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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?

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  • $\begingroup$ If these objections to a variable speed don't convince you, you can only empirically constrain how different it could be. If it had been different by a small enough amount, you wouldn't know. $\endgroup$
    – J.G.
    Commented Jan 28, 2023 at 0:08
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    $\begingroup$ In the SI, the speed of light is defined as fixed: all length measurements are are traceable to time measurements. If you think the speed of light might vary, you must first define how you measure length independently of time. And then, if you found a variation, perhaps it would make more sense that your ruler is varying with time. $\endgroup$
    – John Doty
    Commented Jan 28, 2023 at 0:52
  • $\begingroup$ Related: physics.stackexchange.com/q/523485/2451 , physics.stackexchange.com/q/12805/2451 and links therein. $\endgroup$
    – Qmechanic ♦
    Commented Jan 28, 2023 at 4:15
  • $\begingroup$ Related: What exactly would it take to show (hypothetically) that the speed of light (in vacuum) is not constant? $\endgroup$
    – Sandejo
    Commented Jan 28, 2023 at 6:16
  • $\begingroup$ Since E = mc^2, nonconstant c implies energy nonconservation. This would be a really big deal... $\endgroup$
    – niels nielsen
    Commented Jan 28, 2023 at 7:05

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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!

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  • $\begingroup$ Correct +1 the local speed of light is unity and cannot change. What it dimensional value represents is the scale or how the light second relates to our given distances. For example, if light took 2 seconds in the past for light to reach the Moon, then the Moon was twice farther from the Earth back then. $\endgroup$
    – safesphere
    Commented Jan 31, 2023 at 16:58

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