Chapter 9

Stephen Hawking discusses the concept of time in this chapter and how it is experienced differently by different observers. He posits that there are three different "arrows" of time: the thermodynamic arrow, which dictates that disorder only increases with time; the psychological arrow, in which we remember the past but not the future; and the cosmological arrow, the direction in which the universe expands but does not contract. Hawking suggests that these three arrows currently all point in the same direction, but if the universe were to stop expanding and start contracting, the thermodynamic and cosmological arrows would not point in the same direction and the psychological arrow would become nonexistent. He also reflects on his previous mistake in not considering that disorder would continue to increase during a period of contraction, meaning the arrows would not reverse.