# Why can't we tell, we are moving?

I stumbled upon this question: Can we really not tell if we are moving? and it made me start thinking (probably a bad idea for a non-physics guy).

To my knowledge, moving at relativistic speed means that time passes slower than for a non-moving observer. My question is, can't this be used to determine whether we are moving?

Say, we have people 1, 2 and 3 and we know they move along a certain vector at the same relativistic speed (say, they are oriented along a line in space, they know, one of them is "front" and one is "back" but not who is what, they also know the speed $$v$$ they are going).

All three synchronize their clocks and now 1 and 3 accelerate in opposite directions until they reach $$v$$ relative to 2. After some time, they accelerate again until they match speed with 2 again. We know, one of them has somehow doubled his absolute speed, the other one came temporary to a standstill.

As I know, we cannot tell, we are moving, I am certain, that re-synchronizing their watches will give the same deviation for 1 and 3 relative to 2. But why, as one of them stood still for a certain amount of time, while the other was almost at twice $$v$$ velocity.

• How can you tell 2 is at rest? other observers that move relative to him can say that he is moving. you can tell if you are accelerating though.
– user65081
Commented Sep 4, 2020 at 20:57
• If all 3 people are traveling rapidly in outer space, which must be true if they are to have enough room to move at relativistic speeds, what are you measuring their speed relative to? In other words, for the person who "comes to a stop", he is "stopping" in outer space, but how do you know he is stopped if there is nothing around him? Commented Sep 4, 2020 at 20:58

"To my knowledge, moving at relativistic speed means that time passes slower than for a non-moving observer. My question is, can't this be used to determine whether we are moving?"

Because there is no such thing as an absolutely non-moving observer. Everyone's moving in some reference frame.

If you see a someone moving at a relativistic speed relative to you, yes, you would observe their time to be moving slower. If you switch to their perspective of you, suddenly you are moving relative to them with the same speed and they're the one at rest. And the same rules of time dilation apply again (as all inertial frames are equivalent), meaning that they observe your clock to be moving slower now.

Since the same rules apply to both, so there's no preference here. If physical laws give preference to neither of you, then there's no experiment you can use as an evidence to say that there's something 'unique' happening in your frame that could qualify your movement to be absolute.

The above holds for inertial frames. People in accelerated frames can detect that they're accelerating because they experience pseudo forces.

Your question implicitly assumes that there is a "absolute speed". There is no such thing. As far as 2 is concerned, they start at rest and then 1 and 3 fly off in opposite directions until again coming to rest.

In special relativity, there is a concept called relativity of simultaneity. It says that events that appear simultaneous in one reference frame might not appear simultaneous in another reference frame. In your situation, 1 and 3 will have synchronized clocks in the reference frame of 2. However, 1 and 3 will not have synchronized clocks in the world reference frame.

It is actually a postulate of special relativity that you can never tell whether you are moving at a constant velocity.