Laplace's demon
In the history of science, Laplace's demon was the first published articulation of causal or scientific determinism by Pierre-Simon Laplace in 1814.[1] 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.[2]
English translation
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[3]
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
Due to its assumption of determinism, Laplace's thought experiment is inherently incompatible with quantum mechanical theories, where chance is an essential part of the world's unfolding. The Heisenberg Uncertainty Principle, for example, states that exact measurements of positions and momentum may not be defined (and observed) together with more than a given precision.
John Polkinghorne argues that nature is cloud-like rather than clock-like and points out that, apart from any other problems, uncertainty about the exact position of an electron on the other side of the universe would be sufficient to invalidate a calculation about the position of an O2 molecule in air after 50 collisions with its neighbours (i.e. in about 0.1 ns), even if they were solely influenced by Newton's laws.[4] This is an example of a chaotic system, in which, even with well-defined initial conditions, arbitrarily small changes in those conditions quickly become very large. This is not an argument against Laplace's demon per se, but would require that entity to have perfect knowledge of the Universe, without error.
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, under current theory, thermodynamics (i.e. real processes) are thought to be irreversible in practical terms (compared to the age of the universe, for instance).
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.[5][6] 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.
Recent views
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.[7] 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.
See also
References
- ^ 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
- ^ see John Polkinghorne Quarks, Chaos and Christianity pp. 65–66
- ^ David H. Wolpert (2008). "Physical limits of inference". Physica D. 237 (9): 1257–1281. arXiv:arXiv:0708.1362. doi:10.1016/j.physd.2008.03.040.
{{cite journal}}: Check|arxiv=value (help) - ^ P.-M. Binder (2008). "Theories of almost everything" (PDF). Nature. 455 (7215): 884–885. doi:10.1038/455884a.
- ^ Physical Review Focus (24 May 2002). "If the Universe Were a Computer". APS.