The reason [James Clerk] Maxwell's Demon cannot exist is that it does take resources to perform an act of discrimination. We imagine computation is free, but it never is. The very act of choosing which particle is cold or hot itself becomes an energy drain and a source of waste heat. The principle is also known as "no free lunch."We do our best to implement Maxwell's Demon whenever we manipulate reality with our technologies, but we can never do so perfectly; we certainly can't get ahead of the game, which is known as entropy. All the air conditioners in a city emit heat that makes the city hotter overall. While you can implement what seems to be a Maxwell's Demon if you don't look too far or too closely, in the big picture you always lose more than you gain.Every bit in a computer is a wannabe Maxwell's Demon, separating the state of "one" from the state of "zero" for a while, at a cost. A computer on a network can also act like a wannabe demon if it tries to sort data from networked people into one or the other side of some imaginary door, while pretending there is no cost or risk involved.

It's always seemed like a big mystery how nature, seemingly so effortlessly, manages to produce so much that seems to us so complex. Well, I think we found its secret. It's just sampling what's out there in the computational universe.

Theory is relevant to you because it shows you a new, simpler, and more elegant side of computers, which we normally consider to be complicated machines. The best computer designs and applications are conceived with elegance in mind. A theoretical course can heighten your aesthetic sense and help you build more beautiful systems.

Thirty years ago, we used to ask: Can a computer simulate all processes of logic? The answer was yes, but the question was surely wrong. We should have asked: Can logic simulate all sequences of cause and effect? And the answer would have been no.