I can't believe this node has gotten this far without some one mentioning Heisenberg's Uncertainty Principle. This is the part of quantum mechanics that m_turner inexplicably fails to mention in his/her write-up that is the only one thus far to mention quantum mechanics.

In exact contradiction to the the implications of m_turner's wu (i.e., that infinitely precise measurements are unnecessary) quantum mechanics tells us that infinitely precise measurements of the type needed by the eponymous demon of this node are impossible. Specifically, the demon requires exact knowledge of two non-commuting observables (position and momentum). The Heisenberg Uncertainty Principle states that the product of the uncertainties of these two quantities must be approximately h.

Thus, while Laplace's Demon is an interesting idea that provoked much thought at the time, it is quite irrelevant in these "post-modern" times, except perhaps as a high water mark for classical mechanics.

lj: I respectfully disagree.

If you're mentioning Heisenberg's uncertainty principle, you're missing the point... ...you'd be tripped up by more fundamental problems...

The question Laplace asks is 'Should such a device exist, with a perfect knowledge of the entire universe...

(emphasis mine)

The point of the Heisenberg Uncertainty Principle is that there is no such thing as "perfect knowledge", especially of the whole universe. Thus if the demon where to create another, larger universe outside of our own in which to store his data and computing device, it still could not predict the future. The Uncertainty Principle is more fundamental than the size of the universe. The size of a given universe is variable. As far as we know, the Uncertainty Principle is an absolute. (With varing importance depending on the value of h in your universe...)