On May 11, 7:04 pm, Igor wrote:
On May 11, 5:38 am, Albertito wrote:

Let two bodies, A and B, with equal masses, move inertially
along a straight line. Velocity of A wrt B is V_ab, and velocity
of B wrt A is V_ba. Since both bodies are inertially moving
along a straight line, we assume V_ab = - V_ba will always
hold, so we say both velocities are symmetrical. Suppose
now, body A accelerates during a time t at constant a_A along
the same straight line to yield a final velocity V_ab'. Can we
still claim the new velocity of B wrt A is V_ba' = -V_ab'? IOW,
isn't it reasonable to claim that the new V_ba' is actually not
that new, but V_ba' = - k*V_ab', for a real k 1? If it is true
that
V_ba' = - k*V_ab', for a real k 1, after the acceleration a_A
and V_ab' V_ab, then, can we conclusively say that
acceleration a_A has created an eventual gravitational field,
by claiming that both masses are no longer equal?

Just one question. What exists in empty spacetime that would make
them not be symmetric?

Energy, a lot of energy. Empty spacetime is not
actually empty.