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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.

Laplace strongly believed in causal determinism, which is expressed in the following quote from the introduction to the Essai:

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.

John Polkinghorne 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 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.^[1]

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, under current theory, thought to be irreversible.

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^[2]. 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.)

• In Adam Fawers "Improbable: A Novel", the main character becomes a personification of Laplace's demon.
• A character using the name Laplace no Ma (literally Laplace's Demon) is featured in the second series of the Rozen Maiden anime series.
• Laplace No Ma (Laplace's Demon) is a 1993 Videogame for the SNES.
• The Masoukishin Cybuster from the Super Robot Wars metaseries possesses the 'Laplace's Demon Computer' which gives it the ability to predict the future as well as editing/altering events in the timeline itself. A similar (though less powerful) device is used by another giant robot, the Unicorn Gundam.
• The character Sion Eltnam Atlasia, from TYPE-MOON's PC game Melty Blood and all members of her family are possessed by the demon of Laplace.
• While never referred to as such, the Mainframe Entertainment superhero Action Man possesses a super power that could be considered a limited form of Laplace's Demon.
• In Frank Herbert's Dune, certain, highly trained people are capable of prescience; this feat is limited only by the user's knowledge and focus. Prescients see time as a nexus of causality, and as such, recognize the fallacy of free will. Powerful prescients with greater focus and knowledge can see further into the future, and the presence of other prescients require a great deal more focus to predict the actions of the other prescient.

1. ^ see, e.g., John Polkinghorne Quarks, Chaos and Christianity pp. 65–66
2. ^ [1] Article published by APS

• Maxwell's demon