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C independent of observer and emitter

  1. Oct 8, 2007 #1
    When we say that the the velocity of light, c, is independent of the velocity of emitter or the observer, does it mean that ..

    if we are in an spaceship and measure c with Fizeau arrangement we will obtain the same result, no matter the velocity of the spaceship nor the velocity of the emitter ?

    Fizeau experiment : "http://laser.phys.ualberta.ca/~egerton/c&ether.htm" [Broken] drawing: Fizeaus method

    ( and no matter the angle between the Fizeau device and the direction of the incoming light )
    Last edited by a moderator: May 3, 2017
  2. jcsd
  3. Oct 8, 2007 #2
    Yes, this is correct.

    Last edited by a moderator: May 3, 2017
  4. Oct 8, 2007 #3


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    i dunno what example experiment that Einstien was referring to a century ago, but i always thought that, unlike for our everyday experience of viewing particles emitted by some "particle emitter", it didn't matter who was holding the flashlight when two observers, moving relative to each other at constant velocity, both observering the same beam of light, measure the speed of propagation of that beam of light to be the same. if the flashlight is moving (relative to you) at a speed of c/2, that light does not get a speed "boost" of c/2 as one might have expected prior to the previous century. both observers, the one holding the flashlight (we would expect that guy to measure light's speed to be c), and the one flying by the first at a speed of c/2, when both are looking at the same beam of light, they measure its speed to be the same c.
  5. Oct 9, 2007 #4
    May I ask why you have emphasised the word 'same', here?
  6. Oct 9, 2007 #5


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    if you are standing on the side of the street and i am riding in a car going past you (from west to east) at 10 m/s and we both have our own baseballs that we throw in an easterly direction at 15 m/s, when we observe the same ball that we threw, we see that they are moving (relative to ourself) at 15 m/s. but when you look at my baseball, you see it as moving at 25 m/s easterly and when i look at your baseball, i see it as moving at 5 m/s easterly. if we observe the same ball, we observe different speeds. we both have "ball emitters".

    now replace the car with a spaceship (that i am in) moving at c/4 (relative to you) and we both have flashlights ("light beam emitters"). now, even before Einstein, we would have both expected to measure the speed of light coming from our own flashlights to be the same c (but we are looking at different beams of light).

    but, before Einstein, we may have expected that if we look at the beam of light coming from other's flashlight to different from c. before Einstein, you might expect to measure the beam of light coming from my flashlight to be 5/4 c and i mignt expect to measure the beam of light coming from your light emitter to be propagating at a speed of 3/4 c. we might expect, if we are moving relative to each other and looking at the same beam of light (either both of us are looking at my beam of light or both are looking at your beam of light) for the speed of propagation to be different.

    but the reality is that, even though we both have flashlights and we both think the speed of light coming from our own flashlight is c, when either one of us looks over to the other beam of light (so we are both observing the same beam of light, not two different beams of light, each that originated from the flashlight we're carrying), we observe the same speed c, even though one of us is carrying the emitter so the other might think that that beam of light got a "boost" in speed. but it doesn't, even though, in the case of baseballs, it does.
  7. Oct 9, 2007 #6
    And this has ever been done ?
    I mean, measuring c of light coming from stars, the way Fizeau did ( with a mechanical device, not measuring wavelength ).
    measuring the time that ellapses from one signal, coming from one moving satellite, been received at London and at NewYork.

    Could you tell me about an experiment ?
  8. Oct 9, 2007 #7


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    There's a list of references on experimental tests at http://www.math.ucr.edu/home/baez/physics/Relativity/SR/experiments.html

    one of the classic tests is to look at binary star systems. A google found a translation of de-Sitter's paper here.

    Cosmological tests like the above are what you asked for, but wouldn't necessarily rule out Ritz's "extinction" theory. However, tests have also been done with rapidly moving terrestrial sources generated by particle accelerators that address the "extinction" issue.
    Last edited: Oct 9, 2007
  9. Oct 9, 2007 #8
    One convincing test of the light speed independence on the velocity of the source was performed in

    T. Alvager, F. J. M. Farley, J. Kjellman and I. Wallin, "Test of the second postulate of special relativity in the GeV region", Phys. Lett. 12 (1964), 260

    They measured the speed of gamma quanta emitted by [itex] \pi^0 [/itex] particles moving with high velocities (the gamma factor of pions was greater than 45). The speed of light was measured by a direct time-of-flight technique, and was found equal to c, as expected.

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