That is not actually the case. It turns out there is a fundamental physical limitation on destructive computation; any irreversible operation must produce a sm

That is not actually the case. It turns out there is a fundamental physical limitation on destructive computation; any irreversible operation must produce a small amount of heat. Wikipedia has more at http://en.wikipedia.org/wiki/Reversible_computing http://en.wikipedia.org/wiki/Reversible_computing and the current state of the art last I looked at it (probably in 2003/2004) was that there were a handful of very experimental processor architectures that do this; the one I looked at the most was Vieri's Pendulum. Programming in a reversible fashion is very strange; in his PhD thesis, Vieri's demonstration Fibonacci program turns out to be incomplete and I had to debug and fix it, and it was one of the stranger paradigm shifts I've experienced.