my father who is an MBA not a PhD asked me: Which is it more correct to say Mass curves space and bends light or Gravity curves space and bends light or Mass curves space and gravity bends light or some other phrasing? My answer was/is: Interesting distinction you're seeking Gravity causes space-time curvature and the latter cause light to bend but Gravity has two sources, one of which has nothing to do with mass. Gravity not mass is thus Supreme First consider the Equivalence Principle The Equivalence Principle says that there is an equivalence between a frame of reference in a gravitational field (the person in the elevator feels gravity pulling him onto the floor) and a frame of reference which is accelerating at the right rate (the person still feels pulled to the floor) So you have the feeling by an observer of gravity in the two cases. 1) a nearby mass causes gravity or 2) a constant acceleration causes gravity without a nearby mass The issue is if there is no nearby mass then one wonders what agency or agent is causing a constant acceleration of the reference frame? Clearly it takes a lot of power or energy to constantly accelerate a reference frame (an observing person) so one wonders if mass isn't doing it what is? (A rocket engine could be hurtling the observer through empty space-time far from any masses) Assuming simply that there is constant acceleration (by whatever cause) then it would appear that it is the constant acceleration causing curvature of space which bends light But this way of analyzing it might be incomplete When objects fall or ppl are forced into elevator floors or light bends it is equivalent to all of those objects following curved space time lines To conclude let's define gravity as equivalent to constant acceleration You can have space-time curvature without mass You cannot have space-time curvature without gravity (constant acceleration) So I'd say it was constant acceleration or gravity which causes all these effects: curvature of space, bending of light, ppl being forced into elevator floors Since they all can happen without mass but which then implies some unknown cause for constant acceleration this means that mass cannot be Primal Cause Constant acceleration or Gravity is the Primal Cause for all of these phenomena and matter or mass just is a simple way of achieving Gravity but there is another way to cause Gravity without matter or mass and that is by externally applying a force which results in constant acceleration Big question is if mass or matter isn't present then how in tarnations is constant acceleration being caused or forced ? The answer is a rocket engine delivering propulsion or thrust. If that answer is irrelevant then Gravity alone is Primal Cause but mass or matter isn't! Do General Relativists or other physicists or interested observers of this web forum agree? TO boil it down to its utter simplicity, choose one of the 4 following mutually exclusive and collectively exhaustive statements as being more correct than the others: 1. Mass curves space and bends light. 2. Mass curves space and gravity bends light. 3. Gravity curves space and mass bends light. 4. Gravity curves space and bends light.