apricimo Posted January 6, 2011 Share Posted January 6, 2011 Can someone tell me what the exact experiment was and who performed it. The experiment which shows that speed of light is a constant independent of frame of reference. I understand (I think) that Einstein said it was independent of frame of reference, but how was the experiment performed which showed this? Link to comment Share on other sites More sharing options...
ydoaPs Posted January 6, 2011 Share Posted January 6, 2011 Can someone tell me what the exact experiment was and who performed it. The experiment which shows that speed of light is a constant independent of frame of reference. I understand (I think) that Einstein said it was independent of frame of reference, but how was the experiment performed which showed this? I don't know of the exact experiment, but I do know it falls out of the Maxwell Equations. I also know that it is a vital assumption for relativity and relativity is one of the best tested theories ever. Link to comment Share on other sites More sharing options...
apricimo Posted January 6, 2011 Author Share Posted January 6, 2011 I don't know of the exact experiment, but I do know it falls out of the Maxwell Equations. I also know that it is a vital assumption for relativity and relativity is one of the best tested theories ever. Yeah so that's what I've been trying to just satisfy within myself. Whether this assumption is based on calculations or real experiments. Anyone know this please reply. Link to comment Share on other sites More sharing options...
timo Posted January 6, 2011 Share Posted January 6, 2011 (edited) Several measurements are linked to in the Wikipedia article on "speed of light". With 134 references within this article (not all on measurements, of course) you should have plenty of material to browse through. Since I have good experience with the webpage (on other topics) I can also recommend the first hit (after WP) on Google: http://www.colorado.edu/physics/2000/waves_particles/lightspeed_evidence.html EDIT: Forget the above. I was somehow thinking you were looking for information how the speed of light is measured. In principle, you can of course test if it is constant by repeating the measurements under different conditions, e.g. by moving your laser source around. Edited January 6, 2011 by timo Link to comment Share on other sites More sharing options...
ydoaPs Posted January 6, 2011 Share Posted January 6, 2011 Yeah so that's what I've been trying to just satisfy within myself. Whether this assumption is based on calculations or real experiments. Anyone know this please reply. p->q p q If relativity is a good approximation of reality, then the speed of light is constant in all inertial frames. Relativity is a good approximation of reality(it has passed every test thrown at it), so it is logical to conclude that the speed of light is constant in all inertial frames. Link to comment Share on other sites More sharing options...
TonyMcC Posted January 6, 2011 Share Posted January 6, 2011 (edited) Michelson and Morley in 1887 tried to prove that the speed of light was not a constant and failed. It was soon seen that the reason their experiment had failed was because the speed of light is a constant (which surprised them). http://en.wikipedia.org/wiki/Michelson%E2%80%93Morley_experiment By the way any radar equipment works by determinig the time taken for EM waves to travel a given distance. Edited January 6, 2011 by TonyMcC Link to comment Share on other sites More sharing options...
apricimo Posted January 6, 2011 Author Share Posted January 6, 2011 Michelson and Morley in 1887 tried to prove that the speed of light was not a constant and failed. It was soon seen that the reason their experiment had failed was because the speed of light is a constant (which surprised them). http://en.wikipedia.org/wiki/Michelson%E2%80%93Morley_experiment By the way any radar equipment works by determinig the time taken for EM waves to travel a given distance. It's my understanding that they set out to see whether aether existed or not. By splitting light into two beams they measured if there was a difference in the path length should it travel through aether. They did this by turning their platform in to different angles with respect to "aether" and found there was no difference. How from that does it follow that light is the same speed independent of the observer? Link to comment Share on other sites More sharing options...
swansont Posted January 6, 2011 Share Posted January 6, 2011 It's my understanding that they set out to see whether aether existed or not. By splitting light into two beams they measured if there was a difference in the path length should it travel through aether. They did this by turning their platform in to different angles with respect to "aether" and found there was no difference. How from that does it follow that light is the same speed independent of the observer? M-M disproved the movement through the ether. Bradley, by measuring stellar aberration more than 150 years prior has shown we could not be stationary with respect to the ether. If we aren't stationary in it, and we aren't moving through it, it doesn't exist. No ether = no preferred reference frame against which one would measure the speed. Conclusion: it's the same in all frames. And with what ydoaPs posted; a mountain of experiments based on the assumption that it's true have worked as predicted, to a very high degree of precision. if it wasn't true they would not have worked. 1 Link to comment Share on other sites More sharing options...
apricimo Posted January 6, 2011 Author Share Posted January 6, 2011 And with what ydoaPs posted; a mountain of experiments based on the assumption that it's true have worked as predicted, to a very high degree of precision. if it wasn't true they would not have worked. Well, that's all fine, but it's also a bit of a stretch. Newtons laws worked for how long until they had to be modified. Thanks for the explanation about the prior experiment though. Link to comment Share on other sites More sharing options...
TonyMcC Posted January 6, 2011 Share Posted January 6, 2011 It's my understanding that they set out to see whether aether existed or not. By splitting light into two beams they measured if there was a difference in the path length should it travel through aether. They did this by turning their platform in to different angles with respect to "aether" and found there was no difference. How from that does it follow that light is the same speed independent of the observer? Yes you are correct in that they set out to prove the existence of aether and failed. They expected light to change speed according to movement through the aether. However the results of their experiment had a bearing on the acceptance of Einsteins work as the part of the link headed "Einstein and General Relativity" shows. I knew their experiment eventually had something to do with the speed of light, but sorry I was not completely accurate. http://en.wikipedia.org/wiki/Michelson%E2%80%93Morley_experiment Link to comment Share on other sites More sharing options...
Mike-from-the-Bronx Posted January 11, 2011 Share Posted January 11, 2011 The following link describes several different methods for measuring the speed of light. http://en.wikipedia..../Speed_of_light Scroll down to section 5 Mesurement. Link to comment Share on other sites More sharing options...
alpha2cen Posted January 12, 2011 Share Posted January 12, 2011 (edited) If we are(were) in the Dark Matter halo, the measured value will(would) be the speed of light through the Dark Matter halo. Edited January 12, 2011 by alpha2cen Link to comment Share on other sites More sharing options...
imatfaal Posted January 13, 2011 Share Posted January 13, 2011 Light does not interact with dark matter - or does so incredibly weakly - that's one of the reasons we cannot observe it directly. Link to comment Share on other sites More sharing options...
swansont Posted January 13, 2011 Share Posted January 13, 2011 Light does not interact with dark matter - or does so incredibly weakly - that's one of the reasons we cannot observe it directly. Light does not interact electromagnetically, but there would be relativistic effects; gravitational lensing and the Shapiro delay. Link to comment Share on other sites More sharing options...
John Cuthber Posted January 13, 2011 Share Posted January 13, 2011 I can calculate the speed of light from the measured properties of a vacuum.* How can I have a speed WRT a vacuum? I can't. So those properties must remain constant, no matter how fast I travel. So, no matter what my speed, I calculate that same value for c *Here's how http://en.wikipedia.org/wiki/Maxwell's_equations#Relation_between_electricity.2C_magnetism.2C_and_the_speed_of_light Link to comment Share on other sites More sharing options...
IM Egdall Posted January 14, 2011 Share Posted January 14, 2011 (edited) Can someone tell me what the exact experiment was and who performed it. The experiment which shows that speed of light is a constant independent of frame of reference. I understand (I think) that Einstein said it was independent of frame of reference, but how was the experiment performed which showed this? To answer the original question, the Michelson-Morley experiments did not prove Einstein's light postulate (speed of light independent of frame of reference). It just failed to find the so-called ether. The MM expeririments were conducted in a single frame of reference , the Earth. So they could not verify Einstein's light postulate one way or another. The first experiment to support Einstein's light postulate was the 1913 DeSitter binary star analysis. See link: http://en.wikipedia....star_experiment From a book I am writing on relativity for the non-expert: In the over 100 years since Einstein first proposed his light postulate, "thousands of scientific observations" have confirmed its validity.(i) For example, a vastly more accurate version of the de Sitter experiment was performed by MIT physicist Kenneth Brecher in 1977. His tests using Uhuru satellite images of X-ray pulsars verified Einstein's light postulate to one part in a billion.(ii) See link for more info: http://www.marksmodernphysics.com/ i Victor J. Stenger, Quantum Gods, Creation, Chaos, and the Search for Cosmic Consciousness, p. 74. ii Nigel Calder, Einstein's Universe, The Layperson's Guide, p. 176-178. Edited January 14, 2011 by I ME Link to comment Share on other sites More sharing options...
cos Posted January 16, 2011 Share Posted January 16, 2011 To answer the original question, the Michelson-Morley experiments did not prove Einstein's light postulate (speed of light independent of frame of reference). It just failed to find the so-called ether. I have a copy of a science documentary from some years ago in which Michelson himself refers to his pride in the fact that his experiment ratified special theory. The MM experiments were conducted in a single frame of reference , the Earth. So they could not verify Einstein's light postulate one way or another. I have been informed that the fact that the experiment was repeated six months later, when the Earth was orbiting the sun in the opposite direction, provides another frame of reference and that when the MMX is viewed by a sun located observer he applies the Lorentz contraction factor to the EW leg of same thus comes up with c however I shall reserve my judgment on that claim until such time as I read a paper written by same. The first experiment to support Einstein's light postulate was the 1913 DeSitter binary star analysis. It was then realized that because binary star systems are enshrouded by gas clouds that the quantum absorption-emission factor invalidated their application. Link to comment Share on other sites More sharing options...
Klaynos Posted January 16, 2011 Share Posted January 16, 2011 http://prl.aps.org/abstract/PRL/v99/i5/e050401 Looks like it might be useful... If I get a chance I'll give it a read on Monday... Link to comment Share on other sites More sharing options...
John Cuthber Posted January 16, 2011 Share Posted January 16, 2011 The MM experiments have done on a rotating table, but observed by a stationary observer. The results were the same as those with the table stationary. That's two different frames of reference. Rotating (wrt to Earth) and stationary (wrt Earth). Link to comment Share on other sites More sharing options...
cos Posted January 16, 2011 Share Posted January 16, 2011 (edited) The MM experiments have done on a rotating table, but observed by a stationary observer. The results were the same as those with the table stationary. That's two different frames of reference. Rotating (wrt to Earth) and stationary (wrt Earth). They were done on a rotatable table but I don't think it was rotating when the respective tests were actually carried out as this would involve aberration of the beams and would not ratify special theory as has been claimed (albeit not intended) on the basis of a non-inertial reference frame. Furthermore - the results of the experiments were not determined by observations of the actions of the beams made by an observer but by the evidence provided by the interferometer i.e. a determination, or otherwise, of a frequency shift. His observations of the interferometer determinations would be unaffected by the table's rotation. Edited January 16, 2011 by cos Link to comment Share on other sites More sharing options...
IM Egdall Posted January 17, 2011 Share Posted January 17, 2011 (edited) Imagine two identical guns A and B which shoot bullets at the same speed. One is pointed perpendicular to the other. And, as expected, the speed of bullet A is the same as speed of bullet B. Now we place the two gun set-up on a uniformly moving train and repeat the experiment. Observers on the moving train find speed of bullets A and B are still the same. (Per Galieo's dictum, in a uniformly moving reference frame, its motion has no effect on phenomena within this frame). Let's place the two gun set-up on a table which can be rotated. Again place all this on the uniformly moving train. No matter how you orient the table, bullets A and B still record the same speed for observers on train. Now substitute two light sources for the two guns, photons for the bullets. and a moving Earth for the moving train. You have, in principle, the MM experiment. So the speed was the same value for two perpendicular light beams for observers on the Earth. This is just like the speed of the two perpendicular bullets is the same for observers on the train. MM only showed that there is no "ether wind" slowing down one direction of light more than another. MM tells us nothing about Einstein's light postulate. A number of books on relativity get this wrong. They either imply or state directly that MM demonstrated Einstein's light postulate. But it does not. Edited January 17, 2011 by I ME Link to comment Share on other sites More sharing options...
cos Posted January 17, 2011 Share Posted January 17, 2011 MM only showed that there is no "ether wind" slowing down one direction of light more than another. MM tells us nothing about Einstein's light postulate. A number of books on relativity get this wrong. They either imply or state directly that MM demonstrated Einstein's light postulate. But it does not. "A number of books on relativity?" I venture to suggest that all books which support relativity (including physics textbooks) and which make reference to the MMX, insist that it ratifies SR - albeit not its intention. It was the null result of the MMX which led Fitzgerald and Lorentz to introduce the length contraction concept which, along with time dilation, is an absolutely indispensable aspect of SR and it is, for example, the length contraction factor applied by a sun located observer that is claimed to 'prove' the MMX 'ratification' of SR. I agree that the MMX does not ratify SR however arguing against such 'logic' is a waste of time. Link to comment Share on other sites More sharing options...
Mike-from-the-Bronx Posted January 17, 2011 Share Posted January 17, 2011 "A number of books on relativity?" I venture to suggest that all books which support relativity (including physics textbooks) and which make reference to the MMX, insist that it ratifies SR - albeit not its intention. It was the null result of the MMX which led Fitzgerald and Lorentz to introduce the length contraction concept which, along with time dilation, is an absolutely indispensable aspect of SR and it is, for example, the length contraction factor applied by a sun located observer that is claimed to 'prove' the MMX 'ratification' of SR. I agree that the MMX does not ratify SR however arguing against such 'logic' is a waste of time. I have to disagree with the opinion being expressed here. I have half a dozen physics text books that deal with SR in my bookcase. Not a single one says that MMX ratifies SR. What some say is that the results of MMX are consistent with what SR predicts. Not the same thing. From the book "Special Relativity" by A.P. French Copyright 1966, by Massachusetts Institute of Technology Page 73 "This null result is consistent with the proposition that the speed of light is the same in all directions with respect to a reference frame having an arbitrary (but unknown) motion through space." From the book "Modern Physics for Scientists and Engineers" Copyright 2006 by Brooks/Cole Page 77 "Efforts by Michelson and Morley proved in 1887 that the either the elusive ether does not exist or there must be significant problems with our understanding of nature." P.S. I can't speak for books and articles written by laymen. I don't pay any attention to them. Link to comment Share on other sites More sharing options...
IM Egdall Posted January 17, 2011 Share Posted January 17, 2011 (edited) I should have been more specific. I was talking about popular science books, not text books. I agree wtih Mike-from-the-Bronx. MMX is consistent with special relativity, but it does not ratify it. Good way of saying it. Edited January 17, 2011 by I ME Link to comment Share on other sites More sharing options...
John Cuthber Posted January 17, 2011 Share Posted January 17, 2011 They were done on a rotatable table but I don't think it was rotating when the respective tests were actually carried out as this would involve aberration of the beams and would not ratify special theory as has been claimed (albeit not intended) on the basis of a non-inertial reference frame. Furthermore - the results of the experiments were not determined by observations of the actions of the beams made by an observer but by the evidence provided by the interferometer i.e. a determination, or otherwise, of a frequency shift. His observations of the interferometer determinations would be unaffected by the table's rotation. Sorry, I'm getting my interferometry experiments muddled up http://en.wikipedia.org/wiki/Sagnac_effect I meant this one. Link to comment Share on other sites More sharing options...
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