Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Causal Determinism

  1. Nov 11, 2011 #1
    Please bare with me, I have A-Level physics (barely any Quantum physics) and hence no idea on the subject, just general crap.

    I at the moment am an advocate for causal determinism but have never had a proper answer to why quantum mechanics disproves the idea of determinism.

    What I understand determinism to be is, for there to be an effect, there has to be a cause, and so the whole universe interacts with itself ( i mean the particles within it) in a deterministic fashion.

    now I am not saying that it is possible to ever determine anything, the uncertainty principle is pretty clear (and logical). You can not know BOTH the position and speed of a particle infinitely precise - because you are changing its trajectory with the photons when observing i believe?

    So to the question. are there random effects/actions in the universe (proven to be random or indicative) OR is it a deterministic universe and things ONLY SEEM random because of the uncertainty principle ?

    Any feedback would be greatly appreciated :]
  2. jcsd
  3. Nov 12, 2011 #2


    User Avatar
    Science Advisor

    There exist different interpretations of quantum mechanics, which cannot be distinguished experimentally. Some of them are indeterministic and some are deterministic. I think much of the indeterminism results from the earlier days of QM, which defined the "standard language" and had the focus on human experience. This indeterministic Copenhagen interpretation is probably the mainstream view, but many physicists don't show much interest in the foundations of QM.
    Last edited: Nov 12, 2011
  4. Nov 12, 2011 #3


    User Avatar
    Science Advisor
    Gold Member

    Welocome to PhysicsForums, drupillow!

    According to Bell's Theorem, experiments show: if there is causal determinism, there are faster than light actions occurring in nature. So pick your poison! If you continue to assert determinism, then you may want to read up on Bohmian Mechanics (which is a nonlocal deterministic interpretation).
  5. Nov 12, 2011 #4
    I think you're putting a little too much meaning into Bell's theorem. It states that if there is a hidden variable theory, it can't be both Einstein-local and realistic. So a hidden variable theory must be either nonlocal, nonrealistic or both. You can keep determinism either way. Nonrealistic merely implies that the measurement results are not a property of the measured system before the measurement. That does not exclude a deterministic mechanism that determines the measurement outcome during the measurement interaction.

    To the OP: I think it's helpful to distinguish two meanings of determinism in the discussion. The first one is local determinism. That is what we test in experiments. If we prepare an experimental setup as carefully as possible by controlling all local properties of the system state, then we'd expect that identical preparations result in the same experimental outcome. That does not happen in quantum theory, so we can say it's locally (or observer subjectively) indeterministic.
    Does that imply that the universe is indeterministic, which would be the other meaning of determinism? No. We still have the option to blame our inability to completely prepare all aspects of a quantum system so that no two experiments are truly identical.
    Additionally, it seems questionable if a concept like universal indeterminism even makes sense. To test the universe for determinism we would have access to two universes in exactly the same state and observe different evolutions. Otherwise we can always blame lack of understanding of the dynamical laws of the universe if we don't find our expected evolution. Of course it is impossible in principle to do an experiment like that. So what good is it to talk about universal indeterminism?
    Still, there could be a lot to gain from knowing that the universe is deterministic. If for example the quantum measurement problem could be solved by making the assumption that the universe evolves deterministically and that only local observers see an indeterministic system evolution, then we would have the right to assume that a deterministic universe without contradicting any observations.
    The philosophical consequences would probably be immense, mostly because many philosophers already got used to the idea of fundamental indeterminism and connect that idea to other concepts like "free will".

    I hope this makes sense to you.


  6. Nov 12, 2011 #5

    Ken G

    User Avatar
    Gold Member

    I think the problem is that we often think about determinism in a backwards way. Determinism only makes sense within the context of a physical theory, there is just no other kind of determinism that means anything in physics. So determinism is never an attribute of reality, because reality has no way to be deterministic independently of some physical theory that is deterministic. Thus we see that it is always the physical theory, never the reality, that is deterministic. All we can say is whether or not a deterministic physical theory accomplishes our objectives, and that is all we should ever even try to say.

    Put in that light, we see that quantum mechanics is fundamentally not a deterministic theory, it makes no difference if reality is deterministic because that phrase doesn't even have any meaning (only theories can be demonstrably deterministic or indetermistic). Yes, the Schroedinger equation expresses a deterministic evolution of the wave function, but physics is more than evolution of a wave function-- determinism in physics means you can predict an outcome to a precision that depends only on the precision of the input, such that as the input precision increases arbitrarily, so does the output precision. That's it, that's all deterministic ever meant in any physical theory, and that is the only meaning that the word ever held in physics. Quantum mechanics does not have that property, end of story-- it makes no difference what interpretation is applied to it.

    Now, this does not contradict DrChinese's point that the deBroglie-Bohm interpretation allows quantum mechanics to work in the context of some deeper deterministic theory. The problem is, that deeper deterministic theory does not make verifiable predictions (although some are working on improving that situation, but I'm not convinced they've had much success). So it's kind of an "angels on a pin" theory-- the only theory that actually makes predictions we have verified is quantum mechanics, and it is not deterministic. It just isn't.

    Now, the fact that quantum mechanics is itself not deterministic makes some people ask, does the success of quantum mechanics imply that the universe is not deterministic? No, for two reasons:
    1) the success of a theory never tells you anything about the universe, beyond that the theory is successful for whatever it is successful for, and
    2) as I said above, the universe is never deterministic, theories are deterministic, or they are not deterministic. Any effort to answer the question "is the universe deterministic" can only look like one thing-- finding a deterministic theory and seeing if it works. But then it is still the theory that is deterministic, never the universe.
  7. Nov 12, 2011 #6
    Is causality the same thing as determinism? What's the difference in meaning between determinism and causality? Is causal indeterminism possible? I'm asking because there is some debate in this area whether, in fact, the 2 terms are the same or even if they're in conflict. Just to hi-lite some of these difficulties, consider Carl Hoefer's entry in Stanford Encyclopedia of Philosophy on "causal determinism" where he opts from refraining discussing the "causal" part:


    Even more forcefully consider his paper here:

    Causality and Determinism: Tension, or Outright Conflict?


    Furthermore with respect to QM and determinism he writes (In Stanford entry):

    Last edited: Nov 12, 2011
  8. Nov 12, 2011 #7
    The answer depends on which branch of physics you want to focus on.

    In the blockworld of SR - i'd say - No. Events do not appear to happen because of causal relationships but because somehow time(esp. time that flows according to the known arrow of time) enters the picture and the known objective reality is perceived as taking place.

    In the quantum world - it seems to be bit of both, but never just one or the other. Some events are manifestly deterministic, while others appear manifetly indeterministic, but in the limit qm becomes completely deteministic.

    In classical mechanics - reality is completely deterministic.

    We have to know what reality is to know if it's deteministic or not and this is problematic. I'd say that my observations are completely deterministic, based on the fact that it's probably impossible to fathom of a non-deterministic reality.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook