The concept of "relativity" is a solid one. Einsteins SR and GR are not.


As an example of the correctness of the basic concept of relativity:

If I am traveling on the highway at 70mph I am moving at various relative velocities with other vehicles and have different kinetic energy potentials with them is one real relativity.
With cars traveling the same direction and same speed there is no relative velocity and yet we are moving at 70mph relative to the road itself, for cars traveling the same direction but different velocities the relative velocity is the difference between our velocities and with cars traveling the opposite lane or direction the relative velocity is our velocities added together. here we see a classic Newtonian relativity that he no doubt understood. It gets a bit more complicated than that obviously when the directions of travel of masses are at various angles to each other and not in the same or opposite directions, but trigonometry will suffice in calculating relative velocities by establishing ratios in these cases that can be used to calculate KE and relative velocities.