A Brief History of Time | Study Guide

Stephen Hawking

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A Brief History of Time | Chapter 9 : The Arrow of Time | Summary



In this chapter, Stephen Hawking begins with how a sense of absolute time is no longer possible in situations that involve the speed of light and the vast distances it travels throughout the universe, rendering time relative and personal to the observer measuring it. But Hawking asks, If the laws of science obey the symmetries of C, P, and T, then why is it that time goes only forward (described as the "arrow of time") instead of also backward?

He answers this by using the second law of thermodynamics, which dictates that disorder (or the entropy we observe) only increases with time but never decreases. This distinguishes how we experience a single direction, or "arrow," of time from the past into the future, which (when we are in it) is the present before becoming the past. Hawking further states that not only is there the thermodynamic arrow of time, there is also the psychological arrow of time (by which we remember the past but not the future) and the cosmological arrow of time, the direction in which the universe expands but does not contract.

Hawking posits that all three arrows currently point in the same direction of time, but the no-boundary picture of the universe means that this will not always be the case. If the universe ceases to expand and instead reaches a point at which it begins to collapse (as per the first of Friedmann's models presented in Chapter 3), then the thermodynamic and the cosmological arrows will not point in the same direction, and the psychological arrow of time will become nonexistent because such a universe could not support beings to observe these conditions.

Indeed, Hawking points out that the psychological arrow of time depends on remembering how things in a given order increase entropy, because that is how we measure the passage of time itself. The teacup on the table starts by first falling and then shattering into pieces, not the other way around. Hawking follows this line of reasoning by way of explaining his mistake in not considering that disorder would continue to increase during a period of contraction, so these arrows would not reverse.


This chapter presents some rather humorous images of time running in reverse. Although these are interesting to think about (like how watching a reversed film of a teacup shattering is rather fascinating), Stephen Hawking rains on the parade by pointing out that actually experiencing time in reverse is probably impossible, because it would break the second law of thermodynamics.

Hawking gives the reader an interesting meditation on how to handle being corrected, or how one should adjust after learning they have been mistaken on some point. He uses the instances of Sir Arthur Eddington and Albert Einstein as negative and positive examples, respectively.

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