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My response to an anti-relativist

  1. Mar 27, 2008 #1

    aaj

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    I chanced upon an article http://homepage.mac.com/ardeshir/TrainDisprovesRelativity.html which sought to show through a thought experiment that SR is inherently contradictory.

    Now I am not a pro in Relativity but from whatever understanding I have built up, here is the response that I sent across to the author. Guys, can you comment whether I make a point here or make a fool of myself?

    I would like to point out that the fallacy you are making is the same one as in the commonly discussed 'Twin Paradox'.

    You basic gist is that at the end of the thought experiment, it is possible to determine in an absolute sense whether it was the track/ platform that were moving or whether it was the train that was moving. You conclude on this basis that SR is contradictory since it refutes the concept of absolute motion.

    Let me now explain why your argument is incorrect.

    You believe that the whole thought experiment is symmetrical from the point of view of the platform and from the point of view of the lady riding in the train. Therefore you say that if SR is correct, either of them should not be able to conclude that one of them was moving in an absolute sense. However, you are mistaken in your belief that the situation was symmetrical. In fact, for the lady and the man both to be able to take the two sets of measurements (distance between the marks on the tracks and distance between the ends of the train in the frame of the platform), the train would need to stop and then the two of them would take the measurements. However, here is where the symmetry ends. The train has undergone an 'acceleration' in order to stop to a halt. 'Acceleration' is absolute even according to Relativity. So there is a clear mismatch in the experiences of the lady and the man. The lady in the train felt the train decelerate whereas the man felt nothing of the sort. Hence, since the two observers have not gone through the same experiences, there is nothing contradictory about them concluding that one of them did accelerate. Infact, the observations of the marks would indeed permit them to conclude that one of them accelerated and this would be consistent with what they would have experienced. And as for Relativity, it has no objection to absolute acceleration being detected. Hence your thought experiment does not disprove special relativity.

    A similar confusion is responsible for the Twin 'paradox'. People think both the twins are in a symmetrical position and therefore it is paradoxical for one to be younger when they meet. However, a similar reasoning as above reveals that for the twins to meet again, one of them has to accelerate and this causes the dis-symmetery in experiences. And then, its not paradoxical any longer to see them age differently on return.

    It all boils down to the fact that Relativity does believe in the concept of absolute acceleration even though it does not believe inthe concept of absolute velocity.

    Please let me know your comments.
     
  2. jcsd
  3. Mar 27, 2008 #2

    Ich

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    Well, both. Your argument is ok, but your belief that he will listen to arguments is extraordinarily naive. (How do I produce those dots on the i?)
    Expect to be insulted by him as narrowminded and too stupid to discuss with, at least if you insist on your point.
     
  4. Mar 27, 2008 #3

    jtbell

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    Staff: Mentor

    If you're using a Mac, hit option-u (a.k.a. alt-u) followed by i: naïve.
     
  5. Mar 27, 2008 #4

    JesseM

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    I don't think your response is right--the train thought-experiment, which was discussed by Einstein here and here, does not involve any accelerations in the period of time that's being analyzed, where did you get the idea that it does? Throughout the window of time where the lightning strikes occur and the light heads towards the center of the train, both the train-observer and the track-observer are moving inertially. That the train may have accelerated at some point in the past, prior to this window of time that we're analyzing, is wholly irrelevant.

    I discussed what I thought was the problem with the guy's argument on this thread--basically he seems to miss the entire point of the thought-experiment, which is that if the light hits the observer on the train at two different times, this can still be consistent with the idea that both light beams travel at the same speed in her frame as long as she assumes the two lightning strikes themselves happened at different times (non-simultaneously) in her own frame. This is the relativity of simultaneity, and the point of the thought-experiment is to illustrate why simultaneity must be relative in the theory of relativity.

    I've tried emailing this guy to discuss it, and although he was not particularly rude, he didn't seem to get the point and eventually just stopped responding.
     
    Last edited: Mar 27, 2008
  6. Mar 28, 2008 #5

    Ich

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    Thanks, but I use a pc. It's alt+139 there: ï. It took me a while to find out what it's called: diaeresis.
     
  7. Mar 28, 2008 #6

    aaj

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    JesseM, I think you did not read the full argument of the author in the link that I provided. The argument that was discussed in the thread provided by you was a different one by the same author.

    Here is the link I am referring to again. http://homepage.mac.com/ardeshir/TrainDisprovesRelativity.html
    Here the author is trying to show that an extension of the train experiment allows determining in an absolute sense whether the train was moving with respect to the platform or vice versa.

    It is for this that I provided the response. If you read the argument, you will realize that it depends on the fact that the train had to stop and return to the station. Once that happens, the symmetery between the man and the lady breaks since the lady has not remained in a single inertial frame. There is, then, nothing paradoxical about both of them making observations and concluding about who had not remained an inertial observer for the length of the experiment.
     
  8. Mar 28, 2008 #7

    JesseM

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    Ah, sorry about that, I was indeed thinking of this page by the same author. You're right that on the page you link to, the key is that the train-observer accelerates and the track-observer doesn't. If we allowed the train to continue on inertially (assume this experiment is done in space with zero gravity and no friction, so that the train will continue at constant velocity even if it's not touching the tracks), while instead accelerating the tracks and the track-observer until they are at rest relative to the train, then the situation will now be reversed, with the burn-marks on the tracks being closer together than the burn-marks on the train.
     
  9. Mar 28, 2008 #8

    aaj

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    Exactly. And either way, the observations will enable the man and the lady to correctly deduce whether the train had accelerated in order to stop in the fram of the platform or whether the platform had accelerated to be in the frame of the moving train. And, Relativity has no objection to them being able to deduce absolute acceleration in this manner.
     
  10. Mar 28, 2008 #9
    No ... that's not right, is it?

    First, the part I'm sure we agree on:
    The guy's argument is clearly wrong because he neglects to take into account the fact that the lighting strikes will not be simultaneous to the train. Rather, the observer on the train will first see the lightning hit the front of the train, and then later the rear. The time delay will be such that the rear will have moved forward on the track far enough that the train observer will see the scorch marks on the track separated by a distance equal to the Lorentz-contracted distance that the guy on the track measures (L'/gamma).

    Now, if the track gets accelerated up to the train's speed, this Lorentz-contracted spacing seen by the train observer will expand until the track is at rest w/rt the train and the distance is L', as it was for the observer on the ground. In other words, the spacing on the track grows from the Lorentz-contracted distance to its rest length, i.e L'.

    This is the same result that they got when decelerating the train: scorch marks at distance L in the train frame and distance L'=L/gamma on the tracks. Those are "rest lengths", so they should always measure those values when at rest w/rt to them, regardless of who accelerated. That's where we disagree, I believe.

    The asymmetry has only to do with the frame in which the lighting strikes were defined to be simultaneous - in this case, the frame of the tracks. If the experiment were run such that it was the observer on the train who saw both flashes strike the train at the same time, then the guy on the track would see the rear strike hit first, then the front. In the time interval between, the train would have moved far enough that the distance between the scorch marks on the tracks would be the inverse Lorentz contraction of the train's length. Now when the two come to rest together, they see the scorch marks on the track farther apart than on the train, by a factor of gamma, again.

    The point is that the scorch marks are always closer together in the frame where the strikes were simultaneous, since the other frame's measuring rods are contracted in that frame.

    Did I get that right? I'm a little bleary-eyed at the moment ...
     
  11. Mar 28, 2008 #10
    Well, I just explained my objection to this in my response to JesseM's post, but I'll add just this: When the lightning strikes occur, no one has yet accelerated, so there is not yet any asymmetry due to that. At this point, however, there is are two rest lengths between scorch marks - one for the pair on the train and one for the pair on the tracks, and they are what they are. No subsequent acceleration will change their rest lengths. Thus, when the two pairs are brought into the same frame so that they are at rest with respect to each other, the comparison is simply the comparison between their rest lengths, regardless of who accelerated into whose frame. Acceleration does not physically alter the rest length of anything, only the lengths observed by moving observers.

    The critical asymmetry here is in the selection of the frame in which the strikes were simultaneous. That's what makes the distance on the tracks longer.
     
  12. Mar 28, 2008 #11
    BTW, I see that the author of that web page has "N.D." after his name ... "NonDenominational"? "No iDea"? "Nincompoop Dimentioso"? "Noodle-headed Dope"?

    alas ... probably only "No Degree".
     
  13. Mar 28, 2008 #12

    JesseM

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    Yes, you're quite right belliott, I had forgotten to take into account that the strikes were non-simultaneous in the train's frame, so even if the tracks then accelerate to come to rest relative to the train, it will still be true that the distance between burn marks on the track is smaller than the distance between burn marks on the train, just as it would be if the train acclerated to come to rest relative to the track. The symmetry would be better illustrated if we compared these two scenarios:

    1. Two lightning strikes happen which are simultaneous in the track frame, then the train accelerates to come to rest relative to the tracks. Result: burn marks are farther apart on the train than on the track.

    2. Two lightning strikes happen which are simultaneous in the train frame, then the track accelerates to come to rest relative to the train. Result: burn marks are farther apart on the tracks than on the train (by exactly the same factor as the burn marks on the train were further apart in case #1).

    edit: scratch that, as you said above it doesn't even matter which one accelerates, all that matters is the distance between the two strikes in each object's own rest frame when the strikes actually occur, whichever resting distance is greater, that's the one that will still be greater when one accelerates so that they are at rest relative to one another. And all that matters in determining which one sees a greater resting distance between burn marks is which one observed the strikes to be simultaneous. Thanks for clearing up this issue!
     
    Last edited: Mar 28, 2008
  14. Apr 2, 2008 #13
    the key is that the train-observer accelerates and the track-observer doesn't.
     
  15. Apr 2, 2008 #14
    Well, no ... if you read the previous posts, you'll see that there's some agreement that it's not a matter of which observer accelerates to the other's frame, but rather it's a matter of the frame in which the lightning strikes were simultaneous.

    If you disagree, then please read the previous posts and tell us why.

    thanks.
     
  16. Apr 2, 2008 #15
    I object to the terms "relativist" and "anti-relativist". It seems to imply that people are taking up dogmatic or ideological positions. They are not(usually). People are investigating mathematical models, and their agreement, or disagreement, with experiment.

    I like to think of everyone engaged in such work as a "scientist".
     
  17. Apr 2, 2008 #16
    No the key is to not allow any kind of accelerations at all!
    Any measurements or settings of any kind made before the various frames are established with a constant fixed speed differences between them are of no use in a SR experiment.
     
  18. Apr 2, 2008 #17
    However, here is where the symmetry ends. The train has undergone an 'acceleration' in order to stop to a halt. 'Acceleration' is absolute even according to Relativity. So there is a clear mismatch in the experiences of the lady and the man. The lady in the train felt the train decelerate whereas the man felt nothing of the sort. Hence, since the two observers have not gone through the same experiences, there is nothing contradictory about them concluding that one of them did accelerate. Infact, the observations of the marks would indeed permit them to conclude that one of them accelerated and this would be consistent with what they would have experienced. And as for Relativity, it has no objection to absolute acceleration being detected.

    This is the accepted version provided by the General relativity. However, the problem is this; Is this acceleration of the train-observer and the track-observer with respect to each other or another reference frame. Since acceleration is a change of velocity over time and since all observers have equal right in claiming that they are at rest and the other observer has the relative velocity, they must retain the right of attributing the change of the relative velocity, i.e. acceleration also. The train-observer must therefore have equal right of attributing the acceleration to the track-observer system, despite the commomsence "feeling" of the acceleration which can not be considered as a meaningfull argument in physics. If we are observing the system from a third reference system with adifferent relative velocity, then thing would be quite different such as your thought experiment. This also applies equally to the twin paradox.
     
  19. Apr 2, 2008 #18

    JesseM

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    This is true, but as noted by belliott before, acceleration actually has nothing to do with the fact that when the train and the tracks come to rest relative to one another, the burn marks on the train are farther apart than the burn marks on the track. This will be true regardless of which one accelerates, and the reason is that the strikes were assumed to be simultaneous in the frame of the tracks; if the strikes were simultaneous in the frame of the train, then when they came to rest relative to one another the burn marks on the track would appear farther apart, again regardless of which one accelerated.
     
  20. Apr 3, 2008 #19

    This difference between the burn marks is because we are using a third reference frame, one in which the light signals originate, assumed to have a different relative velocity wrt either the train or the tack, in each scenario of the experiment, in so far as the light signals are either simultaneous or not.
     
  21. Apr 3, 2008 #20

    JesseM

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    Huh? There is no assumption of a third reference frame in this problem, just two lightning strikes which happen simultaneously at different positions in the frame of the tracks.
     
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