54 pages • 1 hour read
Stephen HawkingBrief Answers to the Big Questions
Nonfiction | Book | Adult | Published in 2018A modern alternative to SparkNotes and CliffsNotes, SuperSummary offers high-quality Study Guides with detailed chapter summaries and analysis of major themes, characters, and more.
Chapter 4Chapter Summaries & Analyses
Chapter 4 Summary: “Can We Predict the Future?”
The ancients blamed capricious gods for floods, earthquakes, diseases, and other disasters. The stars, though, behaved in a regular manner, and astronomy became the first science. Science evolved as more regularities were found in nature.
Pierre-Simon Laplace claimed that if we knew the initial positions and velocities of every particle in the universe, we’d be able to calculate their activity at any time, past or future. This “scientific determinism” may be impossible because of the complexity of the equations involved and because of the phenomenon of chaos, which introduces unpredictable variety in the way particles interact.
A second problem is that calculating an atomic particle’s speed and location requires firing other particles at it and measuring the results. A high-energy particle can locate another particle somewhat precisely, but the energy of the interaction causes the observed particle to change velocity in unpredictable ways. Conversely, a low-energy measurement doesn’t impart much of a change to the observed particle’s velocity but also doesn’t determine that particle’s location with much precision: “[T]he more accurately you try to measure the position of the particle, the less accurately you can know the speed, and vice versa” (92). At the atomic level, quantum mechanics can predict only probabilities, not certainties.
Quantum mechanics describes particles in terms of wave functions. These wave functions graph out a tiny, rippling region, the waves of which represent the particle’s frequency, or energy, and of which tallest waves roughly describe the particle’s position. The taller the peak, the more the frequency of the ripples changes near it; thus, the position is well measured—but the energy isn’t. A steady set of ripples with smaller peaks, on the other hand, describes a particle with a known energy but an uncertain position.
Many scientists, including Einstein, considered quantum theories incomplete and thought that a more fundamental theory would clear away the uncertainties. Such “hidden variable” hypotheses have since largely been disproven. Even God thus cannot predict the future.
Chapter 4 Analysis
In this chapter, Hawking continues to explore the theme of Knowing the Universe Through Science. He explains why the fate of humanity is ultimately unpredictable. Two factors delimit our ability to know everything about our world: chaos theory and quantum mechanics.
The author explains that “chaos” is inherent in the world. He offers the example of a butterfly that flaps its wings in Asia and sets off a series of events that result in a rainstorm in New York. According to chaos theory, each interaction by the smallest things—water vapor, air particles, flapping wings—causes changes in nearby things that, in turn, cause more changes in the first things. This creates a feedback loop in which the number of possible outcomes quickly swells toward infinity.
Adding to the challenge of predictability, quantum mechanics holds that knowing the exact position and momentum of any particle is impossible because the particles used to measure other particles impart effects on the measured particles that change their positions and velocities. Thus, knowledge of a particle’s position has an inverse relationship to knowledge of its momentum: The more we know about one, the less we can determine the other. It’s almost as if every particle's location is made up of its own velocity, and its velocity is simply an aspect of its location. Like viewing a globe, which presents only half of itself at a time, we can see only half the nature of a given particle at a given point in time, in some combination of positional and momentum uncertainty.
Science has evolved over the centuries to establish laws that predict a vast assortment of phenomena. In 1814, Pierre-Simon Laplace—a great theorist of celestial mechanics who also made advances in statistics, physics, and other topics (and who predicted black holes)—proposed that the universe is deterministic, a machine of which the entire history could be known, were a mind of sufficient strength able to collect information on every particle’s condition.
For more than a hundred years, scientists operated on this assumption. Scientific determinism finally crashed against the wall of uncertainty governed by quantum mechanics and chaos theory. These aren’t failures: The theory shows that the universe works in ways we would never have guessed, had we not performed the experiments that reveal these surprising principles. If, at best, reality is probabilistic, humans can use that knowledge to predict that there will always be surprises and plan accordingly.
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