A Brief History of Time

by Stephen Hawking

scientists describe the universe in terms of two basic partial theories – the general theory of relativity and quantum mechanics.

As an object approaches the speed of light, its mass rises ever more quickly, so it takes more and more energy to speed it up further. It can in fact never reach the speed of light, because by then its mass would have become infinite, and by the equivalence of mass and energy, it would have taken an infinite amount of energy to get it there. For this reason, any normal object is forever confined by relativity to move at speeds slower than the speed of light. Only light, or other waves that have no intrinsic mass, can move at the speed of light.

prediction of general relativity is that time should appear to run slower near a massive body like the earth.