# Measuring time dilation of SR by a mechanical clock

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.

• 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. Commented Sep 1, 2018 at 5:22

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 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. Commented Sep 2, 2018 at 7:46