QM: Interesting View - Get the Inside Scoop

[A. Neumaier]

That's where SR is today and QM reconstructions have gotten QM to the same place.

You reconstructed only finite-dimensional QM - the subject of quantum information theory. Real particles move and are not covered by this reconstruction.

Thus QM is far from the place where SR is today.

 

[vanhees71]

Agreed. The quantum formalism is flawless, and it's nice that you have found a neater (more economical) presentation. But the question remains what it is that Q(F)T describes. To my mind it's absurd to talk about "quantum objects" that can exist in different states with different properties (observables) at the same time, and that these properties become "real" when "measured". At bottom, QFT describes the correlations between events, and "particle" or "field" are just names we give to special patterns of events in spacetime.

Q(F)T describes Nature as we objectively observe it with our senses and with all kinds of measurement equipment. Everything except the gravitational interaction is described by Q(F)T.

 

[WernerQH]

Q(F)T describes Nature as we objectively observe it with our senses and with all kinds of measurement equipment.

What does it mean to "objectively observe"? Surely you have never watched a photon move through a slit. Everything we talk about is based on inferences. The interpretation of experiments is often compelling, but there is no guarantee that it is the only or the best one. Also the mathematical apparatus is not forced on us (e.g. field operators or functional integrals), and it has a strong effect on our thinking. In my opinion we just haven't yet found the best way to talk about quantum field theory and how it relates to the real world around us.

 

[RUTA]

You reconstructed only finite-dimensional QM - the subject of quantum information theory. Real particles move and are not covered by this reconstruction.

Thus QM is far from the place where SR is today.

I'm not sure what you mean by "real particles move." If you're simply talking about the time-evolution of the state, that is covered by the reconstructions.

 

[vanhees71]

What does it mean to "objectively observe"? Surely you have never watched a photon move through a slit. Everything we talk about is based on inferences. The interpretation of experiments is often compelling, but there is no guarantee that it is the only or the best one. Also the mathematical apparatus is not forced on us (e.g. field operators or functional integrals), and it has a strong effect on our thinking. In my opinion we just haven't yet found the best way to talk about quantum field theory and how it relates to the real world around us.

We objectively observer of course never an electromagnetic field let alone a single photon but its reaction with matter (very often the detection is through the photoelectric effect), but this leaves objective data on the action of the electromagnetic field following the laws described by (Q)ED.

Math is just the language to describe our theories. As far as we know the different formulations of relativistic QFT are equivalent. At least they are equivalent within the robust calculations physicists do. Neither formulation is mathematically strictly defined beyond (resummed) perturbation theory.

The great success of relativistic QFT (aka the Standard Model and effective theories derived from it) shows that we grasp at least a great portion of its meaning in relation to the observables made, which are described by it.

 

[rogeragrimes]

Everyone should read Philip Ball's Beyond Weird. Changed the way I think about quantum mechanics the rest of my life. And some of the things he says, against the Copenhagen interpretation, just make more sense (and are very much supported by the math). What he is saying makes so, so, so much more sense than what most new people are told to think about the way QM works and the Copenhagen interpretation. After you understand what Bell is saying you see the Copenhagen interpretation is almost certainly wrong. Also, check out his articles on Quantum Darwinism.

 

[RUTA]

Everyone should read Philip Ball's Beyond Weird. Changed the way I think about quantum mechanics the rest of my life. And some of the things he says, against the Copenhagen interpretation, just make more sense (and are very much supported by the math). What he is saying makes so, so, so much more sense than what most new people are told to think about the way QM works and the Copenhagen interpretation. After you understand what Bell is saying you see the Copenhagen interpretation is almost certainly wrong. Also, check out his articles on Quantum Darwinism.

Concerning the Copenhagen interpretation see also Adam Becker’s book What is Real?

 

[WernerQH]

We objectively observe of course never an electromagnetic field let alone a single photon but its reaction with matter (very often the detection is through the photoelectric effect), but this leaves objective data on the action of the electromagnetic field following the laws described by (Q)ED.

I'm not calling into question the huge success of QED and the objective reality of the experiments. My concern is the "photon" (or electron) concept and its connotations which are misleading and mostly left implicit. Of course you know that a photon is not a little bullet that moves from A to B. Most people think that it is "something" that exists from the time of its creation to the time of its absorption. I contend that it is more useful (and closer to the actual formalism of QED) to think of a photon as nothing but a pair of emission and absorption events. Only these events are real, and whatever "connects" them in spacetime is useful fiction to capture the ubiquitous correlations between events (aka propagators).

 

[RUTA]

I'm not calling into question the huge success of QED and the objective reality of the experiments. My concern is the "photon" (or electron) concept and its connotations which are misleading and mostly left implicit. Of course you know that a photon is not a little bullet that moves from A to B. Most people think that it is "something" that exists from the time of its creation to the time of its absorption. I contend that it is more useful (and closer to the actual formalism of QED) to think of a photon as nothing but a pair of emission and absorption events. Only these events are real, and whatever "connects" them in spacetime is useful fiction to capture the ubiquitous correlations between events (aka propagators).

That’s the way we think about it in Beyond the Dynamical Universe. See also the physics part of Re-Thinking the World with Neutral Monism in Entropy.

 

[WernerQH]

That’s the way we think about it in Beyond the Dynamical Universe.

The devil is in the details. The mathematical characterization of events requires a bit more than time and location. Does your book expand on this, or is it just discussing general principles?

 

[WernerQH]

After you understand what Bell is saying you see the Copenhagen interpretation is almost certainly wrong.

I dislike the Copenhagen interpretation since the first instant that I encountered it (1973)!
But I must say that I found Ball's talk disappointing. (More disappointing than I have come to expect for talks on this topic anyway.) I also regret to have bought his book. There was nothing that I found illuminating.

 

[vanhees71]

So what's your favorite interpretation?

I think Copenhagen is just fine, if you take away the confusing philosophy by Bohr and (even worse) Heisenberg and just stick to the no-nonsense formulations by Born, Jordan, and Dirac. The only interpretation needed is that quantum states (statistical operators) describe probabilities for the outcome of measurements at a time $t>t_0$ given the preparation of the system at some time $t_0$. There's no need for collapse nor a quantum-classical cut. That's called the "minimal statistical interpretation".

 

[A. Neumaier]

stick to the no-nonsense formulations by Born, Jordan, and Dirac. [...] There's no need for collapse nor a quantum-classical cut.

Dirac explicitly requires the collapse that you dislike!

 

[vanhees71]

That's why I added the last quoted sentence.

 

[A. Neumaier]

That's why I added the last quoted sentence.

But in a generalized form the collapse is also part of quantum information theory, where it figures as a (frequent) special case of a pure quantum channel.

Thus that you don't need the collapse (because you replace it by an equivalent amount of handwaving) does not mean that it is not needed at all!

 

[vanhees71]

Characteristically nowhere in the quoted wikipedia article you find the word collapse. I think there's not more to "collapse" than the special case of state preparation by filtering. E.g., like preparing an ensemble of Ag atoms in a Stern Gerlach experiment with a determined spin-component by dumping all the partial beams not wanted. There's no engimatic "collapse" but just the absorption of particles by matter.

 

[A. Neumaier]

Characteristically nowhere in the quoted wikipedia article you find the word collapse.

This is for two reasons :

1. The first is political, since there are many like you who take offence at that dirty word.
2. The second is pragmatic, since Dirac collapse is somewhat too special for the needs of quantum information applications.

But Dirac's collapse is precisely the special case of a pure quantum channel where the transmission operator $T$ is a projector, corresponding to a von Neumann projective measure-and-prepare situation.

 

[Lord Jestocost]

I dislike the Copenhagen interpretation since the first instant that I encountered it (1973)!

As Ethan Siegel recently puts it:

"Although there are a myriad of interpretations of quantum mechanics that are equally as successful at describing reality, none have ever disagree with the original (Copenhagen) interpretation’s predictions. Preferences for one interpretation over another — which many possess, for reasons I cannot explain — amount to nothing more than ideology."

 

[A. Neumaier]

As Ethan Siegel recently puts it:

"for reasons I cannot explain —

Thus his insight into the problem is quite limited. If he understood, he would not write this.

 

[WernerQH]

So what's your favorite interpretation?

I think Copenhagen is just fine, if you take away the confusing philosophy by Bohr [...]

After the encounter with Ballentine's Rev.Mod.Phys. article I thought I had understood QM. And I'm still convinced that QM is a statistical (stochastic) theory and not deterministic. (I'm astonished that so many people take Schrödinger's equation to be the pinnacle of quantum theory -- it is just a piece of a bigger mathematical machinery that creates the illusion of determinism.)

Of course Bell's inequalities are problematic also for the statistical interpretation if you believe in particles having definite properties at all times. But you don't have to believe in "particles", which are a classical concept anyway. To me, the Aspect et al. experiments show that it is a conceptual dead end to think that "something" travels from the source to the detectors while engaging in superluminal communication. And QFT as a microscopic theory cannot be grounded on a macroscopic concept like "measurement".

 

[Lord Jestocost]

Thus his insight into the problem is quite limited. If he understood, he would not write this.

To my mind, Ethan Siegel's insight mirrors merely that what Paul K. Feyerabend has already remarked some time ago. In his philosophical paper "Problems of Microphysics (1962)" (in "Physics and Philosophy: Volume 4: Philosophical Papers"), Paul K. Feyerabend makes the problem quite clear:

"The issue concerning the foundations of the quantum theory can therefore be solved only by the construction of a new theory as well as by the demonstration that this new theory is experimentally at least as valuable as the theory that is being used at the present time; it cannot be solved by alternative interpretations of the present theory.⁴⁹"

In footnote 49 he adds:

"If I am correct in this, then all those philosophers who try to solve the quantum riddle by trying to provide an alternative interpretation of the current theory which leaves all laws of this theory unchanged are wasting their time. Those who are not satisfied with the Copenhagen point of view must realize that only a new theory will be capable of satisfying their demands." [Italics in original, LJ]

 

[A. Neumaier]

And QFT as a microscopic theory cannot be grounded on a macroscopic concept like "measurement".

Where is your proof of this? All we know about microscopic events is based on macroscopic observations.

 

[A. Neumaier]

To my mind, Ethan Siegel's insight mirrors merely that what Paul K. Feyerabend has already remarked some time ago. In his philosophical paper "Problems of Microphysics (1962)" (in "Physics and Philosophy: Volume 4: Philosophical Papers"), Paul K. Feyerabend makes the problem quite clear:

"If I am correct in this, then all those philosophers who try to solve the quantum riddle by trying to provide an alternative interpretation of the current theory which leaves all laws of this theory unchanged are wasting their time. Those who are not satisfied with the Copenhagen point of view must realize that only a new theory will be capable of satisfying their demands."

The question remains why there are so many competent quantum physicists who disagree. The most likely answer is that the assumption of the quoted paragraph are questionable. Why should he be correct in this? He is just one voice in the myriad of diverging voices. And I bet he is wrong!

 

[WernerQH]

All we know about microscopic events is based on macroscopic observations.

Of course. But how could a microscopic theory ever be found if you insist that it be rigorously derived from classical physics using logical deduction?

John Bell wrote: "And does not any analysis of measurement require concepts more fundamental than measurement? And should not the fundamental theory be about these more fundamental concepts?" (Quantum mechanics for cosmologists)

 

[gentzen]

But how could a microscopic theory ever be found if you insist that it be rigorously derived from classical physics using logical deduction?

But pointing out that our measurements are ultimately based on macroscopic observations is not the same as insisting to derive microscopic theory from classical physics.

I have the impression that you fail to distinguish between "macroscopic phenomena" and "classical physics".

 

[vanhees71]

This is for two reasons :

1. The first is political, since there are many like you who take offence at that dirty word.
2. The second is pragmatic, since Dirac collapse is somewhat too special for the needs of quantum information applications.

But Dirac's collapse is precisely the special case of a pure quantum channel where the transmission operator $T$ is a projector, corresponding to a von Neumann projective measure-and-prepare situation.

There's nothing against this. It's just a special case, depending on the equipment used to prepare the system. It's nothing foundational and it's not anything outside what's described by the quantum formalism.

For me the collapse assumption is the claim that there is "something" (usually not specified by the proponents of collapse interpretations) outside the quantum formalism preparing a system in the eigenstate of the meausured observable. This is not true in most of real-world measurements and also not necessary in the quantum foundations.

If I put a filter, it's simply another macroscopic lump of matter interacting with the quantum system such that it "absorbs" this system under certain conditions and "lets ti through" under other conditions (as in the SG experiment, where the location of the particle is entangled ~100% with the specific spin state, such that I can block particles with those spin states on wanted and keeping only those with one specific spin states. This as not achieved by some magical collapse but simply by the interaction of the blocked particles with the blocking material.

 

[A. Neumaier]

But how could a microscopic theory ever be found if you insist that it be rigorously derived from classical physics using logical deduction?

You might insist; I don't since it is futile. The relation between reality and the mathematics used to describe physics can never be rigorous. Indeed, rigor using logical deduction always entails mathematics, hence is divorced from reality.

The microscopic theory called quantum mechanics was actually found by nonrigorous thinking of physicists like Bohr, Einstein, Born, Heisenberg, and Schrödinger.

John Bell wrote: "And does not any analysis of measurement require concepts more fundamental than measurement? And should not the fundamental theory be about these more fundamental concepts?" (Quantum mechanics for cosmologists)

In practice, the analysis of measurement (as opposed to postulates like Born's rule, which has limited applicability) requires the fundamental theory called quantum field theory. It is about the more fundamental concept of a quantum field. These explain the properties of macroscopic equipment used to define the meaning of measurements to a highly satisfactory extent. Thus we have what John Bell was asking for in this quote.

 

[vanhees71]

After the encounter with Ballentine's Rev.Mod.Phys. article I thought I had understood QM. And I'm still convinced that QM is a statistical (stochastic) theory and not deterministic. (I'm astonished that so many people take Schrödinger's equation to be the pinnacle of quantum theory -- it is just a piece of a bigger mathematical machinery that creates the illusion of determinism.)

Of course Bell's inequalities are problematic also for the statistical interpretation if you believe in particles having definite properties at all times. But you don't have to believe in "particles", which are a classical concept anyway. To me, the Aspect et al. experiments show that it is a conceptual dead end to think that "something" travels from the source to the detectors while engaging in superluminal communication. And QFT as a microscopic theory cannot be grounded on a macroscopic concept like "measurement".

Sure, Ballentine is usually associated with the minimal interpretation, and that's all that's needed to use QT as a physical theory.

I don't believe in "particles having definite properties at all times", because this is disproven by all the Bell tests confirming Q(F)T. QFT is in accordance with all experiments, particularly with these Bell tests and that's why there's no need for the assumption of non-local interactions. One of ther cornerstones of the successful relativistic QFT is locality and microcausality implying the cluster decomposition principle.

The upshot is: The only valid description of properties of particles is what's encoded in the statistical operator describing their "preparation". These described properties are probabilistic and nothing else.

 

[A. Neumaier]

This as not achieved by some magical collapse

This is called collapse by everyone except you. No magic was ever claimed to be involved.

If I put a filter, it's simply another macroscopic lump of matter interacting with the quantum system such that it "absorbs" this system under certain conditions and "lets it through" under other conditions

A polarization filter does not do this. It collapses the state without letting particles through under certain conditions (which conditions would this be?) and absorbing them under all other conditions. A quantum channel accounts for this change of state without explaining how it happens.

Your catch-all explanation of this being due to interaction with a macroscopic lump of matter looks like an intuitive explanation but is not a consequence of the statistical interpretation without collapse.

 

[vanhees71]

An ideal polarization filter let's through the electric-field component in one direction and absorbs the others. It's describable by the interaction of the electromagnetic field with the medium making up the filter. There's no collapse necessary to explain this. It's all described by standard QED.

 

[WernerQH]

Thus we have what John Bell was asking for in this quote.

Obviously we don't. You may be thinking that you've solved the problem. But not everybody shares this view. Why do we have these endless discussions?

 

[Lord Jestocost]

Sure, Ballentine is usually associated with the minimal interpretation, and that's all that's needed to use QT as a physical theory.

Thee are others who usally associate Ballentine with the ensemble interpretation. For example, Maximilian Schlosshauer in „Classicality, the ensemble interpretation, and decoherence: Resolving the Hyperion dispute“ (Found Phys 38, 796–803 (2008)):

"Instead, Wiebe and Ballentine explicitly state that the only role of quantum states is to describe “the probabilities of the various possible phenomena” [13, p. 022109-1]. Thus they presume the ensemble, or statistical, interpretation of quantum mechanics[2].

The key assumption of this type of interpretation is to consider the quantum state as only representing the statistical properties of an ensemble of similarly prepared systems. The ensemble interpretation thus implies that the entire formal body of quantum mechanics (for example, a probability amplitude) has no direct physical meaning, in the sense of having no direct correspondence to the entities of the physical world (see also the comment by Leggett [7]). This interpretation effectively points toward the need for some hidden-variables theory to fully specify the state of individual systems, but it does not actually specify what this “complete theory” would be.“
[bold by LJ]

 

[RUTA]

The devil is in the details. The mathematical characterization of events requires a bit more than time and location. Does your book expand on this, or is it just discussing general principles?

Yes, you still need fields on the lattice (links for gauge fields, nodes for fermion fields). We talk about lattice gauge theory with quantum field theory in general in Chapters 5 and 6.

 

[vanhees71]

Thee are others who usally associate Ballentine with the ensemble interpretation. For example, Maximilian Schlosshauer in „Classicality, the ensemble interpretation, and decoherence: Resolving the Hyperion dispute“ (Found Phys 38, 796–803 (2008)):

"Instead, Wiebe and Ballentine explicitly state that the only role of quantum states is to describe “the probabilities of the various possible phenomena” [13, p. 022109-1]. Thus they presume the ensemble, or statistical, interpretation of quantum mechanics[2].

The key assumption of this type of interpretation is to consider the quantum state as only representing the statistical properties of an ensemble of similarly prepared systems. The ensemble interpretation thus implies that the entire formal body of quantum mechanics (for example, a probability amplitude) has no direct physical meaning, in the sense of having no direct correspondence to the entities of the physical world (see also the comment by Leggett [7]). This interpretation effectively points toward the need for some hidden-variables theory to fully specify the state of individual systems, but it does not actually specify what this “complete theory” would be.“
[bold by LJ]

I can agree with everything except the conclusion in the last bold-faced sentence. Why does this point toward the need for some hidden-variable theory? If one simply takes QT with the probability interpretation of the state a la Born seriously then there is no need for any hidden variables. All there is are the well-known observables, which simply do not take predetermined values but behave as "random variables" in probability theory do, i.e., to analyze there properties, given the quantum state of the system, you have to prepare an ensemble and measure the quantities and do some statistical analysis with the data. That's why you need an ensemble to test QT by experiment. It's implied by the probabilistic interpretation of the quantum state.

So it does not necessarily follow that there must be hidden variables. This only follows under the assumption that the world must behave somehow deterministically on the most fundamental level. The Bell tests tell us that, if this is really true, we'd need a non-local deterministic description. Of course, I cannot disprove yet this possibility, but for sure if one finds any such deterministic theory in accordance with the observations being in accordance with the statistical predictions of QT, it won't be any less strange to our classical-physics-biased "common sense" than QT itself. So even as a philosopher, who has to save his classical prejudices about his worldview, won't be any "happier" with this maybe existing non-local deterministic theory.

For non-relativistic QM we have such a theory, Bohmian mechanics. Though a deterministic reinterpretation of QM, it doesn't bring back much of the "intuitive" classical picture of particles described as mathematical points following precisely predetermined trajectories.

 

[A. Neumaier]

An ideal polarization filter let's through the electric-field component in one direction and absorbs the others. It's describable by the interaction of the electromagnetic field with the medium making up the filter. There's no collapse necessary to explain this. It's all described by standard QED.

No.

Exercise: If the electromagnetic field is in a pure 1-photon state neither parallel nor orthogonal to the polarization direction - does it absorb the field or let it through?