannot usefully determine the temperature during a single year. --Stephan Schulz (talk) 17:11, 27 March 2009 (UTC)

You're mistaken. Though one can measure δO of air, most ice core temperature work refers to the δO of ice where the O is from H2O. Dragons flight (talk) 20:43, 27 March 2009 (UTC)

Aha. I did not know that, thanks! So you can get the temperature with a reasonably high resolution, but the trace gases with a much lower one? --Stephan Schulz (talk) 22:32, 27 March 2009 (UTC)

Yes, though in practice the difficulty in time and labor required for gas work tends to be a bigger factor in influencing the results. It is not uncommon to see only one gas sample for every few meters of core which is even worse for resolution than the gas mixing effect you refer to. By contrast, the modern temperature systems can get ~20 measurements per meter. Dragons flight (talk) 03:22, 30 March 2009 (UTC)

Does all life share some genetic code?

Humans share genetic code with many life-forms. That is very obvious for creatures with skeletons, but it is not obvious to me that we share genetic code with a tree, an insect or a tube worm living on an oceanic thermal vent. Do humans share genetic code with a fungus, a bacteria? a virus? a prion (which may not be considered a "life-form")? If humans share genetic code with all life on earth, could it be assumed that there was a single "original life" from which all life grew and from which all life-forms evolved? Ddcarroll (talk) 16:10, 27 March 2009 (UTC)

Yes to everything except prions, which don't have any DNA or RNA. StuRat (talk) 16:36, 27 March 2009 (UTC)

I agree that all currently existing cells have a shared set of base genetic machinery. However, I'm not sure all viruses necessarily have genes in common with humans. Viruses only need a limited genome because they co-opt the cellular machinery of other organisms. So you might find viruses on a very distant branch of the tree of life (bacteriophages, perhaps?) where their genome had no overlap with humans in particular. Dragons flight (talk) 21:03, 27 March 2009 (UTC)

There are things, like prions and some computer programs, that make giving a definition of the word "life" difficult. For the things you describe, see common descent, which discusses the currently accepted dogma. --Sean 17:35, 27 March 2009 (UTC)

It's unclear whether we share common descent with prions - and it's also unclear whether we should classify them as "life" or "chemicals" (or indeed whether we should even make such distinctions!). However, excluding things with no DNA or RNA at all - there is a very clear set of common genes - a much bigger percentage than you might perhaps expect. This does indeed point towards a single "original" lifeform from which we're all descended. While it may not look like we have much in common with some bacterium or other - or with plantlife - there are a vast number of chemical pathways that we all share - and which represent a much bigger fraction of the genome than the genes for more obvious things like having a head, two arms and two legs. Things are a little more complicated though because it's clear that our genes have large 'insertions' from lesser life-forms that have somehow come to be wedged into our DNA. So we should perhaps strictly be considering all of life as an interlinked 'web' rather than the traditional tree-of-life. But this is a very small matter. The "big picture" is that we're all descended from a common ancestor and our DNA & RNA proves that. SteveBaker (talk) 20:53, 27 March 2009 (UTC)

Well, since all prions are just variant foldings of a host protein, and are encoded in the host protein, they certainly are descended in EXACTLY the same way as that protein in the host. Viruses, on the other hand, do not clearly share descent with their host and their evolutionary path is not so clear (though as agents of horizontal gene transfer they often carry some host genetic material). Going back to the OP, all currently existing cells do not share the same genetic code, though the similarities are greater than the differences and do suggest common descent. By necessity, viruses share the genetic code of the host cell. --Scray (talk) 02:40, 28 March 2009 (UTC)

Sure, obviously the genetics of all living things are not "identical" because if they were, we'd all look identical! But we have to emphasise that the biochemical mechanisms - and a large percentage of the genes - are so spectacularly similar that common descent is the only reasonable hypothesis. It's comforting to hear that Prions are really no exception to that. SteveBaker (talk) 12:19, 28 March 2009 (UTC)

The term "Genetic code" has a precise meaning, and I get the impression that some of the participants in this thread are using the term when discussing something different - the sharing of DNA sequences between organisms. From our article: The genetic code is the set of rules by which information encoded in genetic material (DNA or RNA sequences) is translated into proteins (amino acid sequences) by living cells. A computer analogy: the genetic code is the instruction set by which a cell's ribosomes, t-RNAs, and Aminoacyl tRNA synthetases (together, the CPU) translate genes (the program) into protein (the output). Scray's point above, is that the CPUs of different organisms may have slightly different instruction sets, not the obvious point that there are large differences in the programs (genes) that are used to build the organisms. --NorwegianBlue ^talk 20:37, 28 March 2009 (UTC)

Yes, that was the definition of "genetic code" I was using, and is why I excluded prions, as they contain neither DNA nor RNA. StuRat (talk) 23:03, 28 March 2009 (UTC)

I understand the distinction - but even in organisms with different genetic codes (different "CPU instruction sets" for the computer geeks amongst us) - the difference is only in the interpretation of a few codons - most of the code is read the same way - which (if you think about it) is a necessary consequence of the other part - which is that we do indeed share considerable chunks of genetic information (the software that's being run by the genetic-computer is identical over long stretches). This is not unlike the transition from 8080 computers to Z80 computers. A few instructions were added in the Z80's processor - this didn't hurt anything because software written for the 8080 didn't use those instruction codes. But one or two very obscure instructions changed meaning in very subtle ways. 99% of programs written for the 8080 ran just fine on the Z80 - but some would behave a little differently - or (mostly) just fail entirely and crash the computer. Similarly, organisms with different genetic code interpretations can still have genes in common - but either those genes don't happen to include the codons that have alternative 'meanings' - or perhaps the alteration doesn't affect the consequences of the gene being expressed - or perhaps that change in expression is actually beneficial. Of course in cases where changing the meaning of a particular codon results in "the program crashing" - evolutionary considerations will ensure that such lifeforms die off - so there are unlikely to be cases like that in nature - just as a typical Z80's disk drive would be unlikely to contain any 8080 programs that caused it to crash. SteveBaker (talk) 14:24, 29 March 2009 (UTC)

Make time history from PSD?

Hi, I'm trying to write MATLAB code to generate a representative time history from a spectral density plot. I've found a few things that look useful on the internet, for example this one [1], but I don't really understand the steps involved. For example, in the link above, I don't understand what is meant by a "PSD that you have empirically as samples" - what is a sample referring to in this sense?

If anyone could explain in simpler terms that would be great.

Cheers for any help, LHMike (talk) 17:07, 27 March 2009 (UTC)

It sounds like you have been given the power spectral density, discretely sampled in frequency (see discrete fourier transform). It sounds like an inverse fourier transform should convert that PSD into a representative time-series. You may need to normalize amplitudes based on your detailed problem specifications. Nimur (talk) 09:00, 28 March 2009 (UTC)

Samples of power (i.e. amplitude squared) at discrete frequencies say nothing about the phase at each frequency. You can suppose that the phase at every frequency is the same i.e. zero, and do the inverse Fourier transform of the real-valued samples to get a time history. It will be a possible but not necessarily representative time history because of the equal-phase assumptions. Other phase information would be given by complex amplitude samples. In any case the time series from the IFT is periodic at the frequency of lowest sample. The internet link concerns random value samples in time so phases are not an issue. Cuddlyable3 (talk) 08:37, 1 April 2009 (UTC)

list of self HOMO-LUMO gaps (especially that of chlorophyll a)

Why is it so hard to find self HOMO-LUMO gap values on the internet? If they are relatively easy to calculate using Hartree-Fock and a bit of time, why doesn't Wikipedia have a self HOMO-LUMO gap listed on all major chemical infoboxes?

Decidedly, I need the HOMO-LUMO gap of chlorophyll-a (with the magnesium atom attached) compared to beta-carotene compared to silicon, in eV, if possible, to win an internet argument. John Riemann Soong (talk) 17:33, 27 March 2009 (UTC)

Readers unfamiliar with the expression can begin by referring to HOMO/LUMO. -- Wavelength (talk) 20:17, 27 March 2009 (UTC)

this google search may provide a place to start. I haven't started reading all of these articles for you, but there are some promising hits in this google search. You could also try google-scholar, which will confine the search to research journals and the like. --Jayron32.talk.contribs 01:39, 28 March 2009 (UTC)

Why hasn't anyone set up a database of found HOMO/LUMO gaps (theoretical or otherwise), seeing as how useful the information often is? John Riemann Soong (talk) 19:31, 29 March 2009 (UTC)

What would be the use of HOMO/LUMO gaps? There are plenty of places to find information about absorptions and reduction potentials which are the observed expressions of HOMO/LUMO gaps. As far as listing the results of Hartree-Fock calculations, there is considerable debate to the value of such calculations among computational chemists let alone those with a more experimental bent.--OMCV (talk) 03:07, 31 March 2009 (UTC)

Cuz, I'm not looking for organic chemical reactions per se -- rather possible ways to incorporate highly-conjugated proteins or organic molecules into circuits and semiconductor arrangements (not necessarily directly). John Riemann Soong (talk) 18:46, 31 March 2009 (UTC)

Seasonal affective disorder and sunshine advertising

How common was seasonal affective disorder before the tourism industry started to remind people about the sunshine they were missing? To what extent is the condition actually caused by a lack of sunshine experienced by sedentary people in some places during some seasons? To what extent is it caused by envy cultivated by the tourism industry? -- Wavelength (talk) 19:25, 27 March 2009 (UTC)

We don't know - the condition was only noticed scientifically in the 1980's when we were already neck-deep in tourism adverts. However, they underlying implication that you think this is all nonsense is not backed up by the facts. Firstly, we know that reports of the condition go back to the 6th century - LONG before tourism was commonplace - and secondly, it's been studied carefully and there are even some pretty solid genetic and dietary risk factors that have been nailed down. So yes - it's very real. Fortunately, it's fairly manageable once diagnosed. SteveBaker (talk) 20:44, 27 March 2009 (UTC)

I sense a conspiracy theory. SAD was invented by unscrupulous travel agents and we are all their unwitting dupes.. Cuddlyable3 (talk) 21:58, 27 March 2009 (UTC)

I'm not sure about SAD, but the idea of Blue Monday, that the most depressing day in the year is the 3rd Monday in January, was invented by a travel company to sell holidays.[2]. --Maltelauridsbrigge (talk) 11:25, 30 March 2009 (UTC)