Insights Interview with Astrophysicist: Adam Becker - Comments

[atyy]

The cut is also only in certain flavors of Copenhagen!

The cut is in all flavours of Copenhagen.

There's no clear definition of it,

True, the cut is subjective.

and there's no known limit to the validity of quantum theory also for macroscopic systems. It's only a technical problem of state preparation preventing us from measuring "quantum properties" of macroscopic objects. In any case there are some empirical examples that prove the existence of predicted quantum effects like entanglement, as for example the experiment entangleling vibration modes of diamonds over some distance (working even at room temperature on a usual lab desk).

True, the cut can be shifted, so anything can be moved from the classical side of the cut to the quantum side of the cut.

However, you cannot put the whole universe, including all observers on the quantum side of the cut, with nothing left on the classical side. People try to do so, but that requires an attempted solution to the measurement problem, eg. Many Worlds or Bohmian Mechanics.

 

[zonde]

Well, this review is, however, also a bit besides the point. I've not read Becker's book, but to think that any of the physics problems, some seem still to claim to be existent, could be solved by philosophy is wishful thinking. There's not a single example in the history of science in the modern sense (which with about 400 years is however not that old yet) where philosophical considerations have solved any physical problem.

This is strawman attack. Philosophy is not rival to physics. Philosophy of science is concerned about physics solutions rather than physics problems.

The rest of the interpretation just reproduces the probabilistic statements of the (minimal) standard interpretation, and the minimal standard interpretation (also known as statistical interpretation) is just a flavor of Copenhagen where all the philosophical mumbo-jambo of some of its followers is stripped off bringing bare bones of physically observable facts into the focus, i.e., there is a theory that predicts the probabilistic outcomes of measurements in real-world labs very well, and that's it.

It is interesting that the author of Statistical interpretation clearly differentiates his interpretation from Copenhagen and describes it the way that can be viewed as generic HV interpretation (wavefunction is not a complete description of individual system).

 

[Lord Jestocost]

Copenhagen usually assumes the existence of reality...

Henry P. Stapp in “The Mindful Universe”:

“In the introduction to his book Quantum Theory and Reality the philosopher of science Mario Bunge (1967, p. 4) said:

The physicist of the latest generation is operationalist all right, but usually he does not know, and refuses to believe, that the original Copenhagen interpretation – which he thinks he supports – was squarely subjectivist, i.e., nonphysical.

Let there be no doubt about this point. The original form of quantum theory is subjective, in the sense that it is forthrightly about relationships among conscious human experiences, and it expressly recommends to scientists that they resist the temptation to try to understand the reality responsible for the correlations between our experiences that the theory correctly describes.”

The confusion arises when one begins to reason about “the experience of WHAT” - maybe, you can call the "WHAT" the "REALITY" in a metaphysical sense. Quantum theory is – so to speak - about that what’s in our head, the varying content of our consciousness. It has nothing to say about the WHAT. The WHAT is of inscrutable nature. And the tremendous fallacy to mistake the map – the content of our conscious – with the territory - the WHAT - leads to pseudo-questions at the heart of quantum theory.

 

[atyy]

Henry P. Stapp in “The Mindful Universe”:

“In the introduction to his book Quantum Theory and Reality the philosopher of science Mario Bunge (1967, p. 4) said:

The physicist of the latest generation is operationalist all right, but usually he does not know, and refuses to believe, that the original Copenhagen interpretation – which he thinks he supports – was squarely subjectivist, i.e., nonphysical.

Let there be no doubt about this point. The original form of quantum theory is subjective, in the sense that it is forthrightly about relationships among conscious human experiences, and it expressly recommends to scientists that they resist the temptation to try to understand the reality responsible for the correlations between our experiences that the theory correctly describes.”

The confusion arises when one begins to reason about “the experience of WHAT” - maybe, you can call the "WHAT" the "REALITY" in a metaphysical sense. Quantum theory is – so to speak - about that what’s in our head, the varying content of our consciousness. It has nothing to say about the WHAT. The WHAT is of inscrutable nature. And the tremendous fallacy to mistake the map – the content of our conscious – with the territory - the WHAT - leads to pseudo-questions at the heart of quantum theory.

As Bell said, presumably, you do not buy life insurance.

 

[vanhees71]

The cut is in all flavours of Copenhagen.
True, the cut is subjective.
True, the cut can be shifted, so anything can be moved from the classical side of the cut to the quantum side of the cut.

However, you cannot put the whole universe, including all observers on the quantum side of the cut, with nothing left on the classical side. People try to do so, but that requires an attempted solution to the measurement problem, eg. Many Worlds or Bohmian Mechanics.

Well, then you'd call the minimal interpretation not a Copenhagen flavor. Fine with with me, although I don't think that it is too much different from what's presented as "Copenhagen Interpretation" in standard textbooks. For me the minimal interpretation is mostly this "Copenhagen Interpretation" omitting the collapse (which is not needed and almost never realized in experiments, except it's necessary to take the effort to do so) and the classical-quantum cut, which is anyway not clearly defined as you agree about above. If you call to put a "classically" behaving macroscopic measurement device a "cut", it's just strange language, and that macroscopic measurement devices behave classically for me is rather explained by decoherence than by some fundamental quantum-classical cut.

 

[vanhees71]

This is strawman attack. Philosophy is not rival to physics. Philosophy of science is concerned about physics
solutions rather than physics problems.

My criticism against philosophy in QT is not that it doesn't solve any problems, but that they pretend that there are problems, where there are none and then confusing the subject by unclear definitions of prime notions like "reality". Thanks to philosophy (starting with the unfortunate EPR paper, which according to Einstein has not brought out his main concerns with QT which was more about inseparability due to entanglement, as he wrote in his Dialectica article of 1948 [*]) the word "reality" has almost lost its usability, because it is not clear anymore what exactly an author using it wants to say ;-)).

[*] A. Einstein, Quanten-Mechanik und Wirklichkeit, Dialectica 2, 320 (1948)
https://doi.org/10.1111/j.1746-8361.1948.tb00704.x

It is interesting that the author of Statistical interpretation clearly differentiates his interpretation from Copenhagen and describes it the way that can be viewed as generic HV interpretation (wavefunction is not a complete description of individual system).

Who is "the author"? Please try to cite clearly; if possible, I guess many in the forums appreciate also a link to a legal source of the paper.

 

[Demystifier]

My criticism against philosophy in QT is not that it doesn't solve any problems, but that they pretend that there are problems, where there are none

Scientific method can solve some problems, but scientific method, by itself, cannot determine what is a problem and what is not. Your criticism against philosophy in QT is a philosophy itself.

 

[Peter Morgan]

My criticism against philosophy in QT is not that it doesn't solve any problems, but that they pretend that there are problems, where there are none and then confusing the subject by unclear definitions of prime notions like "reality".

So, going back to your earlier comment,

The outcome is very clear: If there is a deterministic HV theory that could reproduce the probabilistic predictions of QT (which in fact were shown to be correct, and the Bell inequality is violated as predicted by QT!) it must be non-local, and it's obviously hard to produce non-local HV theories in accordance with Einstein causality.

do you consider that whatever nonlocality there is in QM/QFT is not a problem? Of course microcausality is satisfied, so there is not that kind of nonlocality, but still there is, say, Hegerfeldt nonlocality (for references relevant to that, please see https://www.facebook.com/max.derakshani/posts/10103068335632754?comment_id=10103069043593994 and the comments that follow). Personally, I agree that the modern focus of philosophers specifically on "reality", whatever that means beyond hammering the desk, is perhaps excessive — I prefer a rather heavier dose of empiricism and calibrated acceptance of current theories.

 

[Demystifier]

Maybe, there's a masochistic side in me

Many great physicists turn into philosophers later. Maybe you are getting old.

 

[vanhees71]

Scientific method can solve some problems, but scientific method, by itself, cannot determine what is a problem and what is not. Your criticism against philosophy in QT is a philosophy itself.

Well, in physics there are a lot of problems determined within physics itself and some are solved and some are unsolved. That there is a "measurement problem" in QT for me is disproven by evidence since experimentalists and theorists can very well design and analyze experiments using QT. If this is philosophy, that's fine with me ;-))).

 

[Demystifier]

Well, in physics there are a lot of problems determined within physics itself and some are solved and some are unsolved. That there is a "measurement problem" in QT for me is disproven by evidence since experimentalists and theorists can very well design and analyze experiments using QT. If this is philosophy, that's fine with me ;-))).

Define "physics itself". I think there is no such thing.

 

[RUTA]

I’ve been studying, researching, and teaching physics for nearly 40 years with one motive — to make ontological inferences and use those to create new theory. These motives are germane to foundations of physics, so I’ve been participating in that community for the past 24 years. Different physicists have different motives for putting in the hard work needed to do research in physics. Whether or not someone’s motives are “worthwhile” is purely a value judgment. If you’re not interested in foundations of physics, don’t participate in those discussions.

 

[vanhees71]

So, going back to your earlier comment,

do you consider that whatever nonlocality there is in QM/QFT is not a problem? Of course microcausality is satisfied, so there is not that kind of nonlocality, but still there is, say, Hegerfeldt nonlocality (for references relevant to that, please see https://www.facebook.com/max.derakshani/posts/10103068335632754?comment_id=10103069043593994 and the comments that follow). Personally, I agree that the modern focus of philosophers specifically on "reality", whatever that means beyond hammering the desk, is perhaps excessive — I prefer a rather heavier dose of empiricism and calibrated acceptance of current theories.

I couldn't sympathise more with poor Gross. It's hopeless to discuss with philosophers about the fact that local and microcausal relativistic QFT (as is applied with more success than wanted in the Standard Model) do not imply "spooky action at a distance", as claimed about QT in the EPR paper (which in fact Einstein was not quite satisfied with since he felt that his problems with QT are not well represented in this paper; his view becomes much clearer in his article in Dialectica 2, 320 (1948)). In fact, it's the collapse hypothesis of (some flavors of the) Copenhagen interpretation, which clearly contradicts the very construction of standard QFT and the meaning of the S matrix (see the first few chapters in Weinberg, QT of Fields, vol. 1, particularly the chapter on the linked-cluster theorem). Gross is of course referring to the state-of-the-art QFT of the 21st century and has as much problems with making sense of the EPR paper.

I've no clue what "Hegerfeldt nonlocality" is though. Do you have a reference (preferrable a physics one, where one has clear statements and a sufficient math density rather than some unclear philosophical gibberish) ;-)).

 

[vanhees71]

I’ve been studying, researching, and teaching physics for nearly 40 years with one motive — to make ontological inferences and use those to create new theory. These motives are germane to foundations of physics, so I’ve been participating in that community for the past 24 years. Different physicists have different motives for putting in the hard work needed to do research in physics. Whether or not someone’s motives are “worthwhile” is purely a value judgment. If you’re not interested in foundations of physics, don’t participate in those discussions.

I'm very interested in foundations of physics, but I don't think that philosophy helps to formulate the foundations clearly. To the contrary, philosophy tends to obscure clearly-defined notions (as "locality", "causality", etc) which have a very clear meaning and quantitative description in physics in terms of the most fundamental theories (relativistic local and microcausal QFT and GR).

 

[vanhees71]

Define "physics itself". I think there is no such thing.

Of course there are plenty of problems in physics completely unrelated to philosophy. If this was not the case there'd be no necessity for pure physics research anymore. Fortunately we are far from such a sad state!

Take as an example the discovery of quantum theory. There was a well-posed physics problem in the 19th century to find the spectral distribution of black-body radiation, whose solution lead to modern relativistic QFT (which is imho the first complete solution of the problem; Planck 1900 and Einstein 1917 being important steps towards this solution). This is a typical problem within the natural sciences with no philosophical pseudoproblem around: You simply didn't know the distribution of black-body radiation. Then it was measured with high accuracy at the PTR around 1900, and using the data Planck found the correct spectrum as an empirical formula. Then his problem was to derive it from theory, which was not possible using the then established classical electrodynamics, thermodynamics, and classical statistical physics. He found an ad-hoc explanation in terms of "energy quantization" (where energy is meant to be the exchange energy between the em. field and the cavity walls in Planck's idealized oscillator model). This left him (and also Einstein) quite unsatisfied. The next very important step was Einstein's kinetic-theory treatment of 1917, which lead to the discovery of spontaneous emission, which in fact we know today is not explainable other than by field quantization! This was finally the important notion for Dirac to come up with his annihilation-creation-operator formalism in 1927 (although Jordan had already quantized the em. field in the "Dreimännerarbeit" in 1926 before, but that was not noticed by the community; I've to read that paper carefully to figure out, to guess why).

Another example, which is more a theoretical problem, is the discovery of special relativity. The Maxwell theory of electromagnetism was more or less established at the end of the 19th century (mostly due to the creation and detection of electromagnetic waves by H. Hertz in 1887). There was, however, a theoretical problem, because the theory is not Galilei invariant. Of course, the common opinion at the time was the presence of a preferred frame of reference in terms of the restframe of the aether, but the attempts to empirically prove the latters existence failed. That's why many physicists and mathematicians like Fitzgerald, Lodge, Lorentz, Poincare, and finally Einstein were investigating this problem, which although purely theoretical is clearly a problem within physics as a natural science and not one of some philosophy.

 

[Peter Morgan]

I've no clue what "Hegerfeldt nonlocality" is though. Do you have a reference (preferrable a physics one, where one has clear statements and a sufficient math density rather than some unclear philosophical gibberish) ;-)).

From a few comments down the Facebook comment thread after the Facebook comment I mentioned above:

I'd offer either Hegerfedt's https://arxiv.org/abs/quant-ph/9809030 or his https://arxiv.org/abs/quant-ph/9806036, which link to the conference-published papers and which both cite what I think of as rather less clear papers from the 1970s and 1980s. These two papers include a helpfully more abstract presentation as a Theorem, with the conditions better stated.
My own take on this is that Hegerfeldt's nonlocality, by depending on analyticity deriving from a positive energy condition, is sui generis with the Reeh-Schlieder theorem, however it depends on much less mathematical structure than algebraic QFT, LQP, etc, so that even someone dismissive of the mathematics of algebraic QFT should have a hard time dismissing Hegerfeldt nonlocality.
However, as I say above [in the Facebook comment thread], we can face Hegerfeldt nonlocality with a reasonable degree of equanimity because it is compatible with Lorentz invariance. A further aspect, although this is not something that I would expect a QFTist to find compelling (but who knows?), is that boundary and initial conditions, which are by their very nature nonlocal, determine which (Lorentz invariant) propagator should be used in classical physics, with extensive consequences.

I should add that the philosopher I was engaging with there, Max Maaneli Derakhshani, has more-or-less refused to engage subsequently on the more careful, indeed more-or-less axiomatic, characterization of different kinds of locality. Hegerfeldt is essential reading, IMO, although if you know of something that more satisfyingly characterizes different kinds of nonlocality, I'll be very pleased to hear of it. Of course axiomatization is often disdained by physicists as "too much mathematics", which can be almost as much a smear as "too much philosophy". On the other hand, the best philosophy of QFT literature is almost indistinguishable from axiomatic QFT.

Probably I should add that Hegerfeldt is essentially a physicist.

 

[vanhees71]

The first cited paper investigates relativistic classical fields interpreting them in terms of first-quantized wave mechanics a la Schrödinger in the non-relativistic case. I don't think that in the year 2018 we still have to discuss why this doesn't work and why one has to employ relativistic quantum field theory to precisely cure this problem with apparent acausality. It's discussed in any textbook (see, e.g., Peskin-Schroeder).

 

[PeterDonis]

philosophy tends to obscure clearly-defined notions (as "locality", "causality", etc) which have a very clear meaning and quantitative description in physics in terms of the most fundamental theories (relativistic local and microcausal QFT and GR)

The way I read philosophers on this is that they are not so much questioning the precise quantitative descriptions as physics, as questioning whether they properly capture our intuitive sense of the ordinary language terms "locality", "causality", etc.

I agree with you that the latter quest is, in the end, a fool's errand, because if our ordinary language intuitions conflict with the precise quantitative physics that has been confirmed to umpteen decimal places by experiment, then what needs to change is our ordinary language intuitions, not the physics. But philosophers don't seem to like that very much, which is not surprising, since our ordinary language intuitions are the basis of their entire discipline.

 

[zonde]

Who is "the author"? Please try to cite clearly; if possible, I guess many in the forums appreciate also a link to a legal source of the paper.

Author is Ballentine. The book is Quantum Mechanics A Modern Development (1998). p47:
In classical mechanics the word “state” is used to refer to the coordinates and momenta of an individual system, and so early on it was supposed that the quantum state description would also refer to attributes of an individual system. ... However, such assumptions lead to contradictions (see Ch. 9), and must be abandoned.
The quantum state description may be taken to refer to an ensemble of similarly prepared systems. One of the earliest, and surely the most prominent advocate of the ensemble interpretation, was A. Einstein. His view is concisely expressed as follows [Einstein (1949), quoted here without the supporting argument]:
“The attempt to conceive the quantum-theoretical description as the complete description of the individual systems leads to unnatural theoretical interpretations, which become immediately unnecessary if one accepts the interpretation that the description refers to ensembles of systems and not to individual systems.”
and look at chapter 9.3. The Interpretation of a State Vector

 

[Peter Morgan]

The first cited paper investigates relativistic classical fields interpreting them in terms of first-quantized wave mechanics a la Schrödinger in the non-relativistic case. I don't think that in the year 2018 we still have to discuss why this doesn't work and why one has to employ relativistic quantum field theory to precisely cure this problem with apparent acausality. It's discussed in any textbook (see, e.g., Peskin-Schroeder).

One can't talk about relativistic quantum field theory "precisely", at least in 3+1-dimensions, except about free quantum fields, because interacting relativistic QFTs, again in 3+1-dimensions, only exist as asymptotic expansions, for which discussion is necessarily imprecise. In 1+1- or 2+1-dimensions, where there are models of the Wightman axioms, the Reeh-Schleider theorem is effectively the same as Hegerfeldt nonlocality.
To discuss free Wightman fields in 3+1-dimensions, one can consider as a simplest example the variance $\hat\phi_f^2$ of an observable $\hat\phi_f=\hat\phi_f^\dagger$ in the state $\frac{\langle 0|\hat\phi_g^\dagger\hat A\hat\phi_g|0\rangle}{\langle 0|\hat\phi_g^\dagger\hat\phi_g|0\rangle}$, that is, the expression $\frac{\langle 0|\hat\phi_g^\dagger\hat\phi_f^2\hat\phi_g|0\rangle}{(g,g)}=(f,f)+2\frac{(g,f)(f,g)}{(g,g)}$, where $(f,g)=\langle 0|\hat\phi_f^\dagger\hat\phi_g|0\rangle$ is a vacuum expectation value (which is enough to fix the Gaussian free field.)
This expression shows that the variance of the observable $\hat\phi_f$ is modified by the absolute value $|(f,g)|^2$ in the vector state $\hat\phi_g|0\rangle/\sqrt{(g,g)}$. Of course it is the case that measurements $\hat\phi_f$ and $\hat\phi_g$ commute if $f$ and $g$ are at space-like separation, but $|(f,g)|^2$ in general is non-zero. Another way to state this is that $[\hat\phi_f,\hat\phi_g|0\rangle\langle 0|\hat\phi_g]\not=0$ even if $f$ and $g$ are at space-like separation. This simple computation shows that the relationship of state preparation to measurement is different from the relationship between two measurements; it can be dismissed as about free fields, which can be said to be not physically relevant, and the Reeh-Schlieder theorem (which subsumes this simple computation) can be dismissed as about Wightman fields, which can also be said to be not physically relevant, however interacting QFT would agree that $[\hat\phi_f,\hat\phi_g|0\rangle\langle 0|\hat\phi_g]\not=0$ in general, so there seems to me to be a prima facie case for there being some value in identifying and characterizing different kinds of nonlocality, not only repeating "microcausality", powerful though that indubitably is.
Finally, you're right about the first Hegerfeldt paper I cited; in future I will cite only the second paper, which I think enough applies to the relativistic case as well as to the nonrelativistic case to be at least of historical interest to anyone who wishes to understand nonlocality/locality in QFT.

 

[diPoleMoment]

So philosophy seems to be discussing the validity of language and scientific reasoning. For the latter, the exploration in the scientific reasoning for a science that has not had much deductive evidence seems worthwhile. To add to that statement with the change of language, the nuances of what was originally meant to what is understood today could affect the interpretation of what was the original intention. However this is just skimming the surface knowledge that I have gained.

 

[RUTA]

I'm very interested in foundations of physics, but I don't think that philosophy helps to formulate the foundations clearly. To the contrary, philosophy tends to obscure clearly-defined notions (as "locality", "causality", etc) which have a very clear meaning and quantitative description in physics in terms of the most fundamental theories (relativistic local and microcausal QFT and GR).

You’re missing the point, quantum nonlocality and delayed choice experiments are analyzed within experimental limits using non-relativistic QM. So, obviously, Lorentz invariance does nothing to abate these mysteries. Now let’s look at some problems in physics that can actually be resolved with philosophy, i.e., the problematic initial conditions of big bang cosmology known as the low entropy problem, the horizon problem, and the flatness problem.

These are indeed problems in physics, as evidenced by the creation of inflationary cosmology whose practitioners are physics professors at highly regarded institutions. How could mere philosophy resolve these problems? We explain that at length in chapter 3 of our book, but the short answer is that all we have to do as physicists is move from dynamical explanation per the Newtonian Schema Univese to block universe explanation per the Lagrangian Schema Universe. Those problems are created by physicists’ dynamical bias, as pointed out by ... philosophy of physics. You may not like the answer, but it is an answer from philosophy for a problem in physics. If you want to argue about it, we’ll have to take that to another thread. Let’s try to keep this thread on topic, i.e., Adam’s book.

 

[vanhees71]

The way I read philosophers on this is that they are not so much questioning the precise quantitative descriptions as physics, as questioning whether they properly capture our intuitive sense of the ordinary language terms "locality", "causality", etc.

Physics is about objective reproducible quantitative observations in nature, and theoretical physics aims at a mathematical description and the derivation of the observable phenomena from as little assumptions (fundamental Laws of Nature, themselves finally always based on empirical evidence) as possible. This implies also the aim to adapt our intuitive sense for whatever ideas we have about nature. Locality and causality have a very clear and well-defined meaning in local microcausal relativistic QFT, which is the mathematical basis for the Standard Model of elementary particles. It in my opinion and open question, how to incorporate self-consistently gravitation and spacetime structure, i.e., some theory of "quantum gravity", but that's not a philosophical but purely scientific problem, which I doubt very much to be solvable by pure qualitative "philosophical" thought.

I agree with you that the latter quest is, in the end, a fool's errand, because if our ordinary language intuitions conflict with the precise quantitative physics that has been confirmed to umpteen decimal places by experiment, then what needs to change is our ordinary language intuitions, not the physics. But philosophers don't seem to like that very much, which is not surprising, since our ordinary language intuitions are the basis of their entire discipline.

Ordinary language is inadequate for any kind of physics in the natural sense. Already Galileo new that "the book of nature is written in terms of geometry...". This is still true today, even in a much narrower sense. Of course you have to use a modern idea of geometry, which reaches back to Klein's Erlanger program, but that's another story.

 

[vanhees71]

Author is Ballentine. The book is Quantum Mechanics A Modern Development (1998). p47:
In classical mechanics the word “state” is used to refer to the coordinates and momenta of an individual system, and so early on it was supposed that the quantum state description would also refer to attributes of an individual system. ... However, such assumptions lead to contradictions (see Ch. 9), and must be abandoned.
The quantum state description may be taken to refer to an ensemble of similarly prepared systems. One of the earliest, and surely the most prominent advocate of the ensemble interpretation, was A. Einstein. His view is concisely expressed as follows [Einstein (1949), quoted here without the supporting argument]:
“The attempt to conceive the quantum-theoretical description as the complete description of the individual systems leads to unnatural theoretical interpretations, which become immediately unnecessary if one accepts the interpretation that the description refers to ensembles of systems and not to individual systems.”
and look at chapter 9.3. The Interpretation of a State Vector

Yes sure, that's the minimal interpretation, advocated by Ballentine in his famous RMP article and also in his excellent textbook. For me the probabilistic interpretation taking Born's rule as a fundamental postulate (the only logical way, because attempts to derive Born's rule from the other postulates failed so far; see Weinberg, Lectures on Quantum Mechanics, Cambridge University Press) implies that the predictions of QT can only be experimentally tested on ensembles. Formally, a state is defined as an equivalence class of preparation procedures and as such of course refers to individual systems, because in order to create ensembles the state has to refer to a preparation procedure on a single system, since each ensemble consists of many realizations of the same state (in the sense of a preparation procedure). E.g., at the LHC you have well-defined bunches of protons which in a well defined way collide at specified interaction points, where the detectors are located.

 

[vanhees71]

Finally, you're right about the first Hegerfeldt paper I cited; in future I will cite only the second paper, which I think enough applies to the relativistic case as well as to the nonrelativistic case to be at least of historical interest to anyone who wishes to understand nonlocality/locality in QFT.

By QFT I mean what's used in practice. Of course, I'm aware that QFT is not strictly defined in the mathematical sense, but renormalized perturbative QFT is well defined and obeys all the fundamental properties you expect, including locality of interactions and causality (in the sense of the linked-cluster theorem). In Hegerfeldt's paper it's not clear to me, how he defines his observables. You cannot define particles in transient states in the Heisenberg picture at all. A particle interpretation is only possible for asymptotic free states, which makes it pretty clear that relativistic particles are even less localizable as "little billard balls" than non-relativistic particles. This is all well known since Bohr and Rosenfeld and no contradiction to causality.

 

[Lord Jestocost]

My criticism against philosophy in QT is not that it doesn't solve any problems, but that they pretend that there are problems, where there are none and then confusing the subject by unclear definitions of prime notions like "reality".

Philosophy, per se, is not confusing. It’s merely the person itself which gets confused when philosophy questions his/hers implicit assumptions.

 

[atyy]

Physics is about objective reproducible quantitative observations in nature, and theoretical physics aims at a mathematical description and the derivation of the observable phenomena from as little assumptions (fundamental Laws of Nature, themselves finally always based on empirical evidence) as possible. This implies also the aim to adapt our intuitive sense for whatever ideas we have about nature. Locality and causality have a very clear and well-defined meaning in local microcausal relativistic QFT, which is the mathematical basis for the Standard Model of elementary particles. It in my opinion and open question, how to incorporate self-consistently gravitation and spacetime structure, i.e., some theory of "quantum gravity", but that's not a philosophical but purely scientific problem, which I doubt very much to be solvable by pure qualitative "philosophical" thought.

How come it's not ok to talk about "reality", but it is ok to talk about "Nature"?

Is Nature different from reality?

 

[Lord Jestocost]

My criticism against philosophy in QT is not that it doesn't solve any problems, but that they pretend that there are problems, where there are none and then confusing the subject by unclear definitions of prime notions like "reality". Thanks to philosophy (starting with the unfortunate EPR paper, which according to Einstein has not brought out his main concerns with QT which was more about inseparability due to entanglement, as he wrote in his Dialectica article of 1948 [*]) the word "reality" has almost lost its usability, because it is not clear anymore what exactly an author using it wants to say ;-)).

[*] A. Einstein, Quanten-Mechanik und Wirklichkeit, Dialectica 2, 320 (1948)

Einstein believed “that the notions of physics would refer to a real external world and that these ideas would be set by things that claim a "real existence" independent of the perceiving subjects.” And then he tried to force quantum physics into the corset of his conceptions. Everybody knows how successful he was. “Physics” cannot establish that such beliefs are true, but it can establish that such beliefs are not true. But, instead of learning from Einstein’s convoluted and ultimately entirely unsuccessful attempts, some are still on the quest to find some good elements of “objective reality” in quantum theory. And the “interpretative game” goes on. It’s not the word "reality" that has almost lost its usability, it’s the concept of a physical reality that has lost all its usability.

 

[Peter Morgan]

Physics is about objective reproducible quantitative observations in nature, and theoretical physics aims at a mathematical description and the derivation of the observable phenomena from as little assumptions (fundamental Laws of Nature, themselves finally always based on empirical evidence) as possible. This implies also the aim to adapt our intuitive sense for whatever ideas we have about nature. Locality and causality have a very clear and well-defined meaning in local microcausal relativistic QFT, which is the mathematical basis for the Standard Model of elementary particles. It in my opinion and open question, how to incorporate self-consistently gravitation and spacetime structure, i.e., some theory of "quantum gravity", but that's not a philosophical but purely scientific problem, which I doubt very much to be solvable by pure qualitative "philosophical" thought.

atyy's comment,

How come it's not ok to talk about "reality", but it is ok to talk about "Nature"?

is perhaps too much a cute fussing about words, but I'll further note that there are no “objective reproducible quantitative observations in nature” insofar as events never repeat perfectly. Of course pragmatically a given experimenter makes their choice of what is close enough (perhaps quantitatively, a formal choice of a distance between events, but even in the most meticulous experiments there are also judgement calls), there are "good" experimenters who serve as exemplars of best practice, and there are social conventions that have been honed over centuries that make intersubjective seem objective to those who have been trained in those social conventions, but there is a gap. Research is arguably about getting "out of the box" —or, for some, the straightjacket— that we find ourselves trained into, and creating a new and beautiful box for students to have to get out of in their turn. All of us have some groups of outsiders, people who have been trained into different social conventions than those we have been trained into, to whom we pay some attention. We can and should make our own choices, and perhaps it's OK even to disdain some other groups, but, I suggest, philosophers of physics are too diverse a group, at least as I find them, for physicists to dismiss all of them.
I'll also add that there's no such thing as “pure qualitative "philosophical" thought”, except as a straw man. Most of the philosophers I pay attention to engage in quantitative mathematics of one kind or another.

 

[Lord Jestocost]

His view is concisely expressed as follows [Einstein (1949), quoted here without the supporting argument]:
“The attempt to conceive the quantum-theoretical description as the complete description of the individual systems leads to unnatural theoretical interpretations, which become immediately unnecessary if one accepts the interpretation that the description refers to ensembles of systems and not to individual systems.”

Again, one of Einstein’s fallacies, merely based on his psychological predispositions and his desire to return to the ontology of materialism.

In his book “Chemistry, Quantum Mechanics and Reductionism: Perspectives in Theoretical Chemistry“ Hans Primas cites Fock:

“The deeper reason for the circumstance that the wave function cannot correspond to any statistical collective lies in the fact that the concept of the wave function belongs to the potentially possible (to experiments not yet performed), while the concept of the statistical collective belongs to the accomplished (to the results of experiments already carried out) (Fock 1952, 1957).”