# Why don't virtual particles cause decoherence?

by Mukilab
Tags: decoherence, particles, virtual
P: 381
 Quote by maxverywell Of course it has nothing to do with that, It's simply an analogy.
It's a very bad analogy! In the double slit experiment you have particle in a quantum state. In the case of virtual particles, the latter are NOT described by a quantum state. So, the way you interpret the quantum state in the double slit experiment (i.e "the actual particle" goes through one slit or both) is irrelevant. We are not talking about interpreting the quantum state. We are talking about virtual particles that are not described by quantum states in the first place, so you don't have anything to interpret
P: 161
 Quote by maxverywell We simply know that there is an interaction taking place in that process, not that the virtual particles are there and that they are "real". It's your wrong interpretation of the situation, like saying that in the double slit experiment a particle went through one specific slit.
Yes, there is a real interaction. Right? You don't like the term 'virtual particle' -- OK, don't use it! There is a physically real interaction, described by the VEV of the quantum field. The VEV describes a physically real entity -- otherwise there is no interaction! So we agree.
P: 161
 Quote by JK423 We are talking about virtual particles that are not described by quantum states in the first place, so you don't have anything to interpret
This statement presupposes that no part of a theory that is not a quantum state is reasonably subject to physical interpretation. One can assume this restriction on one's thinking if one wishes, but it's unnecessary. Remember that Ernst Mach railed against the idea of atoms because they were supposedly an artificial, abstract, unobservable theoretical construct.

Also, consider an excerpt from Freeman Dyson's essay about how seemingly abstract mathematics has an uncanny way of reflecting the real world: " [Mathematicians] always thought of complex numbers as an artificial construction, invented by human mathematicians as a useful and elegant abstraction from real life. It never entered their heads that this artificial number system that they had invented was in fact the ground on which atoms move. They never imagined that nature had got there first.

-- Birds and Frogs Freeman Dyson, Notices of the American Mathematical Society, February 2009
 P: 161 Saying that there are mediated particles in the scattering process is an attempt to interpret scattering classically, but it's wrong because we don't know what's going on during it. It's totally a quantum process and we simply can compute the amplitudes of the transition from the initial state to the final state. That's it. Similarly in the double slit experiment, we don't know through which slit a particle went. Saying that a particle went through one specific slit is wrong because it didn't. We can only compute amplitudes for the process of interaction of particle with slit.
 Sci Advisor P: 5,451 Ruth, I think you completely missed my post #89 http://www.physicsforums.com/showpos...2&postcount=89
P: 161
 Quote by maxverywell Saying that there are mediated particles in the scattering process is an attempt to interpret scattering classically, but it's wrong because we don't know what's going on during it. It's totally a quantum process and we simply can compute the amplitudes of the transition from the initial state to the final state. That's it. Similarly in the double slit experiment, we don't know through which slit a particle went. Saying that a particle went through one specific slit is wrong because it didn't. We can only compute amplitudes for the process of interaction of particle with slit.
No. This is not a 'classical' ontology; it's a realist ontology. They are not the same thing. A realist ontology need not be classical. Consider this from Ernan McMullin:

“[I]maginability must not be made the test for ontology. The realist claim is that the scientist is discovering the structures of the world; it is not required in addition that these structures be imaginable in the categories of the macroworld.” (1984, ref on request)

So we are discovering the real structures of the world. Nothing I've said requires that these be classical (i.e., expressible in 'categories of the macroworld'). In fact quite the opposite.
 P: 161 Tom, regarding the gauge issue, PTI is based on a direct action theory so see Davies (1971, 1972) for the precise formalism, which uses the Coulomb gauge for QED. If Nature really uses direct action, this fixes the gauge and there is no ambiguity. So the bottom line: it is an empirical matter what the correct gauge is. The feature of theoretical gauge dependence does not necessarily mean that something is 'unphysical'. The basis of the perturbative expansion approach is the idea of mediated interactions between field currents, with increasingly accurate approximation to reality with the addition of each order. I think of this as a fractal process; keep in mind that fractals are ubiquitous in nature already.[http://www.wired.com/wiredscience/wp...fractal_10.jpg If you don't want to use perturbation theory, then use a different theory of interacting fields. You will still have to deal with field propagation in some way. My model is realist about field propagation, that's all. I didn't say this is a classical thing and I didn't say that fields chug along classical trajectories, which they don't. I did already address your criterion for calling something 'real'. Basically you are equating 'real' with 'empirical,' when you say things like: "therefore it does not represent reality (in the sense we can define it when we want to have agreement with observations)". This is where the McMullin quote is relevant.
 Sci Advisor P: 5,451 I think you miss my point. We know that perturbation theory is ill-defined; we know that especially in QCD there is no phenomenon which allowes for purely perturbative formulation; we know that the gauge artifacts are much more severe in non-abelian gauge theories, so a results for QED don't tell us how to treat them. Eq. (2) using the free propagator D(x-y) is wrong in QCD! So for everything which follows after eq. (2) extrapolating QED results is simply not applicable.
 P: 161 Tom --to clarify, my basic ontology does not depend on their being a purely perturbative formulation. In fact in PTI there is *not* a continuum of spacetime points, so lattice gauge theory would be more natural for my ontology and that is already used in QCD. I do hope you will read my book and consider these ideas and not reject them out of hand. Thanks again for your questions and interest!
 Sci Advisor P: 5,451 Could you provide some hints what PTI has to say about QFT? I agree that it's interesting as an interpretation of QM, but afaik there is no equivalent line of discussion for QFT (do you know Esfeld's book on "philosophy of physics"? it seems to me that there are no reasonable interpretations of QFT on the market) In addition: you start reasoning based on your paper, based on propagators and perturbative formalism; once confronted with technical arguments regarding an ill-defined starting point and regarding missing key issues your response is to read a book. Does this book contain a nonperturbative analysis of non-abelian quantum gauge theory? If no, then why should I read that book? I still think that you don't address the key issues. But w/o a detailed analysis of these issues and w/o the construction of a well-defined formalism it does no makes sense to interpret a formalism.
 P: 381 @RKastner You are not talking clearly. I bet not even you have exactly understood what you mean by what you're saying, "it's that, but not exactly that, and it's like this but not exactly like this..." This is how you talk all this time, i haven't seen a single real argument about the reality of propagators. You say that if propagators didn't exist then interactions wouldn't exist. Now, what kind of argument is that? Do you know how it sounds like? If $e^x$ represents a real quantity, then every term in the perturbative expansions ${e^x} = \sum\limits_n {\frac{{{x^n}}}{{n!}}}$ MUST BE REAL because if they didn't exist (i.e. weren't present in the expansion) then $e^x$ wouldn't be what it is! If we start interpreting mathematics like that, in whatever branch of science, we are screwed. Virtual $dollars$ will start popping in finance.
P: 161
 Quote by tom.stoer Could you provide some hints what PTI has to say about QFT? I agree that it's interesting as an interpretation of QM, but afaik there is no equivalent line of discussion for QFT (do you know Esfeld's book on "philosophy of physics"? it seems to me that there are no reasonable interpretations of QFT on the market) In addition: you start reasoning based on your paper, based on propagators and perturbative formalism; once confronted with technical arguments regarding an ill-defined starting point and regarding missing key issues your response is to read a book. Does this book contain a nonperturbative analysis of non-abelian quantum gauge theory? If no, then why should I read that book? I still think that you don't address the key issues. But w/o a detailed analysis of these issues and w/o the construction of a well-defined formalism it does no makes sense to interpret a formalism.
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.

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
 Sci Advisor P: 5,451 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).
 P: 161 Tom --not sure I understand your concern. Please specify exactly what problem you see as not solved.
P: 5,451
Ruth, no single problem regarding non-abelian gauge theories and 'virtual particles' has been addressed.

#16, #32, especially #35, your statements
 Quote by rkastner If one wants to argue that virtual particles (i.e. propagators) don't exist, then one is essentially saying that quantum fields don't exist.
and
 Quote by rkastner [the] presupposing that quantum fields do not exist (and therefore don't propagate -- since you deny that propagators have physical content). So the burden is on you to explain how there can be excitations of something that doesn't exist (quantum states being excitations of quantum fields).
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.
P: 381
 Quote by 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.
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.
 P: 161 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
P: 5,451
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
 Quote by tom.stoer Decohence is due to factorizing the full Hilbert space H in Hsystem, Hpointer and Henvironment and then "tracing out" the environment degrees of freedom. The remaining "subsystem" can be described by an "effective density matrix" which is nearly diagonal in the pointer basis, so it seems as if it collapsed to the a pointer state with some classical probability. Virtual particles are artifacts of perturbation theory, i.e they are not present in the full theory w/o using this approximation. Using virtual particles does not introduce the above mentioned factorization of H. And last but not least virtual particles are not states in any Hilbert space Hsystem, Hpointer or Henvironment , but they are "integrals over propagators".
I think that the discussion over the last days hasn't changed anything. Do you agree?

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