Question regarding light clocks and time dilation.

[amk0713]

Hello. I am trying to gain a more intuitive understanding of relativity, and hopefully someone may be able to help me.

Suppose there are two light clocks on two different spaceships - ship A and ship B. If the two ships are moving past one another with a constant velocity, then an observer on A would see their clock ticking "normally" while the clock on ship B would tick more slowly. However, wouldn't this also be the case for an observer traveling in ship B, in that he observes his own clock to tick normally while the other in ship A ticks more slowly? Who's time actually slows down?

 

[jtbell]

If the two ships are moving past one another with a constant velocity, then an observer on A would see their clock ticking "normally" while the clock on ship B would tick more slowly. However, wouldn't this also be the case for an observer traveling in ship B, in that he observes his own clock to tick normally while the other in ship A ticks more slowly?

Yes.

Who's time actually slows down?

Neither. Or both. Take your pick.

 

[DaveC426913]

Hello. I am trying to gain a more intuitive understanding of relativity, and hopefully someone may be able to help me.

Suppose there are two light clocks on two different spaceships - ship A and ship B. If the two ships are moving past one another with a constant velocity, then an observer on A would see their clock ticking "normally" while the clock on ship B would tick more slowly. However, wouldn't this also be the case for an observer traveling in ship B, in that he observes his own clock to tick normally while the other in ship A ticks more slowly? Who's time actually slows down?

They are both correct. There is no such thing as "actually". "Actually" is synonymous with "absolute" (as in some absolute time frame) - and absolute is antithetical to relativity.


The key is: how do they compare their clocks again after the passage? One (or both) of them needs to turn around. This will change the rates of their clocks as seen by the other observer.

 

[amk0713]

Thanks both for the quick replies.

But what about in the case of actual clocks? Which of the two on ships A and B would read an earlier time if each can claim that they are moving and the other is at rest?

 

[DaveC426913]

Thanks both for the quick replies.

But what about in the case of actual clocks? Which of the two on ships A and B would read an earlier time if each can claim that they are moving and the other is at rest?

Each sees the other's clock moving slower.

Note that neither has any claim to being stationary. All they know is that they are in motion with respect to each other. To decide which one is stationary would require referencing the motion of a third point - which could be another spaceship, a planet or a galaxy. But it's still arbitrary. Who is to say the planet isn't moving a .5c?

 

[Hebob80]

I like to sum up Special Relativity like this:

Everything in the universe believes it is the thing that is not moving.

Or, everything in the universe believes it is the thing which is at rest.

 

[pervect]

Hello. I am trying to gain a more intuitive understanding of relativity, and hopefully someone may be able to help me.

Suppose there are two light clocks on two different spaceships - ship A and ship B. If the two ships are moving past one another with a constant velocity, then an observer on A would see their clock ticking "normally" while the clock on ship B would tick more slowly. However, wouldn't this also be the case for an observer traveling in ship B, in that he observes his own clock to tick normally while the other in ship A ticks more slowly? Who's time actually slows down?

In order to compare the clocks, you need to settle on a method to decide what events are simultaneous.

In pre-relativistic mechanics, there is some universal notion of simultaneity. However, in relativistic mechanics, there is not.

The set of events that a rider on spaceship A assigns as simultaneous to some reading "t=0" on spaceship A's clocks is different from the set of events that a rider on spacehip B assigns as simultaneous to the exact same event.

So the answer to the question of whose clock is slower depends on one's notion of simultaneious events - a notion that is dependent on the observer.

Spaceship A uses their definition of simultaneous events, and decides that events at ta=x on a's clock correspond to events at tb = x / gamma on B's clock.

Spaceship B uses their different definition of simultnaeity and decides that events at tb = x on b's clock correspond to events ta = x / gamma on a's clock.

Both defintions of simultaneity are reasonable and self-consistent - there's no way to pick one over the other.