A Brief History of Time

In this foreword to the updated edition of 'A Brief History of Time', Stephen Hawking reflects on the unexpected global success of the original 1988 publication, noting its longevity on best-seller lists and wide translation. He attributes this success to a general public curiosity regarding the origins and nature of the universe. Hawking outlines the updates made to the new edition, which include a new chapter on wormholes and time travel based on Einstein's General Theory of Relativity, and a discussion on 'dualities' in physics that suggest a unified theory may require multiple formulations rather than a single one. Additionally, he highlights critical observational progress, specifically the COBE satellite's measurement of fluctuations in the cosmic microwave background radiation. He explains that these fluctuations support the theory of a universe without boundaries in imaginary time, though he acknowledges that further observation is required to confirm if the universe is entirely self-contained.

In the Foreword to the 2017 Edition of 'A Brief History of Time', Stephen Hawking reflects on the ongoing nature of scientific discovery, quoting Richard Feynman to emphasize that humanity lives in an era of uncovering the fundamental laws of nature. Hawking notes that since the book's original 1988 publication and its 1996 revision, significant new discoveries in physics have occurred, leading to an emerging new picture of reality. He introduces updates provided in the appendix of this edition, focusing on six key topics including his singularity theorems developed with Roger Penrose, Hawking radiation from black holes, and the no boundary proposal aimed at unifying quantum mechanics with Einstein's work. His primary objective remains to convey the excitement of these discoveries to a general audience interested in the fundamental questions of the universe.

This chapter traces the evolution of human understanding of the universe, moving from early Greek geocentric beliefs to the modern scientific era. It details the transition from Aristotle's spherical but stationary earth and Ptolemy's complex epicycles to the heliocentric models of Copernicus, Kepler, and Galileo, and eventually to Newton's laws of universal gravitation. The narrative progresses to the 20th century, highlighting Edwin Hubble's discovery of the expanding universe and the resulting Big Bang theory. Additionally, the author examines the philosophy of science, arguing that physical theories are provisional hypotheses that can be falsified but never fully proven. The text discusses the current reliance on partial theories—general relativity for large-scale structures and quantum mechanics for the subatomic—and the ongoing scientific quest for a complete unified theory of gravity, justified by both evolutionary adaptation and an innate human desire for knowledge.

This chapter traces the scientific evolution of the concepts of space and time, moving from Aristotle's belief in a preferred state of rest and absolute position to the mathematical laws of Isaac Newton. While Newton formulated the laws of motion and gravity, he struggled to abandon the notion of absolute space due to his religious beliefs. The narrative then shifts to the late 19th and early 20th centuries, where the discovery of the finite speed of light and the failure of the Michelson-Morley experiment to detect the ether paved the way for Albert Einstein's theories. Einstein's special theory of relativity abolished absolute time, integrating space and time into a single four-dimensional entity called space-time. This was further expanded by his general theory of relativity, which posits that gravity is not a force but a consequence of the curvature of space-time caused by mass and energy. The chapter concludes by explaining how this dynamic framework affects the movement of bodies, the path of light, and the passage of time itself.

Chapter Three explores the transition from the belief in a static universe to the discovery of an expanding one. It details Edwin Hubble's observational evidence of galactic red-shift, which demonstrated that galaxies move away from Earth at speeds proportional to their distance. This observational data supported Alexander Friedmann's mathematical models, which predicted a non-static universe, contradicting Albert Einstein's initial use of a cosmological constant to maintain stability. The discovery of cosmic microwave background radiation by Penzias and Wilson further validated the Big Bang model over the Steady State theory. The chapter concludes with the development of singularity theorems by Roger Penrose and Stephen Hawking, which mathematically proved that the universe must have begun with a singularity—a point of infinite density and curvature where general relativity breaks down—suggesting that the theory is incomplete and requires quantum mechanics for a full understanding of the early universe.

Chapter Four explores the transition from the deterministic worldview of the nineteenth century to the probabilistic nature of quantum mechanics. It begins with the Marquis de Laplace's vision of a predictable universe, which was challenged by the 'ultraviolet catastrophe' and Max Planck's discovery of quanta. Werner Heisenberg further dismantled determinism with the Uncertainty Principle, demonstrating that the position and velocity of a particle cannot be measured simultaneously with absolute precision. This led to the development of quantum mechanics by Heisenberg, Schrödinger, and Dirac, which replaces definite trajectories with quantum states and probability distributions. The chapter explains the wave-particle duality, illustrated by the two-slit experiment and the behavior of electrons in atoms, which resolved the problem of atomic collapse. Finally, Hawking notes that while quantum mechanics is exceptionally successful in explaining the micro-world, it remains unreconciled with general relativity, particularly in high-gravity environments like black holes and the Big Bang.

This chapter details the historical and theoretical evolution of our understanding of matter and the fundamental forces of nature. It traces the progression from Aristotle's four elements to the discovery of atoms, nuclei, and eventually quarks and gluons. The text distinguishes between matter particles (spin 1/2) and force-carrying particles (integer spin), explaining the roles of the gravitational, electromagnetic, weak nuclear, and strong nuclear forces. A significant portion of the analysis focuses on the quest for unification, specifically the Weinberg-Salam theory which unified the electromagnetic and weak forces, and the conceptual framework of Grand Unified Theories (GUTs). The chapter also explores the critical role of symmetry and symmetry breaking (C, P, and T), explaining how violations of these symmetries account for the observed asymmetry between matter and antimatter in the universe, while noting that gravity remains a separate challenge for a fully unified theory.

Chapter Eight, titled "Chapter Six - Black Holes," details the theoretical evolution and physical properties of black holes. It traces the concept from 18th-century hypotheses by John Michell and the Marquis de Laplace to the foundational framework of Einstein's general relativity. The text explains the stellar life cycle, noting that a star's final state—whether a white dwarf, neutron star, or black hole—is determined by its mass relative to the Chandrasekhar limit. The chapter explores the mechanics of gravitational collapse, the nature of the event horizon, and the existence of singularities, while introducing the cosmic censorship hypothesis. Furthermore, it discusses the 'no hair' theorem, which posits that stationary black holes are characterized solely by their mass and rotation. The narrative bridges theoretical physics with observational astronomy, citing the discovery of pulsars, the analysis of quasars, and the identification of Cygnus X-1 as evidence for the existence of black holes.

In this chapter, Stephen Hawking details his discovery that black holes are not entirely black, but instead emit radiation—now known as Hawking radiation. This realization began with the discovery that the area of a black hole's event horizon never decreases, leading to the conclusion that black holes possess entropy and temperature. By combining general relativity with the quantum mechanical uncertainty principle, Hawking demonstrated that virtual particle pairs created near the event horizon can be separated, allowing positive energy particles to escape as radiation while negative energy particles fall in, reducing the black hole's mass. This process implies that black holes eventually evaporate, particularly small primordial black holes. Hawking also discusses the implications for the early universe, suggesting that the lack of observed primordial black holes indicates a smooth, uniform early state. Despite initial skepticism from peers like John G. Taylor, the synthesis of quantum mechanics and relativity suggests that singularities might be removable as black holes disappear at the end of their lives.

In this chapter, Stephen Hawking explores the origin and fate of the universe, contrasting the general theory of relativity with quantum mechanics. He details the 'hot big bang model,' tracing the evolution of the universe from a singularity of infinite heat and zero size to the formation of particles, galaxies, and eventually life on Earth. Hawking addresses the limitations of general relativity in explaining the early universe's uniformity and discusses various inflationary models proposed by Alan Guth and Andrei Linde to resolve these issues. He critiques the 'new inflationary model' as scientifically dead while favoring the 'chaotic inflationary model.' Central to the chapter is Hawking's 'no boundary' proposal, developed with Jim Hartle, which utilizes imaginary time and Euclidean space-time to suggest the universe is self-contained and finite without an edge or singularity. This model challenges traditional theological concepts of a creator by proposing a universe that neither began nor ended in a way that requires external initiation.

In this chapter, the author examines the 'arrow of time,' investigating why the past and future are distinguishable despite the time-symmetry of fundamental scientific laws. He identifies three distinct arrows: thermodynamic, psychological, and cosmological. The author argues that the psychological arrow is determined by the thermodynamic arrow, as the process of recording memories increases universal entropy. Utilizing the no boundary condition and quantum gravity, he explains that the universe began in a smooth, ordered state, leading to the current increase in disorder. Hawking corrects a previous belief that disorder would decrease during the universe's eventual contraction, noting that disorder continues to increase regardless of whether the universe is expanding or contracting. Furthermore, he employs the weak anthropic principle to argue that intelligent life can only exist during the expanding phase of the universe, where a strong thermodynamic arrow is present to support the energy conversion processes necessary for life.

The author examines the theoretical possibility of time travel and faster-than-light (FTL) travel through the lenses of general relativity and quantum theory. The discussion covers the discovery of rotating universe models by Kurt Gödel and the 'bridges' (wormholes) proposed by Einstein and Rosen. While general relativity allows for these structures, the author notes that FTL travel implies the ability to travel back in time, which necessitates matter with negative energy density. The Casimir effect is cited as experimental evidence that quantum theory allows for such negative energy densities. However, the author distinguishes between microscopic time travel—evidenced by black hole radiation and the behavior of antiparticles—and macroscopic time travel. To address the paradoxes and causality issues associated with the latter, the author introduces the 'chronology protection conjecture,' suggesting that the laws of physics may conspire to prevent macroscopic bodies from carrying information into the past by creating energy densities that block the necessary space-time curvature.

Chapter 13 explores the scientific quest for a unified theory of physics that can reconcile general relativity with quantum mechanics. The author details the historical failures of this pursuit, including the limitations of Grand Unified Theories (GUTs) and the mathematical challenges of renormalization. The narrative traces the evolution of theoretical frameworks from supergravity to string theory and p-branes, noting that these may be different approximations of a single fundamental theory, a concept supported by the discovery of dualities. The author also addresses the implications of extra space-time dimensions and the anthropic principle, arguing that life requires a specific dimensional configuration to exist. Finally, the chapter examines three possibilities for the future of physics: a final theory, an infinite sequence of theories, or a universe of randomness. Hawking suggests that the Planck energy may provide a limit that makes an ultimate theory possible, though he cautions that such a theory would still be subject to the predictive limits of the uncertainty principle and mathematical complexity.

In the concluding chapter, Stephen Hawking reflects on the human quest to understand the nature of the universe and our place within it. He traces the evolution of 'world pictures,' from early mythological explanations involving unpredictable spirits to the scientific determinism proposed by Laplace. Hawking explains how quantum mechanics challenged this determinism via the uncertainty principle, redefining science as the search for laws that predict events within those limits. He emphasizes the role of gravity in shaping the universe and discusses the implications of combining general relativity with quantum mechanics. Specifically, he explores the 'no boundary proposal,' which suggests a finite, four-dimensional universe without singularities. This possibility significantly limits the role of a Creator, as it implies initial conditions may have been inevitable. Hawking concludes by lamenting the divide between technical science and philosophy, arguing that a complete unified theory would eventually allow all of humanity to engage in the ultimate question of why the universe exists, thereby 'knowing the mind of God.'

This chapter provides a biographical sketch of Albert Einstein, focusing on the intersection of his scientific achievements and his political activism. Hawking describes Einstein's life as being divided between 'politics and equations,' highlighting his early antiwar activities during World War I and his subsequent efforts toward international reconciliation. The text details Einstein's evolving relationship with his Jewish identity and his support for Zionism in response to rising anti-Semitism, despite facing professional attacks and personal threats. The narrative covers his decision to remain in the United States after Hitler's rise to power in 1933 and his paradoxical role in the development of the nuclear bomb—urging the U.S. to develop one to preempt Nazi Germany while simultaneously advocating for international control of nuclear weapons to prevent global war. Ultimately, the chapter portrays Einstein as a man who, despite his political engagements, viewed the eternal nature of mathematical equations as more significant than the transient nature of politics.

This chapter examines the pivotal role of Galileo Galilei in the birth of modern science, emphasizing his shift toward empirical observation of the real world to understand the universe. Galileo championed the Copernican theory that planets orbit the sun, which brought him into direct conflict with Aristotelian professors and the Catholic Church. Despite attempting to reconcile scientific discovery with biblical allegory, Galileo faced ecclesiastical repression, including a 1616 decree banning the defense of Copernicanism. Although he later gained permission to write a balanced dialogue on the two world systems, the Pope eventually viewed the work as a violation of previous decrees, leading to Galileo's lifelong house arrest by the Inquisition. Despite these constraints, Galileo maintained his belief in the independence of science, smuggling his final major work, 'Two New Sciences', to Holland, which laid the foundation for modern physics.

This chapter provides a biographical sketch of Isaac Newton, focusing less on his scientific achievements and more on his difficult personality and contentious relationships. Despite the monumental success of 'Principia Mathematica' and his rise to public prominence as president of the Royal Society and a knighted scientist, Newton was characterized by a penchant for disputes. The text details his aggressive conflict with John Flamsteed over astronomical data and his prolonged, deceptive battle with Gottfried Leibniz over the invention of calculus. In the latter case, Newton manipulated the Royal Society's investigative process to officially accuse Leibniz of plagiarism. The chapter concludes by noting Newton's transition from academia to politics and his role as Warden of the Royal Mint, where he applied his ruthless nature to the prosecution of counterfeiters.

The Appendix serves as a retrospective update on the progress of cosmology since the original publication of A Brief History of Time. Hawking discusses the discovery of dark energy and the accelerating expansion of the universe, which suggests that the 'big crunch' is unlikely and that space will expand forever. He connects this acceleration to Einstein's cosmological constant and the anthropic principle within a multiverse framework. The text further examines the cosmic microwave background radiation, noting how measurements from COBE, WMAP, and Planck support the no boundary proposal and inflation. Hawking also addresses the 2016 detection of gravitational waves by LIGO, which validates his 'area theorem' and provides a new tool for studying black holes. Finally, he revisits the black hole information paradox, admitting his previous error in believing information was lost, and explains how string theory and conformal field theory suggest information is preserved, albeit in a scrambled form.