Special relativity says that different observers qualify two events as simultaneous based on their velocities. But isn't that a statement about information they can measure rather than about reality? In other words, can we say that there is an universal clock, giving absolute time, but that as observers embedded in the universe, there is no experiment that would allow us to read it?
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1$\begingroup$ No. Have you considered why would you want to say that? Are you unsatisfied with Special Relativity in some way? BTW, Special Relativity does not even attempt to provide a universal explanation of the whole universe (think gravitation, amongst many other things), so there is no way to define an absolute clock even if you want to. $\endgroup$– m4r35n357Commented Aug 9, 2022 at 9:32
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1$\begingroup$ @m4r35n357 because I have trouble jumping to the conclusion that there is no universal clock. To me Einstein's theory is merely about information than reality. I don't understand how physicists accept the reality of the phenomenon of relativity of time without questioning that it might just be a feature of accessible knowledge, which seems perfectly reasonable to me. $\endgroup$– qleguennecCommented Aug 9, 2022 at 10:28
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3$\begingroup$ I’m voting to close this question because (as indicated by the OP) it is a question about philosophy, not science $\endgroup$– DaleCommented Aug 9, 2022 at 13:20
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1$\begingroup$ That should tell you something right there. $\endgroup$– DaleCommented Aug 9, 2022 at 13:32
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1$\begingroup$ Actually, the opposite. It tells you that science and philosophy are fundamentally distinct. Science requires input from experiment, so philosophical concepts that are explicitly designed to avoid experimental contradiction are fundamentally non-scientific. E.g. the concept of a "universal clock, giving absolute time, but ... there is no experiment that would allow us to read it" is non-scientific at its very core $\endgroup$– DaleCommented Aug 9, 2022 at 14:10
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3 Answers
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Can we say that there is an universal clock, giving absolute time, but that as observers embedded in the universe, there is no experiment that would allow us to read it?
We can say that, but for the purposes of physics, a universal clock that cannot be read or observed might as well not exist. I can say that the universe is filled with massless, invisible, undetectable pink unicorns - but that statement is not within the realm of physics.
On the other hand, if the universal clock can be read/observed then it establishes a universally stationary frame of reference (stationary observers are those whose local clocks match the universal clock) which contradicts both special and general relativity.
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$\begingroup$ @qleguennec I have expanded my answer. $\endgroup$ Commented Aug 9, 2022 at 10:32
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There is a common confusion about the meaning of the verb observe in relativity. It does not mean what we see i.e. what our eyes tell us. We start with what we see, but then we correct for factors like the time the light took to reach us and the result is the observation. Observation means we assign an event to a point in spacetime $(t, x, y, z)$ where the time $t$ is the time the event happened in our coordinate system, not the time we saw it. There is more on this in The nature of the observer in special relativity.
So when we say that different observers disagree about whether events are simultaneous or not, this does not just mean that the observers saw the events at different times due to the time the light took to reach their eyes. It really does mean the events happened at different times in different frames. That's why there cannot be any universal clock in relativity.
The reason why there is no universal time is because there is no unique way to split up spacetime into time and spatial parts. Suppose we are drawing a coordinate system on the $xy$ plane. There is no unique way to define the $x$ and $y$ axes e.g. we could rotate our axes by any random angle and they would still be a perfectly good way to describe the coordinates on points on the plane.
Now consider the $tx$ plane. Common sense seems to tell us the above argument wouldn't apply because, well, time is special. However special relativity tells us that time is not special, and we can indeed use different $t$ and $x$ axes for our coordinates. Specifically when observers are travelling at different speed their time axes are rotated relative to each other.
For there to be a universal time axis there would need to be a universal speed, in which case the rotation of all other observers' time axes would depend on their speed relative to this universal speed. But no such universal speed exists, and hence no universal time axis exists.
answered Aug 9, 2022 at 9:34
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$\begingroup$ The word "observer" in my question was intended to mean observer "as a coordinate system", not as a being. As of "It really does mean the events happened at different times in different frames.", I don't get how you draw that conclusion. $\endgroup$ Commented Aug 9, 2022 at 9:58
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1$\begingroup$ @qleguennec I have updated my answer to address your comment. $\endgroup$ Commented Aug 9, 2022 at 10:11
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$\begingroup$ Thanks for the effort but it seems to me that you're rather explaining special relativity than answering the question, but I guess that the question is rather about epistemology than physics, so it might not be suited for this forum. Can we at least say that the idea of an universal clock (or an universal ordering of events) is out of the scope of physics because of relativity, rather than denied by relativity ? In other words, can I postulate the existence of an universal clock as a superfluous assumption? Is it ascientific or unscientific? $\endgroup$ Commented Aug 9, 2022 at 10:20
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2$\begingroup$ @qleguennec what I've shown is that the existence of an absolute clock implies the existence of an absolute speed. And, as you say, there is no absolute speed in special relativity since all speeds are relative so that means there can be no absolute clock. $\endgroup$ Commented Aug 9, 2022 at 11:03
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No, you cannot have a universal clock. It is analogous to having a universal up and down axis and a universal spirit-level.
answered Aug 9, 2022 at 11:26
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$\begingroup$ It is- you just don't understand the significance of the analogy. For there to be a universal time, there would need to be absolute simultaneity. Take two events that happen a long distance apart. If there were a universal time, all observers would be able to agree which event occurred before the other. There is no absolute time, so there cannot be a 'master clock' for the universe. $\endgroup$ Commented Aug 9, 2022 at 14:22
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$\begingroup$ It is not a case of relativity forbidding the idea of an absolute time. As far as we know, reality is that spacetime is four dimensional, with no unique time axis. That is directly analogous to 3d space having no unique 'up' axis. Asking whether two events are absolutely simultaneous is like asking whether one event is absolutely higher in space than another. $\endgroup$ Commented Aug 9, 2022 at 14:26