A Brief History of Time

by Stephen Hawking

A theory is a good theory if it satisfies two requirements. It must accurately describe a large class of observations on the basis of a model that contains only a few arbitrary elements, and it must make definite predictions about the results of future observations.

Any physical theory is always provisional, in the sense that it is only a hypothesis: you can never prove it. No matter how many times the results of experiments agree with some theory, you can never be sure that the next time the result will not contradict the theory. On the other hand, you can disprove a theory by finding even a single observation that disagrees with the predictions of the theory.

The eventual goal of science is to provide a single theory that describes the whole universe.

Only light, or other waves that have no intrinsic mass, can move at the speed of light.

We must accept that time is not completely separate from and independent of space, but is combined with it to form an object called space-time.