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Sorry if this is a stupid question or if it's already been answered, but I struggled to find this answered anywhere:

I know that the equations and transformations etc etc show that, from two different perspectives (in different inertial reference frames? I guess is the word), two events could be perceived in one frame as having occurred simultaneously, and not simultaneously in the other frame. And that all the laws of physics are perfectly consistent with both points of view, and there's no way of measuring or determining that one frame was 'correct' and the other one was 'incorrect'.

What I'm unclear on is, even if there's no way of measuring or determining the 'correctness' of one frame over another, is it still possible that in reality, in the "source code of the universe" if you will, that everything really is being calculated from one particular inertial reference frame? Is it just that we can't find out what that 'objective' reference frame is, or is it categorically conceptually impossible for there to be an objective reference frame that underlies physics?

TKoL
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3 Answers3

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... even if there's no way of measuring or determining the 'correctness' of one frame over another, is it still possible that in reality, in the "source code of the universe" if you will, that everything really is being calculated from one particular inertial reference frame ...

If there is no way even in principle to detect something, then in practice it does not physically exist. There's no reason to postulate undetectable things (like the "true" reference frame of the universe).

If that doesn't satisfy you, then consider this: the requirement that the laws of physics be the same for all choices of simultaneity is actually much stronger than assuming that there is some preferred "true" choice of simultaneity. Hence it is more useful in determining the laws -- it imposes an additional symmetry which isn't present in the "only one true simultaneity" situation. The "source code of the universe" is like a distributed program which must work correctly regardless of which system's clock is chosen as the reference. Writing a program that satisfies that requirement is much harder than writing a program that works on just one system (one choice of clock), and therefore there are fewer ways to do it.

Eric Smith
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Your question assumes that there can be some measure of simultaneity that is somehow more absolute than others. That is quite an unnecessary assumption. All simultaneity means is that two or more events share the same t coordinate in some reference frame, no more and no less. It is rather like the concept of a horizontal level. Two points are level horizontally if they share the same z coordinate, which means they are level in some reference frames and not in others. You would not dream of wondering whether there is an absolute or objective sort of horizontal level- you would simply accept that it depends on the direction chosen for a z axis. You should apply the same thinking to time. There is no absolute direction for a t axis, and so there can be no sense in which one person's measure of simultaneity can be more significant than another's.

Professor Sushing
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Consider the same two high precision clocks, with a difference of altitude $h$, which can be as small as one or two centimeters. If at a moment you decide that they are synchronized, later you will observe a relative time shift of $1 + \frac{g h}{c^2}$, with the same synchronization protocol now and later (see for example https://doi.org/10.1038/s41566-020-0654-5).

If, because of the presence of the Earth with lab gravitation acceleration gravitation $g$ two synchronized clocks cannot give the same time, it is clear that the concept of simultaneity has no meaning other than mathematical, with the arbitrary choice of a $(t, x, y, z)$ coordinate system.

Fabrice Pardo
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