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Why should the speed of light be the same in every frame of reference.

  1. Mar 21, 2012 #1
    If we have light, any particle, and maybe car, the particles travel at 0.98c and car let say 50m/s. Let them start moving at the same time to the given point let say 300 metres away. The difference between times of arrival of light and particle will be small compared to that between light and car. This means that light and particles spend more time together than the time light spend with car, hence the speed of light relative to particle is not the same as speed of light relative to car.
  2. jcsd
  3. Mar 21, 2012 #2


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    I have no idea what you could mean by "spend more time together".

    I suspect your error is that you are not taking into account the Lorentz contraction of time for the particles as seen from the car's frame of reference.
  4. Mar 21, 2012 #3

    D H

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    You are implicitly assuming a Newtonian universe here, where length and time are absolute.

    That the speed of light is experimentally the same to all observers was a big clue that this Newtonian view is not how the universe works, at least not at very high speeds. And indeed, length and time are not absolute.
  5. Mar 22, 2012 #4
    Hi HallsofIvy: By saying they spend more time together I mean it take some time for light to leave these particles back if they are travelling at may 0.9c.
  6. Mar 22, 2012 #5
    If you're sitting in the moving car, and you measure the speed of light, and if your sitting on the particle and you measure the speed of light, you will get the same speed.

    This is because time dilates and length contracts in the moving frame of reference WHEN MEASURED BY A FRAME AT REST. When measured from inside the moving frame, time and length are normal. These factors mean that the speed of light is always measured to be the same in all frames, regardless of their relative motion.

    BTW, the light and the particle are only together at the instant they start. The light and the particle spend no time together because the light still moves at c, as measured from the particle.
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