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Why don't virtual particles cause decoherence? 
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#19
Feb1613, 06:32 PM

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13: Your observations are too slow. You observe just longliving particles, which act like longliving particles.
If you try to "catch" a photon in the near field of an emitter, you get polarizations of the field which are impossible for real photons. If you go away, the radiative part gets more and more dominant, but there is no line after which you observe just radiation and nothing else. 


#20
Feb1613, 06:33 PM

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You ignore most of what I am saying.



#21
Feb1613, 06:38 PM

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No, I reduce it to the main point, and adress that.
Anyway, I don't think further discussion will produce anything new here. 


#22
Feb1613, 06:43 PM

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No, the main point are unphysical properties of virtual particles; ignoring these facts does not solve any problem. There are not even shortlived ghosts in reality; and reality is not gauge dependent. So focussing on livetime and offshell is not the main point. It misses nearly everything which characterizes virtual particles.



#23
Feb1713, 10:53 AM

P: 381

Can i ask both of you a question?
The interaction time between two particles is finite. If we could make measurements during this finite period while interactions are still on, what would we see? What are the observables? Is, for example, the number operator of virtual particles (if such thing exists) an observable? Any idea? 


#24
Feb1713, 11:08 AM

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I don't think that the question regarding interaction time makes sense.
Regarding the number operator it's trivial: this operator acts on Hilbert space stars. But virtual particles (internal lines in Feynman diagrams) are propagators, not Hilbert space states. Therefore there is not operator 'counting' virtual particles. 


#25
Feb1713, 11:17 AM

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I haven't seen anywhere a discussion of what happens during an interaction (not just in/out states). Why doesn't it make sense (theoreticaly at least)?



#26
Feb1713, 11:24 AM

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How would those measurements look like? They would be interactions with the particles, and not separable from the process you consider.



#27
Feb1713, 11:29 AM

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#28
Feb1713, 12:29 PM

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All these entities are unphysical simply b/c strictly speaking they are not required; you can find formulations avoiding them, so there is no fundamental reason to introduce them into the formalism (they are artifacts of the formalism) and therefore there is no reason to interpret them as physical entities. 


#29
Feb1713, 01:20 PM

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I was under the impression that "physical" means exchange and interaction of virtual particles(at least according to qft) between....er... objects/real particles or whatever you want to label it. If they are not real(don't exist), is anything real? How would physicalness arise?



#30
Feb1713, 03:51 PM

P: 127

Imagine I develop a new mathematical formalism, that is a good enough mathematical approximation to, say, Newtonian Mechanics, based on a given mathematical Serie.
Imagine I call each element of the Serie, "a little green dwarf", because I like it. Would you say that those "little green dwarfs" are "real" or "physical"? Would you say that gravity exists because of the actions of those "little green dwarfs"? 


#31
Feb1713, 04:38 PM

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If i see a multitude of little green dwarfs all around me for a life time, i might be inclined to believe they are real and exist. The mathematics wouldn't work if it had no resemblance to reality. Why would it work otherwise? Just a happy coincidence? 


#32
Feb1813, 01:08 AM

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I guess we should come back to the question
Let me summarize some additional ideas Last but not least: nobody would assume that any approximation like a Taylor series (or green dwarfs) do introduce additional effects which are not already present in the full theory w/o the approximation (w/o green dwarfs). So if the theory w/o virtual partices green dwarfs) already contains decoherence (gravity) it would be silly to say that decoherence (gravity) is due to virtual particles (green dwarfs). This changes if the theory cannot be formulated w/o virtual particles (w/o green dwarfs), or if the formulation is conceptally simpler (in the sense of Ockham's razor) using virtual particles (green dwarfs). I am not an expert regarding green dwarfs, but I know that perturbation theory is incomplete and misses relevant nonperturbative effects. So I can't see any reason to rely on the interpretation of partially unphysical artifacts due to an incomplete approximation instead of using the full theory. 


#33
Feb1813, 03:12 AM

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There are external lines which are slightly offshell (see e.g. http://en.wikipedia.org/wiki/Scharnhorst_effect ). 


#34
Feb1813, 08:44 AM

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#35
Feb1813, 10:31 AM

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In QCD nearly everything requires nonperturbative methods (even in DIS  using perturbation theory  one probes nonperturbative structure functions) There is one problem, namely that QED is illdefined in the UV (Landau pole), in contrast to QCD which is UV complete. Anyway, most perturbation series (QED, QCD, phi^4 theory, ...) are illdefined and divergent, so perturbation theory does not make sense to arbitrary high order; its radius of convergence is zero. 


#36
Feb1913, 01:52 AM

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