Quantum mechanics is not weird, unless presented as such

[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)?

 

[A. Neumaier]

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

This is probably the reason why the physics community has little interest to change the state of affairs: it makes excellent advertisement for the subject.

I started this thread (and contributed to some related ones) for those who want to get a better understanding, not for those who are content with the current practice of talking about quantum mechanics.

The weirdness is not in quantum mechanics itself (the math and its relation to the applications, which works very smoothly) but only in the way it is translated into ordinary language. So the latter must be changed if things shall improve. My gradual discovery was that it can be changed to a considerable extent, with remarkable consequences.

 

[A. Neumaier]

Or do you think that there's something wrong with the interpretation of these experiments?

Yes. I need to study one in detail to see precisely what goes wrong. But it is most likely the meaning attached to the word ''photon'' that allows these seemingly absurd conclusions.

 

[fresh_42]

I started this thread for those who want to get a better understanding, not for those who are content with the current practice of talking about quantum mechanics.

... which made me thankful for the book you published and linked to.

 

[A. Neumaier]

even though I don't agree with him that they have been fully addressed (by him, or by anyone).

Did you read all my evidence? It is not only what I actually wrote down here. A much more detailed case is made in Chapter 10 of my book and in my Thermal interpretation FAQ.

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.

This is due to a weird notion of ''observable''. The true observables are only the macroscopic ones, given by statistical mechanics. That one only measures macroscopic stuff (pointer readings, pixels on a screen, numbers stored on an electronic memory device, electric currents, and the like) is obvious once one asks the question. Concluding that everything about the microscopic world are inferences - whose translation into ordinary language is faulty - reconciles everything.

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.

The wave function is completely meaningless; it is not even well-defined since its phase is undetermined. The object with a physical meaning is the density matrix. It describes the objective state of the world and/or any well-defined part of it, Our knowledge about the world is only an approximation to this state, and therefore given by a different density matrix (different for each subject or object having knowledge). The extend of the difference is a measure of the quality of this knowledge. This perfectly fits all facts.

 

[stevendaryl]

The wave function is completely meaningless; it is not even well-defined since a phase is undetermined. The object with a physical meaning is the density matrix. it describes the objective state of the world, Our knowledge about the world is only an appoximation to it. This perfectly fits all facts.

Well, the distinction between the wave function and the density matrix is not important for what I'm about to say.

In an EPR-type experiment with spin-1/2 particles, let's suppose that Alice and Bob agree ahead of time to each measure their respective particle's spin along some agreed-upon axis, $\vec{A}$. Suppose that in Alice's rest frame, she measures her particle a few seconds before Bob measures his.

• So, before Alice's measurement, she doesn't know whether Bob will measure spin-up or spin-down.
• Then she measures her particle to have spin-up.
• Immediately afterward, she knows that Bob will measure spin-down.

So, it seems to me that after Alice observes her result, she would describe Bob's particle to be in a definite spin-down state. I don't care whether she describes it using density matrices or wave functions. So the question is: Was it in a definite spin-down state BEFORE Alice performed her measurement? Neither an answer of "yes" nor "no" makes any sense to me.

Answering "yes" would seem to me to be in line with the interpretation of states as reflecting knowledge; Alice's measurement simply reveals the value of a variable that already had a definite value. That seems along the lines of a "hidden variables" assumption, which is inconsistent with Bell's theorem.

Answering "no" would seem to me to be implying that Alice's measurement had an effect on the state of Bob's particle. It was in an uncertain state of spin beforehand, and it was in a definite state of spin afterward. It's not an FTL signal, because Alice can't communicate anything using this change, but it seems nonlocal to me.

 

[A. Neumaier]

it seems to me that after Alice measures her result, she would describe Bob's particle to be in a definite spin-down state.

The change in knowledge and the associated model of the situation is an anthropomorphic step introduced into the objective description. It is precisely this anthropomorphism - absent in classical mechanics (which makes classical mechanics rational) but omnipresent in the discussion of quantum foundations - that causes the problems. Cutting out the anthropomorphic part restores rationality to the quantum foundations.

The rational way to describe a change of knowledge is by going over to conditional expectations. If you know nothing about a situation, you assume a probability model Pr(x) for arbitrary random variables x. If you get to know the value of some random variable y you change your model to a conditional probability model Pr'(x)=Pr(x|y). This takes care of all facts and the change in knowledge. It is not difficult to see that applied to quantum mechanical models, this change of probability model is exactly equivalent to collapse.

Thus the collapse is a necessary element of a rational description of anthropomorphic knowledge (but also of animate machine knowledge) in the quantum domain. Whereas it is absent in an objective description of Nature, which has no notion of knowledge.

Lo and behold - through a more careful choice of language we have reconciled the basic conflict in the quantum foundations by cleanly separating the objective and the subjective.

 

[stevendaryl]

The change in knowledge and the associated model of the situation is an anthropomorphic step introduced into the objective description. It is precisely this anthropomorphism - absent in classical mechanics (which makes classical mechanics rational) but omnipresent in the discussion of quantum foundations - that causes the problems. Cutting out the anthropomorphic part restores rationality to the quantum foundations.

The rational way to describe a change of knowledge is by going over to conditional expectations. If you know nothing about a situation, you assume a probability model Pr(x) for arbitrary random variables x. If you get to know the value of some random variable y you change your model to a conditional probability model Pr'(x)=Pr(x|y). This takes care of all facts and the change in knowledge. It is not difficult to see that applied to quantum mechanical models, this change of probability model is exactly equivalent to collapse.

Thus the collapse is a necessary element of a rational description of anthropomorphic knowledge (but also of animate machine knowledge) in the quantum domain. Whereas it is absent in an objective description of Nature, which has no notion of knowledge.

Lo and behold - through a more careful choice of language we have reconciled the basic conflict in the quantum foundations by cleanly separating the objective and the subjective.

I don't see that clean separation, at all. It seems to me that if there were such a clean separation, then my questions would have clear answers:

1. After Alice measures spin-up, is it true (an objective fact about the world) that Bob's particle is in a spin-down state?
2. If so, was it in that state before Alice performed her measurement?

 

[A. Neumaier]

if there were such a clean separation, then my questions would have clear answers

Indeed, your questions have a clear answer:

• After Alice measures spin-up, is it true (an objective fact about the world) that Bob's particle is in a spin-down state?

• If so, was it in that state before Alice performed her measurement?

It is true in her subjective approximation $\rho'_A$ of the objective state $\rho$. And it wasn't subjectively in that state before since before she performed the measurement, her subjective approximation was $\rho_A$, from which $\rho'_A$ was derived by collapse = conditional expectation.

Symmetrically, Bob has a subjective approximation $\rho_B$ of the objective state $\rho$ that changes into $\rho'_B$ when the knowledge of Bob is updated.

Neither of these changes has any effect on the objective state $\rho$. The latter changes in accordance with the dynamics of the universe, of which it is a projection to a tiny 2-photon subspace. The dynamics of the universe also contains a description of the activities of Alice and Bob, and all details of the measuring equipment, while $\rho$ only contains a description of the cumulative effect on the 2-photon system.

This incompleteness in the 2-photon description is responsible for the perceived randomness in the whole setting - not very different from the randomness introduced in a deterministic chaotic system once some of its variables are discarded. The reduced quantum mechanical only takes into account (an idealized version of) the preparation, and this part accounts for the perfect correlations among the otherwise random results.

 

[Mentz114]

I don't see that clean separation, at all. It seems to me that if there were such a clean separation, then my questions would have clear answers:

1. After Alice measures spin-up, is it true (an objective fact about the world) that Bob's particle is in a spin-down state?
2. If so, was it in that state before Alice performed her measurement?

These questions refer to classical concepts like individual particle, position, time ordering and maybe others. I find that if I abandon those I get a picture of a system whose internal 'state' is unknowable between the initial state and final state. The system must end in a state that does not violate conservation laws. Whatever is required will happen - and those events or processes do not have classical analogues. The extended field relaxes into the final state and that's it.

Applying billiard ball dynamics naturally causes confusion and misunderstanding. A statement like 'a photon is created at (t,x) then moves to (t',x')' is devoid of meaning. How would one ever determine this ? We cannot 'see' it in the classical sense.

All I did to remove all the mysteries was to forget the classical notion of 'before' and 'after'. If the final state is deferred until (classically) every possible measurement order has been tested, there is no quantum mystery, only the (classical) perception that information has come from the (classical) future.

To misquote Prof Neumaier

"You see particles and quantum mystery, I see fields and quantum beauty'.

 

[zonde]

The change in knowledge and the associated model of the situation is an anthropomorphic step introduced into the objective description. It is precisely this anthropomorphism - absent in classical mechanics (which makes classical mechanics rational) but omnipresent in the discussion of quantum foundations - that causes the problems. Cutting out the anthropomorphic part restores rationality to the quantum foundations.

The change of knowledge is only analysis of physical fact that detector clicked. So what do you propose to cut out as anthropomorphic part?
- the idea that "detector clicked is a physical fact"
- any analysis of physical fact that detector clicked

 

[martinbn]

Well, the distinction between the wave function and the density matrix is not important for what I'm about to say.

In an EPR-type experiment with spin-1/2 particles, let's suppose that Alice and Bob agree ahead of time to each measure their respective particle's spin along some agreed-upon axis, $\vec{A}$. Suppose that in Alice's rest frame, she measures her particle a few seconds before Bob measures his.

• So, before Alice's measurement, she doesn't know whether Bob will measure spin-up or spin-down.
• Then she measures her particle to have spin-up.
• Immediately afterward, she knows that Bob will measure spin-down.

So, it seems to me that after Alice observes her result, she would describe Bob's particle to be in a definite spin-down state. I don't care whether she describes it using density matrices or wave functions. So the question is: Was it in a definite spin-down state BEFORE Alice performed her measurement? Neither an answer of "yes" nor "no" makes any sense to me.

Answering "yes" would seem to me to be in line with the interpretation of states as reflecting knowledge; Alice's measurement simply reveals the value of a variable that already had a definite value. That seems along the lines of a "hidden variables" assumption, which is inconsistent with Bell's theorem.

Answering "no" would seem to me to be implying that Alice's measurement had an effect on the state of Bob's particle. It was in an uncertain state of spin beforehand, and it was in a definite state of spin afterward. It's not an FTL signal, because Alice can't communicate anything using this change, but it seems nonlocal to me.

Isn't this just a problem with the language? The two events are spacelike seperated, but you say that Alice measures before Bob, which is meaningless. I know you said in a certain reference frame, but the notion before is meaningless for spacelike separeated events. If you acknoledge that how do you get a problem with the second answer?

 

[lightarrow]

THis is probably the reason why the physics community has little interest to change the state of affairs: it makes excellent advertisement for the subject.

I started this thread (and contributed to some related ones) for those who want to get a better understanding, not for those who are content with the current practice of talking about quantum mechanics.

The weirdness is not in quantum mechanics itself (the math and its relation to the applications, which works very smoothly) but only in the way it is translated into ordinary language. So the latter must be changed if things shall improve. My gradual discovery was that it can be changed to a considerable extent, with remarkable consequences.

Yes, Alfred, I had get it, I only wanted to express my opinion on motivation to the study.
Thanks for your answer.

--
lightarrow

 

[A. Neumaier]

So what do you propose to cut out as anthropomorphic part?

Any talk about knowledge. Knowledge requires a subject, hence turns the problem into something subject-dependent = subjective.

Let us modify the setting a little. A machine makes the decisions and records the responses in the experiment while Alice sleeps. Next morning Alice wakes up and reads the records of all decisions and responses. At this moment her knowledge changes and the states of all the photons collapse, long after they stopped to exist. It is obvious that in this setting the states only refer to Alice's knowledge not to any underlying physical reality. Here approximation $\rho_A$ to the true physical state $\rho$ improves as she draws the consequences of having obtained new information.

This explains the acausality of collapse. One has the same acausality classically under similar circumstances when rolling classical dice. Knowledge = subjective beliefs based on prior information are well-known not to respect causality. This also resolves the problem when

there are experiments which claim to show entanglement between photons which haven't coexisted.

Note that in all these experiments nothing nonlocal happens - something happens in Alice's memory (or interpretation of her memory) only. But this memory is localized in her brain.

 

[A. Neumaier]

Yes, Alfred

Who is Alfred?

 

[A. Neumaier]

what's weird about QM is never addressed in advanced work. It almost never comes into play

Why do you write ''almost''? It never comes into play since it is not objective.

 

[A. Neumaier]

To me, the idea of single quantum object spanning the universe is a bit weird no matter how you slice it.

The universe as a whole is a single quantum object, no matter how you slice it.

It is obvious that talking about the values of some observables of this quantum object as if they depend on the knowledge of tiny parts of it is misguided and therefore prone to all sorts of paradoxes. Weird in, weird out.

 

[martinbn]

The universe as a whole is a single quantum object, no matter how you slice it.

And what do you call the universe as a whole?

 

[A. Neumaier]

what do you call the universe as a whole?

The smallest thermally, mechanically, and chemically isolated physical system that contains the Earth.

There is just one of these, and it is the only system that deserves this name.

 

[Andrew Wright]

The weirdness is not in quantum mechanics itself (the math and its relation to the applications, which works very smoothly) but only in the way it is translated into ordinary language. So the latter must be changed if things shall improve. My gradual discovery was that it can be changed to a considerable extent, with remarkable consequences.

The terminology for discussing quantum mechanics is certainly weird for onlookers. In re-translating quantum mechanics into ordinary language, is there a danger of creating even more weirdness?

 

[martinbn]

The smallest thermally, mechanically, and chemically isolated physical system that contains the Earth.

There just one of these, and it is the only system that deserves this name.

My question was about something else. If you have no preferred time coordinate it makes no sense to talk about the universe. It is a frame depended consept.

 

[A. Neumaier]

In re-translating quantum mechanics into ordinary language, is there a danger of creating even more weirdness?

Like in every translation, one has to choose the best substitute in the language translated to. if it is done well, the content of the original comes through fairly faithfully. if it is done poorly, it comes through in a weird, distorted way.

The fact that the math in quantum mechanics and nonrelativistic quantum field theory is completely unproblematic but the traditional translation into ordinary language is weird shows that the current translations are very poor. The quality of retranslation depends on the care with which the correspondences are made. If it is done well, the amount of weirdnes drops drastically. But if done poorly, it may well end up even more weird.

 

[ddd123]

The smallest thermally, mechanically, and chemically isolated physical system that contains the Earth.

There just one of these, and it is the only system that deserves this name.

Are you assuming its existence?

 

[A. Neumaier]

If you have no preferred time coordinate it makes no sense to talk about the universe. It is a frame depended concept.

No. In flat spacetime (which suffices for the present discussion as nonrelativistic quantum field theory and QED presuppose that), the objective universe is described by 4-dimensional Galilei or Minkowski space and the fields in it. Choosing a frame only reveals the subjective view of the universe a particular observer gets.

The situation is analogous in curved spacetime. Here the objective universe is described by a curved 4-dimensional manifold, and the fields in it. The observer-dependent subjective views at any given time are given by a Cauchy surface and the approximations of the fields that reflect the observer's knowledge or assumptions. (Because of the unresolved issues in quantum gravity it does not make sense to discuss this question assuming quantized geometry - at least not in this part of the forum.)

 

[A. Neumaier]

Are you assuming its existence?

Of course, otherwise we wouldn't be able to have this discussion.

 

[ddd123]

Of course, otherwise we wouldn't be able to have this discussion.

I mean, that of an isolated system. If the universe is infinite, it's never isolated.