The Universe Has a Beginning
Hawking begins with the expanding universe (Hubble's discovery, 1929) and traces it backward: if galaxies are moving apart now, they were closer in the past. Extrapolating back reaches a moment of infinite density — the Big Bang, roughly 13.8 billion years ago. General relativity predicts the universe began as a singularity. The Penrose-Hawking singularity theorems prove this is unavoidable under plausible conditions.
Space and Time May Have No Boundary
The book's most original cosmological claim: the no-boundary proposal (with Jim Hartle). Just as Earth has no edge — you can travel south indefinitely without falling off — Hawking and Hartle proposed that space and time together have no boundary. 'The boundary condition of the universe is that it has no boundary.' The universe did not begin at a singularity; time itself came into existence with the universe. There was no 'before' the Big Bang.
Black Holes and Hawking Radiation
Hawking explains his greatest discovery: black holes emit thermal radiation (Hawking radiation) due to quantum effects near the event horizon. This means black holes slowly lose mass and eventually evaporate — a profound connection between general relativity, quantum mechanics, and thermodynamics. The information paradox (what happens to information that falls into a black hole) is posed but not resolved.
The Arrow of Time
The laws of physics are (mostly) time-symmetric — they work equally well forward and backward. Yet we experience a definite arrow of time: the past is fixed, the future open; entropy increases. Hawking argues three arrows of time coincide: the thermodynamic arrow (entropy increases), the psychological arrow (we remember the past, not the future), and the cosmological arrow (the universe expands rather than contracts).
'The Mind of God'
Hawking ends with the question of a unified theory: if we can combine general relativity and quantum mechanics, 'we would know the mind of God.' He does not mean this literally — Hawking was an atheist. The phrase means: a complete physical theory would answer the ultimate why, and would be complete knowledge of physical reality.
よくある質問
What is A Brief History of Time about?
Hawking's book covers the Big Bang and origin of the universe, black holes and Hawking radiation, the no-boundary proposal (the universe has no beginning or boundary in imaginary time), the arrow of time, and the quest for a unified theory of quantum mechanics and general relativity. It argues that science is approaching a complete understanding of the universe's origin and structure.
What is the no-boundary proposal?
The Hartle-Hawking no-boundary proposal suggests that the universe has no initial singularity — no 'edge' in time any more than the Earth has a geographic edge. In imaginary time, the universe is finite but boundless, like the surface of a sphere. Time itself came into existence with the universe; asking what came before is like asking what is south of the South Pole.
Did Hawking believe in God?
Hawking was an atheist or agnostic who used religious language metaphorically. His reference to 'the mind of God' in A Brief History of Time refers to a complete physical theory, not a deity. He stated publicly in 2010: 'I regard the brain as a computer which will stop working when its components fail. There is no heaven or afterlife for broken down computers; that is a fairy story for people afraid of the dark.'