I think this is a very basic question, but relativity is always not intuitive to me. When we say that time goes slow when moving (viewed by another inertial coordinate system), we just assume that there is a clock to measure time.

However, everyday clocks are mostly mechanical (like quartz clock). I know that quartz clock works using piezo-electric effect, so it's a tuning fork that vibrates when you apply electricity. So it's a mechanical effect, and the accurately tuned frequency depends on many things, among them specifically length of the fork. (Tuning fork is a two cantilevers basically)

So what seems confusing to me is that the length of the fork should be affected by relativity itself, (length contraction) and it would depend on the direction the clock moving. Is this effect real? If so, is it same as time dilation I've learned? But it seems to me that cannot be true, because time dilation is independent of moving direction but this seems not. I know that SR was tested many times, and I've not heard that they used special clocks (like atomic) in the experiments.

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    $\begingroup$ Neither the length of the clock nor its rate is actually changed for the clock itself. What changes is how you see them. Your view is a projection. When the Sun goes up, your shadow becomes shorter, but you don't. As far as the clock is concerned, it is at rest and the fact that you are flying away fast doesn't affect the clock rate or length in any way. $\endgroup$
    – safesphere
    Commented Sep 1, 2018 at 5:22

1 Answer 1


"… we say that time goes slow when moving (viewed by another inertial coordinate system)… " I don't say this, because I don't understand what it means. What I do say – having learnt from some excellent expositors – is this…

The time between two events, as measured in an inertial frame of reference in which the events occur in different places, using synchronised clocks at the locations of the events, is greater (by a factor of $\gamma$) than the time between the same events measured in the inertial frame in which the events occur in the same place (and therefore require a single stationary clock).

I know that this sounds complicated, but once you've grasped it, it's easy to apply and saves a lot of muddle. It brings out the key idea that time dilation is all about the inter-relatedness of time and space, and nothing to do with clock mechanisms being 'affected'. There's nothing to affect them: each clock is stationary in its own inertial frame and the laws of Physics are the same in all inertial frames.

It's true that clocks and the parts of which they are made are contracted in the direction of motion when correctly measured in a frame of reference in which they are moving, but this is quite irrelevant to the phenomenon of time dilation as defined in my second paragraph – even though length contraction, like time dilation, is a manifestation of the inter-relatedness of space and time!

  • $\begingroup$ I've edited my answer to try and answer the question more sharply. $\endgroup$ Commented Sep 1, 2018 at 15:11
  • $\begingroup$ Well, I should say my mind is boggled. I never learned relativity as a separate subject, learned it always as a chapter of college physics, mechanics, and E&M, so I never felt comfortable with it, though I can calculate basic results of it ( not tricky ones). So as far as I understand your answer, SR is not about how one inertial frame of reference measure time in the perspective of other IFR. It's just about a comparison between the measurements of two IFRs. Am I right? $\endgroup$
    – Septacle
    Commented Sep 2, 2018 at 0:13
  • $\begingroup$ I think that "comparison of measurements" is quite a good description –as long as you don't suppose that it's anything to do with weird things happening to the measuring instruments! The second paragraph of my answer tries to make clear that time dilation is all about the interaction of time and space; it matters $where$ in your frame of reference the time measurements are made. $Spacetime\ Physics$ by Taylor/Wheeler$ is excellent for concepts, and quite accessible. $\endgroup$ Commented Sep 2, 2018 at 7:46

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