Relativity, Theory of

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n the early 1900s Albert Einstein (1879-1955) produced two theories that caused a revolution in science. Together, they are known as the theory of relativity.

The first of Einstein’s relativity theo­ries is his Special Theory of Relativity of 1905. This states that:

(1) The laws of physics are the same for people moving at different speeds.

(2) The speed of light is always the same for all observers, no matter how fast they are moving.

These two simple principles produce some surprising effects when speeds close to the speed of light are involved.

There is no such thing as absolute rest anywhere in the universe. Someone standing still on Earth is actually moving because Earth is spinning. The planet also is orbiting the Sun. The Sun orbits the center of the Milky Way (our

galaxy), and the Milky Way moves through space among billions of other galaxies that also are moving. Any observer can say that he or she is at rest and everything else in the universe is moving in relation to—or relative to— him or her. This phenomenon gives relativity its name.

One of the most surprising effects of relativity is that times and lengths depend on who measures them. Imagine that one person stays on Earth while her twin goes on a long spaceflight almost as fast as the speed of light. The person on Earth would see the spacecraft become shorter as it accelerates toward the speed of light. This is called Lorentz contraction. Also, time runs more slow­ly as speed increases. When the astro­naut twin returns to Earth, she would be younger than her earthbound twin. This is known as the “twins paradox.”

The Special Theory of Relativity also predicts that as something goes faster, it becomes more and more difficult to make it go even faster. The more energy it has, the more inertia it has. Einstein also showed that energy

О The twins paradox illustrates Einstein’s theory that if two bodies at the same point accelerate to different speeds and then meet again, they will find that a different amount of time has elapsed for each of them.

О If space were a stretched rubber sheet, a heavy ball on the surface would curve the sheet by sink­ing into it. According to Einstein’s General Theory of Relativity, gravity occurs because smaller mov­ing objects curve toward the larger mass.

and mass are linked. He produced his famous equation, E=mc2, to show how they are related. In this equation, “c” is the speed of light, a huge number, so even a tiny mass (m) is equivalent to an enormous amount of energy (E).

In 1915, Einstein produced a new theory, his General Theory of Relativity, which added the effect of gravity to the Special Theory. Nobody had been able to explain how gravity works. Einstein imagined that space is flat and all objects dent it, like heavy weights sitting on a stretchy sheet. In other words, mass bends space. The paths of moving objects curve toward massive objects, simply because space itself is curved there. Light also is bent by curved space.

TIME CORRECTION

Global Positioning System (GPS) navigation satellites fly around the world at 7,000 miles per hour (11,300 kilometers per hour). According to special relativity, their onboard clocks should run more slowly than clocks run on Earth. According to general relativity, the curving of space caused by Earth has the opposite effect. It makes the clocks run faster than clocks on Earth. The result of these two coun­teracting effects is that the satellite clocks run 38-millionths of a second faster every day than clocks on Earth. It is a tiny error, but GPS clocks have to be more than 1,000 times more accurate than this. So satellite clocks are deliberately set to run slow before they are launched, so that they are correct in orbit.

Relativity is not just a strange set of ideas and mathematics. Its effects are real. Observations and experiments have confirmed many of its predictions. Spacecraft designers and planners of space missions have to allow for them.

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SEE ALSO:

• Einstein, Albert • Global Positioning System • Spaceflight

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