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> "The point is that if physical law is deterministic..." Under current QM, it doesn't appear that things like alpha decay happen predictably or at one time vs

> "The point is that if physical law is deterministic..." Under current QM, it doesn't appear that things like alpha decay happen predictably or at one time vs another for any reason. And that isn't simply a problem of not knowing the state of the system; hidden variable theories have not met with empirical success. So, imagine we could have two universes, both with our laws of physics, and identical up to time=0. Current QM allows that at t=1 in one universe radioactive decay occurs which then produces chain of events A, while in the other universe at t=1 there is no decay and thus chain of events B. Why then call these universes and their physical laws deterministic? They seem rather not.

jessriedel 15y ago
I'm a physics grad student doing work in quantum decoherence, so I'm aware of all this. When discussing free will, "determinism" really is shorthand for the idea that the cause of all events can be traced back to previous events in known physics, including fundamental probabilistic events. The key claim is that there is no "soul" (or any other causal source which cannot in-principle be explained by known physics) producing choices. (Yes, you bundle up all the probabilistic quantum events which occur in your brain and identify that as "free will", but I don't think most people would find that compelling. Not the least of which because then it seems like a plutonium atom has just as much right to the term "free will" as you do.)
- nooneelse 15y ago
  Well, I'm glad then that we get to skip the bit were I have to spell out the facets of the argument I made. (Over on reddit bringing this up usually means me having to gently prod the other person to read up on hidden variable theories so they can tell me if that is what they meant by phrases like "something determines it we just can't see what".) I'm actually a compatibilist about "free will", so I've no need for a ghost in the machine or to be flipping quantum coins in my head. The point of my comment was simply to point out that "our universe is deterministic", full stop, isn't a warranted premise by current evidence. If you only meant "adequately deterministic" by "deterministic", fine and dandy. However, I don't know anything that actually stops the two causal chains A and B in my thought experiment from being macroscopically very different. So skipping the "adequately", I feel, gives those not familiar with the details a misleading impression that we can unproblematically think of ourselves as just in the old clockwork universe. (Thus the kinds of phrasing I see a lot on reddit from the loosely-informed on such matters.) (Edits: silly grammar and spelling stuff.)
  - jessriedel 15y ago
    OK, I support being precise with our terminology. What I was trying to emphasize is that, with regards to discussion of "free will", the important thing about "adequate determinism" isn't predictability. It's that we can trace back all causes to known (possibly intrinsically probabilistic) physical "stuff", with no room for a soul.
    - jaylevitt 15y ago
      Something I've been wondering, and I'm way out of my depth here - would determinism be something that could eventually find an explanation in information theory? My layman's argument against determinism was something like: OK, even if my biological self is making my decisions for me, how does it get the inputs that drive those decisions? Imagine a simple exercise study, conducted without face-to-face contact. Half of the participants are told to exercise, and half are told not to. At the end, the half that is told to exercise will have exercised more often than the half that did not. Therefore, if the world is deterministic, what's "sending" that determinism from the study to the participant? My conclusion was: I dunno, information is flowing? Though I guess that's where quantum mechanics comes in, and I guess there's probably also an information-theoretic aspect to that.
      - jessriedel 15y ago
        Not sure I know what you're asking. If we ignore quantum mechanics, then all of our empirical evidence strongly suggests that the universe is strictly deterministic. (Although this conflicts with our intuition about our own consciousness.) What that means is that if we were to have the position and velocity of every particle at a given moment in time we could (in principle) calculate the position and velocity for any future moment. The important thing is that the known and understood physical "stuff" is just particles, and the only thing that can or need be known about them is their position and velocity. There is no thing-which-is-NOT-made-of-particles which we might call a soul or whatever. In particular, you could in-principle predict what experimental subject Bob would chose based on the configuration of the particles in the universe before he was born. So, is your question "How is that pre-Bob configuration of particles causing Bob's choice?" If so, then you just look to the fundamental laws of classical physics: particles obey certain rules which govern how they move around. Given a certain pre-Bob configuration, we would compute that the particles would all follow certain trajectories which eventually would lead to a configuration where many particles were assembled into Bob himself, and these particles moved in such a way (according to known physics) that Bob (who is composed of the particles) made a certain choice. Or is your question about quantum mechanics?

jessriedel 15y ago

I'm a physics grad student doing work in quantum decoherence, so I'm aware of all this. When discussing free will, "determinism" really is shorthand for the idea that the cause of all events can be traced back to previous events in known physics, including fundamental probabilistic events. The key claim is that there is no "soul" (or any other causal source which cannot in-principle be explained by known physics) producing choices. (Yes, you bundle up all the probabilistic quantum events which occur in your brain and identify that as "free will", but I don't think most people would find that compelling. Not the least of which because then it seems like a plutonium atom has just as much right to the term "free will" as you do.)

nooneelse 15y ago
Well, I'm glad then that we get to skip the bit were I have to spell out the facets of the argument I made. (Over on reddit bringing this up usually means me having to gently prod the other person to read up on hidden variable theories so they can tell me if that is what they meant by phrases like "something determines it we just can't see what".) I'm actually a compatibilist about "free will", so I've no need for a ghost in the machine or to be flipping quantum coins in my head. The point of my comment was simply to point out that "our universe is deterministic", full stop, isn't a warranted premise by current evidence. If you only meant "adequately deterministic" by "deterministic", fine and dandy. However, I don't know anything that actually stops the two causal chains A and B in my thought experiment from being macroscopically very different. So skipping the "adequately", I feel, gives those not familiar with the details a misleading impression that we can unproblematically think of ourselves as just in the old clockwork universe. (Thus the kinds of phrasing I see a lot on reddit from the loosely-informed on such matters.) (Edits: silly grammar and spelling stuff.)
- jessriedel 15y ago
  OK, I support being precise with our terminology. What I was trying to emphasize is that, with regards to discussion of "free will", the important thing about "adequate determinism" isn't predictability. It's that we can trace back all causes to known (possibly intrinsically probabilistic) physical "stuff", with no room for a soul.
  - jaylevitt 15y ago
    Something I've been wondering, and I'm way out of my depth here - would determinism be something that could eventually find an explanation in information theory? My layman's argument against determinism was something like: OK, even if my biological self is making my decisions for me, how does it get the inputs that drive those decisions? Imagine a simple exercise study, conducted without face-to-face contact. Half of the participants are told to exercise, and half are told not to. At the end, the half that is told to exercise will have exercised more often than the half that did not. Therefore, if the world is deterministic, what's "sending" that determinism from the study to the participant? My conclusion was: I dunno, information is flowing? Though I guess that's where quantum mechanics comes in, and I guess there's probably also an information-theoretic aspect to that.
    - jessriedel 15y ago
      Not sure I know what you're asking. If we ignore quantum mechanics, then all of our empirical evidence strongly suggests that the universe is strictly deterministic. (Although this conflicts with our intuition about our own consciousness.) What that means is that if we were to have the position and velocity of every particle at a given moment in time we could (in principle) calculate the position and velocity for any future moment. The important thing is that the known and understood physical "stuff" is just particles, and the only thing that can or need be known about them is their position and velocity. There is no thing-which-is-NOT-made-of-particles which we might call a soul or whatever. In particular, you could in-principle predict what experimental subject Bob would chose based on the configuration of the particles in the universe before he was born. So, is your question "How is that pre-Bob configuration of particles causing Bob's choice?" If so, then you just look to the fundamental laws of classical physics: particles obey certain rules which govern how they move around. Given a certain pre-Bob configuration, we would compute that the particles would all follow certain trajectories which eventually would lead to a configuration where many particles were assembled into Bob himself, and these particles moved in such a way (according to known physics) that Bob (who is composed of the particles) made a certain choice. Or is your question about quantum mechanics?