Shapiro Crapiro

In this animation, light represented by the yellow bar is sent from Earth to Mars where it reflects and returns to Earth. In the frame of reference of the "fixed" stars (represented by the letters "Mars" and "Earth") the light travels in a straight line, but in the frame of reference of the Earth (from which we see Orion in Winter but not in Summer) it follows a curved path.

If spacetime is curved, is the curvature convex or concave?

If at the point A of space there is a clock, an observer at A can determine the time values of events in the immediate proximity of A by finding the positions of the hands which are simultaneous with these events. If there is at the point B of space another clock in all respects resembling the one at A, it is possible for an observer at B to determine the time values of events in the immediate neighbourhood of B. But it is not possible without further assumption to compare, in respect of time, an event at A with an event at B. We have so far defined only an A time'' and a B time.'' We have not defined a common time'' for A and B, for the latter cannot be defined at all unless we establish by definition that the time'' required by light to travel from A to B equals the time'' it requires to travel from B to A. Let a ray of light start at the A time'' from A towards B, let it at the B time'' be reflected at B in the direction of A, and arrive again at A at the A time'' .

In accordance with definition the two clocks synchronize if

We assume that this definition of synchronism is free from contradictions- Albert Einstein.

We don't need to assume Albert Einstein was a ranting lunatic, clearly his "definition" is absurd, obviously it takes more time for light to reach Mars from Earth than it does to return.