Smolin: Realistic and anti-realistic interpretations of QM

[A. Neumaier]

The single system then is described in terms of probabilities.

What do probabilities mean for a single system? How does this show in the postulates of quantum mechanics?

 

[vanhees71]

I'm not sure I understand your point here. There are no gravitational interactions according to GR. Inertia just bends spacetime. GR describes that. It's predictions agree with observations. What else do you want? Any aesthetic considerations should not be relevant to physics, right?

Well, GR has built in its own failure, namely the unavoidable singularities of all physically relevant solutions. When it comes to the singularities of cosmology ("big bang") and black holes ("Schwarzschild, Kerr" of very compact objects), the physical laws of GR break down, and it's expected that an appropriate quantum treatment "cures" these deficiencies of the classical theory.

Further, there's nothing in GR that forbids to think about gravity as an interaction. The geometrization even can be derived from this ansatz (e.g., due to Weinberg; see also Feynman's book, "The Feynman lectures on gravitation", which is a brillant textbook on the subject).

 

[vanhees71]

What do probabilities mean for a single system? How does this show in the postulates of quantum mechanics?

I answered this question a zillion times. We don't need to go into this issue again (I know, I shouldn't have answered to this thread...).

 

[cube137]

I'm reading Smolin Einstein's Unfinished Revolutions and there is something that perflexed me. Quoting Smolin:

At the same time, there are several reasons pilot wave theory is not entirely convincing as a true theory of nature. One is the empty ghost branches, which are parts of the wave function which have flowed far (in the configuration space) from where the particle is and so likely will never again play a role in guiding the particle. These proliferate as a consequence of Rule 1, but play no role in explaining anything we’ve actually observed in nature. Because the wave function never collapses, we are stuck with a world full of ghost branches. There is one distinguished branch, which is the one guiding the particle, which we may call the occupied branch. Nonetheless, the unoccupied ghost branches are also real. The wave function of which they are branches is a beable.

The ghost branches of pilot wave theory are the same as the branches in the Many Worlds Interpretation. In both cases they are a consequence of having only Rule 1. Unlike the Many Worlds Interpretation, pilot wave theory requires no exotic ontology in terms of many universes, or a splitting of observers, because there is always a single occupied branch where the particle resides. So there is no problem of principle, nor is there a problem of defining what we mean by probabilities. But if one finds it inelegant to have every possible history of the world represented as an actuality, that sin is common to Many Worlds and pilot wave theory.

(Note: Rule 1 is simply "Given the quantum state of an isolated system at one time, there is a law that will predict the precise quantum state of that system at any other time." Smolin called this law Rule 1. "It is also sometimes called the Schrödinger equation. The principle that there is such a law is called unitarity."

Why didn't Demysitifer worry about the ghost branches? Are there many kinds of Bohmians with regards to how they treat the wave function? How does Demystifer and other Bohmians treat it compared to Smolin?

 

[stevendaryl]

But the quote is obviously wrong, because we very well can use quantum theory to describe real-world experiments, and there's both the notion of the state, described by the statistical operator, its deterministic (!) time evolution, given the Hamiltonian of the system, and its probabilistic interpretation.

I think that what David Wallace was saying is obviously right. He didn't say that we can't use quantum theory. He was saying that in practice we treat macroscopic quantities different from microscopic in an ad-hoc way. That's true. It doesn't make quantum theory useless, but it makes it "softly inconsistent", to use my own phrase.

The state is determined by a preparation procedure, and it implies that not all observables of the system takes determined values, but a measurement of these observables give random results with probabilities given by the states. There's no contradiction in the sense of logic.

I think it is a contradiction. To me, the following two claims are just logically inconsistent (together with the rest of the quantum formalism)

1. A measurement always produces an eigenvalue of the quantity being measured.
2. Measurement devices and observers are themselves described by quantum mechanics.

 

[zonde]

Well, GR has built in its own failure, namely the unavoidable singularities of all physically relevant solutions. When it comes to the singularities of cosmology ("big bang") and black holes ("Schwarzschild, Kerr" of very compact objects), the physical laws of GR break down, and it's expected that an appropriate quantum treatment "cures" these deficiencies of the classical theory.

These hypothetical singularities can not be observed. Why should we care about them?

Further, there's nothing in GR that forbids to think about gravity as an interaction. The geometrization even can be derived from this ansatz (e.g., due to Weinberg; see also Feynman's book, "The Feynman lectures on gravitation", which is a brillant textbook on the subject).

Of course, just because there is one valid theory does not mean there can't be other theory describing the same phenomena. But we have a theory that correctly predicts observable phenomena. So why bother?

 

[charters]

What do you mean by this question? If somebody starts the question with "what's the nature/mechanism...", usually he or she has a conceptual misunderstanding what the natural sciences are methodologically aiming at

I am asking if you think the wavefunction of, say, an electron, describes 1) a new sort of spatially extended object, 2) a 0D classical point whose position is simply unknown, or 3) nothing but the probability of a classical detector click. I believe from you other comments your view is 3, in which case the measurement problem is akin to wondering: where did all these classical detectors come from in the first place? Why do we say they are made of electrons, if electrons on the quantum scale have no purchase as objectively existing objects?

My sense is your answer is going to be "who cares, the theory works." That's fine, but don't confuse not being interested in a problem with whether others are wrong to identify the problem as legitimate. Math works very well in practice, but Godel's incompleteness theorem is still an issue to contend with. The MP is similar in form.

 

[vanhees71]

I think that what David Wallace was saying is obviously right. He didn't say that we can't use quantum theory. He was saying that in practice we treat macroscopic quantities different from microscopic in an ad-hoc way. That's true. It doesn't make quantum theory useless, but it makes it "softly inconsistent", to use my own phrase.
I think it is a contradiction. To me, the following two claims are just logically inconsistent (together with the rest of the quantum formalism)

1. A measurement always produces an eigenvalue of the quantity being measured.
2. Measurement devices and observers are themselves described by quantum mechanics.

Why is there a contradiction? Is there any empirical evidence that 1. is wrong? If that were the case, there'd be a big crises of QT as a whole, and every theorist would struggle to find an alternative theory ;-).

Concerning 2. it's to some degree a matter of taste whether you except the standard quantum-statistical arguments as "description" of the measurement devices or not. So again, about this you can fight enternelly without coming to any conclusion either.

That there's any necessity to also describe us as quantum systems too to solve the "measurement problem" is somewhat exotic to me, because there's really no more direct interaction between us and the measured system (except in the case that you consider our own senses as measurement device of quantum systems, like the very interesting possibility to use our eyes directly as single-photon detectors which seems to be possible in principle according to new studies on the subject).

 

[vanhees71]

These hypothetical singularities can not be observed. Why should we care about them?Of course, just because there is one valid theory does not mean there can't be other theory describing the same phenomena. But we have a theory that correctly predicts observable phenomena. So why bother?

What other theory you are talking about. GR is GR, no matter whether you describe gravity as an interaction or insist on the quite common interpretation that it's entirely a kinematical effect of curved spacetime. For me the interpretation of gravity as an interaction as all the other fundamental interactions (i.e., electroweak and strong interactions) is of some attraction, because it's simplifying and unifying the picture, but that's again just a matter of personal taste of little importance in the sense of science.

 

[vanhees71]

I am asking if you think the wavefunction of, say, an electron, describes 1) a new sort of spatially extended object, 2) a 0D classical point whose position is simply unknown, or 3) nothing but the probability of a classical detector click. I believe from you other comments your view is 3, in which case the measurement problem is akin to wondering: where did all these classical detectors come from in the first place? Why do we say they are made of electrons, if electrons on the quantum scale have no purchase as objectively existing objects?

My sense is your answer is going to be "who cares, the theory works." That's fine, but don't confuse not being interested in a problem with whether others are wrong to identify the problem as legitimate. Math works very well in practice, but Godel's incompleteness theorem is still an issue to contend with. The MP is similar in form.

The wave function describes probabilities for measurement results no more no less. It's wrong to say an electron is the wave function (the more that at the most fundamental level wave functions do not make much sense but are quantized themselves). So indeed I think 3) describes my point of view best.

The classical detectors come from the physicists' curiosity to learn more about nature. That's why they build with some effort ever better ones (and that's often pretty expensive and we can be lucky to get them financed by tax-payers money).

That matter around us, and thus also measurement devices, are made of atomic nuclei and electrons is the conclusion that we very well understand their properties as many-body systems with atomic nuclei and electrons as the relevant degrees of freedom. That's also known as condensed-matter physics and a very successful application of quantum (field) theory. It's so successful that we have all the funny gadgets like the laptop I'm writing this text on and also to construct ever better measurement devices for all kinds of measurements on quantum systems down to the most fundamental building blocks, as far as we know them, and perhaps one day helping us to find even new ones.

 

[charters]

The wave function describes probabilities for measurement results no more no less. It's wrong to say an electron is the wave function (the more that at the most fundamental level wave functions do not make much sense but are quantized themselves). So indeed I think 3) describes my point of view best.

That matter around us, and thus also measurement devices, are made of atomic nuclei and electrons is the conclusion that we very well understand their properties as many-body systems with atomic nuclei and electrons as the relevant degrees of freedom.

So you claim free electrons do not exist - neither as an extended object nor as a classical point, as in options 1 and 2 in post # 42. But you also claim electrons suddenly do start to exist when composing macroscopic, many body systems.

 

[stevendaryl]

Why is there a contradiction? Is there any empirical evidence that 1. is wrong? If that were the case, there'd be a big crises of QT as a whole, and every theorist would struggle to find an alternative theory ;-).

For a while, maybe. But if no satisfactory alternative theory was found, many of them would shift to pretending that the evidence doesn't REALLY show what it seems to show. In other words, many theorist would just live in denial about it. And that's exactly what we do have.

Concerning 2. it's to some degree a matter of taste whether you except the standard quantum-statistical arguments as "description" of the measurement devices or not. So again, about this you can fight enternelly without coming to any conclusion either.

The reason there is eternal fighting about it is because 1 & 2 are contradictory, and there is no agreement about how to fix it. On the other hand, there is a "rule of thumb" that allows us to get past the contradiction, which is that we have heuristics for when to treat a system as a measurement device obeying 1, and when to treat it as a quantum mechanical system. You can't consistently do both.

Let me sketch a scenario showing that rule 1 is contradictory with rule 2.

Suppose that you have a Stern-Gerlach type situation in which an electron that is spin-up in the z-direction is deflected left, and makes a black spot on the left side of a photographic plate. An electron that is spin-down makes a black spot on the right side.

Now suppose that we set up initial conditions that are precisely left-right symmetric. We send an electron through the Stern-Gerlach device that is spin-up in the x-direction.

According to rule 1, eventually the system evolves into a final state that is not left-right symmetric. Either the electron goes left and makes a spot on the left, or goes right and makes a spot on the right. So the final state does not satisfy the left-right symmetry of the initial state.

That is not possible, if everything obeys the laws of quantum mechanics. If the initial state is left-right symmetric, and the Hamilton is similarly left-right symmetric, then the final state will be left-right symmetric.

 

[DennisN]

I think, as with the question about a consistent QT of gravity, we'd need some empirical evidence clearly indicating that there's a real problem in describing an unanimously observed phenomenon which contradicts QT.

Your quote was specifically about QT of gravity (I think), but I'd like to say that in my opinion the best thing that could happen to quantum physics overall is empirical evidence that contradicts QT. If that would happen, things maybe would start to go in new, interesting directions.

 

[vanhees71]

So you claim free electrons do not exist - neither as an extended object nor as a classical point, as in options 1 and 2 in post # 42. But you also claim electrons suddenly do start to exist when composing macroscopic, many body systems.

I do not claim that free electrons do not exist. Where do you get this from? Of course free electrons exist. They are, of course, neither well described as a classical point particle (there's no consistent description of a classical point particle anyway) nor as a classical extended objects. That's why we use Q(F)T to describe them. According to the standard model they are described as the charged leptons (spin-1/2 Dirac fermions), thus carrying electric and WISO charges but no color charge.

Measurement devices are composed of atomic nuclei (protons and neutrons, which themselves are bound states of quarks and gluons) and electrons. That's what you have asked about not about the existence or non-existence of free electrons. Why should my statement above imply such obvious nonsense of non-existence of free electrons?

 

[vanhees71]

For a while, maybe. But if no satisfactory alternative theory was found, many of them would shift to pretending that the evidence doesn't REALLY show what it seems to show. In other words, many theorist would just live in denial about it. And that's exactly what we do have.
The reason there is eternal fighting about it is because 1 & 2 are contradictory, and there is no agreement about how to fix it. On the other hand, there is a "rule of thumb" that allows us to get past the contradiction, which is that we have heuristics for when to treat a system as a measurement device obeying 1, and when to treat it as a quantum mechanical system. You can't consistently do both.

Let me sketch a scenario showing that rule 1 is contradictory with rule 2.

Suppose that you have a Stern-Gerlach type situation in which an electron that is spin-up in the z-direction is deflected left, and makes a black spot on the left side of a photographic plate. An electron that is spin-down makes a black spot on the right side.

Now suppose that we set up initial conditions that are precisely left-right symmetric. We send an electron through the Stern-Gerlach device that is spin-up in the x-direction.

According to rule 1, eventually the system evolves into a final state that is not left-right symmetric. Either the electron goes left and makes a spot on the left, or goes right and makes a spot on the right. So the final state does not satisfy the left-right symmetry of the initial state.

That is not possible, if everything obeys the laws of quantum mechanics. If the initial state is left-right symmetric, and the Hamilton is similarly left-right symmetric, then the final state will be left-right symmetric.

Obviously I'm to stupid to understand, why there's a contradiction between 1 and 2. Particularly your example of an apparent paradox is completely incomprehensible to me, because it contradicts the standard interpretation of QT. Of course, as Bohr has already analyzed, you cannot perform the SG expeirment with free electrons in practice. So let me put Ag atom instead of electron (because that was the atom Stern and Gerlach used at the time)

If you send an Ag atom with indetermined spin-z component through a Stern-Gerlach apparatus it is deflected with the probabilities given by the corresponding (pure or mixed) state either to the left or the right. That's the point, why Born introduced the probability interpretation of the quantum state in the first place: You cannot split a single Ag atom into pieces, because you never find a smeared classical-field like entity that's smeared according to the wave-function squared, but you always find a single spot on the screen after it run through the SG-magnet. It ends either "to the left" or "to the right" and thus either, through the entanglement between position and spin-z component through the running through the magnet we conclude that the spin component is either up or down, depending on where the electron landed. You can even prepare pure spin-z eigenstates by just looking at the corresponding partial beam (through filtering away all electrons running in the other region of space). I.e., here you have a paradigmatic example for a von Neumann filter measurement. Of course, for the single electron there's no way to predict, which value will be found. You can only say that with the probabilities given by the state the Ag atom is prepared in

In the case of the original experiment it was a beam of Ag atoms from an oven running through a little opening, so it's some mixed state of roughly given by (written as product of the spatial/momentum part and the spin part)
$$\hat{\rho} \propto \int_{\text{aperture}} \mathrm{d}^3 p \exp[-\vec{p}^2/(2m k T)] |\vec{p} \rangle \langle \vec{p}| \otimes \hat{1}_{\text{spin}}/2.$$
Of all Ag atoms the probability for either spin-z component up or down is 1/2, i.e., the symmetric situation you assume. Of course, each single atom will go either the one or the other direction, and for the single atom the situation is not symmetric. Only the probability distribution is symmetric, and that's what's also predicted by QM. This can be even calculated in good approximation analytically (I've still not found the time to write this up completely, but it's really not too complicated).

The final state of the Ag atom is a quantum state again and only describes probabilities, and this distribution obeys the left-right symmetry you rightly assume.

In experimental terms: What's symmetric is the distribution of many Ag atoms, all prepared in the same initial state, running through the setup. Each single atom "breaks" the symmetry of course, landing either "left" or "right" from the symmetry plane.

It's the same with any random experiment. Although a perfectly fair die is symmetric, any outcome is showing 1 of the six edges and thus breaks the cubic symmetry. Only "statistically", i.e., the "average" over many outcomes is symmetric, i.e., the probability for each outcome is the same, 1/6.

 

[charters]

Why should my statement above imply such obvious nonsense of non-existence of free electrons?

Because you reject all possible affirmative claims about where in spacetime the free electron is present. You don't accept it is unsharply, ontologically everywhere that it's quantum state has non-zero amplitude (or amplitude in excess of |0>). You don't accept the Bohmian idea that it is sharply at a single (epistemically uncertain) point. You claim quantum theory is just a theory of detector clicks.

Therefore, you don't believe free electrons are present in spacetime, but somehow pop into spacetime when forming macro objects, and you want to start using chemistry, materials science etc.

 

[PeterDonis]

you reject all possible affirmative claims about where in spacetime the free electron is present

No, he just rejected the two you describe:

You don't accept it is unsharply, ontologically everywhere that it's quantum state has non-zero amplitude (or amplitude in excess of |0>). You don't accept the Bohmian idea that it is sharply at a single (epistemically uncertain) point.

But these by no means exhaust the possibilities. As I understand @vanhees71's position, it is that "electron" is a name for a particular class of states of a particular quantum field (the charged lepton field in the Standard Model). Your description of "unsharply, ontologically everywhere that its quantum state has nonzero amplitude" is a non-relativistic description, so is not a valid description of a quantum field state; and Bohmian mechanics also is a non-relativistic model, so it has the same issue.

 

[charters]

No, he just rejected the two you describe:
But these by no means exhaust the possibilities. As I understand @vanhees71's position, it is that "electron" is a name for a particular class of states of a particular quantum field (the charged lepton field in the Standard Model). Your description of "unsharply, ontologically everywhere that its quantum state has nonzero amplitude" is a non-relativistic description, so is not a valid description of a quantum field state; and Bohmian mechanics also is a non-relativistic model, so it has the same issue.

The particles identified with the particle number eigenstates of free (or asymptotic interacting) QFT is very much an example of unsharp, ontologically extended entities. See the discussion here: https://arxiv.org/abs/quant-ph/0112149

Indeed, it is the impossibility of strict particle localization or even sub-Compton effective localization in QFT (due to Reeh-Schlieder) which partially motivates the picture of unsharp entities.

The options I offered are logically exhaustive. It is a tautology an entity must be either sharply somewhere, unsharply somewhere/everywhere, or nowhere. Each leads to a different aspect of the measurement problem, and I am just trying to walk vanhees to the version most salient to him.

 

[vanhees71]

Because you reject all possible affirmative claims about where in spacetime the free electron is present. You don't accept it is unsharply, ontologically everywhere that it's quantum state has non-zero amplitude (or amplitude in excess of |0>). You don't accept the Bohmian idea that it is sharply at a single (epistemically uncertain) point. You claim quantum theory is just a theory of detector clicks.

Therefore, you don't believe free electrons are present in spacetime, but somehow pop into spacetime when forming macro objects, and you want to start using chemistry, materials science etc.

Well, I like Bohmian mechanics as an alternative interpretation for non-relativistic quantum mechanics. What's still missing is a convincing Bohmian interpretation of relativistic QFT.

Of course, I believe free electrons are present at spacetime. They are well observed and in fact they were the first discovered free elementary particles (Wichert, Thomson 1897). They are described in the Standard model as spin-1/2 Dirac quantum fields. Since they carry no color charge they have asymptotic free states and thus are observable has free particles.

It doesn't make sense to think about them as classical point particles, because this contradicts a plethora of known empirical facts. Despite this, classical relativistic interacting point particles are a mathematical nuissance rather than a simplified description as in non-relativistic physics, but that's another story.

 

[charters]

Of course, I believe free electrons are present at spacetime

Then you have to answer my question of *where* in spacetime you think they are, between measurements. Everywhere/exactly where the quantum state says or somewhere more sharply defined? I have often found when people see the measurement problem as a non-issue, it is because they aren't asking all the relevant questions, such as this one, to fully vet the logical coherence of their views.

Well, I like Bohmian mechanics as an alternative interpretation

But your whole argument has been that the measurement problem is a non-issue, while the purpose of Bohmian mechanics is to try to deal with the measurement problem. What's there for you to like about it?

 

[vanhees71]

To the first point: An electron is described as a Dirac field. A free electron is described as a one-particle Fock state, and it's somewhere. Given the state (i.e., the statistical operator) it is prepared in, there's a probability distribution of its location, no more no less. It doesn't take a determined position, but there's only a probability distribution where it'll be found. I don't see a problem in this, because that's how electrons behave in the lab with high precision.

To the second point: It's funny, how you cut my previous comment. I said, Bohmian mechanics is a nice alternative interpretation of non-relativistic quantum mechanis, but it's incomplete, because there's no convincing non-relativistic version of it and thus is incomplete compared to standard QT.

 

[charters]

It doesn't take a determined position, but there's only a probability distribution

This is a contradiction. A "probability distribution" means the electron has a determined but unknown location. So, I have to ask again, between measurements, do you claim the position is A) determined but unknown or B) objectively undetermined, such that the system is "smeared" across all possible positions?

Alternatively, I would say failing to recognize the significance of this nuance is why you resist the validity of the measurement problem.

To the second point: It's funny, how you cut my previous comment. I said, Bohmian mechanics is a nice alternative interpretation of non-relativistic quantum mechanis, but it's incomplete, because there's no convincing non-relativistic version of it and thus is incomplete compared to standard QT.

But why do you think anything at all is nice about BM/whatsoever care about its completeness, given you deny the measurement problem, which is all BM is about?

 

[PeterDonis]

A "probability distribution" means the electron has a determined but unknown location.

Not if the probability distribution is over measurement outcomes. Such a probability distribution makes no ontological claim at all about the state of the system prior to measurement.

 

[charters]

Not if the probability distribution is over measurement outcomes. Such a probability distribution makes no ontological claim at all about the state of the system prior to measurement.

Correct. This was explicitly anticipated above. It is also where I thought we were heading until vanhees affirmed the ontological claim that free electrons exist in spacetime, taking this option off the table.

 

[PeterDonis]

It is also where I thought we were heading until vanhees affirmed the ontological claim that free electrons exist in spacetime, taking this option off the table.

I don't see why. Saying that there is a probability distribution over measurement outcomes is perfectly consistent with saying free electrons exist in spacetime but don't have a determined position. The first statement, as you agree, makes no ontological claim at all about the state prior to measurement; the second makes an ontological claim about the state prior to measurement that rules out having a determined position, but since the first statement makes no ontological claim, it does not require the electron to have a determined position.

 

[charters]

I don't see why. Saying that there is a probability distribution over measurement outcomes is perfectly consistent with saying free electrons exist in spacetime but don't have a determined position. The first statement, as you agree, makes no ontological claim at all about the state prior to measurement; the second makes an ontological claim about the state prior to measurement that rules out having a determined position, but since the first statement makes no ontological claim, it does not require the electron to have a determined position.

But this isn't where the discussion between vanhees and I is currently focused. In #55 I was careful to ask about the nature of the free electron "between measurements." In #56, vanhees said "there's a probability distribution of its location" not a probability distribution of measurement outcomes. So, I worry this is going to un-focus a discussion in which we were already kind of struggling.

 

[vanhees71]

This is a contradiction. A "probability distribution" means the electron has a determined but unknown location. So, I have to ask again, between measurements, do you claim the position is A) determined but unknown or B) objectively undetermined, such that the system is "smeared" across all possible positions?

Alternatively, I would say failing to recognize the significance of this nuance is why you resist the validity of the measurement problem.
But why do you think anything at all is nice about BM/whatsoever care about its completeness, given you deny the measurement problem, which is all BM is about?

No, it's no contradiction. According to QT the electron doesn't take a determined position, no more no less. The state provides a probability distribution to find an electron at a given position. Where should there be a contradiction? According to QT neither A) nor B) is correct. According to B) the position is objectively determined, but whenever I register an electron it's registered as one electron not some smeared entity.

An interpretation like Bohmian mechanics which doesn't cover relativistic physics is incomplete in comparison to minimally interpreted relativistic QFT. So why should I bother about Bohm's interpretation, which doesn't provide anything except a funny deterministic interpretation of non-relativistic QM but doesn't provide anything more than minimally interpreted QT as far as the physics is concerned and is less complete than the minimally interpreted QT?

 

[Auto-Didact]

According to QT neither A) nor B) is correct. According to B) the position is objectively determined, but whenever I register an electron it's registered as one electron not some smeared entity.

This makes your claim about states in between measurements to either be 1) that such states are purely epistemic, or 2) you outright reject the use of standard logic w.r.t. physics i.e. you are claiming that discussing Nature explicitly requires a more exotic form of logic.

Option 1 is explicitly ruled out by the PBR theorem. Incidentally, Smolin discusses this in the book.

Option 2 has been tried before in a specific implementation known as quantum logic and the consensus is that this form of logic fails, but the issue is still somewhat open. For years I myself believed the need for an exotic logic to be the answer, but I have long since changed my mind; important to note is that such an extension to standard logic seems to be both unwarranted and unnecessary given that there actually are other interpretations which work perfectly well using standard logic and Occam's razor.

 

[member 656954]

First I would recommend the book to anyone who reads or takes part in discussions on QM foundations.

Is it comprehensible for a non-QM expert, do you feel, @Auto-Didact?

 

[Auto-Didact]

Is it comprehensible for a non-QM expert, do you feel, @Auto-Didact?

Yes, I would argue it is. As Smolin makes clear in his talk linked in the first post, he is directly speaking to the public. The book is also written in a style that anyone should be able to read and understand. If you can follow his argument in the talk you should be able to follow his arguments in the book; a minor caveat is that fully understanding a few of the last chapters requires having read his previous books.

 

[member 656954]

Thanks, I'll give it a go

 

[vanhees71]

This makes your claim about states in between measurements to either be 1) that such states are purely epistemic, or 2) you outright reject the use of standard logic w.r.t. physics i.e. you are claiming that discussing Nature explicitly requires a more exotic form of logic.

Option 1 is explicitly ruled out by the PBR theorem. Incidentally, Smolin discusses this in the book.

Option 2 has been tried before in a specific implementation known as quantum logic and the consensus is that this form of logic fails, but the issue is still somewhat open. For years I myself believed the need for an exotic logic to be the answer, but I have long since changed my mind; important to note is that such an extension to standard logic seems to be both unwarranted and unnecessary given that there actually are other interpretations which work perfectly well using standard logic and Occam's razor.

Can you explain, why a purely epistemic interpretation is ruled out by the PBR theorem? PBR assume that there is an "ontic state" beyond the quantum mechanical state. Within QT there is no such thing, but only the quantum state, and its meaning is the probabilistic one given by Born's rule. Whether or not there is something beyond the quantum state or not, is not addressed by quantum theory. If you assume that there's an ontic state, the PBR theorem shows that then the epistemic interpretation of the quantum state leads to contradictions, but I don't assume any such thing as an "ontic state". There's not the slightest hint of something like this in any observation of nature, i.e., I don't see why I need the $\lambda$ of the PBR paper (Nat. Phys. 8, 475 (2012)).

 

[charters]

No, it's no contradiction. According to QT the electron doesn't take a determined position, no more no less. The state provides a probability distribution to find an electron at a given position. Where should there be a contradiction? According to QT neither A) nor B) is correct. According to B) the position is objectively determined, but whenever I register an electron it's registered as one electron not some smeared entity.

I specifically asked about *where* in spacetime you claim the free electron is *between measurements* to avoid this dodge. Saying you believe the ontological premise that the free electron exists in spacetime requires that you commit to a belief about this, independent of any talk about measurements or "finding." I am trying to show that your view is untenable under scrutiny, but you resist the application of this scrutiny by not directly answering plain questions that would lock you to a view and the consequences of it.

So why should I bother about Bohm's interpretation,

That's what I'm asking you. You said you like Bohm, not me.

But ok, I think I'm going to bow out of this discussion, not because you've shown the measurement problem to be trivial, but because there isn't enough recollection of our progress from day to day, so this is going in circles. It will probably be more effective for you to discuss this face to face, I think.

 

[vanhees71]

I don't claim "the electron is between measurements". It doesn't make sense at all. An electron is prepared somewhere in space. It's position is always indetermined due to the Heisenberg uncertainty relation. Thus its about its position there's always only a probability distribution to find it at a given position, no more no less. It doesn't make sense to talk about an electron or any other entity in physics, including macroscopic bodies without talking about their observability.

A measurement is also nothing than the interaction of the electron with other entities, following the fundamental laws of (quantum) physics. Thereby an electron may even be annihilated (e.g., if you let it collide with a positron and in the collision it's annihilated together with the positron into two photons). Thus you can only say an electron has been prepared in some state at time $t$. About its fate, i.e., whether it will still "be somewhere" (necessarily with a more or less uncertain position) or not, I cannot say anything, if I don't know the complete setup. As the example with the positron shows, it can even be annihilated. Then there's no electron left at all. That's all well-described by quantum theory, without any contradictions (neither intrinsic contradictions nor contradictions with any observation, so far).

 

[Auto-Didact]

Can you explain, why a purely epistemic interpretation is ruled out by the PBR theorem? PBR assume that there is an "ontic state" beyond the quantum mechanical state. Within QT there is no such thing, but only the quantum state, and its meaning is the probabilistic one given by Born's rule. Whether or not there is something beyond the quantum state or not, is not addressed by quantum theory. If you assume that there's an ontic state, the PBR theorem shows that then the epistemic interpretation of the quantum state leads to contradictions, but I don't assume any such thing as an "ontic state". There's not the slightest hint of something like this in any observation of nature, i.e., I don't see why I need the $\lambda$ of the PBR paper (Nat. Phys. 8, 475 (2012)).

This post demonstrates a clear misunderstanding of what it means to have an ontology: having an ontology means having an actual existence and being ontic simply means actually existing.

Example: Unicorns (one-horned horses) don't have an ontology (or aren't ontic) in the science of biology.

Similarly, any state that actually exists in any literal sense is by definition an ontic state.

If you don't accept this explanation, you are implicitly committing to option 2 from post #63.