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B Constancy of the Speed of light

  1. Mar 27, 2016 #1
    Constancy of the Speed of light

    (I am not a physicist, but somehow interested to understand some aspects of relativity)

    The constancy of the speed of light in vacuo is an important postulate on which the Special theory of Relativity is based. The speed of ordinary objects depend upon the speed of their source or the receiver. But this is not the case with speed of light- it remains independent of the speed of the source or the receiver. Probably, I think, Einstein formulated the Special theory relativity to explain this very behaviour of light.

    As far as I have tried to aprehend this theory, I have no problem with it. But I do have a little doubt about its necessity to explain the special behaviour of light, which I want to remove with the help of this forum. It is as follows.

    If we are sitting in a moving train with a ball in our hand, our body as well as the ball are stationary in reference to the train, though they are moving in reference to the ground. We know that the ball (and our body too) have an inertia. So when we throw the ball in the train compartment at a particular speed, its speed in reference to the ground enjoys the velocity addition. Probably this is due to the inertia of the ball. Before we threw the ball, when it was in our hands, though it had zero velocity in reference to our hands, it was moving at a velocity equal to that of the train in reference to the ground. But what happens if the train is standing at a plateform. The ball has zero velocity both in reference to our hand as well as to the ground. This is also probably due to its inertia.

    Now let us replace the ball with a torch throwing a flash of light in the direction of the movement of the train. But even if the train is standing still, unlike the ball, the light will not have zero velocity either in refernce to the train, or to the ground. It will still travel at c. With my very limited knowledge and thinking power, I think that since light does not have inertia, or at least has negligible inertia, it cannot have zero velocity even in an standing train. So,when the train is running, the flash of light, having zero or negligible inertia, should remain unaffected by the speed of train. If the standing train has no effect on the speed of light (it did affect the speed of the ball- it remained motionless in refernce to the train or our hand), why should a running train have an effect on it? I think that only the speed of objects having sufficient inertia should experience velocity addition, or, should be affected by the speed of their source or the receiver, and not the objects without inertia.

    Now, I want to know to what extent my thinking is correct, and that why the indifference of the speed of light to the movement of its source or the receiver is not explainable simply by the fact of its negligible inertia, and we have to depend on a complex theory of special relativity?
  2. jcsd
  3. Mar 27, 2016 #2


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    Sorry, no. In all of the scenarios you listed there was no acceleration and since inertia is a measure of resistance to acceleration, inertia plays no role in any of them.
  4. Mar 27, 2016 #3


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    I'm not sure that you quite understand what is meant by the "constancy of the speed of light" in the terms of SR. To use your example of the train, it means that both the person on the moving train and someone on the ground will measure the speed of the light leaving the torch as being c relative to themselves.
    In other words, If the person on the train were to measure how long it takes for the light to travel between two points of the train he will get an answer that indicates that the light is traveling at c with respect to the train and as a result moving at c+v (where v is the speed of the train with respect to the ground) relative to the ground. If a person on the ground were to measure how long it takes for the same light to travel between two points on the ground, he also will get an answer that indicates that the light is traveling at c with respect to the ground and moving at c-v with respect to the train.
    No matter what you assume is the reason behind these differences in measurements, you still have to account for them.
    SR resolves this apparent contradiction in measurements by concluding that the person on the train and the person on the ground measure distance and time differently.
  5. Mar 28, 2016 #4
    Thanks. I shall take time to think about it.
  6. Mar 28, 2016 #5
    Thanks. I shall take some time to fully appreciate your answer.
  7. Mar 28, 2016 #6

    Mister T

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    It is popular to attribute this to some property of the ball (its inertia) but it is instead simply a result of viewing things from a different frame of reference.

    Its speed in unaffected, yes. But suppose you aimed the light from the floor of the train car towards the ceiling so that its path is vertical as viewed from the rest frame of the train. As viewed from the rest frame of the ground it would travel not in a vertical direction, but would instead be tilted at an angle. The faster the train moves relative to the ground the further from vertical the tilt, up to a point. The limit being set by the speed limit of the train, which is the speed of light.

    One of the first conclusions you can draw is that if a speed is the same in all reference frames, then that speed is necessarily the fastest speed possible. Two reference frames can move relative to each other at any speed that's smaller, but never at that fastest speed, and certainly never faster.
  8. Mar 29, 2016 #7
    Thanks. I'll get back in a couple of days.
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