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The constancy of the speed of light is a fundamental principle in modern physics, and it is supported by a wide range of current experimental evidence.

There is no evidence to suggest that the speed of light was different in the past, and the idea that it could have been different is at odds with current scientific understanding.

But how can we test and experiment in the present and definitely rule out that the speed of light was differenet in the past?

Qmechanic
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VVM
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1 Answers1

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Speed of light is a dimensionful quantity. To compute it we use the unit of distance defined by the speed of light and unit of time, so indeed we have a tautology, if in the present the speed of light is one light-second per second then in the past it was again one light-second per second. Hence there's no difference between present and past!

VVM
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