[Physics] Does the fact that we can only measure the two-way speed of light undermine the axiom of invariance


When we measure the speed of light we get the same answer in all directions. This is taken to undermine the aether or absolute motion hypothesis and give support to the proposal that the speed of light is invariant, from which derives the theory of special relativity.

But doesn't the fact that we only measure speed of light 'there and back' undermine this conclusion? Wouldn't we expect this result through an aether?

Best Answer

The bottom line is that this is the wrong question to ask. You don't ever prove an axiom in physics.

You're not quite right about the ether: while the first order effect cancels out in "there and back again" experiments, the second order effect doesn't, which is why the Michelson-Morley experiment stood a chance of testing for the ether. But Michelson-Morley wasn't the end of ether theory, because you could always add fixes to account for the results. For example, the famous Lorentz contraction in special relativity was originally invented for ether theory; the idea was that flowing ether physically squeezed objects smaller. Einstein just took effects like these more seriously.

Ether theory limped on for another 40 years, getting progressively more complicated as more results came in. Ether was not abandoned because it was disproven by experiments or because special relativity was proven by experiments, because this never happens. It was abandoned by the 1930s it could only explain experiments using tons of epicycles, while special relativity just worked perfectly out of the box.

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