Laplace's demon
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In the history of science, Laplace's demon is a hypothetical "demon" envisioned in 1814 by Pierre-Simon Laplace such that if it knew the precise location and momentum of every atom in the universe then it could use Newton's laws to reveal the entire course of cosmic events, past and future.
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[edit] Original quote
Laplace strongly believed in causal determinism, which is expressed in the following quote from the introduction to the Essai:
| 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. |
This intellect is often referred to as Laplace's demon. Note, however, that the description of the hypothetical intellect described above by Laplace as a demon does not come from Laplace, but from later biographers: Laplace saw himself as a scientist that hoped that humanity would progress in a better scientific understanding of the world, which, if and when eventually completed, would still need a tremendous calculating power to compute it all in a single instant. While Laplace saw foremost practical problems for mankind to reach this ultimate stage of knowledge and computation, later interpretations of quantum mechanics, which were adopted by philosophers defending the existence of free will, also leave the theoretical possibility of such an "intellect" contested.
[edit] The demon's downfall
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; thermodynamics, i.e. real processes, however, are irreversible.
It should be noted that the existence in principle of this hypothetical "intellect" depends on a number of assumptions, all of which are highly speculative:
- It is possible to know all the information about the past and present states of the universe. (contra both Quantum Mechanics and Relativity)
- It is possible to know all the natural laws governing the universe.
- These natural laws are fully deterministic, computable and do not underdetermine the physical outcomes.
- That an "intellect" could be capable of computing the future states of the universe faster than they actually occur.
- That such an intellect could exist without being inside the universe (contra Materialism) [1]
John Polkinghorne FRS argues strongly as a physicist 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 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 (ie in about 0.1ns), even if they were solely influenced by Newton's laws [2]
[edit] 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 turns out to be 2130 bits. Accordingly, anything that requires more than this amount of data cannot be computed in the amount of time that has lapsed so far in the universe. (An actual theory of everything might find an exception to this limit, of course.)
