Originally Posted by Sam5

After 13 years of being harassed and kidded all over Europe about it, he finally had to change it. That’s why he added the atomic clocks, the acceleration effects, and the gravity fields in his 1918 paper. That’s why he had to take out the “relative motion” and add some real physical effect that would slow down some kind of clock, such as an atomic clock.

Originally Posted by Sam5

Einstein's 1918 resolution:

"According to both descriptions, it is the clock U2 which lags by a certain amount behind clock U1 at the end of the process considered. With respect to the coordinate system K’ the phenomenon is explained in the following manner: During procedural steps 2 and 4, clock U1, moving at velocity v, has indeed a slower rate than clock U2 which is at rest. But the time lag gets overcompensated by the faster rate of U1 during procedural step 3. Because, according to the general theory of relativity, a clock has a more accelerated rate the higher the gravitational potential is at the clock’s location; and during procedural step 3, U2 is indeed at a location of higher gravitational potential than U1. Calculation shows that this running-ahead amounts to precisely twice as much as the lag-behind during the procedural steps 2 and 4. This analysis clarifies completely the paradox you referred to."