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
yeah moving clocks tick at different rates, GPS had this exact problem, allegedly some scientists didn’t believe it, so they launched the clocks synced to the earth, and GPS immediately started drifting until they applied the correction for relativistic effects
The faster you go, the faster you move through time. Now consider how fast satellites are moving in space.
This is Einstein’s theory of special relativity.
“Earth observation satellites fly in orbits at altitudes of about 200 to 2,000 km, called low earth orbit, at a speed of about 28,800 km per hour.”
> would it then be permanently “behind” unless it’s manually caught up?
Well you’d have to reset the clock, but just to be clear, it won’t keep running slow once it’s back on Earth. It’s not a problem with the physical clock. The clock is just measuring time, and *time itself* is what’s changing relative to the observer. The astronauts on the ISS age more slowly than people on Earth too, and when they return to the surface they will be some milliseconds younger than they would have been had they not gone up.
I think perhaps you’re confusing yourself with talk of clocks running slow. Clocks always run at the same rate of one second per second whatever the circumstances. A person on Earth, a person on the ISS and a person on a half light speed galaxy hopper have exactly the same experience of time passing at a second per second. But their times differ. Here’s what that looks like at the relevant relative speeds (each s is a second passing).
Zero: ssssssssss
Fast: s s s s s s s
Faster: s s s s s s
Fastest: s s s s s
It depends how they meet up.
When I go from point A to point B in a straight line and you go in a curved line your odometer will read more than mine.
Now in space-time this is also true with clocks which are basically the odometer’s of time. That is odometers integrate space and clocks integrate time.
The important Point here is they have to meet up at the end to make a determination who’s clock is slower
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.
Latest Answers