French physicist Pierre Laplace came up with this demon to help show that all events are predetermined.

Laplace's demon has perfect knowledge of one moment of the universe -- in that moment he knows where every atom is and what forces are acting on it. He also has a very strong calculating device.

From the information he has on where things are at that point and what forces are acting on them, he should then be able to predict what will happen in the next moment, even if he has no direct knowledge of this next moment. And from this second moment, he should be able to calculate the one following that. And so on.

This was ment to show that at any one time (even without a demon), all the future events in the universe are determined by the past ones.

"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 any given moment knew all of the forces that animate nature and the mutual positions of the beings that compose it, if this intellect were vast enough to submit the data to analysis, could condense into a single formula the movement of the greatest bodies of the universe and that of the lightest atom; for such an intellect nothing could be uncertain and the future just like the past would be present before its eyes."

-Marquis Pierre Simon de Laplace

Note that the demon was only there to help you get the idea, and is not necessary for determinism. Neither the observation nor the prediction of any part of the system is necessary to show predetermination to be the case.

This type of argument is one of the problems challenging the idea that we have free will.

Pierre Laplace's demon was a hypothetical construct which could calculate the position and motion of everything in the universe and then apply the laws of physics to determine everything that would occur afterwards. It exists as a sort of saint of determinism -- the idea that human thought is determined by biology and chemistry, which can be determined from physics, thus placing even free will under the demon's awareness.

Since Laplace's time, however, the mathematical discipline of chaos theory has made the demon a lot more hypothetical. We now know that even the tiniest inaccuracy in measurement at the start of an experiment can, in time, lead to vast differences between a predicted outcome and the actual thing. In other words, Laplace's demon would have to know the position and motion every particle in the entire universe to infinite precision, thus making it more complex than the universe itself.

See also, Aristotle, who formulated a similar logical argument for the existence of God, namely:

  • Each event must have a preceding cause.
  • Each cause must have been another, preceding, event
  • In order for this chain of events to have been initiated, there must have been a "Prime Mover" - God. Q.E.D. sic
The notion of complete predestination is also referred to in philosophical literature as "The Clockmaker's Universe." Note: all of these (convincing) theories have significant implications for free will and self-determination and subjective consciousness.

Also note: This argument has a lot in common, for several reasons, with the flat earth theory.

Laplace's demon could handle chaos theory and infinite precision - that is what perfect knowledge is all about. It is actually quantum mechanics that throws interesting twists into this. First off, perfect knowledge is possible. Space and time are quantized - the smallest possible distance being the Planck Length, and the shortest possible time being Plank Time. Assuming perfect knowledge, your exact position is known. No infinite stream of decimals to indicate time or place, just use Plank Units.

Indeed, it is a hypothetical construct, just as Maxwell's demon and Schrodinger's Cat. Having a calculator more complex than the universe is not a problem in thought experiments.

Laplace's demon does not make any claims to the existence of God or the number of casual chains necessary for the first cause argument of Aristotle and Saint Thomas Aquinas.

While helping in some ways, quantum mechanics also confuses the issue some. At very small distances and units of time, things become probabilistic rather than deterministic. This makes it difficult to make 100% accurate predictions about the next state of the universe.

This is one of those odd things that I came up with on my own when I was a kid without ever hearing about the esteemed Marquis Pierre Simon de Laplace or any of his demons. Even though it isn't new, it's odd that I thought it up without any help. I remember sitting in bed when I was about 12 and thinking about how computers get faster exponentially, and just how powerful they could be made. If you knew all of the parameters in a system, you could predict what would happen next in that system. Computers predict things all the time, their power just depends on the size of the system it can analyze. Think about IBM's chess computer Deep Blue, which could come up with every possible move it could make and predict the opponent's countering moves. Such a raw efficiency is threatening, because it threatens to render the human mind obsolete.

But that doesn't mean it could happen. mblase makes a good point that such a machine would be more complicated than the universe itself. I also read a book cowritten by Arthur C. Clarke, called Richter 10 (I think), about an earthquake predictor who builds a giant computer to predict the movement of the Earth's crust, thus yielding a place and time for all future earthquakes. One of those things that bends your mind.

I can't believe this node has gotten this far without some one mentioning Heisenberg's Uncertainty Principle. This is the part of quantum mechanics that m_turner inexplicably fails to mention in his/her write-up that is the only one thus far to mention quantum mechanics.

In exact contradiction to the the implications of m_turner's wu (i.e., that infinitely precise measurements are unnecessary) quantum mechanics tells us that infinitely precise measurements of the type needed by the eponymous demon of this node are impossible. Specifically, the demon requires exact knowledge of two non-commuting observables (position and momentum). The Heisenberg Uncertainty Principle states that the product of the uncertainties of these two quantities must be approximately h.

Thus, while Laplace's Demon is an interesting idea that provoked much thought at the time, it is quite irrelevant in these "post-modern" times, except perhaps as a high water mark for classical mechanics.

lj: I respectfully disagree.

If you're mentioning Heisenberg's uncertainty principle, you're missing the point... ...you'd be tripped up by more fundamental problems...

The question Laplace asks is 'Should such a device exist, with a perfect knowledge of the entire universe...

(emphasis mine)

The point of the Heisenberg Uncertainty Principle is that there is no such thing as "perfect knowledge", especially of the whole universe. Thus if the demon where to create another, larger universe outside of our own in which to store his data and computing device, it still could not predict the future. The Uncertainty Principle is more fundamental than the size of the universe. The size of a given universe is variable. As far as we know, the Uncertainty Principle is an absolute. (With varing importance depending on the value of h in your universe...)

If you're mentioning the Heisenberg Uncertainty Principle, you're missing the point. No-one's going to go out and build Laplace's Demon (It certainly wouldn't fit in the garage :). If you were to build one, you'd be tripped up by more fundamental problems, like how to represent the entire universe without needing a memory many times the size of the entire universe, how to calculate the events of a meaningful amount if time before the heat-death of the universe, how to power your machine, etc. etc. Whether it's possible to gather the required information to load such a device is irrelevant.

The question Laplace asks is 'Should such a device exist, with a perfect knowledge of the entire universe, what's to stop it precisely predicting the future actions of myself (and others) which I assume to be the result of free will?' The reason Heisenberg is irrelevent to the discussion is because we don't need to build the machine - There is already a machine that has a perfect knowlege of the entire universe, and calculates its behaviour with perfect accuracy - the universe itself. Laplace's Demon is simply a metaphor for the (hypothesised) deterministic behaviour of the universe.

Heisenberg is only relevent to our attempts to copy the universe - it says nothing about whether or not the universe behaves deterministicly or not. A refutation of the consequences of Laplace's Demon should find some behaviour of the universe that is be impossible, not just intractable, to predict, even given perfect information.

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