# Does time dialation cancellation occur between two observers moving in opposite directions [duplicate]

This is the question that has been on my mind lately: If an object A is moving away from an object B which is presumed stationary by A, then the time observed from the perspective of object B will flow (if you will) faster than the time on the object A, correct?

However in the same way according to my understanding, if the view points are reversed, then the same effect is to take place from the perspective of the observer in the Object A towards object B. From perspective of observer A (granted there is no point of any observable reference besides object B, object B is moving away and object A is stationary. In which case the time dialation applicable to A and the same effect applicable to B effectively cancel each other out.

Would this be an accurate assumption?

And if later A or B or both A and B reversed their direction and met, would the time lapsed in each of their measurement be equal to the time either of them have spent in apart and therefore be of equal duration relative to both A and B?

## marked as duplicate by John Rennie, Kyle Kanos, Jon Custer, user191954, Aaron StevensFeb 21 at 14:33

If an object A is moving away from an object B which is presumed stationary

Presumed stationary by whom relative to what? Is there a third observer or just A and B? If just A and B, then A is at rest according to A and B is at rest according to B.

then the time observed from the perspective of object B

What does "the time" refer to? The time according to the clocks at rest with respect to B or the time according to the clocks at rest with respect to A?

It's true that the clocks at rest with respect to B (A) will, according to B (A), run faster than the clocks at rest with respect to A (B). Is this what you mean?

There is no cancellation of this effect. If A and B have relative uniform motion, each observes the other's 'wristwatch' to run slower than their own.

• No third observer, both presumptions are relative to each other, in the context of time reference as assumed based on above mentioned presumption. – Vlad Darevskiy Feb 20 at 2:56
• I have restated the original question with added clarification. – Vlad Darevskiy Feb 20 at 3:25

then the time observed from the perspective of object B will flow (if you will) faster than the time on the object A, correct?

No. You're using misleading language. Try to restate your idea not using "time that flows" and "the perspective of".

Comply instead to the following rules:

• No "perspective", but well-defined inertial frame of reference.

• No ill-defined "flow of time", but time interval measured between a given couple of events, from two different frames.

Then you'll see that when you interchange frames A and B you're also changing the couple of events, so that no cancellation makes sense as you're referring to two different experiments.