A Quoran asks: What is gravity? Does it work like Sir Newton said or Like Sir Einstein said? Which definition is more accurate?
Gravity is the name we give to the fact that all (uncharged) matter tends to clump together.
For normal objects (including apples, moons, planets and stars) moving at normal speeds it works (so far as we can detect with normal instruments) exactly like Sir Newton said; but for objects moving more than a hundred thousand times the speed of sound (like subatomic particles in a high energy accelerator) or for very precise measurements over long periods of time (like observing the orbit of Mercury over many decades or coordinating the timing of super-accurate atomic clocks on GPS satellites) we can detect subtle differences. Most of these differences are consistent with what ‘Sir’ Einstein said, so that version is more accurate.
But whether or not it really works exactly like that is not certain, and in fact some observations (such as the rotation speeds of some galaxies) may not exactly fit (but we don’t yet know whether that means we need a new theory or just that there is more massive stuff in those galaxies than we thought there was). And also, since it’s not a “quantum” theory it may have to be modified in order to correctly describe how it works with elementary particles.
Isaac Newton first suggested that the way matter clumps together can be described by assigning a number (called “mass”) to each object and then, for each object, adding up acceleration vectors pointing towards all of the other objects whose magnitudes are proportional to the masses of the target objects and inversely proportional to the squares of the distances. This rule works perfectly well for the initial acceleration of objects that are stationary relative to one another – and before the invention of super-accurate atomic clocks, no deviation from it was ever detectable for macroscopic objects moving for short times at any achievable speed.
Albert Einstein noted that if the relationship between electricity and magnetism were to have the same form (and so to give the same apparent speed of light) for all observers, then space and time would have to be related in such a way that events which appear simultaneous to one observer might seem to be at different times for another observer who is moving relative to the first. (This relationship can be interpreted as a kind of “geometry” for the combined four dimensional space-time combination.)
In attempting to come up with a modification of Newton’s theory that fit with this, Einstein was led to a version which can be expressed in terms of how mass affects the relationship between space and time (in a way that can be interpreted as adding curvature to the “geometry”). And, as mentioned above, no observation has yet ruled out the possibility that this theory might really be exact.