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Why don't virtual particles cause decoherence? 
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#91
Feb2413, 07:13 AM

P: 381

@TrickyDicky
A bare electron always interact with the vacuum state of the electromagnetic field, noone says otherwise. This interaction gives rise to the so called "dressing" and renormalization etc. But the vacuum state is not virtual particles.. The latter are not described by the vacuum state or any other state, so it's wrong to think that around the electron there are virtual particles being continuously created and annihilated that "dress it". So we shouldn't play with words here. The physical entity in this case is the vacuum state, so if you want to interpret something just interpret what the vacuum state is, that's fine with me. Sure thing is, that it has nothing to do with the "reality" of internal lines in Feynman diagrams of perturbation theory, since the latter are only mathematical artifacts. "Dressing" exists irrespectively of whether you do perturbation theory or not. 


#92
Feb2413, 08:12 AM

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#93
Feb2413, 08:27 AM

P: 381

This is not an extreme position. If you think that it is, please elaborate so that i can understand what you have in mind. 


#94
Feb2413, 10:02 AM

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#95
Feb2413, 10:34 AM

P: 381

I am not going to get inside people's heads to know what they are thinking. I made a simple question all this time about whether virtual particles are described by quantum states. The answer is "no", and for me that's everything i need to know about "virtual particles". Tom.stoer, Demystifier and Healfix agree on this answer.
Now, whether you can or cannot understand the significance of this fact is another matter. Saying that virtual particles are not described by quantum states during their "supposed existence", is such a great statement, that allows you to see these things as purely mathematical artifacts and stop considering ANY ontological significance that they may have in the real world. As Demystifier said, 1 Apple=+2 Apples + (1) Apple, doesn't make the +2 and 1 Apples real. I learned lots of things from this thread to be honest. I hadn't realized all these things before. Thanks PhysicsForums! ;) 


#96
Feb2413, 11:04 AM

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#97
Feb2413, 11:32 AM

P: 381

I wonder, however, why in QFT textbooks the authors never (to my knowledge) warn the reader about the interpretation of perturbation theory and virtual particles, and talk about them like they are "really there" doing their stuff. Example from Peskin (p. 13): Even when there is not enough energy for pair creation, multiparticle states appear, for example, as intermediate states in secondorder perturbation theory. We can think of such states as existing only for a very short time, according to the uncertainty principle ΔΕΔt=h. As we go to higher orders in perturbation theory, arbitrarily many such "virtual" particles can be created. TrickyDicky you still think that i am fighting a straw man? Peskin completely confuses the reader from the first page. He says that "quantum states" are appearing that satisfy the energytime uncertainty principle, when we said that this is not the case. This thing is a crime to science and i am not exaggerating. Most of the PhD students (on experimental particle physics) that i have talked to about this issue, believe that "virtual particles are actually exchanged down there, real time". That's not their fault, it's scientific community's fault. Feynman, unwillingly, created a huge frustration to the future generation of students with his drawings.. And by the way it's not a coincidence that it's mostly the experimentalists (and not theorists) that are confused about virtual particles. They see diagrams with particles being exchanged for so many years, and at the same time most of them don't have the time to carefully study QFT and see for themselves what these things really are, so i cannot blame them. 


#98
Feb2413, 11:58 AM

Sci Advisor
P: 5,451

The 'explantation' from Peskin is unacceptable.



#99
Feb2413, 12:02 PM

P: 3,043

It is true that it might be misleading, but I don't know many textbooks on complex mathematical or physical matters that are not completely misleading at one point or another. Although it shouldn't be used as an excuse let's agree that writing/teaching is hard. 


#100
Feb2413, 12:27 PM

Sci Advisor
P: 5,451

I don't think that Feynman ever indicated how to interpret his drawings ontologically (I guess he would have hated this word) Feynman diagrams have been invented for bookkeeping.



#101
Feb2413, 12:50 PM

P: 381

[itex]\hat U\left( t \right)\left {vac} \right\rangle = \sum\limits_n {\left\langle n \right} \hat U\left( t \right)\left {vac} \right\rangle \,\,\underbrace {\left n \right\rangle }_{} [/itex] the multiparticle states [itex]\left\{ {\left n \right\rangle } \right\}[/itex] popped out from the vacuum, at finite time t of the interaction. At large times, none may survive [itex]\left\langle n \right\hat U\left( {t \to \infty } \right)\left {vac} \right\rangle = 0\,\,\forall n \ne vac\,[/itex], but still here you can see there is indeed an exchange of energy between the two fields, and real particles popped out from the vacuum and disappeared. Why don't we describe QFT like that? Virtual particles have nothing to do with these real excitations that do take place, so there is no need to talk about them afterall! (Note: In the equations above i have omitted the states of the other field, e.g. electrons) 


#102
Feb2413, 02:27 PM

P: 161




#103
Feb2413, 02:37 PM

P: 161

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). 


#104
Feb2413, 02:47 PM

P: 161

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 2slit 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. 


#105
Feb2413, 02:48 PM

P: 381




#106
Feb2413, 02:58 PM

P: 381

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. 


#107
Feb2413, 03:17 PM

P: 161

JK423, you've misunderstood my point. I'm just noting that it's legitimate to regard 'virtual photons,' or propagators, as representing something physically real. Of course I'm not saying that a 'virtual photon' is 'like' a photon in a 2slit experiment. I believe someone else on this discussion board made that comparison initially.
Also, I've been checking in from time to time, and have certainly been replying to specific questions or challenges, but apparently I missed some. If you have a specific question, please feel free to ask it at my website, rekastner.wordpress.com; that's the quickest way to get hold of me. You can also email me, rekastner@hotmail.com I'll be glad to try to address a specific question about my proposal. But I regret I have limited time to spend on physicsforums  so if I miss a comment or question please don't take it personally. I also think you'll have an easier time understanding my proposed ontology if you just read my papers and/or my book. Before you summarily dismiss what I've said here about 'virtual particles,' note that I have a paper on the interpretation of propagators in the TI picture in FooP (see my website and the arxiv for preprint version). (That paper also discusses the classical limit of the EM field in terms of coherent states, with reference to the work of Breitenbach, which you may find interesting.) If you don't want to buy the book, you can get it via WorldCat through interlibrary loan. The UMCP library is in the process of acquiring it. Tom, you say "In order to have a scattering process, you need states and an interaction Hamiltonian." Yes. All I'm saying is that the physical interaction described by interaction Hamiltonians, in which field propagation plays a crucial part, is a real physical process. Judging by some of the reactions here, you'd think what I'm saying is heretical or bizarre. It's perfectly natural. Perhaps some of you are reading a lot into the phrase 'virtual particle' that isn't necessarily intended. Of course the entity corresponding to the VEV of a quantum field is not the same thing as the entity described by a quantum state  yes, it does violate conservations laws, etc. That doesn't mean that the former does not physically exist. Of course it's not a classical object, it's very strange, and it's hard to conceptualize as a nonclassical object, but it exists. Here's what Berestetskii, Lifgarbagez and Pitaevskii (QED, vol 4 2nd ed) have to say about propagators: "The propagation functions or propagators defined in [sections 73,73] are of fundamental importance in the formalism of [QED]. The photon propagator [Dmn] is a basic characteristic of the interaction of two electrons...." (p.295 in 2004 paperback edition) So all I'm saying is: whatever you want to call the entity described by a quantity that is of 'fundamental importance' in accounting for the interaction of two electrons, it EXISTS  otherwise there would BE NO INTERACTION! OK, suppose you want to argue that you could use a different theory than QED to account for the interaction between 2 electrons described by quantum states. Well, whatever formal object in your theory does the job done by the propagator, the entity described by that formal object exists! I think the only reason this is coming across as controversial is because some might want to ascribe particular kinds of pictures to it, none of which I'm buying into. I don't think ANY quantum entity is a little corpuscle traveling along a trajectory. I make this very clear in my book. Best wishes, RK 


#108
Feb2413, 03:48 PM

P: 161




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