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This does make me wonder: how do we know that random quantum processes are truly random? This gets philosophical very quickly. But for our purposes: independen

This does make me wonder: how do we know that random quantum processes are truly random? This gets philosophical very quickly. But for our purposes: independent, uniform, etc. But how do we know there isn't another (earlier) way to observe the result in some fashion.

variadix 3y ago
I think Bell’s inequality implies the state can’t be deterministic but hidden. I’m not sure how you could evaluate the “quality” of the randomness, or if there are existing results analyzing this.
nyssos 3y ago
We don't. What we know is that we can't have all of 1. locality (no backwards causation / FTL information transfer) 2. determinism 3. counterfactual definiteness (unperformed measurements still have single definite results: we can talk meaningfully about what we would have seen had we done a different experiment) 4. QM gives correct predictions. Objective-collapse interpretations (like the version of Copenhagen described in introductory textbooks) reject 2: collapse is fundamentally indeterministic. Everett (the badly misnamed "many-worlds" interpretation) rejects 3: "measurement" is just what unitary evolution looks like from the inside, and from that perspective always has multiple outcomes. Superdeterminism rejects 4: QM is wrong, but the initial conditions of the universe were for some reason set up in such a way as to prevent us from carrying out any experiments that would demonstrate that.
- cb321 3y ago
  Bricmont does a good job of discussing entropy, arrow-of-time, and irreversibility, information and probability in a purely classical statistical mechanics setting in https://arxiv.org/abs/chao-dyn/9603009 https://arxiv.org/abs/chao-dyn/9603009 . Bricmont has continued to write/expound upon these ideas ever since and more recently "come out" as a non-local hidden variables proponent of the de Broglie/Bohm variety which rejects your (1. locality) for which you left out an example. :-) Sometimes people refer to giving up "locality" in this context with the mouthful "a preferred fundamental foliation of spacetime". { "fundamental" here does the work to block the Cosmic Microwave Background / "average rest frame" which people have no trouble treating as a preferred inertial reference frame that is "merely contingent" not "fundamental", although Ernst Mach might have begged to differ, but that is further afield. :-) }
  - GoblinSlayer 3y ago
    They have to choose preferred reference frame to dodge special theory of relativity, but this produces antirealism: the preferred reference frame have stronger reality than other frames.
- throwawaymaths 3y ago
  Correct me if I'm wrong, but I don't think we even know what "collapse" is. Maybe a better way to say it is that arguments about what is collapse are at the root of distinguishing interpretations
  - nyssos 3y ago
    Collapse is when you go from a generic quantum state to an eigenstate of a given measurement operator. The disagreement is about whether this is an objective feature of reality, an artifact of semiclassical approximation, or not a real physical process at all.
    - throwawaymaths 3y ago
      Yeah of course we know what the math is and it corresponds to observation, I just mean we don't know deeply what it is beyond what we've operationally defined it as.
- karmakaze 3y ago
  My bet is on 1. backwards causation. Light takes the shortest path, by (virtually) taking all of them. Similarly all paths to the future are taken and the present must agree with boundary conditions in the future.
  - nyssos 3y ago
    > the present must agree with boundary conditions in the future. This is true but it's not really what's meant by backwards causation here. The specific thing you need is for what happens from times (-ε, ε) to depend on what will happen at times (1-ε, 1+ε) even after conditioning on the intervening times (ε, 1-ε). Action principles are still local: maximizing the integral of the Lagrangian L(x(t)) over time (by, for instance, taking the shortest path) is equivalent to satisfying dL/dx = d/dt dL/d(dx/dt). > by (virtually) taking all of them. This is a superficially appealing analogy but very misleading. Even if you take Feynman diagrams literally, which you shouldn't, the "path" integral runs over field configurations. The classical notion of a particle following some path simply breaks down here.
lmm 3y ago
> how do we know that random quantum processes are truly random? If it looks like randomness and quacks like randomness, it's randomness, right? We've experimentally verified these things to dozens of decimal places. Asking whether it's "truly" randomness is meaningless without a measurable way to distinguish what's "truly" random or not. > how do we know there isn't another (earlier) way to observe the result in some fashion. Bell's inequalities.

variadix 3y ago

I think Bell’s inequality implies the state can’t be deterministic but hidden. I’m not sure how you could evaluate the “quality” of the randomness, or if there are existing results analyzing this.

nyssos 3y ago

We don't. What we know is that we can't have all of 1. locality (no backwards causation / FTL information transfer) 2. determinism 3. counterfactual definiteness (unperformed measurements still have single definite results: we can talk meaningfully about what we would have seen had we done a different experiment) 4. QM gives correct predictions. Objective-collapse interpretations (like the version of Copenhagen described in introductory textbooks) reject 2: collapse is fundamentally indeterministic. Everett (the badly misnamed "many-worlds" interpretation) rejects 3: "measurement" is just what unitary evolution looks like from the inside, and from that perspective always has multiple outcomes. Superdeterminism rejects 4: QM is wrong, but the initial conditions of the universe were for some reason set up in such a way as to prevent us from carrying out any experiments that would demonstrate that.

cb321 3y ago
Bricmont does a good job of discussing entropy, arrow-of-time, and irreversibility, information and probability in a purely classical statistical mechanics setting in https://arxiv.org/abs/chao-dyn/9603009 https://arxiv.org/abs/chao-dyn/9603009 . Bricmont has continued to write/expound upon these ideas ever since and more recently "come out" as a non-local hidden variables proponent of the de Broglie/Bohm variety which rejects your (1. locality) for which you left out an example. :-) Sometimes people refer to giving up "locality" in this context with the mouthful "a preferred fundamental foliation of spacetime". { "fundamental" here does the work to block the Cosmic Microwave Background / "average rest frame" which people have no trouble treating as a preferred inertial reference frame that is "merely contingent" not "fundamental", although Ernst Mach might have begged to differ, but that is further afield. :-) }
- GoblinSlayer 3y ago
  They have to choose preferred reference frame to dodge special theory of relativity, but this produces antirealism: the preferred reference frame have stronger reality than other frames.
throwawaymaths 3y ago
Correct me if I'm wrong, but I don't think we even know what "collapse" is. Maybe a better way to say it is that arguments about what is collapse are at the root of distinguishing interpretations
- nyssos 3y ago
  Collapse is when you go from a generic quantum state to an eigenstate of a given measurement operator. The disagreement is about whether this is an objective feature of reality, an artifact of semiclassical approximation, or not a real physical process at all.
  - throwawaymaths 3y ago
    Yeah of course we know what the math is and it corresponds to observation, I just mean we don't know deeply what it is beyond what we've operationally defined it as.
karmakaze 3y ago
My bet is on 1. backwards causation. Light takes the shortest path, by (virtually) taking all of them. Similarly all paths to the future are taken and the present must agree with boundary conditions in the future.
- nyssos 3y ago
  > the present must agree with boundary conditions in the future. This is true but it's not really what's meant by backwards causation here. The specific thing you need is for what happens from times (-ε, ε) to depend on what will happen at times (1-ε, 1+ε) even after conditioning on the intervening times (ε, 1-ε). Action principles are still local: maximizing the integral of the Lagrangian L(x(t)) over time (by, for instance, taking the shortest path) is equivalent to satisfying dL/dx = d/dt dL/d(dx/dt). > by (virtually) taking all of them. This is a superficially appealing analogy but very misleading. Even if you take Feynman diagrams literally, which you shouldn't, the "path" integral runs over field configurations. The classical notion of a particle following some path simply breaks down here.

lmm 3y ago

> how do we know that random quantum processes are truly random? If it looks like randomness and quacks like randomness, it's randomness, right? We've experimentally verified these things to dozens of decimal places. Asking whether it's "truly" randomness is meaningless without a measurable way to distinguish what's "truly" random or not. > how do we know there isn't another (earlier) way to observe the result in some fashion. Bell's inequalities.