Why don't virtual particles cause decoherence?

rkastner

The claim in your last sentence regarding the restriction of interpretation to a 'well-defined formalism' is simply not borne out by careful study. I address this in my book as well, with reference to MacKinnon (ref on request) -- who finds that trying to restrict interpretive efforts to a 'rational reconstruction' of a given (messy or imperfect) theory leads to no better insight than simply working to interpret an existing functioning theory, which is what QM is -- however messy and mathematically 'ill-defined'. QM itself, at both the non-relativistic and relativistic levels, has always had an 'ill-defined starting point'. I'm not creating a new theory, I'm interpreting an existing theory: i.e., proposing physical referents for objects in the theory that do computational work leading to good empirical corroboration. See Chapter 2 of my book for details.

Regarding the previous post expressing dissatisfaction with what I've said here, and as for 'missing key issues in my response' -- as I noted earlier, I have very limited time to spend on this forum. I saw an interesting question --the beginning of the thread -- asking about virtual photons, to which I thought I might contribute, since I have published work on this topic and I think the model I'm working with is an interesting and fruitful one. No, the model has not been extended in detail to QCD. But neither, to my knowledge, have the following competing interpretations of QM: (1) many world theories (2) ad-hoc spontaneous collapse theories (3) The Bohmian theory (4) decoherence-based approaches (which, as I've noted, utterly fail to really solve the measurement problem despite their claims), or any other 'mainstream' interpretation of QM of which I'm aware. So why use that as a criticism of PTI? If you yourselves began to explore the PTI model, you might break some new ground and find that it indeed does shed light on some of the issues that you've demanded that I explain here in detail, such as non-Abelian gauge theories. There is a seed here that could be planted and yield fruit, so let's not reject it out of hand.

Regarding the impatience expressed by someone here with my asking that you read my papers and my book before making demands for detailed explanations on PF: again, my time to spend here is limited. I saw a question about virtual particles in the context of decoherence, and provided a suggestion that the PTI model has something useful to say about it -- especially since decoherence arguments are so inadequate to understanding collapse and determinate results in QM. I've already put quite a bit of time and effort into these publications, and I don't quite see how it's a reasonable request for me to rewrite them all here. It's already out there for you to read. If you read it and you still have questions, I'm happy to engage those. But again, I don't think you'll find detailed interpretive treatments of non-Abelian gauge theories in competing interpretive models of QM. Everettian theories are still struggling to explain the Born Rule in their model. So go ahead and explore the PTI model wrt QCD -- I look forward to reading your own work on this.

As for the question about the propagator, as I've said repeatedly, quantities such as VEV values of fields -- more generally, Wightmann correlation functions -- have a well-defined interpretation in PTI as possible offer waves -- that is, entities that have a less-than-unity amplitude for becoming offer waves |X> -- i.e. what you take as well-defined 'quantum systems'. The specific amplitude is given by the relevant coupling constant. I have not 'shifted' my position on this as someone implied above, I've taken note where someone is reading meaning into 'virtual particle' that I did intend. No, of course a 'virtual particle' is not a corpuscle on a worldline so if someone defines a virtual particle that way, that's not, and has never been, what I'm talking about.

Re what PTI has to say about QFT, a detailed QFT discussion applying to the PTI model, although he doesn't himself apply it to the transactional picture, is in Davies' papers:

Davies, P. C. W. (1970). “A quantum theory of Wheeler-Feynman Electrodynamics,” Proc. Cam. Phil. Soc. 68, 751.

___________(1971).”Extension of Wheeler-Feynman Quantum Theory to the Relativistic Domain I. Scattering Processes,” J. Phys. A: Gen. Phys. 6, 836.

____________(1972).”Extension of Wheeler-Feynman Quantum Theory to the Relativistic Domain II. Emission Processes,” J. Phys. A: Gen. Phys. 5, 1025-1036.

And I address this in some detail in my http://arxiv.org/abs/1204.5227 (FoP)

If you read my book, you'll see that PTI does not involve a spacetime continuum, so that's why there is a natural end to the perturbation expansion (if that computational approach is used) and why lattice gauge theory is probably a more accurate underlying framework. In either case, propagators are an essential component of the process and they have a clear ontological meaning in PTI as nascent offer waves (or if you will, 'failed' or 'aborted' offer waves), with the coupling amplitudes quantitatively characterizing their 'nascence' or degree of presence. Yes, these may be unfamiliar concepts, but that doesn't mean they are ill-defined. Remember we are dealing with quantum entities, not the classical world, and remember Ernan McMullin's comment.

So again, I very much appreciate your interest, and I hope you will continue to explore the TI/PTI picture. Remember that everything about PTI with which you have expressed dissatisfaction or reservations (i.e., certain details not fully explained on this forum, no obvious resolution to the challenge of QCD, etc, -- applies even more so to competing QM interpretations. At least TI/PTI can readily explain the Born Rule and the nature of collapse! So put the model to work yourselves, and see what you might do with it, rather than assuming a priori that it will fail.

I am available for discussion by way of my website:

transactionalinterpretation.org

With warm regards,
RK

tom.stoer

Ruth,

I definitly do NEITHER critizise TI/PTI NOR your book which I haven't studied. What I am critizising is that you comment here on a discussion regarding 'existence of virtual particles' w/o addressing the key issues we identified.

In case you have a solution, please elaborate (here).

rkastner

Tom --not sure I understand your concern. Please specify exactly what problem you see as not solved.

tom.stoer

Ruth, no single problem regarding non-abelian gauge theories and 'virtual particles' has been addressed.

#16, #32, especially #35, your statements and where you ignore that QFT can be formulated w/o propagators but not w/o quantum fields (states), #89, #106, #116 which questions that your paper can be used to understand non-abelian gauge theories, #118.

But we go around in circles. Asked what I miss (e.g. a single statement on non-abelian gauge theories) you respond either with papers on QED or with your book.

JK423

It's not proper to speak from Ruth's behalf, but the answer you are probably going to get is that, she is not implying that quantum states are unnecessary. She just proposes that propagators have a reality that goes beyond the reality of states.

rkastner

Tom,

If you'd like me to address a specific formulation of QFT not involving propagators, please provide a reference. I've already noted that there is no specific prima facie reason why PTI can't be extended to non-Abelian gauge theories, and invited you to work on that front; and that competing QM interpretations certainly don't provide any detailed account of non-Abelian gauge theories either (because they are too busy trying to explain what is already easily explained in TI/PTI). So let's have a level playing field.

Thanks again for your interest,
RK

tom.stoer

Ruth,

I'll check TI/PTI in more detail as soon as I have time.

But let's come back to the original question, why virtual particles do not cause decoherence (I do not want to go through all the discussion regarding virtual particles, I simply refer to a perfect summary written by Arnold Neumaier) http://www.mat.univie.ac.at/~neum/ph.../unstable.html

My first answer why virtual particles are irrelevant for decoherence was I think that the discussion over the last days hasn't changed anything. Do you agree?

mattt

Totally agree with everything A. Neumaier wrote there (and like Tom, in the future I will simply refer to that link).

JK423

It's not that simple; as you can see in another thread "Source of virtual particles in space?" not everyone agrees with what Neumaier says. So...

tom.stoer

Anyway, it's irrelevant for decoherence

Mukilab

Thanks, but if virtual particles don't 'exist' or they are not necessary in many quantum theories, why do they exist?

Please understand that I do not understand the large majority of scientific jargon on this thread, would it be possible for you to try to explain this as if to someone that is slightly above a layman (I have knowledge of most concepts, just not the mathematics behind them, which I do not understand). Thank you.

tom.stoer

Virtual particles in the sense I define them - as internal lines in Feynman diagrams - are a mathematical tool used for an approximation - so-called perturbation theory. Unfortunately most quantum field theories like QED, QCD etc. are very complicated and we do not have the mathematical tools to solve them exactly. But fortunately we have several approximations. Perturbation theory is used for weak coupling, where it makes sense to start with free, non-interacting particles and to add small corrections for interactions. Interestingly this works very well in many cases, especially for scattering experiments (but there are other problems like QCD bound states, e.g. protons, neutrons, ... where this approximation is useless).

If you would have mathematical tools to solve quantum field theories exactly, there would be no reason to introduce perturbation theory, there would be no name for the mathematical artifacts, and we would not have these discussions. Before studying QCD I was working on two-dimensional models, fields living on a line = one space dimension + one time dimension. These models a rather simple, a good starting point for beginners. There are exactly solvable models with bound states, there are other approximations, and thefore no reason to use perturbation theory. In QCD there are tools to study non-perturbative aspects, tools like chiral effective theories, lattice gauge theories, ... All these tools do not require Feynman diagrams and therefore - using these tools - there is nothing which we call 'virtual particle'.

In addition there are mathematical reasons against perturbation theory. We know that strictly speaking it is ill-defined, it is something which does not exist mathematically, but nevertheless it seems to work in a very restricted sense. And there are applications where this ill-defined math does produce correct results which agree with experiments (strange, isn't it? we can prove that it does not work, but using it seems to work ...). Now what I am saying is that we can START with a formulation w/o any approximation and w/o virtual particles. Then we have to introduce approximations, but doing this we CREATE several problems, or we apply approximations outside their scope of applicability, so the approximation BREAKS DOWN. Doing this we have mathematical artifacts - virtual particles - but due to the problems we introduce there seems to be no good reason to believe in virtual particles to be more than just limited tools.

Does this help?

rkastner

I would not be quick to dismiss something as only a 'mathematical artifact' just because it is part of an approximation. This is an interesting methodological question that deserves careful study. Remember that the number 'e' is a limiting value for many types of series expressions. It's possible that these 'approximating' series do have specific meaning in themselves; the fact that they have a well-defined limit at a specific irrational number does not negate that the series terms might have physical content if the argument of the exponential has physical content. Remember that the perturbative expansion of QFT has as an argument for the exponential the action of the field, which certainly has physical content. It's a leap to say categorically that terms in the expansion of a quantity with physical content do not themselves have physical content. Now of course we have to be careful about what the physical quantities are. In the earlier example of the radioactive atom, the exponential argument is not a field, it's just a number (decay rate). So that comparison was not a good one to the QFT case.

I appreciate the interesting discussion here but I should let you know that I am currently swamped with various obligations and may not be able to check in for a while. Thanks again everyone for your interest. I hope you will visit my website which presents preview material from my book and explores some of the ideas we've discussed here.

http://transactionalinterpretation.org

Best wishes
RK