Relativity

Relativity is concerned with observing the same events from two different points of view. The key understanding concerns one point of view moving with respect to another; for example, comparing observations from a speeding airplane with those from the ground below the plane. If one point of view is moving steadily with respect to the other, time and space follow laws first derived by Albert Einstein in 1905 with additional ideas from Henri Poincare [French: 1854-1912] in 1906.

Einstein assumed that physical laws remain the same for both the moving observer and the stationary one, including observer and the stationary one, including that the speed of light is the same for both. These simple assumptions have great consequences, called the special theory of relativity. Time merges with space, objects shrink in the direction of motion, the speed of light in a vacuum cannot be exceeded, and mass can be converted into energy or energy into mass.

By 1915, Einstein had worked out the consequences for one observer accelerating with respect to the other. He assumed that gravity is the same as the force that keeps a body moving in a straight line (ignoring friction or other forces). This leads to the general theory of relativity, which implies that gravity results from mass causing space to curve.