What is the mechanism behind Quantum Entanglement?

[Dragrath]

I figured by now that you think so, but I disagree even though the the link is indeed far fetched seen in the light of the current models.

But as for the general link, there are others that is associating entanglement with potential connections to quantum gravity.
https://arxiv.org/abs/1306.0533

I don't see how the unification of gravity and QM is going to happen in a reasonable way unless one considers and reconstructs some of the foundations of QM.I suspect many creative people keep these speculations private or inside their own heads, and only present the polished results, as it makes the process look cleaner than it really is. Noone wants to read the ugly process of creating a theory that may be wrong. Only once proven right, maybe you can read a little bit about it in biographies or so, but even there I think the ugly turns are omitted, to make it look more sexy.

/Fredrik

I fear you are at least partly correct regarding the expectation in physics that theories against the current mainstream paradigm have to reach a threshold of rigor to even consider or warrant a level of discussion this probably has to do with a large number of ill formed ideas having been proposed by those without a sufficient background in the subject and the deeper philosophical leanings humans inquiries are focused on but there is work showing that the current means for sharing ideas which are not yet fully formed academic preprints are not reaching a wide enough audience given the high rates of information scientists have to deal with as well as the publish or perish imperative. This was discussed in Physics today earlier this year (I believe? It was either that or late last year time is all blurring together on me)

Well, the part you skipped is what motivated the title of the paper. Mermin's challenge is the reason the rest of the paper was written.Brukner and Zeilinger also use this language in "Information and fundamental elements of the structure of quantum theory" where they associate a complete set of complementary spin measurements with a particular reference frame. Establishing what constitutes a reference frame is necessary to using the relativity principle aka "no preferred reference frame" (NPRF), which is the foundation of our answer to Mermin's challenge.Each of the three triplet states is rotationally invariant in a particular plane, as we explain in the paper and I explained in the Insight, "Exploring Bell States and Conservation of Spin Angular Momentum." Putting that together with NPRF per the reference frames of complementary spin measurements tells us that the SU(2) invariance of eigenvalues between different spin measurement operators per Information Invariance & Continuity entails the SO(3) invariance of spin measurement outcomes between those different inertial reference frames. Then add the fact that such measurements are actually measurements of Planck's constant h (Weinberg) and we have an exact analogy with the light postulate, NPRF + c, i.e., we have NPRF + h. So, the "mysteries" of time dilation and length contraction are due to NPRF + c while the "mysterious" Bell state correlations are due to NPRF + h. That's our answer to Mermin's challenge. Very simple, right?For those who are interested in how one might actually prepare a Bell triplet state, see this paper: Dehlinger, D. & Mitchell, M. Entangled photons, nonlocality, and Bell inequalities in the undergraduate laboratory. American Journal of Physics 70, 903–910 (2002).Keep in mind that you're simply making a statement of your ignorance here. These and many other highly accomplished physicists did and do discuss issues concerning the understanding of QM. Once you understand what it is that bothers them, then you can address their concerns (if you so choose) rather than simply expressing the fact that you are ignorant of them."I think I can safely say that nobody understands quantum mechanics." Feynman, Probability and Uncertainty; The Quantum Mechanical View of Nature.

"All of modern physics is governed by that magnificent and thoroughly confusing discipline called quantum mechanics. It has survived all tests and there is no reason to believe that there is any flaw in it. We all know how to use it and and how to apply it to problems; and so we have learned to live with the fact that nobody can understand it." Gell-Mann in The Unnatural Nature of Science, p. 144.

"Everybody who has learned quantum mechanics agrees how to use it. 'Shut up and calculate!' There is no ambiguity, no confusion, and spectacular success. What we lack is any consensus about what one is actually talking about as one uses quantum mechanics. There is an unprecedented gap between the abstract terms in which the theory is couched and the phenomena the theory enables us so well to account for. We do not understand the meaning of this strange conceptual apparatus that each of us uses so effectively to deal with our world. ... What the hell are we talking about when we use quantum mechanics? For practical purposes ordinary everyday quantum mechanics is just fine, and what I have to say is of little or no interest. It is my hope to interest those who, like me, are impractical enough always to have been bothered, at least a bit, by not knowing what they are talking about." Mermin, Making Better Sense of Quantum Mechanics. 2019 Rep. Prog. Phys. 82 012002

You may just have to accept the fact that you will never understand what bothered Einstein, Weinberg, Mermin, Gell-Mann, Feynman, and many others about QM. You simply cannot relate, so you have nothing to contribute to such discussions. I wish I could help you!

Fascinating and articulate way of addressing the measurement problem,it may not have a practical measurable effect but for progressing to unify Quantum mechanics with General relativity such a thing I had noticed similarities between frame of reference shift like behavior especially in context with when I had been looking into Wolfram's computational physics model and the large scale limit for a system evolving under a simple Turing machine if all possible orders of operations were allowed to be computed simultaneously and resolve through interference,

Now note for larger context I'm still quite skeptical that even if their model is correct that they would ever be able to isolate the rule or possibly rules which govern such a computational universe but that is a very different problem, the point of interest for me was how naturally in that limit the model appears to result in the Feynman path integral emerging as a solution to the general formalism of the Einstein field equations in a space of Hamiltonian(energy) states which opened up my mind to the prospects of general relativity playing far more of a role in unifying gravity than has been long assumed. Plus with the quantum Zeno effect naturally being a product of time dilation in such a scenario it sound like a quite promising path towards understanding the measurement problem more broadly. In that light superdeterministic behavior becomes so much more natural as reference frame effects with probabilities in principal potentially representing curvature in this state space of the Feynman path integral, i.e. the nonlocal effects are related to the observers frame of reference shifting rather than an intinsic change in the state of the system. Thus it doesn't surprise me that work if finding potential links between relativity and quantum state observations.
Whoops forgot to post my reply on this

Edit adding latter points while still last comment:

It is "ad hoc" because that is what it is (it flows from no more fundamental consideration)
It clearly doesn't bother you so you have answered my question I believe.. I guess part of me thinks the arrow of time should somehow appear on a celestial billboard !

I think you might get something out of Inhomogenous and anisotropic cosmology by Matthew Kleban and Leonardo Senatore
https://iopscience.iop.org/article/10.1088/1475-7516/2016/10/022/meta

This article while stopping far short of exploring the full implications of its result contains a powerful metamathematical proof by self contradiction which shows that for the general Einstein field equations that all nontrivial flat or open universes must have an irreversible arrow of time. More precisely the theorem shows that for any time slice of spacetime there will always be another larger timeslice that is to say that there can not exist any universal inflection point in the direction of expansion or contraction.

The proof of this theorem the No big crunch theorem as it is defined in context to an initially accelerating universe like we appear to live in based off observations, so no closed timelike geodisics are allowed in a flat or open universe which is kind of self explanatory. More striking however is the mathematical formalism for this total volume of space constraint which will be valid for any frame of reference as it mathematically holds the same form as the second law of thermodynamics with volume in place of entropy. This starts to make more sense if you remember that the Einstein field equations are a system of partial differential equations which mathematically by definition must have a unique solution for each and every possible valid initial condition. Given the infinite nature of flat or open universes this volume link starts to make lot more sense as you can think of a volume in which information can or can not have yet propagated in essence defining an associated entropy link for any nontrivial flat or open universe. I should also note for the discussion of entanglement that this has implications here as it means that the contributions to the metric for every single bit of information on the Universes initial conditions is always nonzero. In Bell's words this would be what he defines as the absence of free will a thought which terrified him but here we can see this is merely an enforced conservation law on information.

After all the same mathematics also properly describes a time reversed and contracting nontrivial flat or open universe.

Thus in this larger context both the irreversible arrow of time and the appearance of nonlocality are relatively automatic consequences of the conservation of information in the context of Noether's theorem. And as is one of the foundational postulates of quantum mechanics it should be no surprise that we see nonlocality in quantum mechanics given that we observe our universe is expanding.

This clearly is an incomplete proof as the means by which it was derived are only applicable for flat or open cosmologies but it does suggest a natural avenue towards quantum gravity particularly the gravitational path integral as the sum of all possible and impossible metric contributions for information in the Universe.

 

[Fra]

I fear you are at least partly correct regarding the expectation in physics that theories against the current mainstream paradigm have to reach a threshold of rigor to even consider or warrant a level of discussion this probably has to do with a large number of ill formed ideas

Yes, yet it's perfectly natural. ie. You can't blame others for not sharing your thinking, which is an easy insight from turning the situation around. I wouldn't wish for others to be less critical.

Small modifications of the current main ideas are certainly more "economical" in the perspective of evolving science, so it is rational to examine that first, before you entertain the idea of large reconstructions (revolutions). But now in 2022, I can't help thinking that after all the efforts of the various "mainstream" ideas to unification, such as string theory for example, it hasn't provided much progress on key questions. String theory is self-declared as fairly "conservative" relative to the QFT paradigm. So my own judgement, is that the scientific community HAS examined many ideas of small modifications of current models and by now it may be time to look at some of the admittedly more "expensive" and radical options. Otherwise we also risk searching wasting resources in hopeless loops.

Now note for larger context I'm still quite skeptical that even if their model is correct that they would ever be able to isolate the rule or possibly rules which govern such a computational universe

Do you speak of this? https://arxiv.org/abs/2004.08210 ??

/Fredrik

 

[CoolMint]

I read all 7 pages of this thread and there is an overbearing impression, and which is contrary to popular belief, that the ones who are shouting the loudest about entanglement being a central issue(and nobody taking their opinions seriously) are exactly the ones who espouse a 16th century worldview. Albeit tacitly. This is the reason why you are still struggling.

And Vanhees71 is not.

A mandatory worldview update among the physics community should be introduced.

 

[CoolMint]

Smile reacts incoming from a Newtonian perspective. As much I love Newton and the illusory comfort of that perspective, it is demonstrably wrong.

The world appears Newtonian to the crude eye but is not. Abandoning it(that matter particles are little balls) on the other hand, removes all issues and tensions surrounding the interpretation of QM.
Have you?
I can manage not knowing.

Can you?

 

[CoolMint]

When the waves of potentiality are correlated, then, when they are actualized as particles, their behavior indicates as if they have communicated instantly, without signals. How could they have done that? Because, this is where most of you sporting a Newtonian perspective have issues, the correlation or entanglement made them into one. One does not need a signal to communicate with itself.

 

[Hornbein]

Susskind supposes entanglement could be a wormhole.

 

[gentzen]

A mandatory worldview update among the physics community should be introduced.

You are welcome to do so. Any specific suggestions? Something along the lines of Wheeler?

But when Everett produced his many-universes interpretation for quantum theory you changed your mind for a while. Why was that?
...

What attracted you to this remarkable idea?

... But I also have a deeper objection: the Everett interpretation takes quantum theory in its present form as the currency, in terms of which everything has to be explained or understood, leaving the act of observation as a mere secondary phenomenon. In my view we need to find a different outlook in which the primary concept is to make meaning out of observation and, from that derive the formalism of quantum theory.

So you think that the many-universes approach may still be useful?

Yes, I think one has to work both sides of the railroad track.

But in the meantime you're siding with Bohr.

Yes. As regards the really fundamental foundations of knowledge, I cannot believe that nature has 'built in', as if by a corps of Swiss watchmakers, any machinery, equation or mathematical formalism which rigidly relates physical events separated in time. Rather I believe that these events go together in a higgledy-piggledy fashion and that what seem to be precise equations emerge in every case in a statistical way from the physics of large numbers; quantum theory in particular seems to work like that.

But do you think that quantum theory could be just an approximate theory and that there could be a better theory?

First, let me say quantum theory in an every-day context is unshakeable, unchallengeable, undefeatable - it's battle tested. In that sense it's like the second law of thermodynamics which tells us that heat flows from hot to cold. This too is battle tested - unshakeable, unchallengeable, invincible. Yet we know that the second law of thermodynamics does not go back to any equations written down at the beginning of time, not to any 'built in' machinery - not to any corps of Swiss watchmakers - but rather to the combination of a very large number of events. It's in this sense that I feel that quantum theory likewise will some day be shown to depend on the mathematics of very large numbers. Even Einstein, who opposed quantum theory in so many ways, expressed the point of view that quantum theory would turn out to be like thermodynamics.

It is an excerpt from "The Ghost in the Atom" (1986). The questions are from Paul Davies.

Or more something along the lines of Bohr (or Peierls)?

Perhaps they are just vocal.

They are vocal. In fact, I was asked the other day why it is that so few people are willing to stand up and defend Bohr's view, and I didn't have an answer on the spot. But the answer is, of course, that if somebody published a paper arguing that two and two makes five, there wouldn't be many mathematicians writing papers to defend the conventional view!

I can manage not knowing. Can you?

I have to live with not knowing, independent of whether I can manage it or not. Worse, I have to live with others not knowing. Those others might be engineers, trying to construct the protocols to keep my communication private, or my financial transactions secure. And maybe this "engineering" in the end is the important difference:

Indeed, I think from the theoretical point of view, these questions are answered, and the problem is indeed to realize, e.g., scalable quantum computers, is now in the realm of an engineering problem and as such it's of course a scientific one.

John Stewart Bell saw himself as a Quantum Engineer. Mathematicians indeed do prove that two and two makes four! The logical foundations for such proofs turned out to have an important impact on how computers are constructed and verified. In theory, there is no difference between theory and practice. In practice, there is!

 

[Fra]

Abandoning it(that matter particles are little balls) on the other hand, removes all issues and tensions surrounding the interpretation of QM.

Does it? Some people had the excellent idea to replace the balls with strings, essentially without resolving the quantum mystery much. Except possibly a hint with n'th quantization and n-branes, and some interesting dualities, but it has raised more questions than answers IMO, which isn't necessarily bad though.

Even if you reject 16th century worldviews, those that adopted the 21th century worldview still has back issues :) Wether those aren't as "lound" as the others is prossibly true though.

/Fredrik

 

[vanhees71]

String theory is one of the most illustrative examples for the fact that theories, which are invented instead of discovered based on solid experimental facts, are unlikely to solve any problems in physics. Maybe they have interesting mathematical properties leading to new ideas for mathematics.

In other words, solving some pseudo-problems invented by philosophers rarely lead to something interesting for physics. Admittedly the quibbles of EPR and their unfortunate paper lead to very interesting further developments and can thus be argued to be a counter-example against my skepticism against such an approach to the natural sciences. Nevertheless the breakthrough only came when Bell made the philosophical quibbles a clearly empirically decidable scientific hypothesis by inventing what he called "local realistic theories", which contradict quantum theory and showed how to decide between these kind of theories and QT experimentally. At this time it was pretty on the edge of what technically was realizable, but this lead indeed to the development of also this technology (mostly within quantum optics and AMO physics). With this development the apparent problems with entanglement and inseparability has been finally solved in favor of QT. It is now even in the realm of the engineering sciences to apply these results of pure research, and there are first real-world applications in view (particularly quantum kryptography but also quantum computing).

 

[CoolMint]

You are welcome to do so. Any specific suggestions? Something along the lines of Wheeler?
Or more something along the lines of Bohr (or Peierls)?
I have to live with not knowing, independent of whether I can manage it or not. Worse, I have to live with others not knowing. Those others might be engineers, trying to construct the protocols to keep my communication private, or my financial transactions secure. And maybe this "engineering" in the end is the important difference:

John Stewart Bell saw himself as a Quantum Engineer. Mathematicians indeed do prove that two and two makes four! The logical foundations for such proofs turned out to have an important impact on how computers are constructed and verified. In theory, there is no difference between theory and practice. In practice, there is!

"Quantum mechanics is a theory about the physical description of physical systems relative to other systems, and this is a complete description of the world"

Rovelli, C. (1996), "Relational quantum mechanics", International Journal of Theoretical Physics, 35: 1637–1678.

I like his relational interpretation.

 

[CoolMint]

Does it? Some people had the excellent idea to replace the balls with strings, essentially without resolving the quantum mystery much. Except possibly a hint with n'th quantization and n-branes, and some interesting dualities, but it has raised more questions than answers IMO, which isn't necessarily bad though.

Even if you reject 16th century worldviews, those that adopted the 21th century worldview still has back issues :) Wether those aren't as "lound" as the others is prossibly true though.

/Fredrik

String theory is not going to mend the faulty intuition about the world.
Anyhow, it makes sense to be restrained on the assumptions and the obvious truths.

 

[bhobba]

String theory is one of the most illustrative examples for the fact that theories, which are invented instead of discovered based on solid experimental facts, are unlikely to solve any problems in physics. Maybe they have interesting mathematical properties leading to new ideas for mathematics.

Dyson was a big supporter of string theory, but he thought it was in the wrong area. It should be part of mathematics, not physics. He could be right. Added later: I forgot to mention Dyson knew little of String Theory, except his friend of many years, Feynman, thought it was the wrong track. But his mathematics colleagues thought it was important mathematics.

Thanks
Bill

 

[bhobba]

String theory is not going to mend the faulty intuition about the world.

I have no idea what faulty intuition you mean. I have a certain intuition based on symmetry. But determining if it is faulty or not is beyond me. I rather like Feynman's view where physics makes progress using all sorts of ways - sometimes it can be positivist philosophy, other times symmetry. Who knows where the next leap will come from?

Thanks
Bill

 

[CoolMint]

Great. I don't see you holding a Newtonian worldview. People who have already accepted that the world is fundamentally indeterminate and that this is how the world works, deal better than those who insist and can't live without a Newtonian universe.
Once you move on, entanglement is not bothersome. Maybe the question of what is the nature of everything... but as Vanhees71 says "It's not one of the goals of science". Entanglement is a well understood consequence of QT.
And it was predicted way back in 1931. So obvious was it.

 

[WernerQH]

Entanglement is a well understood consequence of QT.

Why do we need a special word for it? I'd call them correlations. These correlations are special only if you think that physics is about particles (or "quantum objects"). If you think of QFT as a statistical theory describing the correlations between events in space-time there's no need for an extra word.

Once you move on, entanglement is not bothersome. Maybe the question of what is the nature of everything...

Exactly. Every physicist should know what QFT is about. Maxwell thought that electrodynamics is about the aether. Our view of electrodynamics was much clarified after Einstein and Minkowski had done away with the aether. I think QED is easier to understand if you stop talking quantum objects, and turn to their interactions instead.

 

[vanhees71]

You need a special word for it, because it's generically quantum. It cannot be described by any other statistical theory than QT, and it's different, particularly, from what Bell called "local realistic HV theory", which is an unfortunate naming though, because it uses "locality" in a different way than established in relativistic QFT, where it means microcausality (a clear mathematical property of the operators representing observables). Also "reality" is so distorted by philosophers that it lost all its clear scientific meaning. It simply means "determinism" in the strong classical form, i.e., the assumption that all (!) observables always (!) take predetermined values, and the probabilities are describable under this assumption by standard ("non-quantum") probability theory.

Relativistic physics works consistently, already at the classical level, only with field descriptions. Point-particle models cannot be consistently formulated until this day. Already the electromagnetic interaction between two point particles is not fully self-consistently describable. The best resolution of this problem, which is over 100 years old, we have today is the conclusion that the Landau-Lifhitz approximation of the Lorentz-Abraham-Dirac equation is the best approximation we can get within a classical point-particle approach.

It has been pretty clear early in the history of QT that relativistic QT is best formulated as a QFT, because for interacting "particles" creation and annihilation processes are unavoidable. The key is indeed "microcausality", i.e., the demand that physics is decribed by local operators, representing observables, that commute with the Hamilton density at space-like separated spacetime arguments. This leads to most of the key properties needed for a consistent relativistic QT, which finally admits (at least for massive fields) a "particle interpretation" for the asymptotic free states. These key properties are:

(a) Symmetry under the proper orthochronous Poincare group
(b) necessity of the existence of anti-particles (where one can have the case of strictly neutral particles, where the anti-particles are identical with the particle, as a special case),
(c) existence of a stable ground state ("vacuum state")
(d) relation between spin and statistics (half-integer-spin particles are fermions, integer-spin-particles are bosons)
(e) CPT is necessarily an additional discrete symmetry of any such microcausal relativistic QFT
(f) unitarity and Poincare covariance of the S-matrix

 

[WernerQH]

@vanhees71, you keep repeating yourself. I have read such posts of yours dozens of times (and I even agree with most of what you write). At the same time you regularly fail to address the points I'm trying to make. I'm not sure if further elaboration on them will have an effect.

Also "reality" is so distorted by philosophers that it lost all its clear scientific meaning. It simply means "determinism" in the strong classical form, i.e., the assumption that all (!) observables always (!) take predetermined values, and the probabilities are describable under this assumption by standard ("non-quantum") probability theory.

Reality is what physics is about, and the view of what is "real" has undergone considerable evolution over the course of centuries. To say it "simply means determinism" is an arbitrary and completely unjustified restriction on the space of possible theories to be considered. You may think that QFT has reached its final form (it certainly is mature), but it is possible that it carries similar metaphysical baggage as burdened electrodynamics before 1905. We no longer think of the aether as real, have abandoned the search for mechanical models of the aether, and have arrived at a much more satisfactory theory. (Though at heart it is, of course, still Maxwell's theory.) And this process took more than four decades!

I suspect that in the case of QFT the problem is what we think of as real when we talk about quantum fields. You seem to think of a quantum field as something real, continuous in space and time, and of radiation as a continuous process. The equations of motions for the field operators do suggest continuous evolution in time, but in the real world we observe discrete $\gamma$-quanta, or separate grains in a photographic emulsion. Your distaste for the photon concept puts you at disadvantage in explaining how the formalism relates to the real world. (Please spare us the hand-waving about "field excitations" and special states of the radiation field.) If your world-picture is fundamentally continuous, I feel that you are still trapped in classical thinking, and haven't grasped the essence of quantum theory.

 

[PeterDonis]

Reality is what physics is about

Many people, including many physicists, would not agree with this. They would say physics is about constructing models that make accurate predictions, without considering whether the entities in the models are "real" or not, and leaving matters like "reality" to philosophers.

 

[hutchphd]

Reality is what physics is about,

I too disagree with this. Perhaps commonality is what physics is about. We agree to certain common "truths" and how to define them so that we have a defined field of play. I think reality is much too strong a word in this context. I do not know your reality at all.

 

[Vanadium 50]

leaving matters like "reality" to philosopher

There is an excellent text on the subject:

 

[Fra]

Reality is what physics is about

I like what I think is a classic textbook charicature of in particular theoretical physics - how an observer sits outiside a black box and trying to infer what is inside, using only interactions as a tool? Opening it, or peeking inside, just isn't possible. Or as Bohr put famously and wisely puy it "It is wrong to think that the task of physics is to find out how Nature is. Physics concerns what we say about Nature"

This is in a nutshell is physics as inference, constrained by the interaction tools at hand. And when at it's finest it might represent the "optimal inference" that nature allows, as oppose to random human inferences. This inference process is itself more interesting than it's results for me. It's extremely tempting to think that the laws of physics, is simply the optimal rules of inference (in disguise).

I interpret Bohr to mean what we can infer about "atomic world", from the perspective of the classical environment, which roughly speaking leads to QM and QFT as it stands. It's just the cut that is annoying, but still necessary to separate the "black box" from the external inference machinery.

So the rules of physical inference is as close to "reality" that I can come.

/Fredrik

 

[vanhees71]

@vanhees71, you keep repeating yourself. I have read such posts of yours dozens of times (and I even agree with most of what you write). At the same time you regularly fail to address the points I'm trying to make. I'm not sure if further elaboration on them will have an effect.

I repeat myself, because people also repeat again and again statements which contradict the mathematics of relativistic QFT.

Reality is what physics is about, and the view of what is "real" has undergone considerable evolution over the course of centuries. To say it "simply means determinism" is an arbitrary and completely unjustified restriction on the space of possible theories to be considered. You may think that QFT has reached its final form (it certainly is mature), but it is possible that it carries similar metaphysical baggage as burdened electrodynamics before 1905. We no longer think of the aether as real, have abandoned the search for mechanical models of the aether, and have arrived at a much more satisfactory theory. (Though at heart it is, of course, still Maxwell's theory.) And this process took more than four decades!

I'm not interested in metaphysics but only physics. Reading Bell's papers makes clear that what Bell means by "realistic" is the assumption that all observables always take determined values, which is the very difference between his local realistic hidden-variable theories and QT.

I suspect that in the case of QFT the problem is what we think of as real when we talk about quantum fields. You seem to think of a quantum field as something real, continuous in space and time, and of radiation as a continuous process. The equations of motions for the field operators do suggest continuous evolution in time, but in the real world we observe discrete $\gamma$-quanta, or separate grains in a photographic emulsion. Your distaste for the photon concept puts you at disadvantage in explaining how the formalism relates to the real world. (Please spare us the hand-waving about "field excitations" and special states of the radiation field.) If your world-picture is fundamentally continuous, I feel that you are still trapped in classical thinking, and haven't grasped the essence of quantum theory.

The photon is not a massless point particle but the quantum of the massless spin-1-field, called electromagnetic field, and indeed in QT there's nothing discrete since everything is described by differential equations. Of course photons are very specific asymptotic free states of the electromagnetic field and nothing else, and these special states imply that one registers "the photon" either as a whole or nothing. That's the distinctive feature of these states and the probability interpretation of quantum states a la Born resolves the space-particle duality contradiction of old quantum mechanics entirely.

 

[WernerQH]

Perhaps commonality is what physics is about. We agree to certain common "truths" and how to define them so that we have a defined field of play. I think reality is much too strong a word in this context. I do not know your reality at all.

Yes, "my" reality is not the elusive kind of Reality that only philosophers can conceive. Rather, it's what ordinary physicists have in mind when discussing what their theories are about, the inventory of those theories so to speak. And inventory does not mean the mathematical apparatus, but what the mathematical formalism is supposed to describe. Isn't it embarrassing if, what a theory was supposed to be about, turns out to be non-existent, like the aether?

Many people, including many physicists, would not agree with this. They would say physics is about constructing models that make accurate predictions, without considering whether the entities in the models are "real" or not, and leaving matters like "reality" to philosophers.

Of course, this attitude (philosophy?) is well known, not to say wide-spread: instrumentalism ("saving the phenomena"). In most cases it looks bad in retrospect.

I'm not interested in metaphysics but only physics. Reading Bell's papers makes clear that what Bell means by "realistic" is the assumption that all observables always take determined values, which is the very difference between his local realistic hidden-variable theories and QT.

I also have no interest in metaphysics. But theory space is much larger than hidden-variable theories and conventional QT. And Bell would never claim to have considered all possibilities.

 

[vanhees71]

Yes, "my" reality is not the elusive kind of Reality that only philosophers can conceive. Rather, it's what ordinary physicists have in mind when discussing what their theories are about, the inventory of those theories so to speak. And inventory does not mean the mathematical apparatus, but what the mathematical formalism is supposed to describe. Isn't it embarrassing if, what a theory was supposed to be about, turns out to be non-existent, like the aether?

It's not embarassing but a triumph of the scientific method, which is the only method to overcome our philosophical prejudices about Nature. Math is used as a language to talk about our observations of nature, because it's the only language that is precise enough to do so.

Of course, this attitude (philosophy?) is well known, not to say wide-spread: instrumentalism ("saving the phenomena"). In most cases it looks bad in retrospect.

But what you call "instrumentalism" is precisely what you demand above, namely to define what is described by the mathematical models (or even theories). An observable is defined by concrete measurement procedures.

I also have no interest in metaphysics. But theory space is much larger than hidden-variable theories and conventional QT. And Bell would never claim to have considered all possibilities.

Of course not, but the EPR hypothesis, as far as this vague philosophical non-mathematical writing can be made rigorous as Bell has done, is ruled out by all experiments and "conventional QT" confirmed.

 

[WernerQH]

It's not embarassing but a triumph of the scientific method

Sure. But not all contemporaries of Einstein saw it that way. He did not win his Nobel prize for relativity.

 

[WernerQH]

Or as Bohr put famously and wisely puy it "It is wrong to think that the task of physics is to find out how Nature is. Physics concerns what we say about Nature"

If we were to take this seriously, physics should be considered part of linguistics, or, more precisely, socio-linguistics. ;-)

Seriously, I think that language is flexible enough to create all sorts of images. What stands in the way of our better understanding of quantum theory are strong habits of thought. For example that the world around us cannot be described other than in terms of objects moving about.

 

[vanhees71]

There are sometimes quite ironic things in the history of the Nobel prize. Einstein didn't get the prize for his most important achievement (GR) but for the one theory ("old quantum theory") which was obsolete after only some years after he got the prize, and embarrassing for the Nobel committee they even explicitly said so on the prize diploma ;-)). The reason were the doubts from philosophical side (Bergson) about the concept of time.

Another example for a somewhat funny Nobel prize is that for Fermi, who got it for the discovery of transuranes, which however was just not correctly interpreted. In fact Fermi's experiments demonstrated nuclear fission. Of course, Fermi deserved the Nobel prize anyway.

 

[PeterDonis]

Please spare us the hand-waving about "field excitations" and special states of the radiation field.

Those things are not "hand-waving". They are a reasonable description in ordinary language of the QFT concept of "photons" (or indeed of any "particles"). You are getting very close to a warning here.

 

[PeterDonis]

instrumentalism ("saving the phenomena")

What I described is not "instrumentalism". It's a recognition that all models in physics are just that: models. And a recognition that models are not reality. Confusing the map with the territory is a fundamental mistake.

theory space is much larger than hidden-variable theories and conventional QT

If you have a valid reference to another theory that describes quantum entanglement, post it.

 

[PeterDonis]

What stands in the way of our better understanding of quantum theory are strong habits of thought. For example that the world around us cannot be described other than in terms of objects moving about.

Since quantum field theory does not describe the world "in terms of objects moving about", I don't know what you are getting at here.

 

[Fra]

If we were to take this seriously, physics should be considered part of linguistics, or, more precisely, socio-linguistics. ;-)

If we are to take it litteraly perhaps. Buy I am pretty sure that wasn't what Bohr meant. Inference as an abstraction is the general term for induction, deductiion or abduction and it has as much to do with linguistics as "observer" has todo with conscioussness. I suspect the same people are responsible for both misunderstandings. Unlike mathematics, words should not be interpreted litteraly. It has both pros and cons.

/Fredrik

 

[WernerQH]

Since quantum field theory does not describe the world "in terms of objects moving about", I don't know what you are getting at here.

So you wouldn't call quantum fields (or their excitations) objects? And aren't they subject to equations of motion?

 

[PeterDonis]

you wouldn't call quantum fields (or their excitations) objects?

Quantum fields, no. One might say that "objects" are made of excitations of quantum fields, but calling the excitations themselves "objects" is problematic.

And aren't they subject to equations of motion?

No.

 

[vanhees71]

Well, I've never seen operators acting on a Hilbert space in the lab ;-)). Of course all the math is just a description of the "objects" observed in Nature.

 

[WernerQH]

Quantum theory provides a description that makes these objects hard to recognize.