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Here is a system which is travelling left to right at the speed of 0.5c (half of the speed of light). The system is made of two parts. Both the parts are made of a light source and a light receiver or detector; but both are placed opposite to each other (that means the receivers are at the ends of the system).

Then the both sources emit light at the same time and the receivers receive the pulse. According to Maxwell’s equations, the speed of the pulse of light will not be affected by the speed of its source (and the system). We know that the speed of light is same for all observers. So, the receivers will receive the light at the same time. But here rises a problem: how relativity will explain this phenomenon (that the speed of light is constant here too). Here, we should remember that the system is travelling at 0.5c and we can’t use the length contraction and time delay. Why? It is because:
1.      If the system is affected with length contraction and time delay, both the parts will be affected not a single part. So, if we try to explain how the speed of light become constant in the right part with length contraction and time delay, we shall be unable to explain how the speed of light become constant in the left part with the same length contraction and time delay!
2.      If we suppose that the receivers received those pulses at different time, it will become clear that the speed of light is not same and constant for all observers and the theory of relativity will break down!

From
Ritu Lahkar

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