In the history of science, Laplace's demon is a thought experiment described by Pierre-Simon Laplace in a paper published in 1814 involving a hypothetical entity envisioned such that if it knew the precise location and momentum of every atom in the universe then it could use deterministic principles to reveal the entire course of cosmic events, past and future.^[1]

Contents 1 Original Quotation 2 Arguments against Laplace's demon 3 Recent views 4 Laplace's demon in popular culture 5 References 6 See also

[edit] Original Quotation

Laplace strongly believed in causal determinism, which is expressed in the following quotation 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. While hoping that humanity would progress to a better scientific understanding of the world, he recognized that such a complete level of understanding would always be beyond the grasp of human knowledge, as a tremendous calculating power would be needed to take into account every precondition in a given instant. While Laplace considered this to be a mere practical problem, 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] Arguments against Laplace's demon

In a quantum mechanical world, Laplace's demon becomes a clear impossibility. Chance is an essential part of the world's unfolding, and the Heisenberg Uncertainty Principle forbids exact measurements of positions and velocities simultaneously. But even within a hypothetical classical universe, there are reasons to doubt that Laplace's Demon is a meaningful concept.

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.^[2]

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.

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.^[3]

[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 was shown to be about 10¹²⁰ bits^[4]. 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 is, in fact, mandatory since if a Laplace's demon was in the reality that we occupy it would have to account for itself in addition to every other aspect of matter and energy, and the grand total cannot exceed the smaller portion.

[edit] Laplace's demon in popular culture

• The webcomic Dresden Codak references Laplace's Demon in issues 59, "Advanced Dungeons & Discourse" and 60, "Exorcising Laplace's Demon".
• In Rozen Maiden by Peach-Pit, Laplace No Ma is a character that prompts the Rozen Maiden's mission with riddles, and appears only in the N-field. He looks like a white rabbit in a tuxedo but acts more like the Cheshire Cat, an obvious play on both characters from "Alice's Adventures in Wonderland".
• In Adam Fawer's "Improbable: A Novel", the main character becomes a personification of Laplace's demon.
• Laplace No Ma (Laplace's Demon) is a 1993 video game 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 practitioners of her branch of alchemy employ a method for predicting the future modeled after Laplace's demon.
• That individuals can have conscious or unconscious access to knowledge similar to that available to Laplace's demon can be used to provide an explanation, of a sort, for prescience in works of science fiction and fantasy. For example, in Frank Herbert's Dune, certain individuals are capable of prescience (particularly with the assistance of melange). Individuals see time as a vast expanding network of choices leading to outcomes, and as such lose their perception of free will, as they know the definite outcome of any possible action. However their knowledge is imperfect: they cannot see the actions and choices of other prescients, and depending on their abilities may have vast "gaps" in their visions of the future.
• Laplace's Demon appeared, alongside Maxwell's Demon, in Animal Man #86. The Demon does not move and does nothing, driven insane by its task.
• In The Hitchhiker's Guide to the Galaxy series' fifth book (Mostly Harmless), the guide mark II is said to have "no filters" and has the ability to perceive all time, matter and tangent universes similar to Laplace's Demon.
• In the video game Shin Megami Tensei: Devil Survivor, the players are given a device called a COMP that looks like a Nintendo DS. This device is capable of receiving emails from various people, but one particular source is named "Laplace" which always details the events that will happen later that day.
• Cris Johnson, the protagonist of the movie Next played by Nicolas Cage, has the ability to see what events will occur as a consequence of various possible actions.
• Laplace's Demon is the title of an unlicenced Doctor Who ebook written by Craig Charlesworth and published by Jigsaw Publications in 2006. The story concerns a villain who believes that he is able to see future events via the calculations of a powerful computer.

[edit] References

1. ^ Pierre-Simon Laplace, "A Philosophical Essay on Probabilities", Full Text [1].
2. ^ see, e.g., John Polkinghorne Quarks, Chaos and Christianity pp. 65–66
3. ^ P.-M. Binder, "Theories of almost everything", Nature, 455 (2008), 884-885. [2].
4. ^ [3] Article published by APS

[edit] See also

• Maxwell's demon