# does Quantum Mechanics pretty much undermine Determinism **URL:** \ **Category:** In My Humble Opinion\ **Created:** [October 25, 2014, 3:37pm UTC](https://boards.straightdope.com/t/does-quantum-mechanics-pretty-much-undermine-determinism/702502 "2014-10-25T15:37:10Z")\ **Posts on this page:** 11\ **Page:** 1
**Author:** ![thanatic](https://avatars.discourse-cdn.com/v4/letter/t/97f17d/32.png) [@thanatic](https://boards.straightdope.com/u/thanatic)\ **Post date:** [October 25, 2014, 3:37pm UTC](https://boards.straightdope.com/t/does-quantum-mechanics-pretty-much-undermine-determinism/702502/1 "2014-10-25T15:37:10Z")
I often hear Quantum Mechanics/Physics is in conflict with Determinism because everything in QM is Probabilistic? is this true? ---
**Author:** ![davidm](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/davidm/32/225_2.png) [@davidm](https://boards.straightdope.com/u/davidm)\ **Post date:** [October 25, 2014, 4:06pm UTC](https://boards.straightdope.com/t/does-quantum-mechanics-pretty-much-undermine-determinism/702502/2 "2014-10-25T16:06:24Z")
I would say yes, unless [Everett’s many-worlds interpretation is correct.](http://en.wikipedia.org/wiki/Many-worlds_interpretation) ---
**Author:** ![Itself](https://avatars.discourse-cdn.com/v4/letter/i/d07c76/32.png) [@Itself](https://boards.straightdope.com/u/Itself)\ **Post date:** [October 25, 2014, 5:16pm UTC](https://boards.straightdope.com/t/does-quantum-mechanics-pretty-much-undermine-determinism/702502/3 "2014-10-25T17:16:06Z")
Yes. Classical quantum mechanics predicts and experiments have confirmed (for example, see the Stern-Gerlach experiment\*) that the basic Copenhagen interpretation seems to work: A quantum state collapses instantaneously when required to spit out an eigenvalue of an operator, and the probability of collapsing into a particular eigenstate are given by its coefficient in the state’s original wave function. There are some details I’m glossing over (for example, what happens when there are more than finitely many eigenvalues), and you can put in some philosophical and rather unfalsifiable objects to nondeterminism if you really want to; but yeah, that’s pretty much how the universe works. I should also mention that that non-determinism does not imply that anything can happen or that the quantum world is totally random. We can write down and verify through experiment what probability distributions we expect; quantum does make specific, deterministic predictions in certain cases; and certain result (e.g., conservation of energy) appear to be absolute. If you’re looking for evidence against the some of the philosophical ideas about or implications of determinism, you’re probably going to be disappointed. - The point of that experiment is actually something different (namely, whether the eigenvalue corresponding to one operator is somehow internally ‘preserved’ when you force the system into the eigenvalue of a different, non-commuting operator); but still, it’s a pretty clever setup. ---
**Author:** ![Learjeff](https://avatars.discourse-cdn.com/v4/letter/l/94ad74/32.png) [@Learjeff](https://boards.straightdope.com/u/Learjeff)\ **Post date:** [October 25, 2014, 5:31pm UTC](https://boards.straightdope.com/t/does-quantum-mechanics-pretty-much-undermine-determinism/702502/4 "2014-10-25T17:31:41Z")
Philosophically, determinism is replaced by nondeterminism, which isn’t really all that different. In computer science, there’s a big difference, especially in cryptography. As it turns out, achieving effective nondeterminism is harder than you might guess. ---
**Author:** ![Stranger\_On\_A\_Train](https://avatars.discourse-cdn.com/v4/letter/s/13edae/32.png) [@Stranger\_On\_A\_Train](https://boards.straightdope.com/u/Stranger_On_A_Train)\ **Post date:** [October 25, 2014, 6:30pm UTC](https://boards.straightdope.com/t/does-quantum-mechanics-pretty-much-undermine-determinism/702502/5 "2014-10-25T18:30:43Z")
No. Aside from the relative state formulation (a.k.a. “Many Worlds Interpretation”) by Hugh Everett III (who went on to work in nuclear deterrence theory after he was pretty much blown off the the science community) there are entire classes of theories which allow for a partially or purely deterministic mechanism. However, all involved either violations of causality (things that happen in the future will affect the past in order to get to that result) or locality (there are non-local hidden variables or connections between regions of space that should not be able to influence one another), and because we can’t observe or directly test this the result certainly looks like non-determinism from any practical sense. I personally think the Bohmian interpretation (a.k.a. De Broglie–Bohm theory) is the most viable, or at least, esthetically pleasing candidate, and is consistent with many of our observations such as the double slit experiment, quantum entanglement, and other phenomena without having to result to some kind of wackadoo mechanisms involving nonreal universes and so forth, but frankly there is no way at this point to distinguish between any of the plausible interpretations of QM. Stranger ---
**Author:** ![njtt](https://avatars.discourse-cdn.com/v4/letter/n/ecd19e/32.png) [@njtt](https://boards.straightdope.com/u/njtt)\ **Post date:** [October 25, 2014, 6:50pm UTC](https://boards.straightdope.com/t/does-quantum-mechanics-pretty-much-undermine-determinism/702502/6 "2014-10-25T18:50:42Z")
Maybe it does, depending on your interpretation of quantum mechanics, but, even so, not in a way that, to any significant degree, supports the notion of “[libertarian](http://en.wikipedia.org/wiki/Libertarianism_%28metaphysics%29)”\* free will. ¬¬¬¬¬¬¬¬¬¬¬¬¬¬¬¬¬¬¬¬¬¬¬¬¬¬ \*This is “libertarianism” in the metaphysical sense, not in the quite different (and, here, irrelevant) political and economic one. ---
**Author:** ![Stranger\_On\_A\_Train](https://avatars.discourse-cdn.com/v4/letter/s/13edae/32.png) [@Stranger\_On\_A\_Train](https://boards.straightdope.com/u/Stranger_On_A_Train)\ **Post date:** [October 25, 2014, 6:52pm UTC](https://boards.straightdope.com/t/does-quantum-mechanics-pretty-much-undermine-determinism/702502/7 "2014-10-25T18:52:51Z")
“Any sufficiently complex decision-making system is indistinguishable from free will.” Stranger ---
**Author:** ![RaftPeople](https://avatars.discourse-cdn.com/v4/letter/r/6f9a4e/32.png) [@RaftPeople](https://boards.straightdope.com/u/RaftPeople)\ **Post date:** [October 25, 2014, 7:09pm UTC](https://boards.straightdope.com/t/does-quantum-mechanics-pretty-much-undermine-determinism/702502/8 "2014-10-25T19:09:30Z")
> [@Stranger\_On\_A\_Train](#): > > No. Aside from the relative state formulation (a.k.a. “Many Worlds Interpretation”) by Hugh Everett III (who went on to work in nuclear deterrence theory after he was pretty much blown off the the science community) there are entire classes of theories which allow for a partially or purely deterministic mechanism. However, all involved either violations of causality (things that happen in the future will affect the past in order to get to that result) or locality (there are non-local hidden variables or connections between regions of space that should not be able to influence one another), and because we can’t observe or directly test this the result certainly looks like non-determinism from any practical sense. I personally think the Bohmian interpretation (a.k.a. De Broglie–Bohm theory) is the most viable, or at least, esthetically pleasing candidate, and is consistent with many of our observations such as the double slit experiment, quantum entanglement, and other phenomena without having to result to some kind of wackadoo mechanisms involving nonreal universes and so forth, but frankly there is no way at this point to distinguish between any of the plausible interpretations of QM. > > Stranger Any thoughts on this one? [Many Interacting Worlds (10/23/14)](http://journals.aps.org/prx/abstract/10.1103/PhysRevX.4.041013) ---
**Author:** ![Hari\_Seldon](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/hari_seldon/32/5173_2.png) [@Hari\_Seldon](https://boards.straightdope.com/u/Hari_Seldon)\ **Post date:** [October 25, 2014, 11:24pm UTC](https://boards.straightdope.com/t/does-quantum-mechanics-pretty-much-undermine-determinism/702502/9 "2014-10-25T23:24:30Z")
Actually, strong determinism in the sense that if you knew the position and momemtum of every particle in the universe you could predict the future, is **logically** impossible. Here’s why. Suppose I could build such a computer. Then I could build two of them, call them I and II. The program I to look ahead and print out a 0 at midnight tonight if it predicts that II will print a 1 at midnight tonight; otherwise print a 1. Program II to print out a 1 at midnight tonight if it predicts that I will print a 1 at midnight tonight; otherwise print a 0. This does not, of course, refute determinism. What it shows is that no universe can contain a computer (actually two of them) that can model the whole universe including all such computers. ---
**Author:** ![DrCube](https://avatars.discourse-cdn.com/v4/letter/d/a3d4f5/32.png) [@DrCube](https://boards.straightdope.com/u/DrCube)\ **Post date:** [October 26, 2014, 1:28am UTC](https://boards.straightdope.com/t/does-quantum-mechanics-pretty-much-undermine-determinism/702502/10 "2014-10-26T01:28:12Z")
“Determinism” in the philosophical sense is no way in conflict with quantum physics, and in fact is supported by it. If particles can act randomly, but nevertheless according to physical law, how does that affect the philosophical foundation of your decision making? ---
**Author:** ![njtt](https://avatars.discourse-cdn.com/v4/letter/n/ecd19e/32.png) [@njtt](https://boards.straightdope.com/u/njtt)\ **Post date:** [October 26, 2014, 1:39am UTC](https://boards.straightdope.com/t/does-quantum-mechanics-pretty-much-undermine-determinism/702502/11 "2014-10-26T01:39:32Z")
> [@Stranger\_On\_A\_Train](#): > > “Any sufficiently complex decision-making system is indistinguishable from free will.” > > Stranger Maybe in practice, in the sense that you can’t, in practice, predict its behavior, but for most issues that free will is relevant to (such as ethical ones) it is “in principle” that matters, not “in practice”. Anyway, I don’t think what you say (or quote) is even true. Most highly complex physical systems do not display any behavior that looks remotely like free will. As for the human cognitive system, even considered at the cognitive, or “computational” level it is certainly very complex, but there are very good reasons for thinking, even in the absence of anything like a full understanding of it, that it must _lack_ free will (in the libertarian sense). The free will/determinism problem has been around for millennia, and the huge scientific advances made over that period have actually made very little difference to the relative plausibilities of either either side of the argument. Ancient Greeks actually had about equally as good reasons to accept free will, or determinism (in the senses that those doctrines are opposed to one another) as we do now. The only real advance has been the development of compatibilist arguments, which make a good case that determinism is not inconsistent with any concept of free will that is meaningful and not self-contradictory (or, to put it another way, that the libertarian concept of free will is just incoherent), but that development owes nothing to the sciences, not to any sense of the _complexity_ of our decision making systems.