Two observers: 1 and 2 are moving with uniform motion. As long as they are moving with the uniform velocity they will mutually observe the shortening of time in the other observer’s frame (see chapter RELATIVITY OF OBSERVATION) . However, in order to check in which frame the time is really slowing they have to meet and compare clocks. So the observer 2 turns back and catches up with the observer 2 .  

Hence the time indicated by the clock in the observer 1’s frame is equal to: Whereas the time indicated by the clock in the observer 2’s frame is equal to: because we remember (from the previous page) that the time flow along the arc AB is equal to zero. 
Where: and: Therefore: And it can be seen that the shortening of time, though still remaining with agreement with the Relativity Theory, is not an effect of the velocity itself but the effect of the change of velocity – namely the effect of moving along the part of the trajectory with the zerotime flow (it is a theoretical example with infinite acceleration). THE CONDITION THAT IS NECESSARY FOR THE ACTUAL SHORTENING OF THE TIME OF ONE OF THE BODIES IS THE CHANGE OF THE VELOCITY OF THIS BODY THE CHANGE OF THE VELOCITY CAUSES THAT THE BODY PASSES A PART OF ITS WAY ALONG AN ARC ON WHICH THE TIME FLOW IS EQUAL TO ZERO Conclusions: In the Relativity Theory we considered the change of the time flow in moving frames only as the result of the velocity of the bodies. In the FER the most important factor determining the physics of the time dilation phenomenon is not the uniform motion, because then the phenomena appear symmetrically in the systems of both observers, but the moment when one of the bodies changes its velocity. This moment causes problems which, if solved, would give us the knowledge of the mechanism responsible for the physics of the time dilation in the moving body’s frame. This conclusion remains in accordance with similar conclusions resulting from SRT, which were shown in my other paper dedicated to the Hafele and Keating experiment . 