The special theory of relativity (or just special relativity), proposed by Einstein in 1905, deals with the preferred status of the speed of light. We have noted that the speed of light c is the maximum speed attainable in the universe. But there is more to it than that. In 1887 a fundamental experiment carried out by two American physicists, A. A. Michelson and E. W. Morley, demonstrated a further important and unique aspect of light—that the measured speed of a beam of light is independent of the motion of the observer or the source. No matter what our speed may be relative to the source of the light, we always measure precisely the same value for c—299,792.458 km/s.

A moment's thought leads us to the realization that this is a decidedly nonintuitive statement. For example, if we were traveling in a car moving at 100 km/h and we fired a bullet forward with a speed of 1000 km/h relative to the car, an observer standing at the side of the road would see the bullet pass by at 100 + 1000 = 1100 km/h, as illustrated in the accompanying figure. However, if we were traveling in a rocket ship at 1/10 the speed of light, 0.1c, and we shone a searchlight beam ahead of us, the Michelson—Morley experiment tells us that an outside observer would measure the speed of the beam not as 1.1c, as the preceding example would suggest, but as c. The rules that apply to particles moving at or near the speed of light are different from those we are used to in everyday life.

Special relativity is the mathematical framework that allows us to extend the familiar laws of physics from low speeds (that is, speeds much less than c, which are often referred to as nonrelativistic) to very high (or relativistic) speeds, comparable to c. Relativity is equivalent to Newtonian mechanics when objects move much more slowly than the speed of light, but it differs greatly in its predictions at relativistic velocities. For example, special relativity predicts that a rapidly moving spacecraft will appear to contract in the direction of its motion, its clocks

General relativity is what results when gravity is included in the framework of special relativity. In 1915 Einstein made the connection between special relativity and gravity with the following famous "thought experiment." Imagine that you are enclosed in an elevator with no windows, so that you cannot directly observe the outside world, and the elevator is floating in space. You are weightless. Now suppose that you begin to feel the floor press up against your feet. Weight has apparently returned. There are two possible explanations for this, shown in the accompanying diagram. A large mass could have come nearby, and you are feeling its downward gravitational attraction, or the elevator has begun to accelerate upward and the force you feel is that exerted by the elevator as it accelerates you at the same rate. The crux of Einstein's argument is this: there is no experiment that you can perform within the elevator, without looking outside, that will let you distinguish between these two possibilities.

Thus, Einstein reasoned, there is no way to tell the difference between a gravitational field and an accelerated frame of reference (such as the rising elevator in the thought experiment). Gravity can therefore be incorporated into special relativity as a general acceleration of all particles. However, another major modification to the theory of special relativity must be made. Central to relativity is the notion that space and time are not separate quantities but instead must be treated as a single entity—spacetime. To incorporate the effects of gravity, the mathematics forces us to the conclusion that spacetime has to be curved.

In general relativity, then, gravity is a manifestation of curved spacetime. There is no such thing as a "gravitational field," in the Newtonian sense. Instead, objects move as they do because they follow the curvature of spacetime, and this curvature of spacetime is determined by the amount of matter present. We will explore some of the consequences of this view of gravity in more detail in the text.