It is force. Even without Einstein. That is, even without relativity, or relativistic deformations. How were the directions or orientations of time and space decided. They were uniformly in a circle, in all directions, at the Big Bang, and you just happened to be at a certain configuration among them.
Photonic synchronicity is a very curious matter.
For the photon it doesn't matter. Whether it goes back or forth. You have to see the Feynman diagrams for the photon. When it comes back, after having traveled there from one side, it's not the same time at all. Because they're coordinated, but they can't go back.