A Brief History of Time
For the documentary film on Stephen Hawking, see A Brief History of Time (film). For the biographical film on Stephen Hawking, see The Theory of Everything (2014 film).
A Brief History of Time
First edition
Author Stephen Hawking
Country United Kingdom
Language English
Subject Cosmology
Publisher Bantam Dell Publishing Group
Publication date 1988
English physicist Stephen Hawking wrote a book on theoretical cosmology titled A Brief History of Time: From the Big Bang to Black Holes. The first edition came out in 1988. For readers with no prior knowledge of physics, Hawking wrote the book.
Astronomy and modern physics examine the structure, origin, evolution, and ultimate fate of the Universe; Hawking explains these topics in plain language in A Brief History of Time. He discusses fundamental ideas like space and time, as well as the fundamental forces that control the universe and its basic constituents, such quarks. He writes on black holes and the Big Bang as examples of cosmic phenomena. He talks about two important theories: general relativity and quantum mechanics that modern scientists use to describe the Universe. 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 sold more than 25 million copies.
Publication
Hawking initially sent his ideas for a popular book on cosmology to Cambridge University Press's astronomy book editor, Simon Mitton, in the beginning of 1983. All the calculations in the draft manuscript, in Mitton's opinion, would turn off the customers that Hawking hoped to attract from airport bookshops. He managed to convince Hawking to omit all but one equation, though not easily. In the acknowledgements of the book, the author states that he was informed that if he included an equation in every chapter, the number of readers would be cut in half. For this reason, the book only has one equation. The book does use a lot of intricate models, graphs, and other images to explain some of the ideas it covers in detail.
Stephen Hawking provides a non-specialist reader with an explanation of a variety of cosmological topics in A Brief History of Time, including the Big Bang, black holes, and light cones. In addition to trying to explain some difficult concepts, his primary objective is to provide an overview of the topic. Hawking addresses the ideas of time travel, wormholes, and the possibility of a universe without a quantum singularity at the beginning of time in the 1996 and later editions of the book. The book was published in 2017 and has twelve chapters. A synopsis of those chapters is provided below.
Chapter 1: Our Universe as We See It
Hawking begins by outlining the background of astronomical research in the first chapter. He focuses in especially on the conclusions made by the Greek philosopher Aristotle concerning the spherical Earth and the circular geocentric model of the universe, which were later expanded upon by the Greek astronomer Ptolemy in the second century. Next, Hawking describes how the Aristotelian and Ptolemaic models were rejected and how the heliocentric model of the Solar System gradually evolved in the 16th, 17th, and 18th centuries. This model was first put forth by the Polish priest Nicholas Copernicus in 1514, and it was confirmed a century later by Italian scientist Galileo Galilei and German scientist Johannes Kepler (who proposed an elliptical orbit model rather than a circular one). English scientist Isaac Newton provided further mathematical support for this model in his 1687 book on
In this chapter, Hawking also discusses the centuries-long study and debate surrounding the origins of the Universe and time. Aristotle and other early philosophers postulated the Universe's eternal existence, but St. Augustine and other theologians believed it was created at a specific point in the past, and time was a concept that emerged along with the creation of the Universe. German philosopher Immanuel Kant maintained that time had no beginning and continued to do so in the modern era. The expanding Universe was discovered in 1929 by American astronomer Edwin Hubble, which suggested that the Universe was contained in a single, highly dense location between ten and twenty billion years ago. This finding introduced the idea of the start of the Universe within the province of science. Currently scientists use Albert Einstein's general theory of relativity and quantum mechanics to partially describe the workings of the Universe, while still looking for a complete Grand Unified Theory that would describe everything in the Universe.
Chapter 2: Space and Time
Hawking traces the evolution of scientific ideas about the nature of space and time. He begins with the Aristotelian theory that a body's natural state is rest, and that it can only be moved by force, meaning that heavier objects will fall faster. However, Italian scientist Galileo Galilei experimentally disproved Aristotle's theory by observing the motion of objects of different weights and coming to the conclusion that all objects would fall at the same rate. This led to the development of English scientist Isaac Newton's laws of motion and gravity, which implied that there is no such thing as an absolute state of rest or absolute space, as believed by Aristotle.
Hawking goes on to discuss Aristotle and Newton's theories of absolute time, which hold that time may be precisely measured independent of an observer's motion. Hawking notes that this sensible idea is inapplicable at or close to the speed of light. He cites British scientist James Clerk Maxwell's electromagnetism equations, which demonstrated that light moves in waves at a fixed speed, and Danish scientist Ole Rømer's discovery—based on his observations of Jupiter and one of its moons, Io—that light travels at a very high but finite speed. Since Newtonian mechanics abandoned the idea of absolute rest, Maxwell and numerous other scientists contended that light must move through an imaginary fluid known as aether, with its speed being related to
The Michelson-Morley experiment, which demonstrated that the speed of light always stays constant independent of the velocity of the observer, later refuted this. Later, assuming that there is no absolute time, Einstein and Henri Poincaré argued that the motion of light does not require the existence of an aether. This is the foundation of the special theory of relativity, which holds that light moves at a finite speed independent of the observer's speed.
The famous equation, which states that any object with mass must have an unlimited quantity of energy in order to travel at the speed of light (3×10⁸m/s), relates mass and energy. It was possible to define a metre using the speed of light in a new way. "Events" may additionally The Michelson-Morley experiment, which demonstrated that the speed of light always stays constant independent of the velocity of the observer, later refuted this. Later, assuming that there is no absolute time, Einstein and Henri Poincaré argued that the motion of light does not require the existence of an aether. This is the foundation of the special theory of relativity, which holds that light moves at a finite speed independent of the observer's speed.
The famous equation, which states that any object with mass must have an unlimited quantity of energy in order to travel at the speed of light (3×10⁸m/s), relates mass and energy.
Unlike Newton, who saw gravity as a force that matter imposes on other matter, Einstein believed that "gravity" was an illusion created by the warping of spacetime, and his general theory of relativity describes how the course of a ray of light is impacted by this phenomenon. In spacetime curvature, gravitational influences can cause light to appear to curve in three-dimensional space, even though it always moves straight forward in four-dimensional "spacetime". These are geodesics, or straight lines. The twin paradox is a special relativity thought experiment in which identical twins are used to examine whether or not twins can mature differently if they move at different speeds from one another or even if they lived in different places with uneven curvature of spacetime. The foundation of special relativity is arenas.
Hawking and Roger Penrose worked upon this and later proved using general relativity that if the Universe had a beginning a finite time ago in the past, then it also might end at a finite time from now into the future.
Chapter 3: The Universe Is Expanding
Hawking begins this chapter by outlining the methods used by astronomers and physicists to determine the stars' relative distances from Earth. Sir William Herschel verified the locations and separations of numerous stars in the night sky in the 18th century. Edwin Hubble found a way to calculate the distance in 1924 by observing the brightness of Cepheid variable stars from Earth. A basic mathematical formula connects these stars' luminosity, brightness, and distance. He determined the distances to nine distinct galaxies using all of these. With a tremendous number of stars, our galaxy is a rather typical spiral.
Since the stars are so far away from us, we can only see their brightness, which is their main distinguishing trait. This light produces a spectrum when it is passed through a prism. Since every element has a distinct spectrum, we may measure a star's light spectrum to determine its chemical makeup. Every star has its own spectrum. To determine the temperature of the stars, we use their thermal spectra. Scientists discovered that several of the distinctive lines in the star spectrum were moved towards the red end of the spectrum in 1920 while analyzing the spectra of various galaxies. The Doppler Effect provided insights into this process, and it became evident that a large number of galaxies were veering away from Earth.
Given that certain galaxies are red shifted, it was expected that others would likewise be blue shifted. Redshifted galaxies, on the other hand, greatly outnumbered blueshifted galaxies. Redshift is precisely proportionate to relative distance, according to Hubble's findings. This led him to the conclusion that the universe had a beginning and is expanding. Even in the 20th century, the idea of a static universe remained prevalent. Because Einstein was so certain that the universe was static, he created the "cosmological constant" and "anti-gravity" forces, which made it possible for an infinitely old universe to exist. Additionally, a lot of astronomers remained to their static Universe and avoided the consequences of general relativity, with one particularly noteworthy exception: Russian physicist Alexander Friedmann.
Friedmann postulated two extremely basic ideas: homogeneity, or the idea that the universe is the same no matter where we are, and isotropy, or the idea that the universe is the same no matter which way we look. His findings demonstrated that there is motion in the universe. His theories were eventually validated when two Bell Labs scientists, Arno Penzias and Robert Wilson, discovered unexpected microwave radiation coming from everywhere in the sky, and from almost the same amount. As a result, Friedmann's initial theory was validated.
Around the same time, Robert H. Dicke and Jim Peebles were also working on microwave radiation, and they argued that the glow of the early Universe should be observable as background microwave radiation; Wilson and Penzias had already done so, and they were granted the 1978 Nobel Prize; Friedmann's second assumption was also supported by the idea that since our place in the Universe is not exceptional, we should be able to see the Universe as roughly the same from any other part of space. His work was largely unknown until comparable models were created by Howard Robertson and Arthur Walker.
Three distinct categories of universe evolution models emerged from Friedmann's model. The Universe would first expand for a certain period of time. If the expansion rate is less than the Universe's density (causing gravitational pull), the Universe will eventually collapse. Second, the universe would expand. If the universe's density and expansion rate ever equalized, the universe would eventually stop expanding and become mostly static. Thirdly, if the density of the universe is lower than the crucial value needed to maintain the universe's expansion pace, the universe would never stop expanding.
The first model depicts the space of the Universe to be curved inwards. In the second model, the space would lead to a flat structure, and the third model results in negative 'saddle shaped' curvature. Even if we calculate, the current expansion rate is more than the critical density of the Universe including the dark matter and all the stellar masses. 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 (Friedmann's solutions are based in it) also breaks down.
The biblical notion that the universe has a beginning in time rather than being eternal was supported by this concept of the beginning of time, which was first put forth by the Belgian Catholic priest Georges Lemaître. This idea first appeared to be driven by religious views. In order to counter the Big Bang idea, Hermann Bondi, Thomas Gold, and Fred Hoyle presented a new theory called the "steady state theory". Its forecasts agreed with the structure of the universe as it exists today. However, this idea failed and the Big Bang idea was widely accepted since there are many fewer radio wave sources in our immediate vicinity than there are in the distant Universe, and there were many more radio sources then than there are now. Lifshitz Evgeny and Khalatnikov Isaak Markovich also tried to find an alternative to the Big Bang theory but also failed.
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.
Chapter Four: The Uncertainty Principle
In this chapter, Hawking first addresses the strong scientific determinism of Laplace, the nineteenth-century French mathematician, who believed the principles of science might one day be able to forecast the future of the universe with accuracy. Next he talks about the British scientists Lord Rayleigh and James Jeans' hypothesis of unlimited radiation from stars. Max Planck, a German scientist, corrected this theory in 1900 and proposed that energy must radiate in small, finite packets known as quanta.
Hawking goes on to explain the uncertainty principle, which was developed by the German scientist Werner Heisenberg and states that Planck's quantum hypothesis makes it impossible to determine a particle's exact position or speed. Increasing a particle's speed measurement accuracy will decrease its position certainty, and vice versa. Laplace's theory of a fully deterministic cosmos was refuted by this. After that, Hawking explains how Heisenberg, Austrian physicist Erwin Schroedinger, and English physicist Paul Dirac eventually developed quantum mechanics in the 1920s. This theory introduced an irreducible element of unpredictability into science, and despite the strong objections of German scientist Albert Einstein, it has been shown to be very successful in explaining the universe, with the exception of gravity and large-scale structures.
Hawking goes on to explain how Heisenberg's uncertainty principle implies the wave–particle duality behavior of light (and particles in general).
Light interference causes many colors to appear.
He then 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 nice way of visualizing the wave-particle duality. Finally, Hawking mentions 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: Fundamental Natural Particles and Forces
Using Aristotle's four elements, Democritus's idea of indivisible atoms, John Dalton's theories about atoms joining to form molecules, J. J. Thomson's discovery of electrons inside atoms, Ernest Rutherford's discovery of the atomic nucleus and protons, James Chadwick's discovery of neutrons, and Murray Gell-Mann's work on even smaller quarks that make up protons and neutrons are all mentioned in this chapter by Hawking. Next, Hawking talks about quarks' three "colors" (red, green, and blue) and six "flavors" (up, down, weird, charm, bottom, and top). He talks about anti-quarks later in the chapter, which are less common than quarks because of the universe's expansion and cooling.
Hawking then discusses the spin property of particles, which determines what a particle looks like from different directions. Hawking then discusses two groups of particles in the Universe based on their spin: fermions and bosons. Fermions, with a spin of 1/2, follow the Pauli 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, complex structures could not exist.
Bosons, or force-carrying particles, do not adhere to the exclusion principle when their spin is equal to zero, one, or two. Hawking then goes on to describe virtual photons and gravitons. With a spin of two, virtual gravitons are gravitational force carriers. With a spin of one, virtual photons are the carriers of electromagnetic force. Next, Hawking talks about the strong nuclear force, which is carried by the particle gluon and which binds protons and neutrons together to form atomic nuclei, as well as the weak nuclear force, which is responsible for radioactivity and primarily affects fermions. Next, Hawking discusses the phenomena known as color confinement, which keeps quarks and gluons from being discovered on their own (except from at very high temperatures).
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 behavior of this "combined" force are called Grand Unified Theories, which may help us explain many of the mysteries of physics that scientists have yet to solve.
Chapter 6: Black Holes
In this chapter, Hawking talks about black holes, which are places in spacetime where gravity is so strong that nothing can escape from them, not even light. He explains how most black holes form when massive stars, which are at least 25 times heavier than the Sun, approach the end of their lives. He also talks about the event horizon, which is the boundary of a black hole from which no particle can escape to the rest of spacetime. Next, he talks about rotating black holes with axisymmetry and non-rotating ones with spherical symmetry. Finally, Hawking talks about how astronomers find black holes indirectly, by using special telescopes to observe the strong X-rays that are released when a black hole devours a star. Hawking concludes the chapter by mentioning his hypothesis 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 This chapter explores a facet of black hole behaviors that Stephen Hawking discovered in the 1970s. Previous theories held that black holes could only ever get bigger because nothing can escape from them. However, in 1974, Hawking published a new theory that suggested that black holes could "leak" radiation. Hawking conjectured what might occur if two virtual particles appeared close to a black hole's edge; these particles would briefly "borrow" energy from spacetime itself before annihilating one another, returning the borrowed energy, and ceasing to exist. But at the edge of a black hole, one virtual particle might be trapped by the black hole while the other of the second law of thermodynamics, particles are 'forbidden' from taking 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 continue existing forever as scientists had previously believed.
Chapter 8: The Universe's Origin and Destiny
This chapter discusses the beginning and the end of the cosmos.
The majority of scientists concur that the "Big Bang" expansion is where the universe first started. The universe was too hot at the time of the Big Bang to support the production of even the most basic structures, like atoms, let alone complicated ones like stars. A event known as "inflation" occurred at the Big Bang, when the Universe momentarily expanded ("inflated") to a far bigger extent. Certain aspects of the universe that had previously baffled scientists are explained by inflation. The cosmos expanded after inflation, but more slowly. Eventually, the temperature dropped significantly, enabling the development of these 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: Time's Arrow
In this chapter, Hawking discusses the reasons behind the apparent direction of "real time," which he refers to as time as humans view and feel it (as opposed to "imaginary time," which he says is a result of the principles of science). Notably, "real time" appears to be moving from the past towards the future. The three "arrows of time" that, in Hawking's opinion, give time this quality are then covered. The thermodynamic arrow of time, or the direction in which entropy—or what Hawking refers to as disorder—increases is Hawking's first arrow of time. This, in Hawking's opinion, explains why we never witness shattered cup fragments coming together to make an entire cup. The psychological arrow of time, which is Hawking's second arrow, is how our subjective perception of time seems to flow in one direction, which is why we remember the past and not the future.
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 which 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
In this chapter, Hawking discusses whether it is possible to time travel, i.e., travel into the future or the past. 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, and these concepts have become mainstays of science fiction. Einstein–Rosen bridges were proposed early in the history of general relativity research. 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, even if possible for it to form in the first place, 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.
Hawking also describes his own "chronology protection conjecture", which provides a more formal explanation for why faster-than-light and backwards time travel are almost certainly impossible.
Chapter 11: The Unification of Physics
Quantum field theory (QFT) and general relativity (GR) describe the physics of the Universe with astounding accuracy within their own domains of applicability. However, these two theories contradict each other. For example, the uncertainty principle of QFT is incompatible with GR. This contradiction, and the fact that QFT and GR do not fully explain observed phenomena, have led physicists to search for a theory of "quantum gravity" that is both internally consistent and explains observed phenomena just as well as or better than existing theories do.
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 yet to be proven in spite of significant effort (this remains the case as of 2021). String theory proposes that particles behave like one-dimensional "strings", rather than as dimensionless particles as they do in QFT. These strings "vibrate" in many dimensions. Instead of 3 dimensions as in QFT or 4 dimensions as in GR, 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, leaving countless theoretical string theory landscapes which each describe a universe with different properties. Without a means to narrow the scope of possibilities, it is likely impossible to find practical applications for string theory.
Alternative theories of quantum gravity, such as loop quantum gravity, similarly suffer from a lack of evidence and difficulty to study.
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
In this concluding chapter, Hawking summarizes the historical efforts made by humans to comprehend the universe and their place in it. These efforts range from the belief that anthropomorphic spirits controlled nature to the recognition of regular patterns in nature and, finally, the advancement of science in recent centuries, which has led to a far better understanding of the inner workings of the universe. He brings up the idea put forth by the French mathematician Laplace in the nineteenth century that a system of rules with their origins in God's domain could someday provide a clear explanation for the structure and evolution of the Universe. Nonetheless, Hawking claims that the quantum theory's introduction of the uncertainty principle in the 20th century has placed restrictions on how accurately future laws may be predicted.
Though, in Hawking's opinion, the increasingly technical nature of these theories has caused modern cosmology to become increasingly divorced from philosophical discussion, Hawking nevertheless expresses hope that one day everyone would talk about these theories in order to understand the true origin and nature of the universe. Historically, the study of cosmology (the study of the origin, evolution, and end of Earth and the Universe as a whole) has been primarily motivated by a search for philosophical and religious insights, for example, to better understand the nature of God, or even whether God exists at all.
