I Quantum mechanics is not weird, unless presented as such

[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.

 

[nrqed]

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.

I think that it would have been *very* relevant to discuss non locality here, as your point is that there is nothing strange about quantum mechanics. It would have been interesting to see your explanation of non locality that makes it not strange at all.

 

[stevendaryl]

I think that it would have been *very* relevant to discuss non locality here, as your point is that there is nothing strange about quantum mechanics. It would have been interesting to see your explanation of non locality that makes it not strange at all.

It's been diverted to another thread:
https://www.physicsforums.com/threa...d-locality-and-non-locality-weirdness.851342/

 

[A. Neumaier]

It would have been interesting to see your explanation of non locality that makes it not strange at all.

Didn't you read the whole thread? On an informal level appropriate for discussions with nonexperts, I had explained it already in this thread:

There is nothing weird [in the double slit experiment] if you interpret it in terms of fields rather than particles. This was already known to Huygens in the 17th century.
Much of the weirdness comes from forcing quantum mechanics into the straightjacket of a particle picture. The particle picture breaks down completely in the microscopic domain, as witnessed by the many weird things it causes.
On the other hand, the field picture remains valid at all length and time scales.

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.

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 wor ''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.

If you want an explanation on a more technical level, I invite you to read my Thermal Interpretation FAQ.
Some of it is a bit out of date but much of it is still good.

For a fully up to date account that reflects my current thinking you'll have to wait for a few months. In April I'll give a lecture at the Zentrum für Oberflächen- und Nanoanalytik of the University of Linz (Austria), and the slides of my lecture will afterwards be available on my web page (under publications in physics).

 

[ddd123]

That's very interesting but I can't see how it could make things less weird. Consider entangled electrons of which you measure spin. The electron are easily found to be point-like, at least to a large extent. So you detect two point-like entities separated by a distance (which can even be a mile in this year's Bell test, in two separate diamonds!), and you have to consider these as two sides of the same die. It's much weirder than time dilation of SR.

I appreciate the perspective though, it's insightful but I don't think it de-weirdifies QM to any extent.

 

[A. Neumaier]

The electron are easily found to be point-like, at least to a large extent. So you detect two point-like entities separated by a distance

As I had said before, the choice of language for drawing an intuitive picture makes a lot of difference in presenting and perceiving quantum mechanics. It takes a little practice but then you enter a new world, and everything feels different!

You detect two point-like entities and get quantum weirdness, but I detect one coherent, extended electron field and get quantum beauty.

Consider perhaps that there are reasons why quantum electrodynamics, the theory of photons and electrons and their interaction, is referred to as a quantum field theory and not as a quantum particle theory. Fields simply have much more flexible properties than particles can ever have.

 

[A. Neumaier]

which can even be a mile

Consider how little time it takes for light to travel a mile, and you'll realize that for a relativistic theory like QED, this is a very tiny distance.

 

[PAllen]

Consider how little time it takes for light to travel a mile, and you'll realize that for a relativistic theory like QED, this is a very tiny distance.

Well, per theory, it could be a billion light years. In practice, that presents a few difficulties ... To me, the idea of single quantum object spanning the universe is a bit weird no matter how you slice it. However, arguing about what is weird is no more objective than arguing about what color is most attractive.

 

[nrqed]

Didn't you read the whole thread? On an informal level appropriate for discussions with nonexperts, I had explained it already in this thread:

I did not think that non locality was a topic only appropriate to "experts". It is discussed in almost all popular books on QM.

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.

I personally think that the comparison with a small rigid object misses the key point that the measurements on entangled states can be timelike. So even the order of the two measurements is frame dependent. That's very different from any physical connection between small objects and to me is the key aspect making entanglement strange. But hey, what do I know, I am obviously not bright enough to realize that QM has absolutely nothing strange about it.

 

[fresh_42]

So even the order of the two measurements is frame dependent.

Are there experiments in which this plays a fundamental role? I mean in a relativistic framework. I'm just asking as a silly one since it sounds like a connection between the Quantum world and GR worth to examine.
Please feel free to ignore this if it is too stupid.

 

[zonde]

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.

I think it would be appropriate to give here this link:
Can I Send a Signal Faster than Light by Pushing a Rigid Rod?

 

[strangerep]

So once it is accepted that the entangled photon pair is a conceptual unity
[...]
a coherent 2-photon state is a single (in these experiments very
extended) quantum object and not two separate things, [...]

What then is your criterion for (physically) distinguishing between "1 thing" vs "2 things" ?

 

[vanhees71]

What then is your criterion for (physically) distinguishing between "1 thing" vs "2 things" ?

Quantum field theoretically the photon number is an observable for free photons, and a two-photon state is thus well distinguishable from a one-photon state. The usual Bell experiments a la Aspect use polarization-entangled two-photon states.

Another thought about the weirdness of quantum theory is the following: Quantum theory appears weird to us, because we are used to the classical behavior of macroscopic objects. According to quantum theory the classicality of this behavior is emergent and due to our "coarse grained" observation of the objects. Quantum theory itself is, of course, not weird at all but the explanation why the world, including the classical behavior of objects in our everyday experience, is as we know it since quantum theory is a very accurate description of our experience of the world. It's not perfect and complete (at least a full understanding of gravity is missing), but it's much less weird than classical physics, which couldn't even make the stability of the matter surrounding us, plausible. A classical world thus would be much weirder than the quantum world; our very existence wouldn't be possible!

 

[A. Neumaier]

What then is your criterion for (physically) distinguishing between "1 thing" vs "2 things" ?

Coherence, of course.

Decoherence is a kind of quantum equivalence of classically breaking an object into several smaller ones. If one doesn't take extreme care with long distance entanglement experiment, the experiment won't show the desired 100% correlations, or even none at all. This is why one can do the experiment at distances of a few miles but not across the ocean - transportation through standard optical fibers produces far too much decoherence.

It is not so different from the extreme care needed to create a very long classical rigid rod. The longer it is the thicker it has to be and the stiffer the material, and one is soon at the limits of experimental possibilities, even if one wants to enforce rigidity only up to a transverse deviation of a few millimeters.

The quantum world is not so different from the classical world as standard storytelling makes one believe.

 

[A. Neumaier]

I did not think that non locality was a topic only appropriate to "experts".

Neither did I. This is why I explained my non-weird intuition about it in the present thread. (The reason why I had complained about hijacking the thread was not that there was talk about nonlocality, but that this talk was about technical matters unrelated to how the presentation of nonlocality experiments affects its weirdness.)

 

[strangerep]

What then is your criterion for (physically) distinguishing between "1 thing" vs "2 things"?

Coherence, of course.

That's what I suspected your answer would be.

But coherence does not take integer values in general. If you say that the maximally coherent (fully entangled) case counts as 1 thing, while the totally incoherent case counts as 2 things, then presumably a non-maximally coherent case counts as something in between? E.g., 1.3 things, or 1.99654 things, ...

 

[A. Neumaier]

But coherence does not take integer values in general. If you say that the maximally coherent (fully entangled) case counts as 1 thing, while the totally incoherent case counts as 2 things

The number of objects is encoded in the tensor product structure of the density matrix. coherent + entangled means a rank one density matrix that cannot be decomposed as a tensor product. $k$ completely independent objects have a density matrix decomposable into $k$ pieces. And of course there are all shades in between.

One has an analogous situation classically in image analysis - the number of objects visible on an image is a fuzzy number, not a precise integer. I had already mentioned the number of people in a room which is also not always an integer.

Quantum mechanics is a richer theory with many more observables, hence there are many more ways to create shades.

 

[ddd123]

Remember that this year's experiment tested for light separated particles. The measurements were performed before light could reach from one endpoint to the other.

 

[A. Neumaier]

this year's experiment

which experiment are you referring to?

 

[vanhees71]

Coherence, of course.

I don't understand this answer. The photon number is an observable for free photons and as such represented by a self-adjoint operator in Fock space,
$$\hat{N}=\sum_{\lambda \in \{-1,1\}} \int_{\mathbb{R}^3} \mathrm{d}^3 \vec{p} \hat{a}^{\dagger} (\vec{p},\lambda) \hat{a}(\vec{p},\lambda).$$
This means that a one-photon and a two-photon state are always orthogonal and thus well distinguishable. It's not clear to me what this has to do with coherence or incoherence.

 

[ddd123]

which experiment are you referring to?

Whoops, I mean last year's, still in 2015 mode :D

Of course the loophole-free Bell test which was discussed in this forum too and was speculated to become a Nobel prize etc.

 

[A. Neumaier]

This means that a one-photon and a two-photon state are always orthogonal and thus well distinguishable. It's not clear to me what this has to do with coherence or incoherence.

Yes, photon number is determined by any eigenstate of the number operator.

But the question was different, namely how to distinguish whether one should regard a given (pure or mixed) 2-photon state (thus containing exactly 2 photons) as a single quantum object or as two separate objects.

 

[A. Neumaier]

which was discussed in this forum

I don't have the time to read every thread in this forum. Please give a link to the paper describing the scientific part of the experiment.

 

[lightarrow]

Sorry if someone has already written it: don't know if QM is weird or not, but the believe it is, made me much of my push to study it as an amateur

--
lightarrow

 

[kith]

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.

There seems to be something weird regarding the time evolution of these quantum objects to me. As DrChinese posted in the twin thread, there are experiments which claim to show entanglement between photons which haven't coexisted. Doesn't this suggest a picture of non-dynamical objects which extend over time as well as over space?

Or do you think that there's something wrong with the interpretation of these experiments? Personally, I haven't read any substantial texts about them, I know only the head-lines.

 

[vanhees71]

Yes, photon number is determined by any eigenstate of the number operator.

But the question was different, namely how to distinguish whether one should regard a given (pure or mixed) 2-photon state (thus containing exactly 2 photons) as a single quantum object or as two separate objects.

Two photons are indistinguishable bosons and thus any many-photon state is not a product state but a symmetrized product state (or superpositions thereof). So it's hard to tell, how one should define two photons (or indinstinguishable particles) as separate. In a strict sense only two distinguishable particles are clearly separated. The should be distinguishable by at least one intrinsic quantum number (e.g., an electron and a muon are dinstinguishable by there mass and thus clearly separable).

 

[vanhees71]

I don't have the time to read every thread in this forum. Please give a link to the paper describing the scientific part of the experiment.

I guess, it's the following one

Hensen; et al. "Loophole-free Bell inequality violation using electron spins separated by 1.3 kilometres". Nature 526: 682–686
https://dx.doi.org/10.1038%2Fnature15759

 

[ddd123]

I don't have the time to read every thread in this forum. Please give a link to the paper describing the scientific part of the experiment.

http://arxiv.org/abs/1508.05949

 

[Andrew Wright]

Perhaps people find QM weird because they study classical physics first and because it is different. New things always seem weird until you get used to them.

 

[stevendaryl]

Perhaps people find QM weird because they study classical physics first and because it is different. New things always seem weird until you get used to them.

That's what a lot of people say, but I don't think that that's the real reason. Relativity is as much of a shock to our intuitions as QM is, but people pretty much get over the weirdness of relativity after studying it for a year (or less), while getting over QM takes a lifetime (for some people).

The problem that I have with QM is that it is so unclear what its semantics are. Is the wave function a description of the state of the world, or is it a description of our knowledge about the world? Or somehow both? Neither alternative really fits all the facts comfortably. Then there is the discrepancy between the objects described by the mathematical formalism (amplitudes for different possibilities) and what is actually observed (definite values for whatever is measured). Special Relativity similarly shows up a huge difference between what the theory says and what our observations show, but in the SR case, what things look like to an observer can be derived from what they are, at an objective level. In QM, there seems to be a fundamental distinction between observations and the underlying equations of physics, which means that the former is not completely explained by the latter.

I am someone who has worked with QM for many years, and I am pretty competent at the basics (Schrodinger's equation, the Dirac equation, perturbation theory, quantum statistical mechanics), and I am passably familiar with the advanced topics (quantum field theory, at least as far as QED). But that familiarity does nothing to make QM less weird to me. As a matter of fact, what's weird about QM is never addressed in advanced work. It almost never comes into play. You can pretty much do all of QM from the "shut up and calculate" perspective without ever addressing any of the conceptual issues. I give A. Neumaier a lot of credit for taking the time to address these things, even though I don't agree with him that they have been fully addressed (by him, or by anyone).

 

[WWGD]

It is only the author's view, not ''the modern view''. It cannot be the truth because quantum mechanics was in operation on Earth (or the universe) long before the existence of preparation and measuring devices (which is assumed by Hardy at the end of p.1) - a true derivation must explain why certain multi-particle systems called measurement devices work as postulated! Also the number N of degrees of freedom, which he takes to be finite throughout, is infinite already for the harmonic oscillator, which makes his ''derivation'' invalid for any real system except those considered in quantum information theory.

Those who want to see that quantum mechanics is not at all weird (when presented in the right way) but very close to classical mechanics should read instead my online book Classical and Quantum Mechanics via Lie algebras. (At least I tried to ensure that nothing weird entered the book.)

Is this part of QM: You are replying to bhobba _before_ he has even posted (you made both posts 1,2 before he ha a chance to post)?