What Is Special Relativity?
Watch Most people have heard of Albert Einstein and the theory of relativity, actually there are two, Special Relativity and General Relativity, but only Special Relativity is important to us.

Special Relativity introduces the idea that the faster/heavier something is the slower that time will pass for that object. The reverse is also true the slower/lighter something is the faster that time will pass.

For more details see Wikipedia’s entry on Relativity.

Whilst this seems odd, the basic idea is that the underlying atomic processes that govern our existence and all aspects of the universe are affected by the speed/mass of an object.