Einstein was the creator of two theories of relativity which have revolutionized scientists’ way of thinking about the Universe: the special theory of relativity (1905) and the general theory (1915).
Time is relative because it depends where you measure it from (see time). Distances and speed are relative too. If you are in a car and another car whizzes past you, for instance, the slower you are traveling, the faster the other car seems to be moving.
Einstein showed in his special theory of relativity that you cannot even measure your speed relative to a beam of light, which is the fastest thing in the Universe. This is because light always passes you at the same speed, no matter where you are or how fast you are going.
Einstein realized that if light always travels at the same speed, there are some strange effects when you are moving very fast.
If a rocket passing you zoomed up to near the speed of light, you would see it shrink.
In normal everyday life, the effects of relativity are so tiny that you can ignore them. However, in a spacecraft traveling very fast they may become quite significant.
If a rocket passing you zoomed up to near the speed of light, you’d see the clocks on the rocket running more slowly as time stretched out. If the rocket reached the speed of light, the clocks would stop altogether.
If a rocket passing you zoomed near the speed of light, it would seem to get heavier and heavier. Rut it would gradually become so heavy, there wouldn’t be enough energy in the Universe to speed it up any further.
Einstein’s general relativity theory brought in gravity. It showed that gravity works basically by bending space-time. From this theory scientists predicted black holes and wormholes.
In 1919 an eclipse of the Sun allowed Arthur Eddington to observe how the Sun bends light rays, proving Einstein’s theory of general relativity. 1 In a spacecraft traveling almost at the speed of light, everything becomes heavier. Many scientists believe objects will never be able to accelerate to the speed of light because the faster it goes, the heavier it gets. When astronauts went to the Moon, their clock lost a few seconds. The clock was not faulty, but time actually ran slower in the speeding spacecraft.
In a spacecraft traveling almost at the speed of light, time runs slower. So astronauts going on a long, very fast journey into space come back a little younger than if they had stayed on the Earth.
A spacecraft traveling almost at the speed of light seems to shrink. Of course, if you were actually on board everything would seem entirely normal. Instead, it would he the world outside that seemed to shrink, since it is traveling almost at the speed of light relative to you.