Special relativity is a scientific theory describing how matter moves through time and space. When it was first published in 1905 by Albert Einstein, special relativity caused a revolution in the physics community, and made us look at the universe in a new light. Special relativity is one of the most well-confirmed physics theories of all time, and its predictions have been verified to more than twenty decimal places of accuracy.
The two basic postulates of special relativity are that the laws of physics are the same regardless of absolute velocity, and that the speed of light is constant for all observers. If you are in a closed box moving at constant velocity, special relativity predicts that no experiment you do inside the box can tell you how fast the box is moving. Likewise, the speed of light will remain the same for an observer inside the box, even if the box itself is moving at a large fraction of the speed of light.
Special relativity abandons the notions of “absolute space” and “absolute time” developed by Newton. Under special relativity, there is no such thing as a single universal time; rather, time is different for every observer. There is also no single universal measure of space; a single ruler can be longer or shorter depending on who measures it. Finally, special relativity unifies the concepts of space and time into a single four-dimensional structure called “spacetime.”
According to special relativity, if an object is moving at high speed relative to you, the object will appear to behave strangely. Its mass will increase, so that it becomes harder and harder to accelerate as it approaches the speed of light. It will appear to shrink in its direction of motion, becoming more and more distorted as it travels faster. The object's time will also become distorted; if there is a clock on the object, it will appear to tick more slowly. These effects happen to every object, but they only become noticeable once objects approach the speed of light.
Special relativity prohibits any object from traveling faster than the speed of light. If an object appears to go faster than lightspeed for one observer, it must be possible to find an observer who sees the object traveling backward in time. As an object's velocity approaches that of light, its mass and kinetic energy go to infinity. Even information may not travel faster than light, as this would allow messages to be sent backward in time as well.