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Is the Universe deterministic?

  1. Feb 6, 2015 #1
    Hello everyone!

    There's this one question that keeps bothering my mind, but I'm not sure if this is the right section. So if not I apologise and ask for a moderator to move it for me.

    If there was somehow a way to 'reproduce' the universe we exist with ALL the trillions of constants there are. Would I be questioning this question on this forum at this time on the second Universe? Did I mix the scales too much? That would be either the whole Universe is deterministic or it's not. Do we know which one is it? Is that even graspable?

    Have a good day :)
     
  2. jcsd
  3. Feb 6, 2015 #2

    phinds

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    No, quantum mechanics has shown conclusively that reality is probabilistic, not deterministic. This was very upsetting to many people back in the 1920's or so, before QM became strongly established, but that is the way it is.
     
  4. Feb 6, 2015 #3
    Oh what a relief! Dang it haha! I just started my graduation in Physics this year so I still have no idea how quantum mechanics work. Can you give me a quick example so I can have a better understandment? Thank you for your time, sir!
     
  5. Feb 6, 2015 #4

    phinds

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    Read up on the double slit experiment.
    from the Wikipedia article
     
  6. Feb 6, 2015 #5
    Thanks phinds!
     
  7. Feb 6, 2015 #6
    Wow. It's just brilliant. So, nothing is certain, it's a world of probabilities. Damn the Universe is so freaking beautiful!
     
  8. Feb 6, 2015 #7

    phinds

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    Not only that, it provides a legitimate excuse for all kinds of things. Since I found out about QM, I've changed my answer to questions such as "are you going to do the dishes" from "definitely" to "probably" and if I don't do them and get called on it I just point out that it's a probabilistic universe and I'll probably do them eventually. Maybe.
     
  9. Feb 6, 2015 #8
    So in the end that means that the second universe from my anology would be completely different? Because it's hard to concatenate that with the same constants you would get an entirely different result. I mean no matter how complex the algorithm in nature might be, theres always the question: "Why would it happen differently?". Because even though our mathematical basis cannot grasp the complexity of something, it doesn't mean it's not deterministic. Like the flip of a coin for example, it appears probabilistic, but it is surely deterministic. I think thats what quantum mechanics tries to show. This inconceivable thought. Get where I'm coming from? Is this getting too philosophical? haha
     
  10. Feb 6, 2015 #9

    phinds

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    I'm one of those who does not believe in "other universes" and find that line of thought pointless.

    EDIT: and by the way, since QM says that things could not possibly be exactly the same, your question amounts to "if things were different would things be different?"

    Actually, it does. MATH things can be the same every time. Physical things can't although at the macro level they can come very close and for all practical human-level considerations, BE the same.

    No, it is not. It DOES behave about 99.9999% as though it is deterministic at the macro level, but since at bottom thing are probabilistic and so is it.
     
  11. Feb 6, 2015 #10
    Yeah I don't believe in other universes aswell. It's just that I could not find another pratical anology to make. If you have a better example I'd love to hear it!
    So that means that even if we can't understand it at all, it is still probabilistic? Amazing!
    I'm starting to get what you're saying. I have the feeling that I knew that already somehow, but as I started to question the 'randomness' of the Universe, I suddenly found myself in doubt. That the actual random number could not exist. So, maybe I am skipping scales again, but does that mean that a truly random number is possible to achieve? Kinda comes from the sentence that "If you ask the same question you'll get the same answer every time." But if QM is right that means the universe does not follow a deterministic algorithm and cannot be predicted even if you get to understand the whole process of it. My doubts probably comes from my ignorance about the quantum level of the universe. (This is the irony of my nickname. I'm so curious about it, yet I don't know anything about it)
     
    Last edited: Feb 6, 2015
  12. Feb 6, 2015 #11

    phinds

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    But we DO understand it well enough to know that it is probabilistic. Google "Heisenberg Uncertainty Principle"
     
  13. Feb 6, 2015 #12
    "Actually, it does. MATH things can be the same every time. Physical things can't although at the macro level they can come very close and for all practical human-level considerations, BE the same." I think this quote got me to understand it. Thank you very much.
     
  14. Feb 6, 2015 #13

    atyy

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    We don't know whether the universe is random or deterministic. The quantum formalism is random, but we do know that deterministic theories can underlie many quantum theories. For example, Bohmian Mechanics is a deterministic theory that can underlie non-relativistic quantum mechanics, in the sense that Bohmian Mechanics makes quantum mechanics look like classical statistical mechanics, which is presumably explained by deterministic classical mechanics. However, we do not yet know if there is a deterministic explanation for all of quantum mechanics, especially the quantum mechanics of chiral fermions interacting with non-Abelian gauge fields. So whether the universe is necessarily random remains unknown.

    What we do know is that if quantum mechanics is correct, the prediction of quantum mechanics that the Bell inequalities are violated at spacelike separation, means that no local deterministic theory can explain the results of quantum mechanics.
     
  15. Feb 6, 2015 #14
    Right. I'm glad it's at least way more complicated than one can think. Thanks for the help guys.
     
  16. Feb 9, 2015 #15

    Ken G

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    There are only two possibilities here, and they are not "deterministic" vs. "probabilistic." The two possibilities depend on what you mean by "the universe." If you mean:
    1) the universe is something outside of our understanding of it, but we attempt to model it with our theories. In that case, the universe could never be either deterministic or probabilistic, because both those words are attributes of models of the universe, not what you mean by the universe itself. This is easily demonstrable-- you can test if a model is determinstic or probabilistic just by looking at its postulates, but there is no way to test if the universe is, you can only test which model seems to be working best and in which situation.
    2) the universe is however you are thinking about it, what Hawking would call a "model-dependent" universe. In this situation, we use our models to give meaning to what we regard as the universe, and then what we mean by the universe simply inherits the attributes of the models. In that case, the universe can be both deterministic and probabilistic, depending on which model is being used in which context to interpret the behavior of what you mean by the universe.

    As usual in philosophy, what you get out depends on what assumptions you put in, but in neither case would it make sense to say that the universe is either deterministic or probabilistic in some either/or absolute way. To steer this away from philosophy, you must actually be asking, which models work best, deterministic ones or probabilistic ones? That depends on the context, we certainly use both types all the time in physics.
     
  17. Feb 9, 2015 #16

    phinds

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    Yes, but that's all philosophy. As far as physics is concerned, the universe cannot be deterministic, it is probabilistic. I DO understand that you are saying that's just "what the model says", but that's what physics is about ... trying to understand reality through models that can be shown to work. What lies under the models in the "real" universe seems to me to be metaphysics / philosophy, not physics. If we don't believe the models, even though we know they are limited, we have nothing.
     
  18. Feb 9, 2015 #17

    Ken G

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    It sounds like we agree the physics question here is actually, "which models work better, deterministic ones or probabilistic ones?" But the answer to that is, it depends on the context. There are many situations where the physicist will choose a deterministic model. One cannot then say that those are only effective theories, there are more fundamental theories that underpin them, because we should suspect that the more fundamental theories are also effective theories that have more fundamental theories yet to be discovered. So we cannot say the universe is actually probabilistic, just because quantum mechanics is more fundamental than Newtonian mechanics, and the former requires a probabilistic approach by the physicist while the latter requires a more deterministic approach (unless it is classical statistical mechanics, and then we have a deterministic theory underpinning a probabilistic approach!). Indeed, as atyy pointed out, some hold that quantum mechanics will ultimately be underpinned by a more fundamental deterministic theory, and only time will tell.

    At the end of the day, questions like "which model works better" are contextual questions, and are just examples of the arsenal of tools available to the physicist to try to gain predictive power and understanding-- they are not absolute descriptions of "the universe" unless one wishes to do philosophy, which requires subjective assumptions. We as physicists do not need to believe our models, indeed we do better science when we are skeptical of our models. All we need to know is when we can expect the model to work well.
     
  19. Feb 9, 2015 #18

    phinds

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    Yes, you make good points.
     
  20. Feb 9, 2015 #19

    Ken G

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    Thanks, I think we are actually quite in agreement-- our models convey to us a sense of lessons about the universe, and a lesson we can take from quantum mechanics is that the block-world determinism of Newtonian mechanics is not the whole story.
     
  21. Feb 10, 2015 #20

    Chronos

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    If you assume the universe had a 'beginning' [as is popular], the chain of causality necessarily ends at that point. In that sense the universe is not deterministic. You can avoid this problem by assuming 'something', however alien and unintuitive it may seem [as is also popular], has 'always' existed. That, of course, opens up a whole different can of worms. Quoting a well known legal scholar, the answer is - it depends.
     
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