A Brief History Of Time

First published in 1988, A Brief History of Time: From the Big Bang to Black Holes by British physicist Stephen Hawking is a general-audience science book that describes the basic principles of the universe as scientists have come to know them. From the beginning of everything to the fate of the cosmos—with black holes, wormholes, and time arrows in between—the book describes in non-technical language how the universe works.

The 2009 edition contains a forward by the author, which replaces the forward written by Carl Sagan included in the original 1998 edition, and an appendix that updates the science described in the book. A number of diagrams, graphs, and illustrations accompany the text; the ones most relevant to the guide are described in the summaries below. The eBook version of the updated edition forms the basis for this study guide.

Summary

Hawking writes A Brief History of Time in the first-person present tense to craft an accessible history of theoretical physics. Hawking incorporates rhetorical questions and humorous asides in a friendly, approachable tone.

Chapter 1 focuses on humans’ earliest conceptions of the universe. Ancient Greek philosopher Aristotle deduced features of the universe based on his observations of natural phenomena. He is the first to assert that the world is round, a concept that was controversial at the time. He also believed the Earth was at the center of the cosmos. In the early 1600s, Italian scientist and philosopher Galileo Galilei discovered that moons orbited Jupiter. This showed that some planetary objects orbit others, leading people to accept that the Earth revolved around the sun.

Chapter 2 discusses the modern concepts of space and time. In 1687, English mathematician and astronomer Sir Isaac Newton posited that objects in motion stay in motion, that large and small objects fall at the same rate, and that the solar system is held together by the force known as gravity. Newton’s theories also posited that time is constant and linear. Flaws in Newton’s gravitational models became clear when applied to the scale of the universe. If space were infinite, and if there were an infinite number of stars, the gravitational force drawing them toward each other would balance out at vast distances, creating a static universe with no central point. Today, science accepts that the universe operates according to German-born Jewish theoretical physicist Albert Einstein’s theories of relativity, which recognize that objects’ gravitational forces warp the space and time around them. These theories—special and general relativity— fundamentally differ from Newton’s because they reveal that neither time nor space is fixed.

Chapter 3 charts understanding of the expanding universe. In the 1920s, American astronomer Edwin Hubble discovered that the universe is expanding. The most successful explanation is that the cosmos began as a single, infinitely dense point and expanded rapidly as the result of a massive explosion. The event, called the Big Bang singularity, is considered the beginning of time. Hawking helped prove the Big Bang theory in his joint paper with British mathematician and physicist Roger Penrose in 1970, but admits he later changed his mind. The chapter touches on quantum mechanics, which explains the way particles move at the sub-atomic level.

Chapter 4 focuses on the uncertainty principle, developed in 1926 by German scientist Werner Heisenberg. Heisenberg’s principle states that the more accurately one tries to measure a particle’s speed, the less accurately one is able to measure its position and vice versa. Quantum mechanics state that the universe is made of matter and energy that appear as waves or particles at the atomic level, depending on how they’re observed. This proves that the universe is not deterministic, meaning how much one can predict about the future is limited. Science is still searching for a theory that unifies the principles of general relativity with quantum mechanics to explain both the macro and microlevel behaviors of objects in the universe.

In Chapter 5, Hawking discusses particles and forces. The universe contains four forces: gravity, the electromagnetic force, the weak nuclear force, and the strong nuclear force. Grand unified theories seek to explain the workings of particles and forces in the universe. Gravity is generally left out of unified theories because it is such a weak force relative to the others. Hawking’s work on black holes in the 1970s led to ideas about a quantum theory of gravity that could unite general relativity and quantum mechanics.

Chapters 6 and 7 focus on black holes, regions in space that become so dense that they collapse into a singularity. Surrounding this point is a boundary (event horizon) from which no particles can escape. Because of the uncertainty principle, particles sometimes form in pairs out of nothing. Near an event horizon, one particle may drop into the black hole while the other escapes, causing black holes to evaporate over billions of years.

Chapter 8 returns to the origins of the universe. Studying black holes may help cosmologists learn about the first few seconds of the universe after the Big Bang. Scientists believe the expansion was enhanced by a secondary expansion, called inflation, which helped smooth out irregularities and made the universe what it is today. Hawking’s calculations suggest that the cosmos swings back and forth endlessly between expansive big bangs and contraction in “big crunches,” and he posits that the universe has no edge.

Chapter 9 returns to the subject of time and the laws of thermodynamics. Time always moves forward and never backward because, according to the second law of thermodynamics, things go from states of high order to states of lower order over time. Thus, the arrow of time must always point forward, even if the universe were to expand as far as possible and then begin to contract again. There is also the psychological arrow (how we experience time) and the cosmological arrow (the direction of time in which the universe expands rather than contracts). These concepts of time explain why people remember the past but not the future.

Chapter 10 discusses worm holes and time travel. It might be possible to build a wormhole from one place in space to another and travel faster than light. A quick enough return to the starting place might get a person home before they leave, implying that it is possible to reverse time. These events are likely impossible, but the science remains incomplete.

Chapter 11 discusses the difficulties of creating a unified theory of physics.

The theories of gravity, electromagnetics, and the strong and weak forces are “partial theories” that don’t fully fit together. Quantum mechanics has unified electromagnetism, the strong force, and the weak force, but so far no one has been able to describe gravity using quantum theory. String theory posits one explanation, but whether this theory or others will create a grand unified theory remains to be seen.

Several short chapters form the final sections of the book. These include biographical sketches of important scientists and an appendix, added to the 2017 edition, with updates in physics and cosmology that relate to Hawking’s work.