Chapter 1: Our Picture of the Universe '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, 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 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 E = mc^2. 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 since the Big Bang 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). 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 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. needs to be turned around all the way to look the same again, like this arrow. 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. 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. 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. 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 , 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 them. The term black hole was coined by
John Archibald Wheeler in 1969, although the idea is older. The Cambridge clergyman
John Michell imagined stars so massive that light could not escape their gravitational pull. Hawking explains
stellar evolution: how main sequence stars shine by fusing
hydrogen into
helium, staving off gravitational collapse. A collapsed star may form a
white dwarf, supported by electron degeneracy, or a
neutron star, supported by the exclusion principle.
Subrahmanyan Chandrasekhar found that for a collapsed star of more than 1.4 solar masses, there would be nothing to halt 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. He describes the
event horizon, the black hole's boundary from which no particle can escape. He writes: "One could well say of the event horizon what the poet
Dante said of the entrance to Hell: 'All hope abandon, ye who enter here.'" Hawking discusses non-rotating black holes with
spherical symmetry and rotating ones with
axisymmetry. The discovery of
quasars by
Maarten Schmidt in 1963 and
pulsars by
Jocelyn Bell-Burnell in 1967 gave hope that black holes might be detected. Even though black holes (by definition) do not emit light, astronomers can observe them through their interactions with visible matter. A star falling into a black hole would be a powerful source of
X-rays.
Cygnus X-1, a powerful source of X-rays, was the earliest plausible candidate for a black hole. Hawking concludes by mentioning his
1974 bet with American physicist
Kip Thorne. Hawking argued that Cygnus X-1 does not contain a black hole. Hawking conceded the bet as evidence for black holes proved overwhelming.
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. This was similar to
entropy, a measure of disorder which, per the
second law of thermodynamics, always increases. Hawking and his student
Jacob Bekenstein suggested that the area of a black hole's event horizon is a measure of its entropy. But if a black hole has entropy, it must have a temperature, and must emit radiation. 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
the vacuum, 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. 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.
Chapter 8: The Origin and Fate of the Universe Hawking recalls a conference on cosmology at the Vatican, where he was given an audience with
Pope John Paul II. The Pope said it was fine to study the early universe, but scientists should not study the Big Bang itself, as that was the moment of Creation and the work of God. Hawking writes: "I was glad then that he did not know the subject of the talk I had just given at the conference -- the possibility that space-time was finite but had no boundary, which means that it had no beginning, no moment of Creation. I had no desire to share the fate of Galileo, with whom I feel a strong sense of identity, partly because of the coincidence of having been born exactly 300 years after his death!" At 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.
George Gamow predicted that radiation from the Big Bang should still fill the present universe. This was the cosmic microwave background discovered by Penzias and Wilson. The Big Bang created hydrogen and helium, and heavier elements were
forged in stars. The Big Bang model was supported by the redshift of galaxies, the cosmic microwave background and the relative abundance of hydrogen and helium. But mysteries remained: Why is the universe isotropic? Why is the cosmic microwave background so homogenous? Widely separated parts of the universe have the same temperature, but there would not have been time for these regions to have come into contact.
Alan Guth's model of
cosmic Inflation provided an answer to this
horizon problem. Inflation explains other 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 stars. Hawking discusses how the universe might have appeared if it had expanded slower or faster than it actually has. 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. He discusses the
anthropic principle, which states that the universe has laws of physics that allow for the evolution of life because, if it didn't, we wouldn't be here. Hawking suggests the
no boundary proposal: that the universe is finite but has no beginning in
imaginary time. It might merely exist.
Chapter 9: The Arrow of Time Hawking discusses three "
arrows of time". The first is the
thermodynamic arrow of time: the direction in which entropy increases. This is given as the explanation for why we never see the broken pieces of a cup gather themselves together to form a whole cup. The second 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. The third is the cosmological arrow of time: the direction in which the universe is expanding rather than contracting. Hawking claims that the psychological arrow is intertwined with the thermodynamic arrow. 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 . Quantum mechanics and general relativity describe 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 understanding of the inner workings of the universe. 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 on knowledge. 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 has 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 would know the mind of God". == Editions ==