1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

I Laplace's Demon and determinism

  1. Oct 1, 2016 #1
    Not sure if this is the correct forum to post this thread, but here we go.

    I'm a layman in physics and mathematics, and recently, I was reading about the Laplace's Demon and how, according to classical mechanics, the future could be entirely determined if some entity (the Demon) could have complete knowledge about the position and velocity of every particle in the universe.

    That caught my interest. Today, we know that Laplace's Demons isn't true because of the second law of thermodynamics and quantum mechanics (Heisenberg Uncertainty Principle). But I think that the concept of Laplace's Demon (that every single event in the future is exactly determined by just positions and velocities of particles) is very interesting, indeed. So, even if we can show that Laplace's Demon isn't possible for a bunch of reasons, I wonder if there's some similar theoretical construct which follows the same principles (but takes thermodynamics and quantum mechanics into account).

    So I think my question is: could the universe be deterministic, even with entropy and Heisenberg Uncertainty, and what information should the Demon know so as to be aware of all the past and future events (wave functions? I don't know)?
  2. jcsd
  3. Oct 1, 2016 #2

    Simon Bridge

    User Avatar
    Science Advisor
    Homework Helper

  4. Oct 1, 2016 #3


    User Avatar

    Staff: Mentor

    Entropy comes from our inability to completely specify the state of a system, but Laplace's demon isn't subject to that limitation. So it's not a problem here.

    The uncertainty principle is less fundamental a problem than the Born rule. According to quantum mechanics, no matter how much you know about the initial state of a system, you cannot calculate with certainty how it will evolve under all interactions. The best you can get is the probabilities that it will do this instead of that or the other. Thus, even though the demon knows everything, that's not good enough; there are multiple possible outcomes and the demon's knowledge isn't enough to determine which will happen.

    It is possible that quantum mechanics is not the last word here. It might be that there is some deeper theory that explains quantum mechanics, including its apparent randomness, the way that statistical mechanics explains the ideal gas law. If this hypothetical theory were deterministic, then the demon could use it to make deterministic predictions as Laplace imagined.

    However, no such theory is known at this time, so the possibility remains idle speculation. Interesting.... But idle.
  5. Oct 1, 2016 #4

    Simon Bridge

    User Avatar
    Science Advisor
    Homework Helper

    ... another objection I've heard is that the demon can only know everything inside it's past light cone. I'm unclear how strong an objection that is.
    There are some arguments involving "information" passing the event horizon of a black hole that remind me of this.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted

Similar Discussions: Laplace's Demon and determinism
  1. Maxwell Demon (Replies: 6)

  2. Maxwell's Demon (Replies: 6)