• Question: Can we explain the fact that the speed of light is constant in all inertial frames, or do we just have to take it for granted?

    Asked by bolzanoweierstrass to James, Marcus, Martin, Rob, Suzanne on 19 Mar 2012.
    • Photo: Suzanne McEndoo

      Suzanne McEndoo answered on 19 Mar 2012:

      I think we take it for granted. Or, more precisely, we take it as experimentally verified, so we have experiments that tell us that it is, and none that tell us that it isn’t. I don’t know of any good explanation as to *why* it is so though, but maybe someone else has a suggestion?

    • Photo: Marcus Gallagher-Jones

      Marcus Gallagher-Jones answered on 21 Mar 2012:

      I think we are in a situation where it has been verified in many ways. I don’t know if you could measure an infinite number of inertial frames so we’ve probably checked it’s consistency for many and now take it for granted.

    • Photo: Martin Zaltz Austwick

      Martin Zaltz Austwick answered on 21 Mar 2012:

      That law comes from Einstein’s special theory of relativity, and was in response to a series of experiments that happened around the late 1800s-early 1900s. Victorian scientists had figured out that light was a wave, but they didn’t know what medium it moved on (water waves are on water, sound waves on air/other fluids – so what does light move on?). The experiments they did failed to detect a “medium” – and one of the consequences was that light has this strange property of travelling at the same speed in all (inertial) frames of reference. The General Theory of Relativity has more to say about it, but it’s been a very long time since I studied that – but essentially a lot of the “weird” results in Special Relativity come from the properties of the speed of light.

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