A Brief History of Time

For the documentary film on Stephen Hawking, see A Brief History of Time (film). For the biographical film on Hawking, see The Theory of Everything (2014 film).

A Brief History of Time

First edition
Author	Stephen Hawking
Language	English
Subject	Cosmology
Publisher	Bantam Dell Publishing Group
Publication date	April 1, 1988
Publication place	United Kingdom
Media type	Print (Hardcover and Paperback)
Pages	256
ISBN	978-0-553-10953-5
OCLC	39256652
Dewey Decimal	523.1 21
LC Class	QB981 .H377 1998
Followed by	Black Holes and Baby Universes and Other Essays

A Brief History of Time: From the Big Bang to Black Holes is a book on cosmology by the physicist Stephen Hawking. It was first published in 1988.

In A Brief History of Time, 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 unifying theory that describes everything in the universe in a coherent manner.

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

Early in 1983, Hawking first 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. With some difficulty, he persuaded Hawking to drop all but one equation. The author himself notes in the book's acknowledgements that he was warned that for every equation in the book, the readership would be halved, hence it includes only a single equation: $E=mc^{2}$.

Contents

Hawking explains a range of subjects in cosmology. In the 1996 edition and subsequent editions, he discusses the arrow of time and the possibility of wormholes and time travel.

Chapter 1: Our Picture of the Universe

Ptolemy's Earth-centric model about the location of the planets, stars, and Sun

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, particularly 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 development 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 eventually 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 inertial frame of reference of the observer.

Hawking then describes Aristotle and Newton's belief in absolute time, i.e. time can be measured accurately regardless of the state of motion of the observer. However, Hawking writes that this commonsense notion does not work at or near the speed of light. He mentions Danish scientist Ole Rømer's discovery that light travels at a very high but finite speed through his observations of Jupiter and its moons as well as Scottish scientist James Clerk Maxwell's equations of electromagnetism which showed that light travels in waves moving at a fixed speed. Maxwell and many other physicists argued that light must travel through the luminiferous aether, and that its speed was relative to that of aether. This was later disproved by the Michelson–Morley experiment, which showed that the speed of light always remains constant regardless of the motion of the observer. Albert Einstein argued that the aether is superfluous if we abandon absolute time. The special theory of relativity is based on two postulates: the laws of physics are the same for all observers moving relative to one another, and the speed of light is a universal constant.

Mass and energy are related by the equation $E=mc^{2}$, which means that an infinite amount of energy is needed for any object with mass to travel at the speed of light (c = 3×10⁸m/s). A new way of defining a metre using speed of light was developed. A 4-dimensional spacetime is also described, in which 'space' and 'time' are intrinsically linked. The motion of an object through space inevitably impacts the way in which it experiences time. Events can be described using light cones.

Einstein's general relativity explains gravity as the curvature of spacetime. Matter and energy (including light) follow geodesics. The twin paradox, a thought experiment in special relativity involving identical twins, considers that twins can age differently if they move at different speeds relative to each other, or even if they lived in different locations with unequal spacetime curvature. Special relativity is based upon arenas of space and time where events take place, whereas general relativity is dynamic where force could change spacetime curvature and which gives rise to a dynamic, expanding universe.

Chapter 3: The Expanding Universe

The expansion of the universe since the Big Bang

Hawking describes how physicists and astronomers calculated the relative distance of stars from the Earth. In the 18th century, 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 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. The possibility of an expanding universe 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.

Friedmann made two very simple assumptions: the universe is identical wherever we are, i.e. homogeneity, and that it is identical in every direction that we look in, i.e. 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 they should be able to see the glow of the early universe as background microwave radiation. This was what Penzias and Wilson had found. In addition, our place in the universe is not exceptional, so we should see the universe as approximately the same from any other part of space, which supports Friedmann's second assumption.

Friedmann's model gave rise to three different types of models for the evolution of the universe. First, the universe could expand for a given amount of time, and if the expansion rate is less than the density of the universe (leading to gravitational attraction), it would ultimately collapse at a later stage. Second, the universe could expand, and at some time, if the expansion rate and the density of the universe became equal, it would expand slowly and stop, leading to a somewhat static universe. Third, the universe could continue to expand forever, if the density of the universe is less than the critical amount required to balance the expansion rate of the universe.

. The first model included the beginning of the universe as a Big Bang from a space of infinite density and zero volume known as 'singularity', a point where the general theory of relativity breaks down. Hawking writes: "Many people do not like the idea that time has a beginning, probably because it smacks of divine intervention. (The Catholic Church, on the other hand, seized on the big bang model and in 1951 officially pronounced it to be in accordance with the Bible.)" Hermann Bondi, Thomas Gold and Fred Hoyle introduced the steady-state theory as an alternative to the Big Bang. Hawking writes that the steady-state was a good theory in that it made predictions, but those predictions did not match observations, so it was abandoned in favor of the Big Bang.

Roger Penrose used light cones and general relativity to prove that a collapsing star could result in a region of zero size and infinite density and curvature called a black hole. Hawking and Penrose proved together that the universe should have arisen from a singularity. Hawking showed 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 strong belief in scientific determinism, where scientific laws would be able to accurately 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 then 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 disproved Laplace's idea of a completely deterministic theory of the universe. Hawking then describes the eventual development of quantum mechanics by Heisenberg, Erwin Schrödinger and Paul Dirac in the 1920s, a theory which introduced an irreducible element of unpredictability into science, and despite Einstein's strong objections, has been proven to be very successful in describing the universe except for gravity and large-scale structures.

A representation of a light wave

Hawking discusses how Heisenberg's uncertainty principle implies the wave–particle duality of light (and particles in general).

Light interference causes many colours to appear.

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 how interference refined our understanding of the structure of atoms, the building blocks of matter. While Danish scientist Niels Bohr's theory only partially solved the problem of collapsing electrons, quantum mechanics completely resolved it. According to Hawking, 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 which ignores the uncertainty principle and that it has to be reconciled with quantum theory in situations where gravity is very strong, such as black holes and the Big Bang.

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 notion of indivisible atoms, John Dalton's idea of atoms combining to form molecules, J. J. Thomson's discovery of electrons, 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.

A particle of spin 1 needs to be turned around all the way to look the same again, like this arrow.

Hawking then discusses the spin property of particles. Particles can be divided into two groups. Fermions, with a spin of 1/2, follow Wolfgang Pauli's exclusion principle, which states that 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, with a spin of 0, 1, or 2, do not follow the exclusion principle.

A proton consists of three quarks, which are different colours due to colour confinement.

Gravity is thought to be carried by gravitons, massless particles with spin 2. The electromagnetic force is carried by photons. Hawking discusses the weak nuclear force, responsible for radioactivity and carried by W and Z bosons and the strong nuclear force, which binds quarks into hadrons and binds hadrons together into atomic nuclei and is carried by the gluon. Hawking writes about the phenomenon of color confinement which prevents the discovery of quarks and gluons on their own (except at extremely high temperature) as they remain confined within hadrons.

Hawking writes that at extremely high temperature, the electromagnetic force and weak nuclear force behave as a single electroweak force, giving rise to the speculation that at even higher temperatures, the electroweak force and strong nuclear force would also behave as a single force. Theories which attempt to describe the behaviour of this "combined" force are called Grand Unified Theories, which may help us explain many of the mysteries of physics.

Chapter 6: Black Holes

A black hole, showing how it distorts its background image through gravitational lensing

Hawking discusses black holes, regions of spacetime where extremely strong gravity prevents everything, including light, from escaping from within them. The Cambridge astronomer John Michell imagined stars so massive that light could not escape its gravitational pull. Hawking explains stellar evolution, how main sequence stars shine by fusing hydrogen into helium. This staves off gravitational collapse. AA collapsed star may form a white dwarf, supported by electron degeneracy, or a neutron star, supported by the exclusion principle. Subrahmanyan Chandrasekhar found that a collapsed star more than 1.4 solar masses could not stave off gravity, and there would be nothing to hatl complete gravitational collapse. He was dissuaded from this thinking by Arthur Eddington, though it later won him the Nobel Prize in Physics. The critical mass is known as the Chandrasekhar limit. Hawking describes how black holes are formed during the collapse of massive stars (at least 25 times heavier than the sun) approaching end of life. He writes about the event horizon, the black hole's boundary from which no particle can escape to the rest of spacetime. Hawking discusses non-rotating black holes with spherical symmetry and rotating ones with axisymmetry. Hawking then describes how astronomers observe a black hole through its interactions with visible matter, such as the powerful X-rays emitted when it consumes a star. Hawking ends the chapter by mentioning his famous bet made in 1974 with American physicist Kip Thorne in which Hawking argued that black holes did not exist. Hawking lost the bet as new evidence proved that Cygnus X-1 was indeed a black hole.

Chapter 7: Black Holes Ain't So Black

Hawking discusses an aspect of black holes' behavior that he discovered in the 1970s. According to earlier theories, black holes can only become larger because nothing which enters a black hole can come out. However, in 1974, Hawking published a new theory which argued that black holes can emit radiation. He imagined what might happen if a pair of virtual particles appeared near the edge of a black hole. Virtual particles briefly 'borrow' energy from spacetime itself, then annihilate each other, returning the borrowed energy and ceasing to exist. However, at the edge of a black hole, one virtual particle might be trapped by the black hole while the other escapes. Because of the second law of thermodynamics, particles are forbidden to take energy from the vacuum. Thus, the particle takes energy from the black hole instead of from the vacuum, and escape from the black hole as Hawking radiation.

According to Hawking, black holes must very slowly shrink over time and eventually "evaporate" because of this radiation, rather than existing forever as scientists had previously believed.

Chapter 8: The Origin and Fate of the Universe

The Big Bang and the evolution of the universe

The beginning and the end of the universe are discussed in this chapter.

Most scientists agree that the universe began in an expansion called the "Big Bang". At the start of the Big Bang, the universe had an extremely high temperature, which prevented the formation of complex structures like stars, or even very simple ones like atoms. During the Big Bang, a phenomenon called "inflation" took place, in which the universe briefly expanded ("inflated") to a much larger size. Inflation explains some characteristics of the universe that had previously greatly confused researchers. After inflation, the universe continued to expand at a slower pace. It became much colder, eventually allowing for the formation of such structures.

Hawking also discusses how the universe might have appeared differently if it grew in size slower or faster than it actually has. For example, if the universe expanded too slowly, it would collapse, and there would not be enough time for life to form. If the universe expanded too quickly, it would have become almost empty.

Hawking ultimately proposes the conclusion that the universe might be finite, but boundless. In other words, it may have no beginning nor ending in time, but merely exist with a finite amount of matter and energy.

Chapter 9: The Arrow of Time

In this chapter Hawking talks about why "real time," as Hawking calls time as humans observe and experience it (in contrast to "imaginary time," which Hawking claims is inherent to the laws of science) seems to have a certain direction, notably from the past towards the future. Hawking then discusses three "arrows of time" which, in his view, give time this property. Hawking's first arrow of time is the thermodynamic arrow of time: the direction in which entropy (which Hawking calls disorder) increases. According to Hawking, this is why we never see the broken pieces of a cup gather themselves together to form a whole cup. Hawking's second arrow is the psychological arrow of time, whereby our subjective sense of time seems to flow in one direction, which is why we remember the past and not the future. Hawking claims that our brain measures time in a way where disorder increases in the direction of time – we never observe it working in the opposite direction. In other words, he claims that the psychological arrow of time is intertwined with the thermodynamic arrow of time. Hawking's third and final arrow of time is the cosmological arrow of time: the direction of time in which the universe is expanding rather than contracting. According to Hawking, during a contraction phase of the universe, the thermodynamic and cosmological arrows of time would not agree.

Hawking then claims that the "no boundary proposal" for the universe implies that the universe will expand for some time before contracting back again. He goes on to argue that the no boundary proposal is what drives entropy and that it predicts the existence of a well-defined thermodynamic arrow of time if and only if the universe is expanding, as it implies that the universe must have started in a smooth and ordered state that must grow toward disorder as time advances. He argues that, because of the no boundary proposal, a contracting universe would not have a well-defined thermodynamic arrow and therefore only a universe that is in an expansion phase can support intelligent life. Using the weak anthropic principle, Hawking goes on to argue that the thermodynamic arrow must agree with the cosmological arrow in order for either to be observed by intelligent life. This, in Hawking's view, is why humans experience these three arrows of time going in the same direction.

Chapter 10: Wormholes and Time Travel

Hawking discusses whether time travel is possible. He shows how physicists have attempted to devise possible methods by humans with advanced technology may be able to travel faster than the speed of light, or travel backwards in time. Kurt Gödel presented Einstein with a solution to general relativity that allowed for time travel in a rotating universe. Einstein–Rosen bridges were proposed early in the history of the theory. These wormholes would appear identical to black holes from the outside, but matter which entered would be relocated to a different location in spacetime, potentially in a distant region of space, or even backwards in time. However, later research demonstrated that such a wormhole would not allow any material to pass through before turning back into a regular black hole. The only way that a wormhole could theoretically remain open, and thus allow faster-than-light travel or time travel, would require the existence of exotic matter with negative energy density, which violates the energy conditions of general relativity. As such, almost all physicists agree that faster-than-light travel and travel backwards in time are not possible.

Chapter 11: The Unification of Physics

The fundamental objects of string theory are open and closed strings.

Quantum mechanics and general relativity describe the physics of the universe with astounding accuracy within their own domains of applicability (atomic and cosmic scales, respectively). However, these two theories run into problems when combined. For example, the uncertainty principle is incompatible with Einstein's theory. This contradiction has led physicists to search for a theory of quantum gravity.

Hawking is cautiously optimistic that such a unified theory of the universe may be found soon, in spite of significant challenges. At the time the book was written, superstring theory had emerged as the most popular theory of quantum gravity, but this theory and related string theories were still incomplete and had not yielded testable predictions (this remains the case as of 2021). String theory proposes that particles behave like one-dimensional "strings", rather than as dimensionless particles. These strings "vibrate" in many dimensions. Superstring theory requires a total of 10 dimensions. The nature of the six "hyperspace" dimensions required by superstring theory are difficult if not impossible to study.

Hawking thus proposes three possibilities: 1) there exists a complete unified theory that we will eventually find; 2) the overlapping characteristics of different landscapes will allow us to gradually explain physics more accurately with time and 3) there is no ultimate theory. The third possibility has been sidestepped by acknowledging the limits set by the uncertainty principle. The second possibility describes what has been happening in physical sciences so far, with increasingly accurate partial theories.

Hawking believes that such refinement has a limit and that by studying the very early stages of the universe in a laboratory setting, a complete theory of Quantum Gravity will be found in the 21st century allowing physicists to solve many of the currently unsolved problems in physics.

Conclusion

Hawking summarises the efforts made by humans through their history to understand the universe and their place in it: starting from the belief in anthropomorphic spirits controlling nature, followed by the recognition of regular patterns in nature, and finally with the scientific advancement in recent centuries, the inner workings of the universe have become far better understood. He recalls Laplace's suggestion that the universe's structure and evolution could eventually be precisely explained by a set of laws whose origin is left in God's domain. However, Hawking states that the uncertainty principle introduced by quantum theory has set limits to the predictive accuracy of future laws to be discovered.

Hawking comments that historically, the study of cosmology has been primarily motivated by a search for philosophical and religious insights, for instance, to better understand the nature of God, or even whether God exists at all. However, for Hawking, most scientists today who work on these theories approach them with mathematical calculation and empirical observation, rather than asking such philosophical questions. In his mind, the increasingly technical nature of these theories have caused modern cosmology to become increasingly divorced from philosophy. Hawking nonetheless expresses hope that one day everybody would understand the true origin and nature of the universe. "That would be the ultimate triumph of human reason—for then we know the mind of God".

Editions

• 1988: The first edition included an introduction by Carl Sagan that tells the following story: Sagan was in London for a scientific conference in 1974, and between sessions he wandered into a different room, where a larger meeting was taking place. "I realized that I was watching an ancient ceremony: the investiture of new fellows into the Royal Society, one of the most ancient scholarly organizations on the planet. In the front row, a young man in a wheelchair was, very slowly, signing his name in a book that bore on its earliest pages the signature of Isaac Newton ... Stephen Hawking was a legend even then." In his introduction, Sagan goes on to add that Hawking is the "worthy successor" to Newton and Paul Dirac, both former Lucasian Professors of Mathematics.

The introduction was removed after the first edition, as it was copyrighted by Sagan, rather than by Hawking or the publisher, and the publisher did not have the right to reprint it in perpetuity. Hawking wrote his own introduction for later editions.

• 1994, A brief history of time – An interactive adventure. A CD-Rom with interactive video material created by S. W. Hawking, Jim Mervis, and Robit Hairman (available for Windows 95, Windows 98, Windows ME, and Windows XP).
• 1996, Illustrated, updated and expanded edition: This hardcover edition contained full-color illustrations and photographs to help further explain the text, as well as the addition of topics that were not included in the original book.
• 1998, Tenth-anniversary edition: It features the same text as the one published in 1996, but was also released in paperback and has only a few diagrams included. ISBN 0553109537
• 2005, A Briefer History of Time: a collaboration with Leonard Mlodinow of an abridged version of the original book. It was updated again to address new issues that had arisen due to further scientific development. ISBN 0-553-80436-7

Film

Main article: A Brief History of Time (film)

In 1991, Errol Morris directed a documentary film about Hawking; although they share a title, the film is a biographical study of Hawking, and not a filmed version of the book.

Apps

"Stephen Hawking's Pocket Universe: A Brief History of Time Revisited" is based on the book. The app was developed by Preloaded for Transworld publishers, a division of the Penguin Random House group.

The app was produced in 2016. It was designed by Ben Courtney and produced by Jemma Harris and is available on iOS only.

Opera

The Metropolitan Opera commissioned an opera to premiere in the 2015–2016 season based on Hawking's book. It was to be composed by Osvaldo Golijov with a libretto by Alberto Manguel in a production by Robert Lepage. The planned opera was changed to be about a different subject and eventually canceled completely.

Reception

A Brief History of Time was included on Time magazine's list of the 100 best nonfiction books since the magazine's founding. Jeffrey Kluger wrote:

The genius of Hawking was to understand that while people weren’t losing much sleep over such matters as event horizons, space-time geodesics and stellar contraction, they were deeply, primally interested in such questions as “Why does the universe go to all the bother of existing?” as he elegantly put it. In that riddle lives the smaller but more anthropocentrically relevant one, Why do we exist? Hawking provided answers — with hard physics, gentle metaphor, and ideas so big they fill up space itself. And you know what? We got it — and we’re smarter and better because of that.

See also

• General relativity § Further reading
• Hawking Index – a mock mathematical measurement of how far people will read a book before giving up, named in reference to Hawking's book.
• List of textbooks in thermodynamics and statistical mechanics
• List of textbooks on classical mechanics and quantum mechanics
• Turtles all the way down – a jocular expression of the infinite regress problem in cosmology that appears in Hawking's book
• The Road to Reality (2004), a guide to physics by Roger Penrose

External links

• Photos of the first edition of A Brief History of Time