A Briefer History of Time

Theoretical physicist Stephen Hawking presents this book as a more concise and accessible reformulation of his 1988 bestseller A Brief History of Time, updated with recent developments in string theory, observational cosmology, and the ongoing search for a unified theory of physics. The book traces humanity's evolving understanding of the universe from ancient cosmology to the frontiers of modern physics, organized around guiding questions: What do we know about the universe? Where did it come from? Did it have a beginning? What is the nature of time?

The authors open with an anecdote about a scientist who, after giving a public astronomy lecture, is told by an audience member that the world rests on "turtles all the way down." This image frames the book's central concern: how human conceptions of the cosmos have progressed from the intuitive to the scientific.

The authors trace cosmology from the ancient Greeks forward. Around 340 B.C., the philosopher Aristotle argued that the earth is spherical. The Greek astronomer Ptolemy's geocentric model of the second century A.D. gave way to the Polish priest Nicolaus Copernicus's simpler heliocentric model in 1514. Nearly a century later, the Italian astronomer Galileo Galilei observed moons orbiting Jupiter, and the German astronomer Johannes Kepler showed that planets move in ellipses, undermining the Ptolemaic system. In 1687, the English physicist Isaac Newton published Principia Mathematica, presenting his law of universal gravitation and unifying celestial and terrestrial motion for the first time.

The authors define what makes a scientific theory valid: It must describe a large class of observations with few arbitrary elements and make testable predictions. They invoke philosopher Karl Popper's principle that a theory must be falsifiable, meaning capable of being disproved by observation. Two great partial theories now describe the universe: general relativity, governing gravity and large-scale structure, and quantum mechanics, governing extremely small scales. These theories are inconsistent with each other, and the search for a unifying quantum theory of gravity is the book's major theme.

Newton's laws imply there is no absolute standard of rest, yet both Aristotle and Newton accepted absolute time, the idea that the interval between two events is the same for all observers. The authors foreshadow that twentieth-century physics would overturn this assumption. In 1865, the British physicist James Clerk Maxwell showed that electromagnetic waves travel at a fixed speed matching the speed of light. Scientists proposed a substance called the ether through which light supposedly traveled, but in 1887, the American physicists Albert Michelson and Edward Morley found no evidence for it. In 1905, the physicist Albert Einstein proposed special relativity, postulating that the laws of science are the same for all freely moving observers. This required abandoning absolute time: If observers agree on the speed of light but disagree on the distance it travels, they must also disagree on elapsed time. Einstein introduced space-time, a four-dimensional framework combining space and time, and showed through his equation E=mc² that mass and energy are equivalent.

Einstein resolved the conflict between special relativity and Newtonian gravity in 1915 with general relativity, which reinterprets gravity as a consequence of space-time curvature caused by mass and energy. The theory predicts that light bends near massive objects, confirmed during a 1919 solar eclipse, and that time runs more slowly near massive bodies, confirmed in 1962 using precisely calibrated clocks. General relativity transforms space and time from a passive stage into dynamic quantities shaped by events within them.

The authors describe the discovery of the expanding universe. In 1924, the American astronomer Edwin Hubble proved galaxies exist beyond the Milky Way. Astronomers found that light from distant galaxies is shifted toward the red end of the spectrum, a consequence of the Doppler effect indicating the galaxies are receding. In 1929, Hubble showed that a galaxy's red shift is proportional to its distance, meaning farther galaxies recede faster. In 1965, Arno Penzias and Robert Wilson at Bell Telephone Laboratories accidentally discovered cosmic microwave background radiation, the remnant glow of the hot early universe. Recent observations indicate the expansion is accelerating, suggesting Einstein's cosmological constant, a built-in antigravity term he once called his "greatest mistake," may be real.

The book addresses the big bang and the evolution of the universe. All solutions to Einstein's equations containing the observed amount of matter imply that roughly 13.7 billion years ago, the universe was compressed into a point of zero size and infinite density. As the universe expanded and cooled, particles and elements formed in sequence. Over millions of years, denser regions collapsed under gravity to form galaxies and stars. Stars eventually exhaust their fuel; massive ones may collapse into black holes, regions where gravity is so strong that nothing can escape past the boundary known as the event horizon. Massive stars sometimes explode as supernovas, scattering heavier elements into space. The sun and Earth formed about five billion years ago from such debris, and primitive life on Earth eventually produced oxygen, enabling complex organisms.

General relativity predicts a singularity at the big bang, a point of infinite density where the theory breaks down, signaling the need for quantum gravity. The authors trace the development of quantum mechanics from its origins. In the early 19th century, the Marquis de Laplace, a French scientist, argued that complete knowledge of the universe's state and laws would allow perfect prediction of its future. In 1900, the German physicist Max Planck proposed that electromagnetic energy is emitted in discrete packets called quanta, resolving paradoxes in classical physics. In 1926, the German physicist Werner Heisenberg formulated the uncertainty principle: The more precisely one measures a particle's position, the less precisely one can know its velocity. This undermined Laplace's deterministic vision, replacing exact predictions with probabilities. The American physicist Richard Feynman later developed the sum over histories approach, in which a particle reaches its destination by every possible path at once.

The authors propose extending Feynman's approach to the universe itself through the no-boundary proposal: Space-time might be finite yet have no singularity or boundary, much like the surface of the earth. As Hawking writes, "the boundary condition of the universe is that it has no boundary" (103). The universe would be completely self-contained, requiring no external initial conditions.

The authors explore time travel, noting that forward time travel is permitted by relativity. Wormholes, tubes of space-time connecting distant regions first described by Einstein and the physicist Nathan Rosen in 1935, could theoretically allow shortcuts, but maintaining one would require matter with negative energy density. The authors present the chronology protection conjecture: The laws of physics may prevent macroscopic backward time travel, as quantum effects could counteract the required space-time warping.

The book surveys the four fundamental forces: gravity, electromagnetism, the weak nuclear force (responsible for radioactive decay), and the strong nuclear force (which binds quarks within protons and neutrons). The 1967 unification of the electromagnetic and weak forces by physicists Abdus Salam, Steven Weinberg, and Sheldon Glashow inspired attempts at a grand unified theory, but gravity remains outside this framework because general relativity does not incorporate quantum mechanics. String theories, which replace point particles with tiny vibrating strings, offer a potential path forward but require 10 or 26 dimensions, with the extras curled up too small to detect. The anthropic principle, a form of observer-selection reasoning about why conditions permit life, offers one explanation for why three spatial dimensions and one time dimension are large: Fewer or more would destabilize atoms and planetary orbits, making complex life impossible. Since the mid-1990s, discoveries of dualities—correspondences between apparently different theories of physics—suggest different string theories may approximate a single, more fundamental theory not yet found.

The authors conclude by reflecting on the intellectual journey from ancient cosmology to the threshold of a unified theory. Even if such a theory is discovered, the uncertainty principle and computational difficulty would prevent prediction of all events. They express cautious optimism that a unified theory may be found within some readers' lifetimes. The book closes with the hope that answering why the universe exists "would be the ultimate triumph of human reason, for then we would know the mind of God" (142).