A Brief History of Time

Our picture of the universe began to change dramatically in 1924, when the American astronomer Edwin Hubble demonstrated that ours was not the only galaxy. There were many others, with vast tracks of empty space between them.

In order to predict how the universe should have started, one needs laws that hold at the beginning of time. If the classical theory of general relativity is correct, there must have been a singularity of infinite density and infinite curvature of space-time at the beginning of the universe.

The general theory of relativity is what is called a classical theory; that is, it does not take into account the uncertainty principle of quantum mechanics. The theory tells us that the universe started with a singularity — but this singular point cannot be described by the equations of classical theory.

The ultimate theory must be quantum mechanical in nature. It must incorporate the uncertainty principle and not allow the laws of physics to break down completely, as general relativity predicts the singularity would require.

Stephen Hawking spent much of his career working on the unification of quantum mechanics and general relativity — the two great pillars of twentieth-century physics that remain stubbornly incompatible with each other at the most fundamental levels of reality.