# Observers at rest and simultaneity

Suppose we have two observers A and B and they are at rest. Observer A observes two objects falling from height H (A has same distance between the two objects). Does observer B will measure different times for the duration of falling of the two objects ? (because the two object are not in the same location therefore a finite time interval must exist for the information of their position to reach B).

• I think that this question is basically caused by the misunderstanding that I just answered here. – md2perpe Jul 18 at 14:46

## 1 Answer

The time that A measures between the events "A sees the object start to fall" and "A sees the object land" is in general different from the time that B measures between the events "B sees the object start to fall" and "B sees the object land".

But the time that A measures between the events "the object starts to fall" and "the object lands" is the same as the time that B measures between the events "B sees the object start to fall" --- because $$A$$ and $$B$$ are smart enough to correct for the finite speed of light.

If A and B were in motion with respect to each other, they would measure different times between the events "the object starts to fall" and "the object lands" --- even after correcting for the finite speed of light.

• But from perspective of B the object which is nearest to him it will be observed to have reach first the land right ? Also when we speak about simultaneity in relativity do we mean the observation of simultaneous events or the backwards work (according to finite speed of light) we do to find when the event happen ? In other words with the term simultaneity we mean the observation of two event happen in the same time at the same reference frame ? – ado sar Jul 18 at 11:35
• Relativity of simultaneity means that if two events happen simultaneously in A's reference frame, then they do not happen simultaneously in B's reference frame. This refers to the timing of the events themselves, not to the timing of light from the events reaching A and B. – WillO Jul 18 at 11:42
• "happen simultaneously" you mean observed simultaneously ? – ado sar Jul 18 at 11:44
• No. "Happen simultaneously" means "happen simultaneously". "Observed simultaneously" is a different thing altogether. – WillO Jul 18 at 11:46