(Preposterous Universe) – Sean Carroll:
The fact that entropy increases defines the arrow of time. It is a statistical phenomenon, valid for large systems, and the Second Law looks very different from the “microscopic” laws of physics, which generally don’t care which direction time is running in. Of course, the only reason we see that entropy increases is because it used to be small in the past. Once the gas fills the box, it essentially stays there forever (apart from rare fluctuations).
So, as cosmologists, we have an issue to address – why was the entropy of our early universe so small? If high-entropy states are “natural,” why don’t we live in one? You might think to appeal to the dreaded anthropic principle, and argue that life couldn’t exist in a state with really high entropy. But that turns out not to be good enough; the entropy of our universe is much much lower than it needs to be to support the existence of life. So we are faced with the “arrow of time problem.” …
preposterousuniverse…2004/10/27/the-arrow-of-time
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