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Special Relativity, 2nd postulate, and a dilemma

  1. Feb 12, 2008 #1
    Re-reading "On the Electrodynamics of Moving Bodies" by A. Einstein from 1905 (English translation I found on the internet). Contrary to much of what I've seen (the speed of light is the same for all observers or whatever) the 2nd postulate actually states that light is propagated in empty space with velocity c, and this velocity is independent of the state of motion of the emitter. I've been playing around with some thought experiments employing very basic emitters and receivers (observers). I have a point emitter of light "A". the wavefronts of light emanating from "A" will be a sequence of expanding spheres with "A" inside the spheres. is this concept agreeable to all? makes sense to me.
    Now the question is, can "A" move away from the center of these spheres. The 2nd postulate seems to say yes, once the light energy is travelling in space its propagation is independent of anything "A" does after that particular wavefront was emitted. again, agreed?
    However if I am "A", can I move toward the spheres in one direction causing in effect my own Doppler shift? I picture "A" as seeing itself always at the center of its propagating wavefront spheres. an independent observer "B" can receive light waves propagated from "A" and discern it as coming from a particular direction, that is where "A" was when that wave was emitted. "A" may have moved since then relative to "B" but the wave source direction doesn't change (see "Planetary Aberration" - the sun moves in the time it takes for the light to get here so what we really see is where it was). But does this effect define a point fixed in space where the sun was, where the present wavefront sphere is centered? I keep coming back to the idea that even SRT has, hidden within it, a point-of-view related to fixed space. in fact go back and re-read the 2nd postulate. light propagates with respect to "space", and the emitter can move with respect to "space", therefore "space" seems to be a fixed or defineable reference.
    thoughts?
    rg
     
    Last edited: Feb 12, 2008
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  3. Feb 12, 2008 #2

    Doc Al

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    Staff: Mentor

    Here's a quote from a translation of that paper:
    1. The laws by which the states of physical systems undergo change are not affected, whether these changes of state be referred to the one or the other of two systems of co-ordinates in uniform translatory motion.

    2. Any ray of light moves in the "stationary'' system of co-ordinates with the determined velocity c, whether the ray be emitted by a stationary or by a moving body.​
    Sounds pretty clear to me that the second postulate states that the speed of light is the same with respect to any observer (who is then the "stationary" system), regardless of the speed of the emitter.

    Reference: ON THE ELECTRODYNAMICS OF MOVING BODIES
     
  4. Feb 12, 2008 #3
    That's the one I've been reading. Those are "reflexions" starting section 2 based on the introductory postulates. I am focused (sorry) mainly on the emitter and trying to sort out what its relationship is with the light waves it has been emitting. Are these spheres? for a point emitter in free space em energy must propagate equally in all directions. I see spheres. can the emitter then move after some spheres have been launched into space? of course. there could be acceleration, but there could also be constant translatory motion. What effect does this motion have on the positions in space of the spheres and the emitter? seems to depend on an observer. I arrive at two different effects on the waves of light, from POV of "A" and POV of "B" if they are moving relative to each other.
    rg
     
  5. Feb 12, 2008 #4

    Doc Al

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    Staff: Mentor

    Not exactly. While Einstein certainly refers to the postulates in the introduction, the paragraph introducing the two principles that I quoted above states:
    The following reflexions are based on the principle of relativity and on the principle of the constancy of the velocity of light. These two principles we define as follows:​
    The two restated postulates are the "principles" referred to; the "reflexions" are his conclusions about "the Relativity of Lengths and Times", the subject of that section.

    I suspect I know what's bugging you. Imagine an emitter in an inertial frame A moving at some speed with respect to observer B in another inertial frame. That emitter emits a spherical shell of light at the exact moment that observer B passes by the emitter. In frame A, the light forms a sphere about the emitter; in frame B, the light forms a sphere about observer B.

    See this: The constancy of the speed of light is paradoxical
     
  6. Feb 12, 2008 #5
    Thanks. I'll check that out.
    The site looks familiar but I didn't have it bookmarked. Pictures should help (I'm basically a tech after all).
    rg
     
    Last edited: Feb 12, 2008
  7. Feb 12, 2008 #6

    rbj

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    it depends on whos point-of-view. from B's POV, yes. from A's POV, no.


    see, but the thing is this "space" is empty. no nothing at all in it. not even a something that you can measure movement or velocity against (what they might call an "aether"). you can measure movement and velocity relative to yourself, the observer.

    if there was an aether, like the wind we could sorta feel it breezing past our faces. knowing that sound needs a medium to propagate, if you can feel that wind blowing past your face from left to right, you will measure the speed of sound to be faster from your left than your right. but if you can't feel that aether blowing past your Michaelson-Morley inferometer, then you can't tell that your moving through it, and maybe you can't tell your moving through it because "it" isn't there.

    so, if it ain't there, if there is no meaning to the concept of this vacuum of nothingness blasting past your face at 0.9 c, then either you at A or the other guy at B have equal claim to being "stationary" (and it's the other guy who's moving) and you have to measure identical physical quantities identically. one of those quantities is the speed of propagation of the EM field, which, for both you and the guy at B is [itex]1/\sqrt{\epsilon_0 \mu_0} [/itex].
     
  8. Feb 15, 2008 #7
    Apparently my query is not an original thought (not that I believed it was . . . ).
    Reading wiki entry for Lorentz Ether Theory, there is a reference under the heading Ether where H. Poincare' said in 1900 that the ether explains where the ray of light is after leaving the emitter and before it reaches the receiver/observer. that's the basic concept of what I was asking about. Is there more info on this point of view?
    rg
     
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