Clever Demons and Hungry Black Holes

The French scholar Pierre-Simon Laplace once told the story of a demon. The demon knows all the laws of physics, and is so smart that he can do an infinite number of calculations in his head. If you told him the exact state of the universe at one point in time, then he would be able to predict with certainty the exact state of the universe at some later time. He would always win bets.

He could also use his physics knowledge to turn the clocks back, and deduce, given the state of the universe at some time, the state it had at some earlier time. If you wanted to destroy a document containing information you’d rather no one ever find out, and, say, burned it, you still wouldn’t be safe. The demon could look at the smoke coming off the flames, and use it to deduce what was on the page.

Laplace told this story in order to convey the idea that

“We may regard the present state of the universe as the effect of its past and the cause of its future.”

This seems like a pretty sensible way to view nature to most physicists. The universe is in principle predictable. If it wasn’t the case, what’s the point in physics?

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