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1. Mar 30, 2018

### atyy

Copenhagen usually assumes the existence of reality. There is the classical/quantum cut, and the classical side (measurement outcomes) is reality. The terminology is bad, so one could also call the cut the macro/micro cut or the real/non-real cut.

2. Mar 30, 2018

### Staff: Mentor

... expressing it with a machine that relies on QM and is on the brink of a revolution, which will rely even more on it.

3. Mar 30, 2018

### vanhees71

No, it's a well-established fact by a plethora of high-accuracy measurements of all kinds of systems from the high-energy-particle experiments at the LHC over quantum optics, atomic, nuclear physics to condensed-matter physics. That's not simply a personal interpretation of a single physicist!

4. Mar 30, 2018

### vanhees71

The cut is also only in certain flavors of Copenhagen! There's no clear definition of it, 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).

5. Mar 30, 2018

### Peter Morgan

One way to do something about this, vanhees71, is to ask for a manifestly Lorentz invariantly constructed random field that is equivalent to a quantum field. One finds that Einstein locality is indeed violated, but it's hard to object to a construction that is Lorentz invariantly constructed and that is equivalent to an empirically successful quantum field (specifically, quantized EM). I'll upload to here a current draft of a paper that derives from my EPL 87, 31002(2009) (the arXiv version is a couple of months old, as of now, and there's been lots of useful feedback from people on Facebook and from other correspondents since then; I intend to submit the paper to JMathPhys soon).
Once one knows how, one can say that it's not so hard.
I gotta say that I think philosophy does something more than nothing for physics, though as in anything there's a lot that doesn't do much for me.

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6. Mar 30, 2018

### atyy

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.

7. Mar 31, 2018

### zonde

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

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8. Mar 31, 2018

### Lord Jestocost

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.

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9. Mar 31, 2018

### atyy

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

10. Mar 31, 2018

### vanhees71

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.

11. Mar 31, 2018

### vanhees71

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

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.

12. Mar 31, 2018

### Demystifier

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.

13. Mar 31, 2018

### Peter Morgan

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.

14. Mar 31, 2018

### Demystifier

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

15. Mar 31, 2018

### vanhees71

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 ;-))).

16. Mar 31, 2018

### Demystifier

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

17. Mar 31, 2018

### 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.

18. Mar 31, 2018

### vanhees71

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) ;-)).

19. Mar 31, 2018

### vanhees71

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).

20. Mar 31, 2018

### vanhees71

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.

21. Mar 31, 2018

### Peter Morgan

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.

22. Mar 31, 2018

### 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).

23. Mar 31, 2018

### Staff: Mentor

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.

24. Mar 31, 2018

### zonde

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

25. Mar 31, 2018

### Peter Morgan

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.