In the history of science, Laplace's demon was the first published articulation of causal or scientific determinism by Pierre-Simon Laplace in 1814. According to determinism, if someone knows the precise location and momentum of every atom in the universe, their past and future values for any given time are entailed; they can be calculated from the laws of classical mechanics.
We may regard the present state of the universe as the effect of its past and the cause of its future. An intellect which at a certain moment would know all forces that set nature in motion, and all positions of all items of which nature is composed, if this intellect were also vast enough to submit these data to analysis, it would embrace in a single formula the movements of the greatest bodies of the universe and those of the tiniest atom; for such an intellect nothing would be uncertain and the future just like the past would be present before its eyes.—Pierre Simon Laplace, A Philosophical Essay on Probabilities
This intellect is often referred to as Laplace's demon (and sometimes Laplace's Superman, after Hans Reichenbach). Laplace himself did not use the word "demon", which was a later embellishment. As translated into English above, he simply referred to: "Une intelligence... Rien ne serait incertain pour elle, et l'avenir comme le passé, seraient présent à ses yeux."
Arguments against Laplace's demon
According to chemical engineer Robert Ulanowicz, in his 1986 book Growth and Development, Laplace's demon met its end with early 19th century developments of the concepts of irreversibility, entropy, and the second law of thermodynamics. In other words, Laplace's demon was based on the premise of reversibility and classical mechanics; however, Ulanowicz points out that many thermodynamic processes are irreversible, so that if thermodynamic quantities are taken to be purely physical then no such demon is possible as one could not reconstruct past positions and momenta from the current state. Maximum entropy thermodynamics takes a very different view, considering thermodynamic variables to have a statistical basis which can be kept separate from the microscopic physics.
Due to its canonical assumption of determinism, Laplace's demon is incompatible with mainstream interpretations of quantum mechanics, that stipulate indeterminacy. Whilst indeterminacy is the majority position amongst physicists, the interpretation of quantum mechanics is still very much open for debate and there are many who take opposing views (such as the de Broglie-Bohm interpretation).
Chaos theory is sometimes pointed out as a contradiction to Laplace's demon: it describes how a deterministic system can nonetheless exhibit behavior that is impossible to predict: as in the butterfly effect, minor variations between the starting conditions of two systems can result in major differences. While this explains unpredictability in practical cases, applying it to Laplace's case is questionable: under the strict demon hypothesis all details are known and therefore variations in starting conditions are non-existent.
In 2008, David Wolpert used Cantor diagonalization to disprove Laplace's demon. He did this by assuming that the demon is a computational device and showing that no two such devices can completely predict each other. If the demon were not contained within and computed by the universe, any accurate simulation of the universe would be indistinguishable from the universe to an internal observer, and the argument remains distinct from what is observable.
There has recently been proposed a limit on the computational power of the universe, i.e. the ability of Laplace's Demon to process an infinite amount of information. The limit is based on the maximum entropy of the universe, the speed of light, and the minimum amount of time taken to move information across the Planck length, and the figure was shown to be about 10120 bits. Accordingly, anything that requires more than this amount of data cannot be computed in the amount of time that has elapsed so far in the universe.
Another theory suggests that if Laplace's demon were to occupy a parallel universe or alternate dimension from which it could determine the implied data and do the necessary calculations on an alternate and greater time line the aforementioned time limitation would not apply. This position is for instance explained in David Deutsch's The Fabric of Reality, who says that realizing a 300-qubit quantum computer would prove the existence of parallel universes carrying the computation.
- Hawking, Stephen. "Does God Play Dice?". Public Lectures.
- Pierre-Simon Laplace, "A Philosophical Essay on Probabilities" (full text).
- Laplace, Pierre Simon, A Philosophical Essay on Probabilities, translated into English from the original French 6th ed. by Truscott,F.W. and Emory,F.L., Dover Publications (New York, 1951) p.4
- Stanford Encyclopedia of Philosophy, "Causal Determinism"
- David H. Wolpert (2008). "Physical limits of inference". Physica D 237 (9): 1257–1281. arXiv:0708.1362. doi:10.1016/j.physd.2008.03.040. full text
- P.-M. Binder (2008). "Theories of almost everything". Nature 455 (7215): 884–885. doi:10.1038/455884a.
- Physical Review Focus (24 May 2002). "If the Universe Were a Computer". APS.