Editing A Brief History of Time (section)

=== Chapter 9: The Arrow of Time ===
Hawking discusses three "[[Arrow of time|arrows of time]]". The first is the [[Entropy (arrow of time)|thermodynamic arrow of time]]: the direction in which entropy increases. This is given as the explanation for why we never see the broken pieces of a cup gather themselves together to form a whole cup. The second is the [[Time perception|psychological arrow of time]], whereby our subjective sense of time seems to flow in one direction, which is why we remember the past and not the future. The third is the cosmological arrow of time: the direction in which the universe is expanding rather than contracting. Hawking claims that the psychological arrow is intertwined with the thermodynamic arrow. According to Hawking, during a contraction phase of the universe, the thermodynamic and cosmological arrows of time would not agree.

Hawking then claims that the "no boundary proposal" for the universe implies that the universe will expand for some time before contracting back again. He goes on to argue that the no boundary proposal is what drives entropy and that it predicts the existence of a well-defined thermodynamic arrow of time if and only if the universe is expanding, as it implies that the universe must have started in a smooth and ordered state that must grow toward disorder as time advances. He argues that, because of the no boundary proposal, a contracting universe would not have a well-defined thermodynamic arrow and therefore only a universe that is in an expansion phase can support intelligent life. Using the weak anthropic principle, Hawking goes on to argue that the thermodynamic arrow must agree with the cosmological arrow in order for either to be observed by intelligent life. This, in Hawking's view, is why humans experience these three arrows of time going in the same direction.