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An extended look at light in SR

  1. Jan 9, 2007 #1
    Hey guys

    Ok, after my last whimsical attempt at expressing my lack of understanding regarding c being constant in all reference frames, i did some digging and have come back equally bewildered but with some more ammo.

    Ok, so as far as i know it goes like this. Galilean relativity states that the laws of nature are the same in all [intertial] reference frames, i accept that, proof is all around us and it is logical. Then some guys found that Maxwells equations were not invariant under a Galilean transformation but were under Lorentz. This showed that either absolute time was wrong or relativity was wrong. They threw out relativity and proposed the aether. They then assumed that Maxwells equation using the permiability and permiativity of free space to come up with the speed of light was in reference to the aether as an absolute base speed. The idea of the aether was then thrown out by the big man himself and replaced with variable time and restoring the ideas of relativity.

    Now i get that. I understand why this had to be thrown out if that was true etc. What i DONT get is the bit with Maxwells theoretical measurement of c. It is a simple equation (c=1/sqrt(ue)) with no spacial or temporal variables and so one would think, at a glance, that this measurement is indeed independant of various transformations and so is a relativistic invariant.

    My question is this: would the measurement of these constants not produce different values in different reference frames? or do these differences cancel each other out to produce the all purvading speed of light we all know and love?


    EDIT: I am also aware of a.) the Michaelson Morley experiment and b.) that the equation i used is derived from Maxwells equations which use can be transformed using Lorentz transformations
    Last edited: Jan 9, 2007
  2. jcsd
  3. Jan 9, 2007 #2
    No. Constants are not reference frame dependent. This is why they are called constants.
  4. Jan 9, 2007 #3
    Well youre likening constant to relativistic invariant and im not sure they are exactly the same.

    I understand that person a conducting the experiment gets an answer which is the same as person b moving relative to person a when they stop relative to each other and compare answers.

    But if person A is watching person B conduct there experiment as theyre moving, surely they would witness a different outcome?
  5. Jan 9, 2007 #4
    What makes you think that that they are not the same?

    Not in the case of light speed. Are you trying to learn relativity one post at a time?
  6. Jan 9, 2007 #5


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    Clearly, yes. Your experimental results are only relevant to your reference frame. If you are watching someone else do the experiment, you are not doing it yourself. Making sense of (or predicting) the difference in results is kinda the point of the transformation equations (and the theory itself). Reread your own statement about what Relativity is!
  7. Jan 9, 2007 #6
    First off i think that was a little harsh naku. Secondly i well understand SR and the point of the transformations in getting from one reference frame to another and i have no problem with that. The problem arrises when the transformation concerns something that gives rise to an invariant.

    Sure if youre measuring mass it should be different and you can use Lortenz's equations to find the rest mass (relative to observer). But if the measurement gives rise to a different constant, as observed by person at rest, then they would measure the speed of light to be different for the moving persons reference frame. This, as far as i can see, violates the idea that light is a relatavistic invariant as you shouldnt need to use lortentz transformations to convert anything relating to the speed of light from one frame to another (unless as i stated before the differences cancel out)

    I appologise if im missing something obvious or if its gibberish but snidey comments are not the way to go.

  8. Jan 9, 2007 #7
    Well what sort of explanation are you looking for? I'm not sure this approach will help. I like to think c is at infinity (because of the difficulties of transforming to that frame) and that the commonsense galilean transformations are just incomplete.
  9. Jan 9, 2007 #8
    I can justify to myself in my head why light behaves as it does, but thats not what this threadis about, thats the other thread i wrote. This is about understanding how other people go to this conclusion.
  10. Jan 9, 2007 #9


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    Staff Emeritus
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    But the measurement doesn't give rise to a different constant.
    The person at "rest" doing the same experiment gets the same results as the person in the "moving" frame gets, thus he arrives at the same constant.
  11. Jan 9, 2007 #10
    Well it's an experimental fact...

    (I guess there's been times when I also didn't know exactly what I didn't understand.)
  12. Jan 9, 2007 #11
    Ok i know im kind of going in circles but ill continue anyway. I understand that measurements done in different reference frames will appear equal to the people in their respective frames, ie it all changes in proportion (a crude explination i know) Thats fine, i have no problem with that so long as it applies to something that IS measured differently in different frames. Ie person moving appears heavier than when hes at rest. I have a problem with it when its concerns light which is supposed to not only be the same in each reference frame, but observed to be the same OUTSIDE that frame, ie observing someone whos moving releasing light etc

    am i making sense or not?

  13. Jan 9, 2007 #12
    No , you are not. Because you didn't read the experimental proof I gave you earlier HERE [/QUOTE]
  14. Jan 10, 2007 #13
    I am not after experimental proof as i already stated.
  15. Jan 10, 2007 #14


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    The speed of light is the same whether is is released by someone who is moving with respect to you or not. What the two frames will disagree on in the difference between the speed of the light and the object releasing it. The object releasing it(frame A) measures it as c and the observer in the other frame, B (the one in which the object is moving), measures it as the difference between c and the velocity of the releasing object.

    Even then, an observer in frame B would agree that an observer traveling in frame A would measure the difference between the object and the light as c. Because, according to B, A's clocks run slow, A's meters are contracted, etc.
  16. Jan 10, 2007 #15
    Then you are out of luck, there is no answer to incorrectly phrased questions.
  17. Jan 10, 2007 #16
    I understand the first part, and in the 2nd part are you saying that the information i was trying to describe is irrelevant unless its transformed to the relavent frame?

    Thats not what i said. I said i was looking for an explanation not an experimental proof.

    Anyway nevermind, ill figure it out. Thanks for your responses but ill have to sit down with my lecturer and try to work out how to convey my question better.

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