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In Newton’s model, gravity is the result of an attractive force between massive objects. Although even Newton was troubled by the unknown nature of that force, the basic framework was extremely successful at describing motion. Although general relativity is not the only relativistic theory of gravity, it is the simplest such theory that is consistent with the experimental data. Several physicists, including Einstein, searched for a theory that would reconcile Newton’s law of gravity and special relativity. Only Einstein’s theory proved to be consistent with experiments and observations. Since everything in the elevator is falling together, no gravitational effect can be observed.
Roughly speaking, the principle states that a person in a free-falling elevator cannot tell that they are in free fall. Objects are falling to the floor because the room is resting on the surface of the Earth and the objects are being pulled down by gravity. Objects are falling to the floor because the room is aboard a rocket in space, which is accelerating at 9. The objects are being pulled towards the floor by the same “inertial force” that presses the driver of an accelerating car into the back of his seat.
Conversely, any effect observed in an accelerated reference frame should also be observed in a gravitational field of corresponding strength. Einstein’s master insight was that the constant, familiar pull of the Earth’s gravitational field is fundamentally the same as these fictitious forces. In 1907, Einstein was still eight years away from completing the general theory of relativity. Nonetheless, he was able to make a number of novel, testable predictions that were based on his starting point for developing his new theory: the equivalence principle. Aboard such a ship, there is a natural concept of “up” and “down”: the direction in which the ship accelerates is “up”, and unattached objects accelerate in the opposite direction, falling “downward”. Einstein argued that such frequency shifts must also be observed in a gravitational field. This is illustrated in the figure at left, which shows a light wave that is gradually red-shifted as it works its way upwards against the gravitational acceleration.
Two bodies falling towards the center of the Earth accelerate towards each other as they fall. The equivalence between gravitational and inertial effects does not constitute a complete theory of gravity. But a freely falling reference frame on one side of the Earth cannot explain why the people on the opposite side of the Earth experience a gravitational pull in the opposite direction. A more basic manifestation of the same effect involves two bodies that are falling side by side towards the Earth. Earth, the effect is large.
For gravitational fields, the absence or presence of tidal forces determines whether or not the influence of gravity can be eliminated by choosing a freely falling reference frame. In the summer of 1912, inspired by these analogies, Einstein searched for a geometric formulation of gravity. Einstein formulated a geometric description of gravity in which Minkowski’s spacetime is replaced by distorted, curved spacetime, just as curved surfaces are a generalization of ordinary plane surfaces. 1915, culminating in his final presentation on November 25, 1915. What this means is addressed in the following three sections, which explore the motion of so-called test particles, examine which properties of matter serve as a source for gravity, and, finally, introduce Einstein’s equations, which relate these matter properties to the curvature of spacetime. In the absence of gravity and other external forces, a test particle moves along a straight line at a constant speed. These paths are certainly not straight, simply because they must follow the curvature of the Earth’s surface.
But they are as straight as is possible subject to this constraint. The properties of geodesics differ from those of straight lines. For example, on a plane, parallel lines never meet, but this is not so for geodesics on the surface of the Earth: for example, lines of longitude are parallel at the equator, but intersect at the poles. But still there are crucial differences between them and the truly straight lines that can be traced out in the gravity-free spacetime of special relativity. In special relativity, parallel geodesics remain parallel. In a gravitational field with tidal effects, this will not, in general, be the case. If, for example, two bodies are initially at rest relative to each other, but are then dropped in the Earth’s gravitational field, they will move towards each other as they fall towards the Earth’s center.