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Atomic clocks tick differently with different gravity. That's a fact. This implies either one of the following statements: (1) Gravity affects time itself OR (2) Gravity affects time measurement only

Relativity supports hypothesis 1, but I wonder: is there any experimental evidence against hypothesis 2? Or the two are equivalent?

Example: a clock on the Earth measures 1 minute while a clock in the space measures 2 minutes. Why do we say that "time is slower on Earth" instead of just saying "clocks tick slower on Earth"? How can we be experimentally sure that time-dilation is real, and not just an illusion due to the side-effects of gravity on instrumentation?

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    $\begingroup$ Most physicists simply consider time to be what clocks measure. If you believe clocks don’t measure time, how do you propose to measure it? $\endgroup$ – G. Smith Jun 10 '19 at 22:42
  • $\begingroup$ Do you think different types of clocks will measure different times because they are affected by gravity differently? $\endgroup$ – 511mev Jun 10 '19 at 23:44
  • $\begingroup$ Maybe. Let's imagine an "human clock". Will he perceive time differently than an atomic clock in his same reference frame? Will he perceive time in an "absolute way" and see the clock run slower/faster? I mean: the theory says he wouldn't but this cannot be experimentally proven. Aside from my "human clock" example, I'm just asking: is time-dilation experimentally confirmed by more than one type of time measurement? $\endgroup$ – agdev84 Jun 11 '19 at 0:00
  • $\begingroup$ Yes, by the decay times of unstable particles. The “internal clock” of every kind of unstable particle runs slow by the same $\gamma$ factor when the particle is moving fast. For example, this is why muons produced in the upper atmosphere have enough time to reach the surface. $\endgroup$ – G. Smith Jun 11 '19 at 0:21
  • $\begingroup$ Thanks, I was missing this piece of the puzzle $\endgroup$ – agdev84 Jun 11 '19 at 0:52
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That's a matter of philosophy. You can always say that a system that doesn't seem to have a property really does have it. For example, there doesn't seem to be any notion of absolute velocity, but you could always assume the ether exists, thereby defining absolute velocity. For this to not contradict observation, you would have to assume the ether is completely unobservable. Same thing goes for your notion of absolute time; you can just assume that some absolute time exists but that no clocks can measure it. While you're at it, you might as well also say that the Sun really orbits the Earth, but some mechanical conspiracy perfectly hides this fact so that it looks like the Earth orbits the Sun.

Physicists are not eager to bolt such unobservable and useless things on to their theories, because it's already hard enough to understand the theories without this extra complication. Imagine if some synesthate came up to you and told you that every word had a true color, and if you didn't remember that "clock" was yellow and "time" was blue, you would be wrong. Since memorizing all the colors would be useless, I bet you wouldn't bother, and the same goes for physicists and absolute time.

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  • $\begingroup$ Wittgenstein might have a few choice words about your color argument... $\endgroup$ – Jon Custer Jun 10 '19 at 22:30
  • $\begingroup$ Ok, between your answer and the comments to my question, I got it. It makes no sense to talk about "time itself" vs "measured time", because there's no privileged reference frame having an absolute time, and there's no privileged clock unaffected by gravity. The latter implies that even an "human clock" (an human observer) would experience time in a way which is consistent with any clock in his reference frame. Maybe an absolute time exists, but we cannot measure it, neither experience it, so it becomes "matter of philosophy". $\endgroup$ – agdev84 Jun 11 '19 at 0:47
  • $\begingroup$ @agdev84 I think you are close but it's a more direct thing in my opinion: Natural Philosophy and Materialism, which are the philosophical craddles of scientific thinking, consider anything that is impossible to measure or observe non-existent. The meaning of "it exists" is bounded to an empirical revelation of its presence by an interaction with the material world. If you consider something to exist but that thing doesn't interact with the physical reality so it can theoretically be measured, then that thing does not exist for science. It is outside of the natural world. It is metaphysics. $\endgroup$ – Swike Aug 3 '20 at 12:42

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