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Einstein's Derivation, why is light velocity c-v and c+v?

  1. Aug 20, 2005 #1
    Why is light velocity c-v and c+v in Einstein's derivation, On the Electrodynamics of Moving Bodies (Annalen der Physik vol XVII 1905 p 891-921)?
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  3. Aug 20, 2005 #2


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    If you are referring to
    § 2. On the Relativity of Lengths and Times
    [tex]t_B - t_A = \displaystyle \frac{r_{AB}}{c-v}[/tex] and [tex]t'_A - t_B = \displaystyle \frac{r_{AB}}{c+v}[/tex]
    (see http://www.fourmilab.ch/etexts/einstein/specrel/www/ ),
    light velocity is c.
    [tex]t_B - t_A = \displaystyle \frac{r_{AB}}{c-v}[/tex] comes from [tex]c(t_B - t_A) = r_{AB} + v(t_B - t_A)[/tex],
    where [tex]c(t_B - t_A)[/tex] is the distance that light travelled, and [tex]v(t_B - t_A)[/tex] is the distance that the moving-observer (the rod) travelled.
    Note that all of these variables are measured in the "stationary system".
  4. Aug 20, 2005 #3
    Thanks. I got that. Now what's the meaning to the expression, "If we place x'=x-vt, it is clear that a point at rest in the system k must have a system of values x', y, z, independent of time" and the sentences that come next? What is being set up for the big equation that follows? What is [tex]\tau[/tex]? I can't get past Einstein's tortured explanation.
  5. Aug 22, 2005 #4


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    A while ago on this usenet thread I posted my own attempt to follow Einstein's derivation of the Lorentz transform in his original 1905 paper. Here it is:
  6. Mar 11, 2008 #5


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    are you Daryl McCullough? or no, what is a different post in that thread? i thought this post from Daryl was interesting (answering essentially why it's [itex]8 \pi[/itex] instead of [itex]4 \pi[/itex], where that extra factor of 2 comes from). i just thought it would be cool to point that out to Daryl if you're the same.
  7. Mar 11, 2008 #6


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    No, my post was this one.
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