Timeline for Is the non-existence of a rest frame for photon in vacuum a consequence of the second postulate?

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Dec 20, 2017 at 2:10	comment	added	The_Sympathizer		(cont'd) that this is an additional assumption, while the derivation can rest purely on geometry alone. Even without causality, $K$ will be a limit to speed achieved by acceleration , but there may also be particles that always move with speed higher than $K$ (and cannot decelerate to below it), these are called "tachyons". But we have not seen that, and furthermore observe strict causality directly, so this assumption holds empirically.
Dec 20, 2017 at 2:09	comment	added	The_Sympathizer		@GRrocks : Thanks :) Another thing to point out is that it does not follow from purely geometric constraints that the invariant speed $K$ must be a limiting speed (indeed in the Euclidean case it's not a speed anything can travel at at all as it's imaginary, and there is no speed limit.). That $K$ is a limiting speed follows from the additional imposition of the requirement of unidirectional causality, that effect must always temporally precede cause or, that an "arrow of time" exists. This also rules out the Euclidean case as well - but it's important to point out (cont'd)
Dec 19, 2017 at 16:33	comment	added	GRrocks		Thanks a lot! What fascinated me the most is the 2nd part of your answer, the part about deriving SR from the 1st postulate entirely. Never heard of that one. I'll try to read up on it!
Dec 19, 2017 at 16:32	vote	accept	GRrocks
Dec 19, 2017 at 11:05	history	answered	The_Sympathizer	CC BY-SA 3.0