Quantum mechanics is not weird, unless presented as such

[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 (Schrödinger'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)?