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.

At this time the universe would have contained mostly photons, electrons, and neutrinos (extremely light particles that are affected only by the weak force and gravity) and their antiparticles, together with some protons and neutrons.

This is an example of the application of what is known as the anthropic principle, which can be paraphrased as, ‘We see the universe the way it is because we exist.’

Einstein once asked the question: ‘How much choice did God have in constructing the universe?’