A Brief History of Time | Study Guide

Stephen Hawking

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A Brief History of Time | Summary



Matter, Energy, Space, and Time

The first chapters of A Brief History of Time can be divided into sections of investigation that focus on matter and energy on a tiny scale and space and time on a large scale. Chapters 2, 3, 6, and 7 discuss the merging of space and time into two interrelated dimensions, applicable to the vast regions of the universe. Chapters 4 and 5 examine the exchanges of matter and energy at subatomic levels. It is in Chapters 8, 10, 11, and 12 that Stephen Hawking conceptually explains how space–time dimensions, combined with an understanding of the relationships between matter and energy in and around black holes, point the way toward a grand unified theory to explain everything. This has been the elusive goal of physicists of the 20th and early 21st centuries.


In his brief introduction to A Brief History of Time, Hawking states that this revised and updated edition responds to a general interest in universal human queries about the nature of existence—past, present, and future. His approach is from two directions: observations and theories. New observational results that lead to speculation on time travel and wormholes (structures that connects points separated by time and space) have also been addressed in a new chapter. Hawking also comments on correspondences between different theories, which suggest there may be a single theory to explain "everything."

Chapter 1|Our Picture of the Universe

This chapter begins with a historical overview of developments in astronomy by working through progressive models of the solar system. Alterations in these models had to be made to match observations of the properties of gravity, time, and space. In the realm of modern physics, Hawking lays out the properties of theoretical examination as expressed in two incompatible sets of theories. One is quantum mechanics (the investigation of the tiniest units of material existence) and the other is the theory of relativity (the investigation of the vast scale of the universe).

Chapter 2|Space and Time

In this chapter, Hawking introduces the role of the observer and the theory of relativity in our perception of space and time, showing that there is no absolute point in either dimension. Hawking describes the difficulties in establishing an absolute point in space or time that an observer can use to accurately measure the movements of objects. Sir Isaac Newton's laws of motion hold true for the most part, even though both time and distance in any given event can be different, depending on the observer. But while time and space seem separate when measuring position and speed "with things like apples, or planets that travel comparatively slowly," it is only in thinking of space and time as two relative points of reference that things moving at speeds approaching or rivaling that of light can be measured.

Chapter 3|The Expanding Universe

This chapter explains the way in which the universe appears to be expanding. Hawking points out that this expansion is observable in the behavior of light as it travels great distances from stars in other galaxies to Earth. On the basis of Einstein's theory of relativity, the advances of the last 50 years prove that the universe had a beginning in time.

Chapter 4|The Uncertainty Principle

While Einstein's theory of relativity holds true for the laws governing the behavior of star systems and galaxies in the universe on a large scale, the rules governing matter on the smallest scale of quantum mechanics are very different. To understand matter at very small scales, where the motion of a single atom or lone electron could change the state of matter, we need quantum mechanics. Like relativity, quantum mechanics relies on the observer, but in an even more subtle way—the very act of observation can determine the state of matter. Here, particles live in an indeterminate state, exhibiting wave–particle duality that represents probabilities of what it may be. When a measurement is made, the particle somehow will exhibit one of those outcomes, behaving like a particle or like a wave, depending on the method of observation. Although quantum mechanics creates a wide range of possibilities, it does not yet answer all our questions about how the universe works and how it began.

Chapter 5|Elementary Particles and the Forces of Nature

This chapter addresses the discovery of subatomic particles and the forces that act between them. So far, the smallest known particles are quarks, which have several classifications, but scientists continue to search for yet smaller particles. Hawking explains the four types of force—gravitational, electromagnetic, strong nuclear, weak nuclear—and the efforts to discover a grand unified theory (GUT) that explains them all. So far, all but gravity have been explained within one model. Hawking introduces and explains the combined symmetries of charge, parity, and time (CPT) that inform this cluster of partial theories. These symmetries help to explain some aspects of the universe as we know it, but the search for a quantum theory of gravity continues.

Chapter 6|Black Holes

In this chapter, Hawking traces the history of recent accumulating evidence (and the oppositions to it) for the presence of black holes in our universe. He traces the evolution of how a star collapses into a black hole and what happens to matter, light, and time approaching the intense gravitational forces of one. Determining the properties of primordial black holes (those formed at the same time as the universe) may indicate the progression of events immediately following the big bang.

Chapter 7|Black Holes Ain't So Black

Based on the idea that black holes do not automatically absorb everything that comes near them but may hold light trapped at their very edges, Hawking began a new line of thinking about their behavior. He found that light forms an event horizon at the edge of a black hole, trapping the light in its gravitational field, from which it does not escape. However, radiation emitted from a black hole indicates that its gravity is not as absolute as previously thought.

Chapter 8|The Origin and Fate of the Universe

Hawking speculates that various partial theories imply several specific properties. A single unified theory of quantum mechanics and relativity would need to express these properties. The idea that space-time might look something like the surface of Earth, a finite configuration that has no boundary, is proposed. If this is so, it is uncertain whether the universe had a beginning or will have a future end.

Chapter 9|The Arrow of Time

This chapter begins with questions about how time seems to move in only one direction and why it is we remember the past but not the future. Hawking presents three arrows of time that separate the past from the future in an expanding universe:

  • The thermodynamic arrow: disorder increases with time
  • The psychological arrow: time is perceived to flow in one direction
  • The cosmological arrow: disorder increases with the expansion of the universe

Hawking explains how he once thought these arrows might reverse direction when the universe begins to contract, but he later rejects this idea.

Chapter 10|Wormholes and Time Travel

This chapter provides an overview of the likelihood that time travel, which is one of the most popular technologies in science fiction, has any basis in scientific fact. The paradox principle suggests that time travelers to the past would have no free will in changing the past. However, the idea of many alternate universes indicates that travelers to the past might get to a past place but would have no ability to return to their original universe. In any case, based on our current models using quantum mechanics, Hawking believes that time travel is not possible in any sense that would be useful to humans.

Chapter 11|The Unification of Physics

Hawking outlines the steps necessary to arrive at the unification of physics into one comprehensive theory. The key is in how things respond to gravity, which is negligible on a quantum level but significant with matter of large mass subject to relativity. Such unification would have to be applicable to both ends of the spectrum of existence. It is unclear whether a unified theory will ever be found or if we will just continue to create better approximations and partial theories.

Chapter 12|Conclusion

The final chapter of A Brief History of Time brings together the wide range of speculations people have had on the nature of being, where life comes from, and where it is all heading in the future. Hawking states that physics grows ever closer to accurately describing the laws governing the what of the universe but also that it will take thinkers of many diverse disciplines to weigh in on the why of it.

Back Matter|Three Scientists

The back matter of the book includes a brief personality description of three key scientists. Albert Einstein made few friends with his pacifist stance and support of Zionism or a Jewish state in Palestine. Galileo Galilei struggled against both the scientific community and the Catholic Church in his determination to support the truth of observation. Sir Isaac Newton is described as a contentious and disagreeable man.

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