I Quantum mechanics is not weird, unless presented as such

[stevendaryl]

You are changing the context. I was only discussing your statement

the problem is that it is inconsistent to assume that all possible observables have values at all times.​

But when I made that statement, what I had in mind was the sort of choice of observables as spin direction measurements in EPR. In that case, fuzziness or infinite precision doesn't seem relevant.

 

[stevendaryl]

When people say that the problem in understanding QM is because it is too far removed from human experience and human intuition, I don't agree. To me, what's weird is the parts (1) and (3) above, and what's weird about them is that they seem much too tightly tied to human actions (or to humanly comprehensible actions). Nature does not have preparation procedures and measurements, so it's weird for those to appear in a fundamental theory.

I don't believe that decoherence completely solves the problem. What decoherence basically tells us is that certain observables are in practice impossible to measure, because of entanglement. We can observe a dead cat, and we can observe a live cat, but there is no way we can observe a cat to be in the state:

\frac{1}{\sqrt{2}} |dead\rangle + \frac{1}{\sqrt{2}} |alive\rangle

But I find that a less than complete resolution. It still seems to be putting measurement into the fundamental physics.

 

[A. Neumaier]

what I had in mind was the sort of choice of observables as spin direction measurements in EPR.

Well, I can't read your mind, but only respond to what you write down. But...

the startling fact isn't that Alice and Bob's measurements of spin are fuzzy--it's that they are very precise.

Yes. But what is the problem here? It is in this respect no different from the very ordinary fact that casting a die always gives very precise numbers. A classical probabilistic model for predicting the die does not give precise predictions for this number, but only for the mean value after a long sequence of casts. Similarly, the quantum probabilistic model does not give precise predictions for Alice's measurements, but only for the mean value after a long sequence of casts.

The only seemingly startling fact in entanglement experiments is that the quantum probabilistic model predicts 100% correlations between the unpredictable results of Alice and Bob. But conceptually, this is no more startling than that if one records together with the value of each die cast (Alice) also the value of the invisible face (Bob) of the same die [aka entangled photon pair]. Comparing the predictions of the classical stochastic model of the die with the
observations of Alice and Bob gives a perfect prediction of 100% correlations: The values of Alice and Bob add up to 7 in the classical analogue.

So once it is accepted that the entangled photon pair is a conceptual unity of the same kind as a die (and indeed careful preparation avoiding decoherence is needed to ensure the former!), the analogy is complete. Thus there is nothing startling at all in predicting precise correlations in an otherwise random experiment.

The only startling fact remaining is that the two faces of the die are close and rigidly connected, while Alice and Bob in the quantum experiment may be very far away. But this has nothing to do with measurement or probabilities. Hence it has nothing to do with the conceptual clarity of quantum mechanics independent of any assumed quantum-classical correspondence.

Therefore, quantum mechanics is a complete and consistent theory independent of the need for any classical concepts related to measurement.

 

[vanhees71]

''So'' doesn't follow, since there might be an underlying deterministic theory from which quantum mechanics is derived.

Even though I don't believe that Bohmian mechanics is the right mechanism, I do believe that God doesn't play dice.

The main reason is that the notion of inherent randomness is conceptually problematic, and I believe even ill-defined, especially since quantum mechanics obviously applies to unique objects such as the Earth or the Sun.

Well, but which observation tells you that they do not behave probabilistically? Some coarse-grained observables behave classical to the accuracy relevant for any practical purpose, but if you believe that QT applies to macroscopic objects like the Sun and the other bodies, then this implies that they are not deterministic (not even determined precisely at any instant of time).

Whereas the appearance of randomness through chaos and limited knowledge is well-founded and mathematically well-understood, without any of the philosophical problems associated with classical probability.

For a thorough discussion of these problems, see the very informative books by
T.L. Fine,
Theory of probability; an examination of foundations.
Acad. Press, New York 1973.
and
L. Sklar,
Physics and Chance,
Cambridge Univ. Press, Cambridge 1993.

Thanks for the references. I'll have a look at them.

 

[A. Neumaier]

At some level, (1) and (3) are just complicated physical processes, so that should be included in (2).

And they are, when - rather than starting with postulates assuming an external classical world - one analyzes the measurement process in terms of statistical mechanics. See, e.g.,
A.E. Allahverdyan et al., Understanding quantum measurement from the solution of dynamical models. Physics Reports, 525 (2013), 1-166. http://arxiv.org/abs/1107.2138

 

[ddd123]

The only seemingly startling fact in entanglement experiments is that the quantum probabilistic model predicts 100% correlations between the unpredictable results of Alice and Bob. But conceptually, this is no more startling than that if one records together with the value of each die cast (Alice) also the value of the invisible face (Bob) of the same die [aka entangled photon pair]. Comparing the predictions of the classical stochastic model of the die with the
observations of Alice and Bob gives a perfect prediction of 100% correlations: The values of Alice and Bob add up to 7 in the classical analogue.

So once it is accepted that the entangled photon pair is a conceptual unity of the same kind as a die (and indeed careful preparation avoiding decoherence is needed to ensure the former!), the analogy is complete. Thus there is nothing startling at all in predicting precise correlations in an otherwise random experiment.

Well no, come on, the weirdness in entanglement experiments shows up when considering non-commuting observables. As Scott Aaronson puts it:

Perhaps the best way to explain local realism is that it’s the thing you believe in, if you believe all the physicists babbling about “quantum entanglement” just missed something completely obvious. Clearly, at the moment two “entangled” particles are created, but before they separate, one of them flips a tiny coin and then says to the other, “listen, if anyone asks, I’ll be spinning up and you’ll be spinning down.” Then the naïve, doofus physicists measure one particle, find it spinning down, and wonder how the other particle instantly “knows” to be spinning up—oooh, spooky! mysterious! Anyway, if that’s how you think it has to work, then you believe in local realism, and you must predict that Alice and Bob can win the CHSH game with probability at most 3/4.

Even having to give up counterfactual definiteness to avoid the problem is weird. It's weird however you put it.

 

[stevendaryl]

Yes. But what is the problem here? It is in this respect no different from the very ordinary fact that casting a die always gives very precise numbers.

Yes, but for two very distant throws of the dice to always give the SAME numbers is pretty weird.

The only seemingly startling fact in entanglement experiments is that the quantum probabilistic model predicts 100% correlations between the unpredictable results of Alice and Bob. But conceptually, this is no more startling than that if one records together with the value of each die cast (Alice) also the value of the invisible face (Bob) of the same die [aka entangled photon pair]. Comparing the predictions of the classical stochastic model of the die with the
observations of Alice and Bob gives a perfect prediction of 100% correlations: The values of Alice and Bob add up to 7 in the classical analogue.

Yes, that makes it more understandable, but has the undesirable quality that it's based on a falsehood. That's explaining the correlation in terms of hidden variables, which are inconsistent with quantum predictions.

So once it is accepted that the entangled photon pair is a conceptual unity of the same kind as a die (and indeed careful preparation avoiding decoherence is needed to ensure the former!), the analogy is complete.

I don't agree. Your analogy would make sense if you can imagine that for every possible choice of measurement angle, there is a corresponding "Alice end" and "Bob end" of the dice. But that's the sort of predetermined result that Bell's inequality proves is impossible. So relying on this analogy seems to me to be relying on something that's provably false.

 

[gjonesy]

I remember how weird it was to me when I learned that, in a vacuum, a feather and hammer would fall at the same rate

Experiments like that are very straight forward and to me "not weird" it follows basic logic. But when you get to the micro level and things behave differently then an intelligent person would logically expect. Logic kinda flies out the window and we are left with mathematical equations and very complex diverse ideas that on paper make sense but that the language of mathematics and some of us don't speak that very well unfortunately. How ever they have no commonsensical equivalent at a macro level. Its very complex to translate that mathematical language that I feel we lack the complete English language to explain. Perhaps one day a bright physics professor will right a book and include complementary dictionary explaining the simple English language used to describe these experiments in detail.

 

[A. Neumaier]

Well, but which observation tells you that they do not behave probabilistically?

They may behave random when viewed as one item in the ensemble of all planets and stars, respectively. But we observe a lot of nonrandom, fairly accurate facts about materials and processes of the unique Earth and Sun, and although they are properties of unique quantum objects (namely our Earth and our Sun), many of them are predictable with quantum mechanics to the accuracy we can measure them! I'd call this a startling fact!

Of course there are also a lot of detailed, fairly accurate facts about Earth and Sun that are not predictable by quantum mechanics. But in any classical model of a mechanical system there is also a lot unpredictable, since most of the details depend on their initial conditions (the classical state), which is not fixed by theory and must be learned by observation. Thus this is nothing specific to quantum mechanics. Both theories only predict correlations between past and present observations. And this is what works in the numerical quantum models for the interior of the Sun as well as it works for numerical classical models for water running in a pipe.

So I see nothing intrinsically strange in the foundations of quantum mechanics - one doesn't need to shut up and calculate, but one can calculate and at the same have a consistent intuition about how to interpret everything!

 

[stevendaryl]

And they are, when - rather than starting with postulates assuming an external classical world - one analyzes the measurement process in terms of statistical mechanics. See, e.g.,
A.E. Allahverdyan et al., Understanding quantum measurement from the solution of dynamical models. Physics Reports, 525 (2013), 1-166. http://arxiv.org/abs/1107.2138

Thanks for the pointer. It's a long paper, with lots of mathematics, but from skimming, what I believe that they are discussing is the way that a system can be a measuring device, and that such devices can be described using ordinary physics (statistical mechanics, since they necessarily involve many, many particles). I can understand that. The measuring device is a complex system in a metastable "neutral state", which then makes a transition into a stable pointer state through interaction with the microscopic quantity that is being measured. That's understandable. It's exactly what happens in classical mechanics, and is the reason that we can get discrete outcomes ("heads" or "tails") from continuous Newtonian dynamics.

But it's the pairing of distant measurement results in a correlated pair such as EPR that is mysterious. Alice's device is in a metastable state, and when it interacts with a spin-1/2 particle, it falls into a stable pointer state. Similarly for Bob's device. But to describe the transition using statistical mechanics seems to make the fact that Alice's and Bob's results are perfectly anti-correlated even more mysterious. If the measurement process is inherently statistical, then how does perfect anti-correlation come about?

The way that people argue that there is nothing mysterious about QM is by showing that the various features (perfect anti-correlation, discrete outcomes to measurements, etc.) have unmysterious analogies in pre-quantum physics. But the different analogies, taken together are mutually inconsistent. If you understand perfect anti-correlation in terms of "Alice and Bob are seeing opposite sides of the same die", that picture is inconsistent with Bell's theorem. If you understand the measurement process in terms of the decay of a meta-stable state, that picture is inconsistent with the perfect anti-correlations. Or it seems to me.

It seems to me that the various ways of explaining away the mystery of QM is akin to trying to prove to somebody that a Mobius strip is actually a cylinder. You point to one section of the strip, and say: "There's no twist in this section." You point to another section of the strip, and say: "There's no twist in this section, either." Then after going over every section, you conclude: "Clearly, there are no twists anywhere. It's a cylinder." The fact that it's twisted is a nonlocal property, you can always remove the twist from anyone section.

 

[stevendaryl]

Of course there are also a lot of detailed, fairly accurate facts about Earth and Sun that are not predictable by quantum mechanics. But in any classical model of a mechanical system there is also a lot unpredictable, since most of the details depend on their initial conditions (the classical state), which is not fixed by theory and must be learned by observation.

But that's exactly the type of nondeterminism that Bell shows cannot serve as an explanation for quantum statistics.

 

[A. Neumaier]

it's based on a falsehood. That's explaining the correlation in terms of hidden variables

I stated true facts about dice and true facts about entanglement experiments, and made the analogy to show that predicting exact correlations in a quantum setting is in itself not more mysterious than predicting exact correlations in a classical setting.

There is no falsehood anywhere. My analogy shows that your pointing to the exact correlations to debunk my statistical arguments justifying that quantum mechanics is independent of a classical context is an unfounded argument.

for two very distant throws of the dice to always give the SAME numbers is pretty weird.

This does not happen in my analogy; you argue against a straw man.

If you reread what I wrote, you can see that I acknowledged that there is something startling only in the distance of Alice and Bob in the quantum experiment. But as I had explained this has nothing to do with the foundations! It is a phenomenon of the same kind as the startling fact that a sufficiently accelerated observer reads completely different clock times than a resting one. But I haven't seen anyone claiming that this means that the basic concepts of general relativity are not sound.

My way of making this intuitively understandable is the realization that a coherent 2-photon state is a single (in these experiments very extended) quantum object and not two separate things, in a similar way as the small, rigid die is a single classical object. The only stretch of imagination needed is then to accept that invisible objects can be as strongly united as small rigid objects of our everyday experience. This is a comparatively minor step of about the same difficulty as accepting length contraction and other well-known classical relativistic effects that are outside our everyday experience. And it is supported by the experimental fact that very extended entangled state are quite fragile objects, easily broken into pieces: The more distant Alice and Bob are, the more difficult it is to ensure that the 2-photon states remain coherent since decoherence strongly works against it. Once coherence is lost, the two photon statistics are completely independent.

Once the possibility of strong unity (this is what the word ''coherence'' conveys) across large distances (and how easy it is to break it) is developed as part of one's intuition, one can get a good intuitive understanding of entanglement phenomena. This is my answer to the weirdness part of your setting.

But I emphasize again that this has nothing to do with problems in the foundations that we had originally discussed in the context of your defense of Landau's statement

that it is impossible to formulate the basic concepts of quantum mechanics without using classical mechanics

My main arguments were targeted at showing that, today, this statement (which is completely independent from any specific experimental conclusions predicted by the resulting theory) is no longer tenable.

 

[A. Neumaier]

If the measurement process is inherently statistical, then how does perfect anti-correlation come about?

The two are not logically contradictory, not even classically, as I showed with the examples of the perfectly correlated die readings, in a much simpler and fully understandable context. That it is possible in principle can be seen from the classical example, the details of how it comes about are of course specific to the quantum experiment. But they are given by the math, which has to (and does) follow the quantum rules - where a single particle is in principle infinitely extended since it is just a semiclassical conceptual simplification of an infinitely extended wave (with a wave function with unbounded support).

For me this is enough to reconcile intuition with the formalism, to the same extent as I can reconcile intuition about relativistic classical effects outside of my experience with what I obtain from calculations. I think it is unreasonable to expect more. A level of intuitive understanding that cannot even be achieved in the classical domain should not be made a requirement for understanding in the quantum domain.

 

[stevendaryl]

The two are not logically contradictory, not even classically, as I showed with the examples of the perfectly correlated die readings, in a much simpler and fully understandable context.

Yes, but in light of Bell's theorem, that sort of "Bertlemann's socks" explanation is known not to work.

Clearing a subject of mystery is worth doing, but not if requires grasping analogies that are known to be wrong.

 

[stevendaryl]

I stated true facts about dice and true facts about entanglement experiments, and made the analogy to show that predicting exact correlations in a quantum setting is in itself not more mysterious than predicting exact correlations in a classical setting.

I don't think the analogy works, because of Bell's theorem.

I understand that if you roll a classical die, and Alice sees one side, and Bob sees the other, then even though both get a random result, their results are perfectly anti-correlated. But if you try to extend that the quantum case of two spin-1/2 particles, it doesn't work. Rather than a single die, it's as if you have a different die for every possible detector orientation. But that's the kind of deterministic function of a random "hidden variable" that Bell proves is impossible. So it doesn't seem clarifying to bring up the classical analogy, it just seems like a distraction.

 

[A. Neumaier]

But that's exactly the type of nondeterminism that Bell shows cannot serve as an explanation for quantum statistics.

But I am not explaining quantum mechanics by a classical model - I am arguing that it is unreasonable to apply different standards to arguments about quantum mechanics and arguments about classical mechanics - in order to make quantum mechanics appear more problematic than classical mechanics.

To state it without any reference to determinism and classical arguments:

• The logical explanation for quantum statistics is the quantum mechanical formalism.
• The quantum mechanical formalism is mathematically consistent and can be interpreted consistently and applied to observations in a quantum world consisting of quantum objects only, without any reference to classical objects or other classical concepts.
• Once the quantum-classical framework is dropped, all philosophical obstacles (beyond those already in the classical concept of probability) are dissolved.
• A satisfying understanding can be developed, both of the interpretation (no need to ''shut up'') and of the formal side (''and calculate'').
• The resulting quantum theory makes a huge number of predictions that confirm our everyday experience.
• In particular, it explains the properties of water and ice, the color of gold, that mercury is a fluid metal, why chemicals undergo reactions, the laws of hydrodynamics, and much else.
• In addition, as any - classical or quantum - theory that makes predictions under conditions that we don't usually are exposed to, quantum theory also makes some predictions that are outside our everyday experience, therefore violate our untrained intuition (and invite heated debates such as the present one).
• These predictions follow from the impeccable mathematical basis together with its interpretation that tells how to relate the mathematics and the observable world.
• To the extend that they deviate from our native intuition (e.g., in the case of large distance entanglement), this is not a defect of the theory or its foundations.
• Instead it is a limitation of our experience and the resulted limited intuition.
• To improve the intuition, one can train oneself by developing useful analogies rooted in our experience but reflecting key properties of the formalism, while remaining aware of the limits of any such analogies.

This is what I mean when I say that quantum mechanics is not weird unless presented as such.

 

[A. Neumaier]

I don't think the analogy works, because of Bell's theorem.

An analogy is not subject to logic, hence not bound by theorems - it is an appeal to intuition and imagination.

 

[ddd123]

It seems you're confusing "weird" with "inconsistent". Granted, weirdness is subjective to an extent, but Bell's theorem: nobody saw that coming. Counterfactual definiteness was so obvious before quantum physics that it wasn't even stated as a concept. So even if there's an amount of subjectivity involved, we can generally say it's weird, like we say Mozart's music is beautiful. You can disagree and say it's all smooth for you but it comes out as idiosyncratic imho.

 

[A. Neumaier]

grasping analogies that are known to be wrong.

Analogies are not logical objects; they need to match only particular aspects under discussion.

Already calling a photon a particle is making an analogy through the choice of language, although the analogy is faulty in many ways. Nevertheless it is universally used.

 

[stevendaryl]

To state it without any reference to determinism and classical arguments or analogies:

• The logical explanation for quantum statistics is the quantum mechanical formalism

How is that an "explanation"?

• The quantum mechanical formalism is mathematically consistent and can be interpreted consistently and applied to observations in a quantum world consisting of quantum objects only, without any reference to classical objects or other classical concepts

I just don't think that's true. I don't think that you, or anyone else, has done that.

 

[A. Neumaier]

How is that an "explanation"?

A mathematical derivation is an explanation - more than any talk about it. Thus I refer to any book about quantum statistical mechanics. Or for the measurement problem to the big article I had referred to earlier.

 

[stevendaryl]

I just don't think that's true. I don't think that you, or anyone else, has done that.

I'm starting to sound argumentative, and I definitely don't want to be, but I just don't agree with what is being said. The mysteries of quantum mechanics are not due to the way that they are presented, and they aren't cleared up by presenting things in a different way. What is possible, is to put the mysteries aside and get on with doing science without worrying about them.

 

[A. Neumaier]

The mysteries of quantum mechanics are not due to the way that they are presented

But whether they are perceived as mysteries depends on the way they are presented. One can make the distance to the ordinary big or small depending on how one puts quantum mechanics into words and pictures. Good science (and good popularization of science) should minimize this distance.

 

[Lord Crc]

I'm starting to sound argumentative, and I definitely don't want to be, but I just don't agree with what is being said. The mysteries of quantum mechanics are not due to the way that they are presented, and they aren't cleared up by presenting things in a different way. What is possible, is to put the mysteries aside and get on with doing science without worrying about them.

This is what I tried to say in my earlier post about Gabriel's Horn. The math is clear, I can work with it fine, but infinity is to me inherently weird and thus so is the horn.

In QM entanglement is for me inherently weird. The math used to describe it is not.

 

[bhobba]

In QM entanglement is for me inherently weird. The math used to describe it is not.

This is where English fails us.

Check out the following:
http://arxiv.org/abs/0911.0695

That transformations between pure states should be continuous is highly intuitive, yet entanglement is not. But they are logically equivalent.

QM is not alone in that. For example conservation of angular momentum is very intuitive, but what goes on when a person is put on a rotating platform with a spinning bicycle wheel is not - yet it follows from angular momentum conservation. But after a while you get used to it and start thinking it is intuitive as well.

After many years of thinking about QM I find much of it like the rotating platform with bicycle wheel.

I always remember one of my calculus lecturers during first year. Whenever something new and strange came up he always said like anything new it seems strange at first but after acquaintance it becomes easy and natural.

Thanks
Bill

 

[nrqed]

No, the more I think about quantum mechanics, the less weird it is. I have written a whole book about it, without any weirdness; see post #2.

It is weird only in the eyes of those who take the talk about it too serious and neglect the formal grounding which contains the real meaning.

I don't think that anyone spending some time about understanding, say, quantum eraser experiments can possibly NOT find QM weird. Even Feynman and Bohr say that the theory is weird. Sure, one can define things mathematically and axiomatize it and from a purely mathematical perspective, it may seem nothing special. But I cannot see how anyone can think about t QM as a way to describe the world we live in, in contrast to a mathematical construct, and not find it extremely weird and, by the same token, incredibly fascinating and exciting.

 

[A. Neumaier]

anyone spending some time about understanding, say, quantum eraser experiments can possibly NOT find QM weird

Don't generalize from yourself to anyone; people and their intuition can be very different.

I understand quantum eraser experiments by questioning the meaning attached to the classical words, which I find inappropriately applied to the quantum situation discussed. Once the experiments are described in a language more faithful to the quantum situation the weirdness disappears.

Most of the weirdness in quantum mechnaics as often presented comes from an inappropriate choice of language, which suggests misleading analogies to classical situations.The weirdness is not in quantum mechanics itself but imposed on it by those interested in weird stories. People who actually work on applications of quantum mechanics rather than analyziing [real or thought] experiments on pure foundations know that thy need an appropriate intuition and - lo and behold - they find a very rational view of the matter.

extremely weird and, by the same token, incredibly fascinating and exciting.

I fully agree. Precisely as fascinating and exciting as science fiction, which indeed it is.

 

[andrewkirk]

I find myself agreeing with @A. Neumaier. I was, in a sense, a little disappointed that QM did not shock me once I got around to learning it.
Whoever said 'Any one who is not shocked by the quantum theory doesn't understand it' perhaps knew plenty about QM, but not much about the magnificent variability of human nature.
I think the reason many people find QM shocking is because it challenges some of the most fundamental, intrinsic metaphysical assumptions that people have - things like object permanence and cause-effect relations. If one is emotionally attached to such assumptions then one will likely be shocked by QM. But if one is perfectly open-minded and prepared to let any such assumptions go, there is no need for shock.
Also, because there's so much pop-science discussion of QM undermining long-held assumptions, these days one enters study of QM expecting any long-held assumption not to hold. So it's no big deal when one finds that some of them don't.
I don't find the idea of entangled particles any weirder than the preposterous pre-QM idea that this grey keyboard on which I am typing, which is quite obviously solid matter, made of some sort of solid grey stuff, is actually made up almost entirely of empty space.
Perhaps the problem with QM is that it is not nearly weird enough. To explain more of the many unexplained things out there, and especially to achieve unification with gravitational theory, I think it needs to get a whole lot weirder than it currently is (or queerer, as Haldane puts it, see sig).

 

[A. Neumaier]

Rubi and Zonde,

it seems that you hijacked this thread by filling it with discussions about the meaning of nonlocality, wheras the topic is whether or not quantum mechanics can be presented so that it doesn't look weird. Please discuss technical nonlocality issues elsewhere.

 

[rubi]

Sorry, I didn't mean to hijack the thread. Maybe some mentor can split it, so we can discuss it somewhere else.

MENTOR NOTE: I have split the thread into an alternative reality localized here:

https://www.physicsforums.com/threads/quantum-mechanics-is-not-weird-alternative-discussion.851342/

for those members who are interested in discussing locality and non-locality weirdness.