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1. Mar 31, 2018

### Demystifier

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

2. 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.

3. 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) ;-)).

4. 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).

5. 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.

6. 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.

7. 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).

8. 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.

9. 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

10. 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.

11. Mar 31, 2018

### 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.

12. Mar 31, 2018

### RUTA

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.

13. Apr 1, 2018

### vanhees71

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.
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.

14. Apr 1, 2018

### vanhees71

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.

15. Apr 1, 2018

### vanhees71

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.

16. Apr 1, 2018

### Lord Jestocost

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

17. Apr 1, 2018

### atyy

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

Is Nature different from reality?

18. Apr 1, 2018

### Lord Jestocost

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.

19. Apr 1, 2018

### Peter Morgan

atyy's comment,
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.

20. Apr 1, 2018

### Lord Jestocost

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).”