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About Complete relativity.

  1. Oct 23, 2009 #1
    As we everybody know,in this universe, every body is in motion... Then there are no frames of references..Then how could we estimate the speed of light accurately. What about the relative speed..
  2. jcsd
  3. Oct 23, 2009 #2


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    Science Advisor

    Postulate 1 of special relativity: The speed of light is the same in all inertial frames.

    That's how.
  4. Oct 23, 2009 #3
    Let me see if I can explain what Einstein had in mind when he came up with special relativity.

    Now in special relativity, Einstein in one of his thought experiments envisioned a traveler on a train traveling at the speed of light. He had wondered if the person aboard the train looking into a mirror would catch up to the photons being reflected off of his face and not be able to see his own reflection. Of course, this made absolutely no sense to Einstein because if this were true, then various velocities at which people would travel would have to be added to the speed of light which would mean that our visual perception of things around us would be a jumble on nonsensical visual data. And clearly this is not the case. So he realized that the speed of light, c had to be constant for all observers independent of their frame of reference or to put it another way, be independent of the velocity at which they may be traveling.

    Now, in the train thought experiment, Einstein realized that to preserve the integrity of the constant of the speed of light, an observer standing on a train platform and the observer traveling on the train traveling at the speed of light would perceive the event differently. To the observer on the platform the length of the train would appear to be squeezed down to the size of a hair width. Also to the observer on the platform, if he were magically able to see the clock aboard the train, it would appear to have stopped. But to the observer traveling on the train, space would seem unaffected and he would perceive time flowing at the same rate as it was before he boarded the train. They would only notice a difference in time when they looked at their watch and compared it to a clock not on the train after he had disembarked from the train.

    Now why does space have to contract and time stop for a person traveling at the speed of light? Because velocity is a function of both distance and time. Therefore in order to preserve the constant velocity of light for a traveler traveling at the speed of light, the distance the light has to travel would have to decrease to nothing and the time it took to travel that distance would have to be reduced to zero. The traveler would notice no change. To him space and the flow of time would be the same as before he boarded the train.

    But, this is a local phenomenon that affects only a person traveling at velocities close to the speed of light. The closer he gets to the speed of light, the slower time passes for him and the more contracted his local space becomes without him even noticing any difference. But this does not in anyway affect the spacetime outside his particular frame of reference.
  5. Oct 23, 2009 #4


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    Staff: Mentor

    Yes. And every body is also stationary.
    Nonsense: you just said every body is in motion. In order to find motion, you have to define a frame of reference from which to measure it!
    The speed of light is measured (not estimated) relative to a stationary observer. Any stationary observer: they all measure the same speed.
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