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JollyOlly's book 'A Roller Coaster Ride through Relativity'

  1. Jan 21, 2014 #1

    ghwellsjr

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    I'm concerned about your book. I got up to this part:

    And I got very concerned. Although what you say is true, the fact that you put the last part of the quote in a box, like you did for what you called the Fundamental Principle and its restated version, gave me the impression that you believe that part of the quote is his second postulate. But you are specifically stating that there really is only one principle or postulate and this misrepresents not only what Einstein said but also the fundamental basis for Special Relativity.

    The actual statement of his second postulate from the introduction of his 1905 paper is:

    Or later at the beginning of section 2:

    When he says the "definite velocity c" or the "determined velocity c", he is referring to his comment at the end of the first section where he defines c to be the measured two-way speed of light.

    But his second postulate is about the unmeasurable one-way speed of light which cannot be part of the first postulate. That's what he means by "propagated" or "ray". He's defining the one-way speed of light to be identical to the two-way speed of light in any system of co-ordinates.

    This is my concern about your book. Do you eventually clarify this issue?
     
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  3. Jan 21, 2014 #2
    I agree that I have misrepresented (I would prefer to say reinterpreted!) Einstein's original postulates.

    I suppose I have never really understood the need for the second postulate. After all, why should the speed of light depend on the speed of the emitting body in the first place? If I proposed a new law of physics which said that 'the speed of sound is independent of the speed of the emitting body' I would be laughed out of court!

    I am afraid I do not understand your distinction between one-way and two-way speeds of light. Please explain.
     
  4. Jan 21, 2014 #3

    ghwellsjr

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    I don't think Einstein would like this.

    Now I know Einstein wouldn't like this.

    Einstein published his theory in 1905 before De Sitter performed his experiment showing that it was true. But without an experiment or a postulate, he couldn't just leave it hanging. However, the competing theory at the time, Lorentz Ether Theory (LET) would also claim this part of Einstein's second postulate.

    The assertion that light propagates at c in any inertial frame is what distinguished Einstein's new (in 1905) theory from LET which claimed that light propagates at c only in one universal, absolute, but unidentifiable, ether rest frame.

    Maybe you should read One-way speed of light.
     
  5. Jan 21, 2014 #4

    PAllen

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    Newton thought the speed of light depended on the source. So did Ritz, with his emission theory. Galilean relativity can be trivially maintained in such a theory without giving up absolute time.
     
  6. Jan 21, 2014 #5
    gwellsjr - Thank you for the reference to one-way speed of light. If I have understood correctly, the statement that the one-way speed of light is the same as the two-way speed (as implied by Einstein's second postulate) is equivalent to the statement that the one-way speed of light is isotropic - ie the same in all directions.

    While a rigorous account of SR might have to take into account the possible anisotropy of space and time, in an introductory account I have assumed without question that the laws of physics are isotropic and the same everywhere in space.
     
  7. Jan 21, 2014 #6

    WannabeNewton

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    Yes.

    There's no real need to consider clock synchronization conventions, for inertial frames, other than Einstein's in a standard SR textbook because other clock synchronization conventions in inertial frames will result in (at the best) non-orthogonal simultaneity hyperplanes and (at the worst) curved simultaneity hypersurfaces; the resulting formulas for Lorentz transformations etc. will be absolutely horrendous when compared to the standard formulas obtained using Einstein's synchronization convention. However IMO it is important to distinguish between the one-way and two-way speeds of light since the latter can be measured without any clock synchronization convention whereas the former can only be measured after such a convention is defined and adopted-the two-way speed of light depends only on local time whereas the one-way speed of light requires a global time coordinate constructed from a consistent method of clock synchronization.
     
  8. Jan 21, 2014 #7

    bcrowell

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    Many authors present relativity with the two 1905 postulates telescoped into one. It's perfectly logical. IMO it's silly to make an affectation out of presenting things in the author's original words.
     
  9. Jan 21, 2014 #8

    A.T.

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    Without the second postulate you just have Galilean Relativity. The emission theory is compatible with the 1st postulate, but not with the 2nd. You need the 2nd postulate to distinguish SR from such theories.
     
  10. Jan 21, 2014 #9
    Thank you bcrowell, your support is appreciated
     
  11. Jan 21, 2014 #10

    Nugatory

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    This is a long-running debate, and there's an element of personal taste to it.

    A fairly representative earlier thread would be: www.physicsforums.com/showthread.php?t=670253 and a search of this forum for the keyword "postulate" will find many more.
     
    Last edited: Jan 21, 2014
  12. Jan 21, 2014 #11

    PAllen

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    Note that even the pure relativity + homogeneity + isotropy frameworks result in either Galilean relativity or SR. Emission theory accounts for MM experiment, and (with extinction) almost all experiments until the 1970s. So to choose, you have to say:

    - based on a series of sophisticated experiments, our universe appears to be SR, with light speed as the invariant speed.

    or, you add a light speed is independent of emitter speed as a postulate; or that Maxwell's equations describe EM for all inertial frames; or something equivalent.

    But you have to add something, and the something is not 'trivial'.
     
  13. Jan 21, 2014 #12

    Nugatory

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    I find it most natural to assume that Maxwell's equation describe E&M for all inertial frames - I'm on record as paraphrasing the second postulate as "And I really mean the first postulate, even when it comes to E&M". But as I said above, there's an element of personal taste in that preference.

    I agree that it's not trivial; the half century between Maxwell and Einstein demonstrates that.
     
  14. Jan 22, 2014 #13
    Also, before you go calling something a subterfuge you really ought to fully understand what "most scientists" said and meant in a technical sense. Most EM specialists from 1890 to 1920 were exceptionally adept and could speak, interpret and create EM theory to the extent that today's educational system arguably does not produce with equal quality.
     
  15. Jan 22, 2014 #14
    I had in mind the theory that motion through the aether would cause a real physical contraction in the length of one of the arms of the MM interferometer and that the absence of a measurable shift in the fringes implied that the Earth was 'dragging the aether along with it'

    I am sorry if my use of the word subterfuge caused offence; please bear in mind that my book is intended to explain the amazing predictions of SR to the intelligent layperson and is not intended to be an authoritative exposition of the historical antecedents of Einstein's theory.
     
  16. Jan 22, 2014 #15
    Would you have some quick references to experiments which indicate inadequacies of Emission theory + Extinction? Spontaneously, I wonder if SU(2) topology must be further invoked, such as to account for the Sagnac effect.
     
  17. Jan 22, 2014 #16

    PAllen

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    One example is essential invariance with respect to emitter speed of neutrino speed. Extinction does not apply to neutrinos. Sagnac effect experiments conducted all in vacuo (first done in the mid 1960s, I believe) also act as refutations. Also, around this time (mid 60s) versions of the Majorana experiment all in vaccuum were done.
     
  18. Jan 22, 2014 #17
    Thanks. I think the Majorana vacuum experiment(s) indicate that macroscopic extinction doesn't account for relativistic effects, but it doesn't eliminate microscopic (single particle generated) extinction in receivers as a cause.
     
    Last edited: Jan 22, 2014
  19. Jan 22, 2014 #18

    PAllen

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    How is single particle extinction (that also includes neutrinos) different from assuming an emitter independent invariant speed? Seems just like changing the words you use at this point. The point is that macroscopic extinction is an an observable phenomenon. After this is ruled out as an explanation of SR effects, you are left with games equivalent to LET: a set of formal definitions that can be adjusted to match any experiment based on effects that can never be directly observed.
     
  20. Jan 22, 2014 #19
    Single particle extinction would theoretically leave some tiny residual field components from the emitter that couldn't be detected in the motion or momentum of the receiving particle. Those don't exist in the assumption of emitter independent invariant speed (or anything associated with LET). But right, with our current technology we're unable to make those potential observations.
     
  21. Jan 22, 2014 #20

    PAllen

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    You keep ignoring the neutrino experiments. This show that high speed emitters of neutrinos have no effect on neutrino speed.
     
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