How Weird is Quantum Mechanics?

If quantum mechanics hasn’t profoundly shocked you, you haven’t understood it yet” (Niels Bohr)

Last week on the way to a baseball game, Nates, David and I began arguing about science and philosophy.  This argument spilled over from brunch to a cab ride (with a surprisingly well informed cab driver, who recognized that we were (a) talking about particle physics, (b) also discussing philosophy, and (c) would not meet many girls at the baseball game).  Two of us being married, that was fine.

We didn’t come to any great conclusions – I think it’s fair to say there was a good amount of hand-waving, as well as a fair quantity of table-thumping.  Rather than make an argument here, I’ll just pose a question, and then some possible answers.

Question: Does quantum mechanics (meaning the two-slit experiment, Schrodinger’s cat, the wave-particle duality, the Heisenberg uncertainty principle, etc.) pose a problem for traditional theories of knowledge, metaphysics, ontology and/or philosophy of science and if so, what sort of a problem does it pose?

Four possible types of answers.  They proceed up from less to more problematic, building on one another.

(1) It poses no problem.   Quantum physics explains everything in a manner consistent enough with earlier scientific conclusions.

(2) It poses an epistemological problem –  quantum physics means we do not know where, at any given time, a photon wave-particle is.  Perhaps one day, we will be able to, but now we don’t.

(3) It poses an in-principle epistemological problem – quantum physics means we not only do not know where, at any given time, a photon wave-particle is, but also that we can never know.

(4) It poses an ontological/metaphysical problem – quantum physics means not only that we do not, and cannot know where a given photon wave-particle is, but that in fact there is no truth of the matter about its location, at least no truth in any traditional sense of the word.

I think I’ve neutrally described the alternatives.  I open it up to our readers to make arguments for any of these alternatives, or any other alternatives I’ve overlooked.  You can, also, argue with the terms in which I’ve cast the question (I expect this, given how little I know about the actual science).  I have some vague notion that the wave-particle duality and the Heisenberg uncertainty principle were founding problems for both the Vienna Circle and those that criticized its work.  I’m also sure a lot has already been written on this question, but I haven’t read it.

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14 Responses to How Weird is Quantum Mechanics?

  1. Nates says:

    This is helpful, Josh. I’m assuming no one thinks (1), right? As I understand it–which is probably not very well, Einstein hoped for (2), but feared (3). And (3) seems to be what our best current theory holds.

    As far as our ballpark debate, I guess the crux of the issue is whether (3) implies (4). From my perspective, it just looks like a peculiar form of human vanity to insist that anything we can’t know must not exist.

  2. Josh says:

    This is a rudimentary stab at how something like 4 could be argued for:

    The conclusions of quantum physics hold that the existential status of the smallest things are inherently unstable. All of the “we don’t know where it is but we know it’s there” reasoning presupposes something like a particle-based ontology. Waves are the sorts of things that aren’t in any particular place; particles are. If something isn’t a particle, is it somewhere?

  3. David says:

    I agree with Nathan–this is a helpful summary of the conversation and the available positions.

    You ask: “If something isn’t a particle, is it somewhere?” It’s in a particular *region* of space, right? And hence ‘somewhere.’

    Anyway, isn’t this wave-particle duality stuff tangential to our original question, which was whether X could have a property that we cannot in principle know?

  4. Nates says:

    David, I think the wave-particle distinction is somewhat relevant here. This is because the waves they’re talking about are probability waves–that is, the curve measures the probability of some quantum state being realized. So to say that the fundamental elements of quantum physics have these wave properties just is to say that probability is built into the scientific ontology.

    All that said, I still don’t think this point supports the move Josh wants to make. Wave-particle duality isn’t further evidence for the ontological status of the uncertainty principle–it’s just another way of making the same point.

    Josh, if the claim David and I were defending was that we have good reason to believe that the fundamental nature of reality is exact and determinate, then you’d be right to object. That would be dogmatic, perhaps for the reason you state: naively assuming a particle-based reality. But we’re not saying that. We’re simply saying that the inference (from not being able to know quantum states determinately to their not being determinate) lacks justification. In fact, I’m inclined to say that the inference is just obviously and self-evidently flawed. [Pounds table!]

  5. Josh says:

    I was honestly more asking the question than arguing for a position. I suppose my questions might have sounded rhetorical, but I was really just curious.

    While I’m not defending a position per se, I am convinced there’s a bit more complexity here than we’re giving it credit for. That is somewhat just based on the fact that there is a large literature around this question, which seems odd if it’s just a simple category error (not that that never happens – our whole first-year seminar felt like it presupposed a similar category error). I’ve started reading a book called Quantum Theory and the Flight from Realism by someone named Christopher Norris (2000 – its full text is available online from the U of C library).

    I have no idea if this is a book central to the debate or not – what little I know about relevant “important” contemporary philosophers is both (a) 10 years out of date, and (b) confined to questions of an ethical/political nature.

    It sounds like it will defend some variant of (2): “…one such alternative, the ‘hidden-variables’ theory … is a version of the pilot-wave hypothesis … according to which the particle is ‘guided’ by a wave whose probability amplitudes are exactly in accordance with the wellsupported QM predictions and measured results. Where it challenges the orthodox theory is in … that the particle does have precise simultaneous values of position and momentum, and furthermore that these pertain to its objective state at any given time, whatever the restrictions imposed upon our knowledge by the limits of achievable precision in measurement” (2-3, my emphasis).

    From here, he says he will “sort out the various deep-laid philosophic confusions—especially that between ontological and epistemological issues—which characterize Niels Bohr’s writings on the topic, and which can still be seen in a great many present-day treatments of QM theory” (3, my emphasis).

    But it also says it intends to review the recent history of the problem and consider “the premises of orthodox quantum theory, as formulated most influentially by Bohr and Heisenberg, and its impact on various later philosophical developments. These include various proposals advanced by W V Quine, Thomas Kuhn, Michael Dummett, Bas van Fraassen, and Hilary Putnam.”

    This suggests to me that something in quantum mechanics bears on the arguments (especially of Quine, Kuhn, and Putnam) all of whom stand in an ambiguous relationship to Realism (with or without the capital R, with or without a human face).

    Also – (3) may be more controversial than you’re giving it credit for. I’m especially interested in the idea that an “in-principle epistemic constraint” poses at least somewhat of a problem for realism, of whatever stripe. Perhaps to use in-principle epistemic constraints to argue in favor of naive anti-realism is bad (and doubtlessly that’s one of the principal target of Norris’ book).

    But if it’s not just a “because our instruments aren’t good enough” contingent epistemic constraint, but a real in-principle one (if such an idea is coherent) this changes the sort of “realism” that can be defended. It would also seem to pose a problem unparalleled in the history of science and our thought about it, unless there are other things that science itself has said are in-principle unknowable, which I don’t think there are.

  6. Nates says:

    Sounds like that’ll be a helpful book for our discussion. I look forward to hearing your take on it!

    I certainly agree that in-principle epistemic constraints are philosophically interesting–more so than contingent ones. But it seems to me that we do have a useful historical precedent for dealing with them. After all they basically play the same role as Kantian-style antinomy arguments. There he attempts to show that certain things (e.g., a first cause) are unknowable in principle–because attempts to do so founder on incoherency. But notice that Kant doesn’t make the mistake of saying that there is no first cause. He simply says that we can’t make a knowledge claim here. In other words, agnosticism is the appropriate response. And I haven’t yet heard anything in the quantum physics example that would lead us to think a different response is appropriate.

  7. Josh says:

    The Kantian point is a good analogy, though also a bit different. But – while agnosticism isn’t atheism (here, anti-realism), it also isn’t theism (i.e., realism). It seems like that question bears somehow on metaphysical/ontological questions – even if it’s an insistence upon neutrality, that’s definitely different from saying it’s of no ontological or metaphysical significance at all.

    Also, Kant was deriving the in-principle epistemic constraint philosophically (I guess, based on the abstract notion “of all possible experience” or something ; physicists are doing so scientifically (presumably based on observation [i.e., specific experience] and inference based on that experience.

    I don’t mean to beg the question – is there a difference between a philosophically demonstrated in-principle epistemic constraint and a scientifically derived one?

    One other question – it still seems strange to say that specific physical observation could give rise to *any* sort of conclusion about the realism/anti-realism debate, doesn’t it?

  8. Nates says:

    Yeah, I agree that the Kantian example is a little different, since it’s philosophically motivated rather than scientific, but I think it’s still somewhat helpful in considering how to respond to these constraints.

    By the way, we’ll be playing poker with a (former) philosopher of physics in the near future, so we should ask him some of these questions!

  9. David says:

    Ok, I’ll try to avoid using terms from quantum physics, as I know shockingly little physics, quantum or otherwise.

    Josh, you write: ‘I’m especially interested in the idea that an “in-principle epistemic constraint” poses at least somewhat of a problem for realism, of whatever stripe.’

    Do you mean that there is something odd or fishy about countenancing the reality of things we admit that we cannot in principle (and not merely contingently) know? I’m not so sure about that. For example:

    Suppose Nagel was right that we cannot know what it is like to be a bat. This is presumably an in principle epistemic constraint–Nagel might himself make the point, I don’t recall, that this isn’t a matter of not having sufficiently sensitive instruments, but rather a logical point about understanding subjective experience from an objective point of view.

    In any event, I certainly don’t want to infer that there is nothing it is like to be a bat from the fact that I cannot, in principle, know what is like to be a bat. Bat subjectivity is a part of reality, but it is a part of reality that science cannot in principle provide access to.

    But if in that case, why not others?

  10. Josh says:

    I am not implying that there is just such a thing as “being a bat” because I cannot experience it. I am more wondering about the nature of scientific inquiry that discovers internal constraints on even objective knowledge. Nagel’s example is meant more to highlight (I think anyway) the irreducibility of subjective experiences. But if, internal to objective, empirical observation about the physical world, we find other such unknowable facts, that’s different, isn’t it?

    “I cannot know what it’s like to be a bat, but I know there is some way that it feels”

    seems much different from

    “I cannot know where the electron orbiting the nucleus of that atom is, but I do know that it is somewhere.”

    The first suggests a different sort of limit of scientific inquiry than the second, don’t you think?

  11. Josh says:

    I’ve just read a little more about the problem of action-at-a-distance.

    This isn’t so much a constraint on complete knowledge, which might be better seen as epistemological than ontological in origin.

    The problem with action at a distance, within quantum theory, is more metaphysical I suppose. The issue is: to some at least, it is impossible to understand some quantum phenomena (which I don’t understand) as making any sense unless two particles are somehow seen as “communicating” with one another. The state of one appears causally dependent on the other, and will be so EVEN when we can be sure that their causal dependence isn’t a result of initial conditions. Changing a property of one appears to change the property of the other, though they don’t do anything like “bump into” each other.

    The metaphysical question at least (don’t want to be misunderstood as making a statement again) is – how could this make sense? It requires a severe addition to the ordinary actions-and-events metaphysics we tend to take as common sense. You don’t have to think for too long about such a quandary before you start to reminder Leibniz’s idea of pre-established harmony.

    So the question this idea raises is – assuming such a result can be experimentally reproduced (and apparently it can) – do we need to accept some sort of addition to our common sense understanding of objects, events, cause, effect, etc.?

  12. David says:

    Josh, you ask: “So the question this idea raises is – assuming such a result can be experimentally reproduced (and apparently it can) – do we need to accept some sort of addition to our common sense understanding of objects, events, cause, effect, etc.?”

    Suppose quantum reality is every bit as bizarre as quantum physicists think it is. We might still insist, I think, that our common sense understanding of objects, events, cause, effect, etc., derives from, and applies to, the macroscopic world of objects and events which we experience and engage with everyday. So far as I understand–and yes, that’s not very far–the bizarre stuff happening at the quantum level NEVER happens at the macro level. If that’s right, then quantum physics might become increasingly weird without having any clear revisionary implications concerning our common sense conception of the world.

    You write: “The state of one appears causally dependent on the other, and will be so EVEN when we can be sure that their causal dependence isn’t a result of initial conditions. Changing a property of one appears to change the property of the other, though they don’t do anything like “bump into” each other….The metaphysical question at least (don’t want to be misunderstood as making a statement again) is – how could this make sense? It requires a severe addition to the ordinary actions-and-events metaphysics we tend to take as common sense.”

    Just a thought, but the first thing that occurred to me is that this ‘action-at-a-distance’ stuff doesn’t seem to cause any problems at all for deflationary accounts of causation like Hume’s. If changing the property of X always causes changes in the properties of Y–so that there is a constant conjunction between these events–then we will infer and may say that changes in X cause changes in Y, regardless of whether X and Y are directly or indirectly in contact with each other. I guess it becomes more puzzling when we think about causation as, essentially, a process in which energy is transferred from one thing to another….but I’m out of my depths, here.

  13. David says:

    So our conversation in May and your subsequent blog posts, Josh, led me to buy a book called “How to Teach Physics to your Dog” by a physicist at Union College named Chad Orzel. The book’s really good, and has taught me a lot about the main differences between ‘Classical’ and ‘Quantum’ physics. Anyway, as you’d expect, there’s a lot on the metaphysical/philosophical interpretation of modern ideas like uncertainty, superposition, wave-particle duality, etc..

    So at one point Orzel says something that sounds like what you were claiming when we talked about this stuff in Chicago–viz., that quantum physics has destroyed the ‘classical’ idea that everything has a definite location/position. And Orzel is quite clear, as you insisted, as well, if memory serves, that the point is not merely epistemological–for instance, the point is not that everything has a definite location/position, but with respect to some things, we cannot possibly know what it is. Rather, it’s a metaphysical point: the idea is that some things really do not have a single and definite location/position–for example, some things may exist in two different locations/positions at the same time.

    So what are the ‘experiments’ that could possibly confirm such a bizarre claim? Orzel refers to something called a ‘quantum eraser experiment,’ in which a beam of photons is—well, beamed—at two narrow slits and patterns are observed on the other side of the slits. There is something called an “interference pattern” when and only when photons pass through both slits at the same time. If one of the two slits is closed, there is no interference pattern. Orzel claims that in these experiments we discover that “each photon has passed through both slits, at the same time.”

    At this point Orzel’s dog—there are parts of the book in which Orzel and his dog engage in conversation—raises a good question: “I thought the interference was between two different photons—one that went through the left slit, and one that went through the right slit?” In other words, how does the experiment show that each individual photon passes through both slits at the same time?

    Orzel responds that it’s a natural question to ask, as many photons are beamed out at the same time. But, says Orzel, we get interference patterns even “by sending light at the slits one photon at a time.”

    Ok, now that WOULD confirm the claim that one and the same photon can be at two different places at the same time, but, oddly, when Orzel goes on to elaborate on this claim he seems to back off it, and doesn’t seem to realize that he’s backing off it (more likely, of course, I’m just not understand things). The dog ‘asks’ “How can an individual photon give an interference pattern?” and Orzel replies “It doesn’t,” but that “if you repeat the experiment over and over again, and keep track of all the photons, you’ll seem them add up to form an interference pattern.”

    Ok, I get THAT, but that confirms quantum indeterminacy, and not the much more radical idea of quanta occupying two positions at one and the same time, right? I’m sure I’m missing something, but I’m not sure what, yet.

  14. Josh says:

    “Ok, I get THAT, but that confirms quantum indeterminacy, and not the much more radical idea of quanta occupying two positions at one and the same time”

    Either way, things are pretty strange. The idea that probability seems to govern individual instances, as though they were part of groups behaving in an interactive way. The idea is that individual photons act like they were actually bouncing off of other photons even when they’re not.

    Also to respond to your earlier query – about whether QM affects things at the macro-level? I think it’s misspeaking to say it has no affect at the macro-level. The idea is more that quantum effects cancel one another out at the macro level. It’s just extremely statistically unlikely that you’ll temporarily (or permanently) end up on Mars. It’s not that it’s impossible, it’s that it’s so low probability it doesn’t enter our decision calculus (or our perceptual apparatuses) both of which evolved to react to high-probability events.

    The analogy to relativity helps here – whenever you’re travelling faster than the objects around you, time is moving more slowly for you than for those objects. It’s just moving very minutely more slowly, such that you can’t perceive it, and it doesn’t seem to affect anything. Both QM and GR mean our world isn’t quite what it feels like, it’s just that in the middle realm we inhabit, neither really big/fast or really small, that doesn’t pose a problem.

    We do intellectually know that Newtonian mechanics is just an approximation, that has dependent variables produced by both QM and GR that are simply too miniscule to substantially effect the calculations we make. If either QM or GR depart from common-sense realism of objects and events, that’s still something that needs to be explained, even if it doesn’t affect our day-to-day lives (also – it does – both QM and GR are used in different ways in many electronic devices we use. For example – if you don’t consider the effect that GR predicts, your GPS would inaccurately report your location. And if QM didn’t work the way it did, I think most electronic devices wouldn’t actually be able to transmit digital signals).

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