They will agree as to the speed at which they are approaching or receding from each other, but the time at which they record the happening of some outside occurrence will not necessarily be the same for both of them.
"This has led to some curious misconceptions (due to insufficient knowledge) which have crept into print. There is the story that a person travelling at great speed to Sirius and back will take less time than we think that it does and will come back younger in age than expected. This is just bunkum. The only difference is that we will see him dashing towards Sirius, while he will think that Sirius is dashing towards him, but the distance, speeds and times are all the same to both of us.
"The question whether the contraction in length is 'real' or not is meaningless. It depends on what is meant by 'real.' To one observer the length 'appears' unchanged, to another it 'appears' to be contracted, and this is all that can be said."
Theory is divided into two parts, "special" or "restricted" theory, which deals with relative velocities, and "general" theory, which deals with relative accelerations.
Explains Einstein's reasoning as expressed in Die Naturwissenschaften 6:697, 1918. (See Item 67).
In a previous paper the author worked out a law of areas for Einsteinian motion in a field considered as Euclidean. In the present one he extends it so as to find a law for cosmic time and the periods of revolution. This enables corrections to be applied to the data of observation; which corrections are extremely minute.
Chapter II is devoted to the physical aspects of time including brief mention of space and time.
The time-traveler paradox is related on p.211-213.
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