In 1905, Albert Einstein published his special theory of relativity. According to this theory, no matter in the universe can attain a speed greater than the speed of light in a vacuum, which thus serves as the speed limit of the universe.

This has been verified in many experiments. However, space and time are no longer absolute. Two observers moving relative to one another do not agree on the length of objects or the passage of time. The mechanics of objects based on Newton's laws of motion, although remarkably accurate even for speeds of many thousands of miles per second, begin to fail when the relative motion between objects approaches the speed of light in a vacuum. Moreover, the special theory of relativity reveals a fundamental limitation of Newton's laws.

According to Newton's laws of motion and Newton's law of gravitation, all actions happen instantaneously. Since Einstein's special theory of relativity states there is a speed limit in the universe, such instantaneous action happening over a finite distance is not fundamentally possible.

In 1915, Einstein proposed a solution to this problem in the general theory of relativity, in which he formalized the principle of equivalence in mathematical terms. According to the theory, gravitation is not a force between two objects; instead, it is an effect of the two objects on the space-time around them, which in turn determines their dynamics.

In the special and general theories of relativity, space and time are treated on an equal footing. The curvature is not of space alone but of the combined entity ‘space-time.'

For weak gravitational fields, the results of general relativity do not differ significantly from Newton's law of gravitation. However, for intense gravitational fields, the results diverge, and general relativity has been shown to predict the correct results. These effects have been observed in our Sun's relatively weak gravitational field at the distance of Mercury's orbit. Since the mid-1800s, Mercury's elliptical orbit has been carefully measured. However, although it is elliptical, its motion is complicated by the fact that the perihelion position of the ellipse slowly advances. Most of the advance is due to the gravitational pull of other planets, but a small portion of that advancement could not be accounted for by Newton's laws. There was even a search for a “companion” planet that would explain the discrepancy at one time. However, general relativity correctly predicts the measurements.

This text is adapted from Openstax, University Physics Volume 1, Section 13.7: Einstein's Theory of Gravity.