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How was it established that the speed of light is a constant?

The speed of light is said to be a constant; if it is an absolute how is this established? Or is it really relative?
Derek Stevens from Pembrokeshire (Aged 55+)

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One Response

  1. Hi Derek,

    You don’t seem to be getting any answers so I’ll have a go for you. As I’m a psychologist you might want to verify my answer!

    First, Einstein’s predictions concerning relativity follow from the assumption that the speed of light is constant in all frames of reference. Those theories have been shown to hold up very well in actual observations, so presumably the premise is correct.

    More simply, you could look up the famous (and very old) Michelson-Morley experiment. It was setting out to see if there was an aether to ‘carry’ light waves, but it’s effectively a speed of light experiment. They sent light in two directions – North-South and East-West. The latter interacts with the rotation of the Earth, so if that modifies the speed of light (as would happen to the bullet of a gun fired on a moving vehicle) the transit time should be different. Comparing arrival times showed no difference.

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