Predicting Gravitational Waves

General Theory of Relativity

Over the next few years, Einstein continued to work out the details of the Special Theory of Relativity. As he did this, he began to wonder how Newton’s definition of gravity fit in with this theory.

Einstein realized that a person falling to the earth does not feel the effect of gravity, just like astronauts in space. He also realized that a rocket increasing in speed at a constant rate would feel the same force of “gravity” as one sitting on the earth.

Einstein’s General Theory of Relativity is based on this idea that objects on earth feel the same gravitational force as identical objects that are far from heavy objects and speeding up at a constant rate. Einstein realized that since these two forces are the same, the laws of physics have to hold in both cases. This required a change in the definition of gravity.

Einstein showed that gravity is not the “force” Newton thought it was. He explained that objects are attracted to each other because heavy objects curve spacetime and other objects take the shortest path through this curved spacetime. Mathematically, Einstein found that spacetime is similar to a stretchy fabric, much like a trampoline.

Curved Space Around a Massive Object

Imagine a bowling ball in the middle of a trampoline. The weight of the ball makes the trampoline sink in the middle. A light object, like a tennis ball, on the edge of the trampoline will follow a curved path toward the bowling ball – like a planet orbiting the sun.

The sinking of the trampoline represents how heavy objects curve spacetime. The path taken by the tennis ball illustrates an object taking the shortest path through curved spacetime. Newton thought gravity was a mysterious force acting between two objects, but Einstein explained that it is the curving of spacetime.

One prediction of this theory is that light should bend near heavy objects. Heavy objects should curve the spacetime surrounding them and everything, even light, should follow a curved path through spacetime.

In 1919, Sir Arthur Eddington tested this prediction during a solar eclipse by measuring starlight bending around the sun. His result exactly matched Einstein’s prediction. This was the first experimental confirmation of Einstein’s theory and made him immediately famous among scientists and the public.