Wikipedia:Reference desk/Archives/Science/2013 August 13

= August 13 =

Who burns more calories
I was reading calories burnt per hour lists of various sports and it got me thinking, would a fat out of shape person spend more calories than a fit person in the same activity even if he did less of it? For example if a fat guy runs for 10 minutes and is absolutely exhausted and faints would he spend more energy than a fit person who runs 20 minutes and does not even feel tired? — Preceding unsigned comment added by 88.195.215.49 (talk) 07:34, 13 August 2013 (UTC)
 * (Note: Wikipedia does not offer medical advice - this is not authoritative) There is a distinction to make between the amount of fat burned and energy spent. The answer is that it depends, based on how efficient that fat person is, the weights of both, the exercises involved, the conditions, and the exact lengths of time.--Jasper Deng (talk) 07:45, 13 August 2013 (UTC)
 * Your question can't be answered as asked, but a few things to help you on your way:
 * How exhausted a person feels is not a measure of how much energy they've used. If two persons ride exercise bikes and spend the same amount of calories in the same time the person in better shape will feel less exhausted even if they spent the same number of calories.
 * Generally speaking, a heavy person will burn more calories per mile running, but it depends very much on how efficient that person's running style is, and to some extent on the speed. Sjö (talk) 08:22, 13 August 2013 (UTC)


 * I'm aware of formal measurements that are made on people with physical disabilities, such as cerebral palsy, that demonstrate that such people use considerably more energy than able bodied people to perform the same task. It's reasonable to guess that obese people would too. HiLo48 (talk) 08:14, 13 August 2013 (UTC)


 * See here for some figures. Count Iblis (talk) 12:49, 13 August 2013 (UTC)


 * Muscles have a much higher resting metabolism than fatty tissue, burn more calories, and require a specific diet to maintain at a weightlifting level. See Basal metabolic rate. μηδείς (talk) 23:30, 13 August 2013 (UTC)
 * I remember an experiment we did at school which is different but related. Everyone had to run up a few flights of stairs and was timed doing it. We calculated how much energy you burned (weight times height) and divided it by the time, the theory being that the more energy you can produce in the shorter amount of time is basically how "fit" you are. Turns out, the most overweight people, even thought they were slower, were actually the most "fit" and the most skinny people were the least fit. Of course this is purely by people's energy output, it says absolutely nothing about health. Vespine (talk) 02:20, 14 August 2013 (UTC)
 * That power must be divided by the weight to obtain the power/weight ratio, which is a good emasure for fitness (You need to be above 3 Watt/kg), so the speed itself is actually a good measure for fitness although one should look at exertions that last for more than 12 minutes (so, you could look at how fast people are able to climb the Empire State Building). Count Iblis (talk) 17:51, 14 August 2013 (UTC)

Seat collision for swing ride caused by wind - how?
WindSeeker is closed during high winds, evidently because its seats, probably even despite the dampers present, collide when the air is in motion. The article, not unexpectedly, doesn't discuss the physics.

So, I did a thought experiment for a very simple case, where the velocity field of the wind is constant, and the air density is assumed constant. I know these are approximations that are probably not valid in real life, but I first figured I should start with this special case, where the simple drag equation is at least approximately applicable. I then made the assumption that the hinges attaching each seat's support bar to the rotating gondola allowed complete freedom of motion (again, the dampers are there to hinder it, but again, I have to simplify the problem a bit), thus not considering the friction of the hinge. With this, I began doing the vector addition of the wind's and seat's velocity fields so I could then calculate the drag force with the resultant relative air velocity. However, doing it mentally, it still didn't quite dawn on me why this should cause the seats to collide, nor the maximum safe wind velocity predicted by my simple model. I don't have a very advanced understanding of fluid dynamics, so I wish the relatively simple drag equation will suffice to model this situation.

While I'm willing to accept a vector calculus explanation (and half-expect one), I'm sure the physics can't be that hard (although with a non-constant force field, I've already fixed my mind on the need to perform integration here).--Jasper Deng (talk) 08:09, 13 August 2013 (UTC)


 * I think it's simply that your assumption of a constant wind isn't valid. Air, like water, contains currents, where winds are higher or lower, and in different directions, and modeling this is necessary to understand the problem. StuRat (talk) 09:31, 13 August 2013 (UTC)


 * Even starting with a 'constant velocity wind', you can end up with oscillations - see vortex shedding. AndyTheGrump (talk) 22:11, 13 August 2013 (UTC)

Theory of Relativity
We know that maximum speed of a body can be as high as the speed of light. Now my question is that - Imagine a train is running with the speed a light and a boy runs in the train. An observer outside the train sees that boy. Then what is the speed of the boy with respect to the  observer outside? If you are thinking to add the speed of the train and the boy, then it would be more than the speed of the light that is impossible.Publisher54321 (talk) 09:49, 13 August 2013 (UTC)


 * For the boy to run in the train, he would have to accelerate himself. But since he already has infinite mass, he can't.  The situation cannot arise. 1.122.88.140 (talk) 09:59, 13 August 2013 (UTC)
 * And just to be clear, the reason that he cannot accelerate while he has an infinite mass, is because to accelerate any infinite mass, you need an ifinite amount of energy, which is not available in this universe. --Lgriot (talk) 11:09, 13 August 2013 (UTC)


 * The above are remarkably misleading answers for what is the most fundamental thought experiment that underpins all of special relativity. The notion that that boy cannot accelerate coz he is nearing infinite mass only begs the question as to why that is so? The simple answer is that at close to light speeds, velocity is not cumulative as in a normal train. We always see light moving at the same speed regardless of whether it is coming from an object moving towards us or away from us. Because we never see something move faster than speed of light, then, in the boy in train case, we see time in the train slow down, and the boy seems to be running in slow motion as the train approaches the speed of light. If the train could achieve light speed, then the boy would be like a photo or statue of himself, totally unmoving, and both he and the train would be moving at the same speed - light speed. To the boy, tho, nothing has changed inside the train, and he moves as per usual. The time dilation and mass increase business are consequences of the unchanging nature of the speed of light. Of course, all this has been confirmed in many experiments since Einstein. Calculations to get the geo-positioning satellites to orbit correctly must include factors that take account for the fact that their time slows down as they pick up speed. Myles325a (talk) 02:18, 18 August 2013 (UTC)


 * Those answers are a bit misleading. In the reference frame of the train, the boy can accelerate in a perfectly normal way.  But in the reference frame of an observer who sees the train moving at nearly the speed of light, time dilation causes the boy's acceleration to appear very small. Looie496 (talk) 14:52, 13 August 2013 (UTC)
 * No I beleive that your answer is misleading, Looie, becuase you didn't properly read the (poorly worded, fair enough) question. The OP did not ask about a train going near the spead of light, but at the speed of light. --Lgriot (talk) 08:21, 14 August 2013 (UTC)
 * The relativistic answer (for speeds below c) is given by Velocity-addition_formula. AndrewWTaylor (talk) 11:54, 13 August 2013 (UTC)
 * It also work for speeds at c. Dauto (talk) 14:40, 13 August 2013 (UTC)


 * Has anyone noticed that this is a slight modification of the very thought experiment used by Einstein when he created the theory of relativity? Plasmic Physics (talk) 11:59, 13 August 2013 (UTC)
 * I wouldn't draw any inferences. — Quondum 12:09, 13 August 2013 (UTC)
 * Strangely, I don't recall the flashlight-on-the-train explanation actually being concocted by Einstein, even though "everybody" knows that Einstein used a flashlight on a train as his example for special relativity. That example doesn't appear in the famous 1905 papers, (available in English translation on Wikimedia and at Mr. Walker's excellent website).
 * So, how did this bit of historical corruption occur? Who concocted a flashlight-on-a-train gedankenexperiment, and why does everybody attribute it to Einstein!?  It would seem that science, like religion, is easily corrupted over the course of history, by people's relentless insistence to deify individual contributors and give them credit for creation of everything.
 * Einstein wrote many things, and originated many ideas. Einstein wrote about electromagnetic wave dynamics, and corpuscles of light, and some very convoluted mathematics about geometric identities between geodesics and inverse-square-law forces; (and plenty more about incorrect theoretical arcana about spectral lines that nobody ever mentions any more because it was totally wrong).
 * But so did thousands of other physicists. Those more-anonymous physicists had lesser egos, worse publicists, and more comprehensible writing.  I put forward the bold statement: nearly everything that we know today about the photoelectric effect, the mechanisms of special and general relativity, electrodynamics, didn't come from Einstein.
 * So, a challenge to the reference desk: which scientist or author first published a parable of a flashlight on a train (or a person running on a train) moving with relativistic velocity? Nimur (talk) 12:45, 13 August 2013 (UTC)


 * Oh, Nimur. You can be such a grouch. Dauto (talk) 14:40, 13 August 2013 (UTC)
 * Sorry, I'll try to be more cheerful! Happy Tuesday, here's a bit of fun science-fiction to brighten your morning... actually it's probably the reason I'm so grouchy today.  Let's all just conveniently forget to account for the energy consumption of the thousands of jet-engines you need to evacuate a 700-mile metal tube, and pretend that you can put an airtight metal tube in California's central valley and model its air-temperature at 68° F!  Also multiply the air-compressor's required horsepower by ten in every sixth paragraph and neglect to update your overall energy budget!  And vomit-inducing 3-mile-radius turns at 760 miles per hour!  At least the cartoons are pretty.  Nimur (talk) 15:39, 13 August 2013 (UTC)
 * Thanks! I was looking for the tech details earlier but missed them.  Curious to observe the walls are 0.8-0.9 inch thick.  Wnt (talk) 22:54, 13 August 2013 (UTC)


 * So you admit it, not even you know who said it first? Well, I hate to pop your bubble, but I never said Einstein was the first either. So, I may yet be vindicated. Plasmic Physics (talk) 19:13, 13 August 2013 (UTC)
 * I admitted nothing except being slightly grouchy! Nimur (talk) 20:41, 13 August 2013 (UTC)


 * So, I will not be vindicated, nevertheless, my preceding statement still remains true. Plasmic Physics (talk) 21:39, 13 August 2013 (UTC)
 * I don't know if the question has been answered yet. I would add that just because something is not theoretically impossible, does not mean it is possible in practice. I suspect that even though relativity says mass can't travel FASTER then the speed of light, I bet that in practice mass (more then a handful of particles anyway) can't travel AT or even very near to the speed of light either. Vespine (talk) 02:05, 14 August 2013 (UTC)
 * "Near the speed of light" is easy: just use a reference system in which the thing has a sufficiently high speed.--Wrongfilter (talk) 12:13, 14 August 2013 (UTC)
 * I actually don't think I really get that yet, it hurts my brain. Vespine (talk) 22:56, 15 August 2013 (UTC)
 * However, I think my point is that nothing actually occurs in those "reference systems" apart from maybe cosmic rays. Things that are more then a molecule or a few, relative to eachother in the whole universe only differ by a few percent of the speed of light, as far as I can gather from some quick googling Vespine (talk) 23:37, 15 August 2013 (UTC)
 * Vespine said 'I suspect that even though relativity says mass can't travel FASTER then the speed of light, I bet that in practice mass (more then a handful of particles anyway) can't travel AT or even very near to the speed of light either. ' Actually light itself (photons) have mass (no rest mass) but do travel at and near the speed of light.  Technically, since space actually has matter in it, and matter slows down light, even light cannot travel at 'c', but that is no longer in the theoretical realm.  Jokem (talk) 23:50, 18 August 2013 (UTC)

Asthma
If a non-asthmatic person uses a Metered-dose inhaler, will it aid them in completing a marathon? Pass a Method  talk  12:49, 13 August 2013 (UTC)
 * If it would give an appreciable advantage it would be banned from use in the sport. So, if it hasn't been banned then it has no effect. I guess it'd be treating a condition the runner doesn't have so would not improve the running.  (But it could make it much worse and be unsafe.)  The inhaler is for reducing constriction of the airways; do marathon runners have constricted airways as a limit on their running?   RJFJR (talk) 16:14, 13 August 2013 (UTC)
 * Obviously that depends on what's in it. There are quite a variety of drugs that can be absorbed into the lining of the lungs - and any one of them might be dispensed this way.  The more common anti-asthma drugs are corticosteroids - and steroids are pretty much banned in events like marathons - so I'm guessing they would be illegal.  Whether they'd help or not is different matter.  I doubt it...but I could easily be wrong. SteveBaker (talk) 16:23, 13 August 2013 (UTC)
 * Inhaled steroids are corticosteroids, not anabolic steroids, and I'm not sure the former would be performance-enhancing. Inhaled beta agonists might or might not help depending on the athlete and conditions. -- Scray (talk) 21:35, 13 August 2013 (UTC)
 * T%he standard emergency inhaler is Salbutamol - our article covers its ban in sports and the contradictory studies of its effect on healthy subjects. Rmhermen (talk) 13:07, 14 August 2013 (UTC)
 * Everyone has constriction of the airways to some degree. When it gets to the point where it becomes an illness, then an inhaler or some other medical aid is necessary.  How much it opens up the airways and how much it aids in the exchange of blood gasses would tell you how much of an advantage it gives.  Jokem (talk) 00:13, 19 August 2013 (UTC)

Origins Of The Universe: Steady State & Big Bang Theories
What are some of the current problems and limitations of the steady state and big bang theory? 220.233.20.37 (talk) 12:57, 13 August 2013 (UTC)


 * For the Big Bang theory, you shoud read the article Inflation (cosmology). The steady state theory has long been debunked, you can read our Wiki articles on that subject. Perhaps also articles on Fred Hoyle (had it not been for Hoyle, steady state would have died decades earlier). Count Iblis (talk) 13:07, 13 August 2013 (UTC)


 * Right now, steady-state is busted, dead, gone, impossible - it's up there with "Adam and Eve" and "the Great Arklesiezure". Not even worth discussing.  The Big Bang is really the only theory that explains everything - and the discovery of the cosmic microwave background radiation more or less sealed the deal.  The problems are really details that are unrelated to the core fact - What caused it?  What was before it?  How will the universe end?  What the heck is it with all of this "Dark" stuff?   Those would be problems with whatever theory you might come up with though, it's not entirely clear whether the fact of the big bang has any impact on those questions. SteveBaker (talk) 16:20, 13 August 2013 (UTC)
 * Adam and Eve still works as a sociological metaphor, so the Steady-State theory is even less credible than Adam and Eve. ←Baseball Bugs What's up, Doc? carrots→ 22:31, 13 August 2013 (UTC)


 * There are still some die-hards out there such as Jayant Narlikar who maintain that the steady-state theory is viable. In his book The Primeval Universe he postulates that the microwave background radiation is due to blackbody radiation associated with helium production in stars (of necessity, the steady-state theory requires something to be making all the helium that is otherwise accounted for by the big bang).  Mr. Baker, please don't shout at me, I didn't say I supported this theory.  Spinning  Spark  00:46, 14 August 2013 (UTC)

In relativity, whether actual mass changes or not
According to Einstein's theory of relativity, mass of a body increases when its speed or motion increases. Therefore, there is a considerable increase in mass of a body moving with a speed of 100000 km/sec. On the other hand, we know, motion of any body in Universe is relative. The body moving with the speed of 100000 km/sec must be at rest with respect to some other object in Universe, hence according to this situation there should be no change in mass. I am confused because in the first case its mass seems to change, but in the second case its mass seems to be constant. Again, this body is in motion as well as in rest with respect to two different reference frames. What should I conclude from this? Whether the mass of the object changes or remains constant? I think the correct explanation for this would be '''the change in mass due to speed or motion of an object depends on the relative motion of that object with respect to the reference frame from which the motion of that object is being observed. Hence, the mass of that object would be different for different frames having different relative speed with respect to that object.''' Correct me if I am wrong. Thanks for bearing me! Publisher54321 (talk) 18:08, 13 August 2013 (UTC)


 * Someone will be along shortly to explain the details, but I'm pretty sure the answer to your question can be gained from a careful reading of relativistic mass and rest mass. SemanticMantis (talk) 18:12, 13 August 2013 (UTC)


 * Changing to a different reference frame changes the object's velocity thus its kinetic energy (and mass per mass-energy equivalence) does indeed differ, but remember that the coordinate change applied to the object also must be applied to all other objects, thus it doesn't matter whether the ball has the energy that hit the ground or the ground it... (I wonder if it will be friends with me...) the dynamics are the same. You said the object gained mass accelerating, but incorrectly stated this: "The body moving with the speed of 100000 km/sec must be at rest with respect to some other object in Universe, hence according to this situation there should be no change in mass." However, it would have been decelerating in the new reference frame thus losing speed and mass-energy in the process until it was at rest. -Modocc (talk) 18:53, 13 August 2013 (UTC)


 * ( In the absence of an expert ... ) The rest mass (as measured in an inertial frame at rest relative to the body) remains the same in all reference frames. The preferred view is to ignore the apparent "increase in mass" because looking at the situation this way does not always give the right answer, and instead to consider the 4-D momentum vector ( but it's too long since I studied this for me to attempt an explanation! )  Einstein wrote: "It is not good to introduce the concept of the mass $$M = m/\sqrt{1 - v^2/c^2}$$ of a moving body for which no clear definition can be given. It is better to introduce no other mass concept than the ’rest mass’ m."    D b f i r s   20:39, 13 August 2013 (UTC)
 * Relativistic mass is used to calculate total mass-energy (or total energy) and, for that purpose, it seems to give correct answers. Modocc (talk) 21:23, 13 August 2013 (UTC)
 * If you heat a body up the atoms will move faster and so the body will be heavier. Yes there is more mass. Dmcq (talk) 21:18, 13 August 2013 (UTC)
 * That increase of mass is due to energy in internal degrees of freedom of a composite body. Its mass (rest mass if you insist) will not change if you move the body as a whole. Similarly, the mass excess of a nucleus (possibly the clearest manifestation of E=mc2) is due to the fact that the nucleus has internal degrees of freedom. --Wrongfilter (talk) 21:30, 13 August 2013 (UTC)

Hawking radiation and the formation of black holes.
For the first part of my question, assume an observer falling in to an existing black hole which is emitting Hawking radiation. As the observer approaches the horizon, my understanding is that the observed temperature of radiation would increase, tending towards infinity at the horizon (or at least the Planck temperature shortly before the horizon). As a result, the infalling observer would observe a greater emitted power from the hole as they fell toward the horizon, and as a consequence a greater mass loss rate than an observer at infinity. So would the infalling observer see the black hole radiate itself away before they actually crossed the horizon? If so, would they see it radiate itself away before the radiation reached trans-Planckian temperatures (or, at least, would the observation of trans-Planckian temperatures be delayed until the observed remaining mass was on the order of the Planck mass)?

For the second part of my question, section 3.8 (on page 26) of this article states that a collapsing object that does not collapse to within its Schwarzschild radius will emit Hawking radiation until the collapse stops (actually it doesn't quite state that directly, it states that the object will stop emitting Hawking radiation when the collapse stops, the article is agnostic as to whether Hawking radiation is emitted from any object in the first place, but the implication seems to be that if Hawking radiation is ever emitted, it will be emitted by collapsing objects as long as the collapse continues).

So the question then is whether, as the radius of a collapsing object approaches its Schwarzschild radius, Hawking radiation in the vicinity of the event-horizon-to-be will be intense enough that the object will lose mass quickly enough that its radius is never smaller than its Schwarzschild radius? In other words, if Hawking radiation in fact occurs, does it prevent the interior portion of the Schwarzschild geometry from ever actually forming, so that objects that are asymptotically close to being black holes in terms of exterior spacetime geometry might exist, but no black holes per-se actually do exist? — Preceding unsigned comment added by Linguofreak (talk • contribs) 21:58, 13 August 2013 (UTC)


 * I don't understand it, but it is tied in with the Unruh effect somehow. The event horizon is a line that is continually accelerating against the rest frame.  The Unruh radiation is seen because it is accelerating.  If you're falling in, you're not accelerating, so what looks like an infinite energy vacuum to the event horizon looks like a just plain vacuum to you.  I think.  Note that there is also some absurd time dilation going on in the event horizon time frame - all that extra Hawking radiation isn't piling up in the region between event horizon and infinity, so the amount going on at any given moment (faraway rest frame time) at the event horizon must actually be very small. Wnt (talk) 23:08, 13 August 2013 (UTC)


 * From the point of view of the observer falling in the hole there is no Hawking radiation. Dauto (talk) 17:22, 14 August 2013 (UTC)


 * The answers above are correct but I want to add that the high temperature near the horizon doesn't translate to a higher rate of heat loss. The Hawking temperature is the temperature of the radiation at infinity. If you're closer than infinity (and stationary) you will measure a higher temperature, but the emitted power doesn't increase just because you made the measurement. This is the same as any other situation where there's a temperature gradient.
 * I'm not sure whether non-black-holes lose mass to Hawking radiation. I can't remember ever reading a text that discussed it. On the one hand, the distant gravitational field is the same and it's hard to see how the nature of the object at the center could matter that much. On the other hand, Hawking and Unruh radiation "come from" an event horizon, and there's no event horizon in this case. -- BenRG (talk) 05:31, 16 August 2013 (UTC)

AM radio reception (unconventional)
The AM radio station WLW once broadcast at 500kW, and could be heard across much of the globe. Our article says there are reports from people living near the tower, who claimed to hear broadcasts through the coils of their mattress springs:WLW. The cited ref acknowledges that these reports are to be treated skeptically. But I wonder: is it conceptually possible for a very high power AM broadcast at 700kHz to stimulate audible vibration in a spring? More far-fetched: is it possible for for a spring to act as antenna and speaker, so that the actual signal being sent is demodulated and heard faithfully? I've seen lots of "yes" (and "no") answers via googling, but few that were explained scientifically or inspired much confidence in their reliability/authority. Thanks, SemanticMantis (talk) 22:05, 13 August 2013 (UTC)
 * Seems to me there have been stories of people picking up radio frequencies in the fillings of their teeth, so anything's possible, especially if you're close to the source. ←Baseball Bugs What's up, Doc? carrots→ 22:28, 13 August 2013 (UTC)


 * The scientific explanation you are looking for is magnetostriction. Close to a powerful transmitter almost any piece of ungrounded metal will mechanically vibrate in sympathy with the RF transmission.  I recall a case during refurbishment of the BBC's medium wave Brookmans Park transmitting station (in the 1980s iirc) that Radio 2 came blaring out of the newly replaced heating radiators because the pipework had not been properly RF earthed.  All test equipment brought onto the site had to be specially custom modified for RF decoupling otherwise it would pick up the transmission rather than measure what it was supposed to be measuring.  It could even be destroyed.  The perimeter fence around powerful transmitting stations have to be earthed with ground spikes at regular intervals to prevent dangerous voltages being induced.  Spinning  Spark  00:07, 14 August 2013 (UTC)


 * ...and I see that they are still causing problems for the local residents 30 years later.  Spinning Spark  00:20, 14 August 2013 (UTC)


 * Lucille Ball always claimed that she could hear radio stations through the fillings in her teeth (There is a brief mention of the story in Lucille_Ball - and a more complete rundown over on Snopes). I suspect she added much to the claim that this happens - but her story doesn't exactly hold water.  She claimed to have heard a Japanese spy network transmitting somewhere in California and that her report led to them being captured.  Sadly, there doesn't seem to any actual information that it really happened - so it's very likely to be a complete fabrication.  But I think that her public retelling of this story in numerous books and interviews is where the whole thing started. SteveBaker (talk) 02:10, 14 August 2013 (UTC)


 * I challenge the claim that WLW could ever have been "heard across much of the globe." Maybe it could have been heard across much of the continental US, not including Alaska and Hawaii, but including parts of Mexico and Canada, if local stations with the same frequency engaged in a "Silent Night" wherein they shut down for an hour or so to allow local listeners to DX, as was sometimes done in the 1920's. The WLW AM frequency is probably too low for the short wave skip propagation which allows reception half the world away. And 50 kilowatts is more likely as a US "clear channel" station than the claimed 500 kW, though I stand ready to be corrected by reliable sources. Some Mexican stations might have transmitted at 500 kW. Edison (talk) 06:03, 14 August 2013 (UTC)
 * You may be to a certain extent wrong. The wikipedia article states that the 500 KW transmitter was used to broadcast to US troops in Europe at the close of World War 2.  With tube radios featuring low intermodulation distortion, and the generally less noisy conditions before the proliferation of home elctric goods in the 1950's, 50 kW AM stations could be heard up to 2000 km away.  Using 10 times the power with top loaded antennas that suppress high angle radiation, you get a bit better than root-10 times the range, ie about 7000 km in this case.  While sky wave propagation requires frequencies above the AM band, the lower frequency you go, the better the ground wave follows the curvature of the Earth.  Once the ground wave gets to an ocean, it follows the curvature of Earth with somewhat less attenuation.  So coverage of at least half the surface of the Earth is possible, perhaps more.  1.122.88.140 (talk) 06:54, 14 August 2013 (UTC)


 * Thanks all, especially Spinning  Spark . So, it seems that getting audible vibration is fairly reasonable with the right setup. I understand why e.g. fences would need grounding to shunt off induced currents, and the basic idea behind magnetostriction. But I'm still confused on how a single spring or radiator (no resistors, transistors, etc) could act to demodulate an AM signal, so that the actual broadcast is heard, and not just some rhythmic "vibration in sympathy". Wouldn't it have to do something similar to that described at Demodulate? I suspect that this would rely on some really good luck regarding the nature of the spring, and the frequency of broadcast. That is, I don't expect to have any old spring act as a complete radio whenever it's sufficiently close to a powerful broadcast. Does that sound right? Bonus question: could I "tune" a spring to work this way for a specific broadcast tower/frequency, provided that I can get rather close to a 50kW AM tower? SemanticMantis (talk) 15:37, 14 August 2013 (UTC)
 * Magnetostriction is inherently non-linear. In mechanical filters that use magnetostrictive transducers, small magnets are sometimes placed near the transducers to bias them into a near linear region, much as DC bias is used to bring a transistor into a linear region.  If you are looking to make the mechanical equivalent of a diode in your spring experiment you could attach a rod to one loop of the spring and then place a stop to catch the rod in one direction.  This effectively acts as a mechanical diode.  Spinning  Spark  19:41, 14 August 2013 (UTC)
 * Thanks again! SemanticMantis (talk) 20:40, 14 August 2013 (UTC)