The Theory of General Relativity
Time is relative. This is because it depends where you measure it from.
Distance and Speed are relative too.
E.g. If you are in a car and another car whizzes past you, the slower you're travelling, the faster the other car seems to move.
Einstein published this theory in 1915.
This theory redefined the Laws of Motion.
It says that it is impossible to tell the difference between gravity and the force of inertia from a moving object.
For example, if you were to climb inside a spinning spacecraft, the inertia will cause you to move towards the outside walls in a way that would feel just like gravity.
Large objects cause outer space to bend in the same way a marble laid onto a large thin sheet of rubber would cause the rubber to bend.
The larger the object, the further space bends. Just like a bowling ball would make the rubber sheet bend much more than the marble would.
Einstein realisedthat if light always travels at the same speed, there are some weird effects that take place.
If a rocket passes you near the speed of light, you would see it shrink.
If a rocket accelerates towards the speed of light, its mass increases dramatically. This is because momentum equals mass x speed. The rocket has the same momentum so it must be gainig mass.
If a rocket passing you zoomed up to the near speed of light, you'd see clocks on the rockets running more slowly as time stretched out.