If you knew everything, could you compute the future?

That time, that place

A century after Newton the heavens were computed to frightening accuracy — but one problem remained. Jupiter and Saturn tug on each other, and their orbits appeared to drift.

Newton himself thought the solar system would eventually fall out of order, and that God adjusted it from time to time.

Why this question

Laplace showed by calculation that the drift does not accumulate in one direction but swings back over very long cycles. No adjustment needed.

The story goes that when Napoleon remarked that his book made no mention of a creator, he replied that he had had no need of that hypothesis.

What was found

He then went further: an intelligence knowing the position and velocity of every particle at this instant could compute the entire past and future.

This is Laplace's demon — the world as clockwork. It was the confidence of nineteenth-century science.

It fell twice. Heisenberg said you cannot know position and velocity at once; Lorenz said that even if you could, the tiniest error destroys the prediction. His other legacy, probability and statistics, is very much alive.