Thursday, May 13, 2010

Laplace's Demon

Pierre Simon-Laplace published a paper in 1814 that took the idea of Newton’s ordered universe to its logical conclusion. He wrote:

“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.”

The idea is that if someone or something could know the location and momentum of every single particle in the universe at a given time, then that entity, nicknamed “Laplace’s demon”, would know the future. The reasoning is that all particles obey the same laws of physics. If I throw a baseball up, then I know it’s going to come down (barring any interference). That fact is determined by gravity, a fundamental physical law. If I had that kind of knowledge about every single aspect of the universe, then what could I predict?

There are different, nuanced views concerning whether a wholly determined future is truly the consequence of physical laws, and I hope to have a good discussion about that in a subsequent post. But right now, let’s look at the idea that if we knew everything about the current state of the physical universe, and we could process that information, then we could know the future.

Does this work?

Imagine an entity with all this knowledge, the knowledge of everything. It would then know the future....but if it knows the future, what is there to prevent the entity from changing the future? If the entity is an embodied being, and it knows that it will turn to the left, why couldn’t it then decide to turn to the right?

We arrive at similar paradoxes to those associated with time travel. If a person travels back in time, and she changes something, then she has changed the causal chain that created her. The time traveler would not only no longer exist, but the person (and the past she experienced which would lie in the future and which led her to make this change!) would not have ever existed! If Laplace’s demon knows the future, then any changes it could make will change the causal chain that brings about this future. So then LD (Laplace’s Demon) would not know the future.

Some people think that we should think of LD as a computer, not a being. Fair enough. But surely a computer could print out the results - then people would read it.....and would that knowledge lock them in to certain actions? Surely not. Imagine reading that you are about to sit down in 10 seconds. Why not keep standing for 15 seconds? Then the computer’s printout would be falsified.

So knowing all available information, and therefore the future, would have to make you into a “prisoner”, following a pre-set script. But surely this is impossible when considering all the mundane details of our lives.

So perhaps nothing can predict the future if it lies within the world it is attempting to predict. Perhaps to be a player in that world, whether conscious on the level of human being or not, is to be made up of elements that the player cannot ultimately apprehend. (One of these days I am going to try to understand Godel’s Theorem which is related to this stuff).

So what about God? And I mean God in the classical theistic sense - an observer existing outside of our universe. If God knew all information, then He could know the future. But this would preclude God from interacting in the universe at all, for that would break the natural, causal chain and determinism would be out the window. We would be left with a divine indeterminism. Of course if God is the creator or programmer of the universe, and He intervened, we might ask why He did not put any changes He made into the original program. Why would an omniscient, omnipotent entity need to change the program when he knew from the beginning how it would go? But that is another subject, getting at some of the trouble with determinism and classical theism.

But what if determinism is simply not true? And I mean determinism in its classical definition - that there is only one possible future for our universe. What if the arrow of time has many possibilities?


  1. Deus sive natura?

    "But this would preclude God from interacting in the universe at all, for that would break the natural, causal chain and determinism would be out the window."

    Unless God was part of the natural causal chain and not separate from it. And God's actions would be determined by his nature.


  2. Yeah, that would change things up as I was mentioning God in the more traditional immutable, eternal, unmoving, etc. sense. The way you phrased it would eliminate the possibility of miracles, so that would be a challenge to classical theism.

    In the Spinoza sense the idea of God becomes very interesting and even more so when Whitehead comes along. I don't know a ton about this stuff, but I find it attractive.