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.

Great definition of a theory.

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

Today

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

The general theory of relativity describes the force of gravity and the large-scale structure of the universe, that is, the structure on scales from only a few miles to as large as a million million million million (1 with twenty-four zeros after it) miles, the size of the observable universe. Quantum mechanics, on the other hand, deals...

The fundamental postulate of the theory of relativity, as it was called, was that the laws of science should be the same for all freely moving observers, no matter what their speed.

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.

So general relativity predicts that light should be bent by gravitational fields.

Now, the apparent brightness of a star depends on two factors: how much light it radiates (its luminosity), and how far it is from us.

A precise statement of this idea is known as the second law of thermodynamics. It states that the entropy of an isolated system always increases, and that when two systems are joined together, the entropy of the combined system is greater than the sum of the entropies of the individual systems.