Re: As long as 2 objects are in a state of *uniform relative motion* (no change of speed OR direction) the situation is totally reciprocal.
ie, each sees the other’s clock ticking out time more slowly than his own, as each can rightfully claim THEY are stationary and the other moving. It’s only when energy is added into the system causing a change in motion or direction that this symmetry breaks. An object will continue in a straight line at a constant rate of motion (inertial motion) until acted upon by an outside force.
Is this true for objects with no mass ?