Virtually every theoretical physicist on the planet agrees that any definition of simultaneity is "conventional." This simply means that the adoption of any particular view of simultaneity is a matter of arbitrary, agreed upon, convention, NOT one that is based on testable empirical fact. Kinda like the length of a yard is arbitrary convention, and not one that has been proven by any empirical facts to be a particularly useful or meaningful as an acceptable unit of length.
"Relative simultaneity" is one such possible "convention," and it is adopted by adherents of special relativity.
"Absolute simultaneity" is another possible convention, and it is also adopted by many highly qualified physicists.
Why pick one over another, if it's all just "arbitrary?"
There are many good reasons to prefer a convention of absolute simultaneity over relative simultaneity.. I won't try to list them all here, but they include such things as consistency, simplicity, lack of "paradoxical" implications, superior predictive ability, more expansive scope of permissible application, and meaningful content. Of course an avoidance of conflict with logic and empirical experience is also a necessary condition when choosing any such convention.
Unfortunately, far too many people actually believe that the relativity of simultaneity is a necessary product of empirically "proven" fact, because that is what SR uses and argues for.
Among other things, SR must resort to imputed and unreasonable subjective perceptions to/of "observers" in or order to "make it's case" for being adopted as the better "convention." It is that flaw, it particular, that I am addressing in this thread.
At one time, the accepted convention for astronomy was to posit that the earth was motionless and that all other objects in the universe revolved around it. Astronomy was "geocentric" then.
After Galileo precipitated the "copernican revolution" a heliocentric view of planetary motion eventually became the accepted convention. So conventions can and do change, as is deemed desirable. As a theoretical matter, there is no way to absolutely prove that one is better than the other. For that matter, there is no way to absolutely prove that any theoretical scientific hypothesis is "correct." But those abstract theoretical considerations don't, as a practical matter, deter anybody from confidently claiming that the earth revolves around the Sun (the solar barycenter, to be precise), and not vice versa.
Every theory must start with some fundamental postulates, and these, by definition, can never be "proven." They are merely assumed and (at least tentatively) accepted without proof. It would be nice if we could "prove" our postulates, of course, but we can't, so we do with what we can. Any given postulate can, in theory, be disproven, but never proven.
Here again, the average "believer" in special relativity way too often assumes that it's premises are, and have somehow been proven to be, "correct." That is unfortunate.
Edited by Moronium, 16 April 2018 - 03:53 PM.