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by Q-Q
943 days ago
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>to see why you would assume an invariant speed, the assumption isn't just pulled out of thin air at the start It's not out of thin air, it's from a very empirically successful theory: Maxwell's electrodynamics. The problem back then was that this theory was not relativistic, i.e. the speed of electromagnetic wave in Maxwell's equations was the same under all reference frames. So you either abandon the idea that laws of physics remaining the same under all reference frames, OR abandon Galilean velocity addition. Einstein's approach was modifying the latter so that it fits with the former, by imposing invariant speed. This was written in Einstein's original paper. It's not a mystery assumption. It's also a very common procedure: two empirically successful theories have conflict and you need to resolve them by building something larger than both and reducing to both under limit. I also agree we have gained insight into how kinematic structure is derived from algebra + physical constraint. Though you still need the physical insight to choose which physical constraint. |
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That was the source of the assumption, yes--as you point out, Einstein said so in his original paper. But from the standpoint of mechanics, as opposed to electrodynamics, it was pulled out of thin air. There was no reason based on mechanics to make any such assumption. In fact, everyone else except Einstein that was working on the problem was looking at ways to modify electrodynamics, not mechanics--in other words, to come up with a theory of electrodynamics that was Galilean invariant, rather than to come up with a theory of mechanics that was Lorentz invariant.
> you either abandon the idea that laws of physics remaining the same under all reference frames, OR abandon Galilean velocity addition
Or, as above, you look for a Galilean invariant theory of electrodynamics. Of course we know today that that is a dead end, but that wasn't known then.