If someone is on Earth, and the other is in space, why would their wrist-watch show a different time?

Question

I understand that space travel affects time, but why would it affect the mechanisms of a watch? doesn’t it just go tiktik u know?

In: 11

Answer 1

You have a lot of comments here, but I’m not sure you have a satisfactory answer yet. It’s a tricky thing to summarise succinctly and in layman’s terms, but I’ll try my best…

Space and time are inextricably linked. It took the genius of Einstein just over 100 years ago to realise this. Part of that realisation was that both are malleable. The thing that IS fixed is the speed of light in a vacuum. Sounds crazy, but hear me out here…

A repercussion of our 4-dimensional spacetime is that *every single object in the universe* is ‘moving’ at the same ‘speed’ when you add the space and time components together. The faster an object moves, the slower time *must* pass.

Now, another realisation – and one that helped him move from Special Relativity in 1906 to General Relatively in 1916 – is that to an observer, acceleration and gravity are indistinguishable. If I put you inside a stationary lift on Earth, you would experience a 10m/s/s force pulling you down (we are so used to this we don’t notice it). If I teleported that lift into an area of space far away from any strong gravitational field and pulled you at 10m/s/s it would be impossible to differentiate.

For you and your watch, whatever acceleration/gravitational force is being applied, your *perception* of the passage of time would remain the same, BUT, because we can’t escape the fact that the spacial and temporal components must always equate to the same value, the faster through space you move (or in the case of your example, simply removing the gravitational acceleration of Earth), the slower time must move. Your watch will appear to tick once per second *to you*, but anyone capable of looking at your watch will witness it ticking at a different rate.

In this example, it’s us on Earth who are accelerating, and your watch which isn’t – its movement through space is zero, therefore it passes through time at the maximum rate. We on Earth are slowed slightly by being in a weak gravitational well, so will perceive your watch to be ticking slightly faster than our own.

Please do ask more questions if you have them – it is incredibly difficult to explain in a few paragraphs.