After discussing the expansion of the universe, Greene addresses a common misconception, that this expansion defies the second law of thermodynamics. First, he explains the law, which states that in a closed system, such as the universe, the total amount of entropy will always remain the same. Some physicists pose the dilemma that because the universe is expanding, and the matter in the universe is becoming more sparse, the total entropy in the universe decreases, which defies the law. The misunderstanding behind this idea lies in the definition of entropy.
A word commonly used as synonymous with entropy is chaos. More specifically, entropy is defined as the lack of organization of matter in a system. As the matter in the universe becomes less dense, it can be seen as decreasing entropy, as more densely packed matter formations tend to be more chaotic. However, much like a child spreading his peas to make it look like he ate more, the appearance of decreasing entropy is only an illusion. This disproves the misconception behind the second law of thermodynamics.
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