Notwithstanding all the technical jargon and seemingly complicated, interrelated competing claims, it is not hard to understand the difference between relative simultaneity and absolute simultaneity.
1. Relative simultaneity "measures" the time and distance (and hence speed) by determining how one or more objects are moving relative to each other.
2. Absolute simultaneity "measures" the time and distance (and hence speed) by determining how one or more objects are moving relative to a standardized (preferred) frame.
We know that, just like changes in speed, changes in temperature can cause the length of objects to contract and the rate of physical processes to slow down (to nothing at absolute zero). Do we therefore say that for every fraction of a degree of temperature change, we must establish a whole new standard for measuring clock rate and length?
Of course not. We always keep the same standard. If a person were put in suspended animation for a century, then revived, would we say that "no time has passed," just because, in a subjective sense "no time passed for him?" Of course not. Many people would have been born and died and had (collectively) many billions of experiences and adventures during that century and those wouldn't be non-existent just because one guy was "suspended." Likewise, the whole universe doesn't disappear when you die (even if you're a solipsist who believes otherwise).
The boiling point of water changes with altitude (atmospheric pressure). Do we therefore say that we must create a whole new celsius thermometer for every faction of an inch we go up? Of course not. We keep our standard for the boiling point (sea level). In every realm of measurement, we set a standard, however arbitrarily, and then refer all differences (in time, length, temperature, whatever) to that standard. Without maintaining such standards, we can never make meaningful comparisons of differences.
That's all absolute simultaneity does--it sets a standard for measuring time and distance. This is essential to science. It's not necessarily a universally applied standard, and it need not be. You don't need to discover an "ether" to employ absolute simultaneity. "Absolute" does not mean "universally valid" in this context. It just means frame-independent. And all that means is that objects in motion relative to each other will agree, rather than disagree (as in SR), about which of the two is moving faster and hence whose clock has slowed down. That's nothing new or unusual. We have always, both before and after the advent of SR, treated accelerating clocks in this way, because accelerated motion is, even by SR's definitions, absolute, not relative, motion. An accelerating clock will be treated as though it (not the other's guy's clock) has slowed down.
Whenever projects (like the GPS and the H-K experiment, for examples) use the ECI as the frame to which the time of every other object is compared (and measured with respect to), they are utilizing absolute simultaneity, which results in radically different (but accurate) measurements than relative simultaneity would yield.
Edited by Moronium, 23 April 2018 - 08:26 AM.