A Brief History of Time: From the Big Bang to Black Holes is a book on cosmology by the physicist Stephen Hawking, first published in 1988.
Hawking writes in non-technical terms about the structure, origin, development and eventual fate of the universe. He talks about basic concepts like space and time, building blocks that make up the universe (such as quarks) and the fundamental forces that govern it (such as gravity). He discusses two theories, general relativity and quantum mechanics that form the foundation of modern physics. Finally, he talks about the search for a unified theory that consistently describes everything in the universe.
The book became a bestseller and has sold more than 25 million copies in 40 languages. It was included on Time's list of the 100 best nonfiction books since the magazine's founding. Errol Morris made a documentary, A Brief History of Time (1991) which combines material from Hawking's book with interviews featuring Hawking, his colleagues, and his family.
An illustrated version was published in 1996. In 2006, Hawking and Leonard Mlodinow published an abridged version, A Briefer History of Time.
Publication In 1983, Hawking approached Simon Mitton, the editor in charge of astronomy books at Cambridge University Press, with his ideas for a popular book on cosmology. Mitton was doubtful about all the equations in the draft manuscript, which he felt would put off the buyers in airport bookshops that Hawking wished to reach. In the acknowledgements, Hawking says he was warned that for every equation in the book, the readership would be halved. So it includes only a single equation: .
Contents Chapter 1: Our Picture of the Universe
Hawking begins with an anecdote about a scientist lecturing on the universe. An old woman got up and said, "What you have told us is rubbish. The world is really a flat plate supported on the back of a giant tortoise." The scientist asked what the tortoise was standing on. She replied, "You're very clever young man, very clever. But it's turtles all the way down!" Hawking goes on to explain why we know better.
He discusses the history of astronomy, starting with Aristotle's conclusions about a spherical Earth and a circular geocentric model of the universe, later elaborated upon by the second-century Greek astronomer Ptolemy. He discusses the development of the heliocentric model of the Solar System by the Polish astronomer Nicholas Copernicus in 1514. A century later, the Italian Galileo Galilei turned a Dutch spyglass to the heavens. His observations of Jupiter's moons provided support for Copernicus. The German astronomer Johannes Kepler formulated his laws of planetary motion, in which planets move in ellipses. Kepler's laws were explained by English physicist Isaac Newton in his Principia Mathematica (1687).
Hawking discusses how the subject of the origin of the universe has been debated over time: the perennial existence of the universe hypothesized by Aristotle and other early philosophers was opposed by St. Augustine and other theologians' belief in its creation at a specific time in the past. Immanuel Kant argued that time had no beginning. In our time, the discovery of the expanding universe implied that between 10 billion and 20 billion years ago, the entire universe was contained in one singular extremely dense place, and that it doesn't make sense to ask what happened before. He writes: "An expanding universe does not preclude a creator, but it does place limits on when he might have carried out his job!"
Chapter 2: Space and Time Hawking describes the evolution of scientific thought regarding the nature of space and time. He starts with the Aristotelian idea that the naturally preferred state of a body is to be at rest, and it can only be moved by force, implying that heavier objects will fall faster. However, Galileo experimentally disproved Aristotle's theory by observing the motion of objects of different weights and concluding that all objects would fall at the same rate. This led to Newton's laws of motion and gravity. However, Newton's laws implied that there is no such thing as absolute state of rest or absolute space: whether an object is 'at rest' or 'in motion' depends on the observer's inertial frame of reference.
Hawking describes the classical belief in absolute time, that observers in motion will measure the same time. However, Hawking writes that this commonsense notion does not work at or near the speed of light. That light travels at a finite speed was discovered by Ole Rømer through his observations of Jupiter and its moons. Scottish scientist James Clerk Maxwell's equations unifying electricity and magnetism predicted the existence of waves moving at a fixed speed, the same speed that had been measured for light. Physicists believed that light must travel through a luminiferous aether, and that the speed of light was relative to that of the aether. The Michelson–Morley experiment, designed to detect the speed of light through the aether, got a null result. Michelson and Morley found that the speed of light was constant regardless of the motion of the source or the observer. Combining this with a postulate that the laws of physics are the same for all observers moving relative to one another, Albert Einstein argued that the aether is superfluous if we abandon absolute time. His 1905 paper became known as the special theory of relativity.
Hawking discusses two remarkable consequences of this new theory. The first is the equivalence of mass and energy: they are related by the equation . This also means that no object with mass can reach the speed of light (c = 3×108 m/s). The second consequence is a fundamental change in our ideas of 'space' and 'time'. Hawking points to the definition of the fundamental unit of length in terms of time as one example.
In 1915, Einstein published general relativity, which explains gravity as the curvature of spacetime. Matter and energy (including light) follow geodesics. Einstein's theory of gravity predicts a dynamic universe.
Chapter 3: The Expanding Universe
Hawking describes how physicists and astronomers calculated the relative distance of stars from the Earth. Sir William Herschel confirmed the positions and distances of many stars in the night sky. In 1924, Edwin Hubble discovered a method to measure the distance using the brightness of Cepheid variable stars as viewed from Earth. The luminosity and distance of these stars are related by a simple mathematical formula. Using this, he showed that ours is not the only galaxy.
In 1929, Hubble discovered that light from most galaxies was shifted to the red, and that the degree of redshift is directly proportional to distance. From this, he determined that the universe is expanding. This possibility had not been seriously considered. Einstein was so sure of a static universe that he added the cosmological constant to his equations. Many astronomers also tried to avoid the implications of general relativity, with one notable exception: the Russian physicist Alexander Friedmann.
In 1922, Friedmann made two very simple assumptions: the universe is identical wherever we are, (homogeneity), and that it is identical in every direction that we look, (isotropy). It follows that the universe is non-static. Support was found when two physicists at Bell Labs, Arno Penzias and Robert Wilson, found unexpected microwave radiation coming from all parts of the sky. At around the same time, Robert H. Dicke and Jim Peebles were also working on microwave radiation. They argued that radiation from the early universe should be detectable as the cosmic microwave background. This was what Penzias and Wilson had found.
In 1965, Roger Penrose used general relativity to prove that a collapsing star could result in a singularity. Hawking and Penrose proved together that the universe should have arisen from a singularity. Hawking later argued this need not be the case once quantum effects are taken into account.
Chapter 4: The Uncertainty Principle Hawking begins by discussing nineteenth-century French mathematician Laplace's belief in scientific determinism, where scientific laws would be able to perfectly predict the future of the universe. A crack in classical physics appeared with the ultraviolet catastrophe: according to the calculations of British scientists Lord Rayleigh and James Jeans, a hot body should radiate an infinite amount of energy. In 1900, the ultraviolet catastrophe was averted by Max Planck, who proposed that energy must be absorbed or emitted in discrete packets called quanta.
Hawking discusses Werner Heisenberg's uncertainty principle, according to which the speed and the position of a particle cannot be precisely known due to Planck's quantum hypothesis: increasing the accuracy in measuring its speed will decrease the certainty of its position and vice versa. This overturned Laplace's idea of a completely deterministic theory of the universe. Hawking describes the development by Heisenberg, Erwin Schrödinger and Paul Dirac of quantum mechanics, a theory which introduced an irreducible element of unpredictability into science, and despite Einstein's strong objections, has proven to be very successful in describing the universe at small scales.
Hawking discusses how Heisenberg's uncertainty principle implies the wave–particle duality of light (and particles in general). He describes the phenomenon of interference, where multiple light waves interfere with each other to give rise to a single light wave with properties different from those of the component waves, as well as the interference within particles, exemplified by the two-slit experiment. Hawking writes that American scientist Richard Feynman's sum over histories is a useful way of visualize quantum behavior. Hawking explains that Einstein's general theory of relativity is a classical, non-quantum theory as it ignores the uncertainty principle and that it has to be reconciled with quantum theory in situations where gravity is very strong, as in a singularity.
Chapter 5: Elementary Particles and Forces of Nature Hawking traces the history of investigation into the nature of matter: Aristotle's four elements, Democritus's indivisible atoms, John Dalton's idea of atoms combining to form molecules, J. J. Thomson's discovery of the electron, Ernest Rutherford's discovery of the atomic nucleus, James Chadwick's discovery of the neutron and finally Murray Gell-Mann's theorizing of quarks which constitute protons and neutrons (collectively called hadrons). Hawking discusses the six different "flavors" (up, down, strange, charm, bottom, and top) and three different "colors" of quarks (red, green, and blue). Later he discusses anti-quarks, which are outnumbered by quarks due to the expansion and cooling of the universe.
Hawking introduces the spin of particles. Particles can be divided into two groups. Fermions, or matter particles, have a spin of 1/2. Fermions follow Wolfgang Pauli's exclusion principle: they cannot share the same quantum state (for example, two "spin up" protons cannot occupy the same location in space). Without this rule, atoms could not exist. Bosons, or the force-carrying particles, have a spin of 0, 1, or 2 and do not follow the exclusion principle.
Gravity is thought to be carried by gravitons, massless particles with spin 2. The electromagnetic force is carried by photons. The weak nuclear force is responsible for radioactivity and is carried by W and Z bosons. The strong nuclear force, which binds quarks into hadrons and binds hadrons together into atomic nuclei, is carried by the gluon. Hawking writes that color confinement prevents the discovery of quarks and gluons on their own (except at extremely high temperature) as they remain confined within hadrons.