The special theory of relativity, proposed by Albert Einstein in 1905, drastically changed our understanding of space and time. According to special relativity, measured distances and times depend on the frame of reference of the observer. Einstein realized that time and space are linked in a space-time system, whose properties appear most dramatically when objects move very fast and are observed by two different observers moving with respect to one another. In other words, space and time are not absolute.

The first tenet of special relativity is that the physical laws governing the way things move and interact are the same for two observers moving at constant velocity with respect to one another. The second tenet is that the speed of light is invariant, meaning it is the same for any observer.

Einstein realized that in order for the laws of physics to be the same and the speed of light to be constant, time and space must be quite different from the way we normally think of them. For example, we typically understand the relative motion of ordinary objects by adding speeds in different reference frames. Imagine pitching a baseball from a car that is moving toward home plate at 50 mph. From the point of view of an observer at home plate, it appears that the ball moves toward him with the speed of the pitch plus the speed of the car.

Now imagine you were to pitch light instead of a baseball. In this case, from the point of view of an observer at home plate, the moving car would not change the speed of your pitch—the speed of light is constant. How is that possible? If space and time were absolute, there would be no way to explain why the speed of light is constant for any observer. However, Einstein's special theory of relativity presented a new theory of space and time that could explain it.

Einstein showed, mathematically, that space and time are related and perceived differently by different observers. One consequence of special relativity is time dilation, which means that time in a moving reference frame passes more slowly than time in a stationary reference frame. In other words, a clock that is moving with respect to a stationary observer is perceived by the observer to run more slowly than a stationary clock. Another consequence is length contraction—the observed decrease in length of objects traveling relative to an observer. As part of the special theory of relativity, Einstein also reconceived our understanding of momentum and energy. As the speed of an object approaches the speed of light, its inertia, or resistance to acceleration, also increases. In other words, its mass increases. Additionally, the theory yielded the famous equation E=mc², which expresses the relationship between energy and mass.

Print Background Essay