A Brief History of Time | Study Guide

Stephen Hawking

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A Brief History of Time | Chapter 1 : Our Picture of the Universe | Summary



The first chapter of A Brief History of Time presents the big questions about the universe with the story of a woman who challenged a lecturer on astronomy with an almost childlike assertion that holding Earth in position in space was an unending stack of tortoises. While this idea may seem ridiculous to most people today, Hawking uses it as a point of reference to begin the discussion of how it is we know what we know about the universe. He credits rapid advances in technology for equally rapid leaps in physics, allowing a much greater range of observations possible in just the last century.

This has enabled scientists to make increasingly accurate theories based on those observations. He cautions that not all theories based on observation actually pan out. While the Greeks concluded that Earth must be a sphere instead of a flat disk (with or without tortoises supporting it) as Aristotle's book On the Heavens expressed in 340 BCE, they also believed that observation firmly placed Earth as fixed and immobile.

Within the cosmological model based on this concept, refined by mathematician and philosopher Ptolemy in the 2nd century CE, Earth occupied a fixed and stationary position in the center, with eight concentric spheres around it. This geocentric (Earth-centered) model of the universe was generally accepted as fact for the next 1,300 years, in part because the Christian church took it up as the scripturally correct model. This was the status quo until Nicolaus Copernicus, a Polish priest, challenged it in 1514. His heliocentric (sun-centered) model was later supported by observations made by astronomers Johannes Kepler and Galileo Galilei, but it was not until 1609 before the Aristotle/Ptolemy model was finally deemed unsupportable. In that year, Galileo observed satellites orbiting Jupiter using the recently invented telescope.

The start of modern physics, according to Hawking, dates from the 1687 publication of Sir Isaac Newton's Philosophiæ Naturalis Principia Mathematica, which established three laws of motion and the law of universal gravitation. Newton's work led to the idea that the universe might be infinite, rather than be contained by an outer sphere of fixed stars. The idea of an infinite universe reopened discussions about the beginning of the world, which had largely been the realm of theologians to this point. Following Newton's work suggesting the universe could not be static, the philosopher Immanuel Kant's Critique of Pure Reason (1781) tackled the problem of its beginning and end, implying that the universe has, does, and will undergo dynamic changes.

The thought the universe might be expanding, however, was not considered much until 1929, when American astronomer Edwin Hubble observed that distant galaxies are moving in all directions away from us. Consequently, the idea that objects like galaxies are traveling away from each other suggests that sometime in the past, all matter in the universe had been much closer together. This led to the idea that there may be physical reasons for the universe to have had a beginning. If this is the case, the when and how might be discernible through observation, leading to the development of the big bang theory.

The chapter concludes with some points on the use of theories and observations. Hawking discusses the limits of observations leading to any provisional theory (or partial theory, also called a hypothesis), noting that a theory cannot be definitively proven, but it can be disproven. He goes on to comment on the use of theories and how new theories are often extensions or alterations of existing theories, building upon prior knowledge. The goal, according to Hawking, is to devise a theory that describes everything. However, scientists typically divide the problem into more manageable parts, leading to a set of partial theories. Today, we have two leading theories, relativity and quantum mechanics, and the search for a universal theory to encompass both partial theories continues.


The anecdote in which an old lady asserts confidently that the world is a flat plate supported by a giant stack of tortoises illustrates the importance of scientific inquiry. If we all took what was taught us as truth and did not test it with observation and theorization, the world at large would still believe giant tortoises support the world, and the discoveries that enable modern technology would never have been made.

Stephen Hawking makes the point that humanity's view of the universe has undergone many changes. When the first humans looked up into the sky and wondered what it was all about, they got busy with explanations (theories or models) that would coincide with their observations. And it is the two-point efforts—combining observation and theory—that physicists have used to the present day.

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