## Why don't virtual particles cause decoherence?

Saying that virtual particles exist in reality is quite the same as saying that in the quantum double slit experiment a particle goes through one specific slit.

In quantum mechanics, it can't be known - in principle - what's going on "inbetween" (or prior to a measurement).

 Quote by maxverywell Saying that virtual particles exist in reality is quite the same as saying that in the quantum double slit experiment a particle goes through one specific slit. In quantum mechanics we simply don't know what's going on "inbetween" (or before a measurement).
No, we don't know what a photon is doing between the slit plane and the detection plate -- but we know it's THERE.

Similarly, we don't know where a 'virtual particle' is or what it's doing, but we know it's THERE because otherwise the two scattering fermions would not know about each other and there would be no scattering when they encountered each other. Now, by 'virtual particle' I don't mean a little tiny corpuscle moving around (just as there isn't a little tiny corpuscle in the 2-slit experiment). But there is a physical entity described by the vacuum expectation value of the relevant field (i.e. propagator). If you want to say that these entities don't exist then you have to explain what is doing the measurable work when particles scatter.

 Quote by maxverywell Saying that virtual particles exist in reality is quite the same as saying that in the quantum double slit experiment a particle goes through one specific slit. In quantum mechanics, it can't be known - in principle - what's going on "inbetween" (or prior to a measurement).
No, it has nothing to do with that!! A particle in a superposition of states has nothing to do with the internal lines of Feynman diagrams!

 Quote by rkastner No, we don't know what a photon is doing between the slit plane and the detection plate -- but we know it's THERE. Similarly, we don't know where a 'virtual particle' is or what it's doing, but we know it's THERE because otherwise the two scattering fermions would not know about each other and there would be no scattering when they encountered each other. Now, by 'virtual particle' I don't mean a little tiny corpuscle moving around (just as there isn't a little tiny corpuscle in the 2-slit experiment). But there is a physical entity described by the vacuum expectation value of the relevant field (i.e. propagator). If you want to say that these entities don't exist then you have to explain what is doing the measurable work when particles scatter.
Since when propagators do work?
In order to have a scattering process, you need states and an interaction Hamiltonian. There are your physical entities, states, that interact!
Edit: The fermions interact with the vacuum state of the E/M field, both of them. That's how they "know about each other".

By the way, rkastner, both tom.stoer and I have asked you for clarifications in previous posts. So, if you want a conversation to actually continue you have to respond and not just "throw something in the air" and leave afterwards.

 Quote by JK423 No, it has nothing to do with that!! A particle in a superposition of states has nothing to do with the internal lines of Feynman diagrams!
Of course it has nothing to do with that, It's simply an analogy.

 Quote by rkastner Similarly, we don't know where a 'virtual particle' is or what it's doing, but we know it's THERE because otherwise the two scattering fermions would not know about each other and there would be no scattering when they encountered each other.
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.

 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

 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.

 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

 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.
 Recognitions: Science Advisor Ruth, I think you completely missed my post #89 http://www.physicsforums.com/showpos...2&postcount=89

 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.

 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.
 Recognitions: Science Advisor 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.
 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!
 Recognitions: Science Advisor 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.
 @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.