The Open Curriculum

The Mind · Level 2

A Brief History of Time

Stephen Hawking · 1988

Thesis

The universe can be understood through a small number of physical laws, and the great unfinished task of science is to combine the two pillars of modern physics, general relativity and quantum mechanics, into a single theory that would explain how the universe began, how it behaves and perhaps why it exists at all.

The main ideas

A book for everyone

Stephen Hawking was a theoretical physicist at the University of Cambridge, known for his work on black holes and the origin of the universe. A Brief History of Time, published by Bantam in 1988, was his attempt to explain these subjects to readers with no training in science.

He was told that every equation he included would halve the book's sales, so he included only one: Einstein's E=mc². The book became one of the best-selling works of science ever written, selling more than 10 million copies and spending 237 weeks on the Sunday Times bestseller list in Britain.

A tenth-anniversary edition in 1998 added a chapter on wormholes and time travel, and in 2005 Hawking and Leonard Mlodinow published A Briefer History of Time, a shorter, more accessible rewrite.

Implication: the hardest ideas can reach a wide audience when they are explained in words rather than formulas.

Space, time and the expanding universe

The book opens with the history of cosmology, from Aristotle and Ptolemy's Earth-centered universe to Copernicus, Galileo and Newton. Newton's laws described a universe in which space and time were fixed and absolute.

Einstein's theories of relativity overturned this. Space and time are woven together into a single space-time, which is curved by the matter and energy within it. Gravity is the effect of that curvature.

In the 1920s, Edwin Hubble found that distant galaxies are moving away from us, and the farther away, the faster. The universe is expanding. Running the expansion backward suggests that everything was once compressed into an extremely hot, dense state: the big bang.

Hawking and Roger Penrose showed that, if general relativity is correct, the universe must have begun in a singularity, a point where density and the curvature of space-time become infinite and the laws of physics break down.

Implication: the questions of how the universe began and how it will end are now questions of physics, open to evidence.

Uncertainty and the forces

Quantum mechanics, the physics of the very small, introduces a fundamental limit. Heisenberg's uncertainty principle holds that the more precisely one measures a particle's position, the less precisely one can know its velocity, and the reverse. This ended the hope, associated with the scientist Laplace, of predicting the future exactly from complete knowledge of the present.

Hawking describes the particles that make up matter and the four forces that act between them: gravity, electromagnetism, the weak nuclear force and the strong nuclear force. Physicists had partly unified the last three, but gravity resisted.

The mechanism is that relativity governs the very large and quantum mechanics the very small, and the two theories are incompatible. Where both matter, at the big bang and inside black holes, neither alone is enough.

Implication: when two well-tested theories conflict, the place where they meet is where the next discovery is likely to be found.

Black holes ain't so black

A black hole forms when a massive star collapses so far that its gravity prevents anything, even light, from escaping. Its boundary is called the event horizon.

Hawking's most famous result came from applying quantum mechanics to black holes. He showed in the 1970s that black holes should emit radiation, now called Hawking radiation, because of quantum effects near the event horizon. As a result, black holes lose mass and could eventually evaporate. The smaller the black hole, the faster this happens.

The chapter's title, "Black Holes Ain't So Black," reflects the surprise. A black hole, defined by the fact that nothing escapes it, turns out to glow.

The mechanism is that combining relativity and quantum mechanics, even partially, produced a result neither theory could reach alone.

Implication: the most surprising discoveries often come from applying a theory to a domain where nobody had thought to apply it.

The arrow of time and no boundary

Hawking asks why we remember the past but not the future. He distinguishes three arrows of time: the thermodynamic arrow, the direction in which disorder, or entropy, increases; the psychological arrow, the direction in which we remember; and the cosmological arrow, the direction in which the universe expands. He argues that they point the same way.

He also presents his own proposal, developed with James Hartle, about the universe's origin. Using a mathematical concept called imaginary time, which behaves like a direction in space, he suggests the universe may be finite but have no boundary, like the surface of the Earth, which has no edge. In this picture, the universe would have no singular beginning that required special conditions to be set from outside.

Implication: some questions that seem to demand an answer, such as what came before the beginning, may dissolve when the framework is changed.

The mind of God

The book's final goal is a complete unified theory that would describe everything in the universe. Hawking was optimistic that such a theory might be found.

He ends by suggesting that if it were found, it would be the ultimate triumph of human reason, "for then we would know the mind of God." The line drew enormous attention. Hawking later made clear that he did not believe in God, and that the phrase was a figure of speech for complete understanding.

Implication: understanding the laws of nature is a different question from understanding why there are laws at all, and the book raises the second without answering it.

Limits

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