Regarding “light clocks” and how moving clocks run slow: Does this mean that, if you were on the ISS or some other satellite, clocks on Earth run slow?

Question

I’m still very confused by how clocks could end up out of sync in this way. For example, if you were to send a clock into space, it “runs slow” for awhile, then bring it back to Earth, would it then be permanently “behind” unless it’s manually caught up?

In: Physics

Answer 1

This is known as the [twin paradox](https://en.m.wikipedia.org/wiki/Twin_paradox). According to special relativity, us on Earth should see the ISS’s clocks as slow relative to their clocks, and the people on the ISS should see Earth’s clocks also run slow relative to the ISS clocks. They can’t both be slow.

But special relativity only applies to inertial observers. The people in space (in this case aboard the ISS) are not following an inertial path. At some point they do need to accelerate in order to come back. Because of this, we could apply the laws of general relativity to compute that it is in fact the accelerating ISS’s clocks that run slow.

If you simplified the problem to a moment of instantaneous acceleration for the clock in space, you could make a spacetime diagram and keep an eye on the planes of simultaneity for the outgoing and return trips, from there the answer falls out neatly. This avoids needing to understand general relativity, it just needs a thorough understanding of special relativity.