What would you see if you could travel at 186,000 miles per second—the speed of light? Einstein spent a lot of time working this out. The things he discovered were so strange that people had to start thinking about the universe in a completely new way.
Einstein found that space and time are connected, and that it is better to talk about them as one thing – spacetime (Light Cone). He showed that objects get heavier as they travel faster, and that their size shrinks in the direction they are moving. Time also passes slower on a clock traveling at high speeds than on a clock at rest (Clock, Train, Twin Paradox). We don’t see these effects in everyday life because cars and bikes don’t go fast enough to experience changes that we can measure.
This work is part of Einstein’s Special Theory of Relativity. It is called the Special Theory because it only applies when objects are moving at a constant speed. Einstein later created the General Theory of Relativity that talks about what happens when objects speed up or slow down (Black Holes).
The famous equation E=mc2 is a result of the Special Theory of Relativity. This equation shows that energy and mass are two forms of the same thing, and that a little mass can turn into a whole lot of energy. This knowledge helps us understand how the stars power themselves (Sun) and how to make nuclear power (Radioactivity).