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Why don't virtual particles cause decoherence?

by Mukilab
Tags: decoherence, particles, virtual
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JK423
#91
Feb24-13, 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.
TrickyDicky
#92
Feb24-13, 08:12 AM
P: 3,012
Quote Quote by JK423 View Post
@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".
Perturbative vacuum state(vanishing VEV) or the one associated to zero-point energy and that has measurable effects(Casimir effect) is what I usually identify with what is misnamed as "virtual particles". Do you know any other meaning for "virtual particles"?

Quote Quote by JK423 View Post
So we shouldn't play with words here. 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.
I get the impression that most of this thread is just that. You seem to be interested in imposing a clear cut semantic separation between existence-non existence and virtual-real, without realizing that taking that extreme position all particles must be considered as idealized mathematical artifacts.


Quote Quote by JK423 View Post
"Dressing" exists irrespectively of whether you do perturbation theory or not.
I made clear I was not including the non-perturbative vacuum of quarks for instance.
JK423
#93
Feb24-13, 08:27 AM
P: 381
Quote Quote by TrickyDicky View Post
Perturbative vacuum state or the one associated to zero-point energy and that has measurable effects(Casimir effect) is what I usually identify with what is misnamed as "virtual particles". Do you know any other meaning for "virtual particles"?
You can call the artifacts of perturbation theory however you want, that's fine with me. But the fact that you give them a fancy name doesn't make them real. The only relevant physical system is the vacuum state, and this has nothing to do with virtual particles since the latter are not described by a quantum state.

Quote Quote by TrickyDicky View Post
I get the impression that most of this thread is just that. You seem to be interested in imposing a clear cut semantic separation between existence-non existence and virtual-real, without realizing that taking that extreme position all particles must be considered as idealized mathematical artifacts.
Your impression is correct, because i am a very annoyed with all these misconceptions that are taught even in Universities and fool so many students, with "virtual particles" popping out from the vacuum etc. THERE IS a clear cut in this case, things that are described by states exist (well not always, depends on the model, see ghosts) and things that are not described by quantum states do not exist (in any model). This is such a clear definition of existence, which allows you to throw the notion of virtual particles as "something that exists" out of the window forever. They are only mathematical artifacts.
This is not an extreme position. If you think that it is, please elaborate so that i can understand what you have in mind.
TrickyDicky
#94
Feb24-13, 10:02 AM
P: 3,012
Quote Quote by JK423 View Post
You can call the artifacts of perturbation theory however you want, that's fine with me. But the fact that you give them a fancy name doesn't make them real. The only relevant physical system is the vacuum state, and this has nothing to do with virtual particles since the latter are not described by a quantum state.
If you only consider virtual particles as the pop-sci nonsense portraits, then you are right, otherwise you should know that Feynman internal lines have something to do with the perturbative vacuum state. Besides you are permanently using "quantum state" as synonym of "real" but that by itself is a matter of interpretation in quantum theory that opens a can of worms I'm not going to even try to address.
Quote Quote by JK423 View Post
Your impression is correct, because i am a very annoyed with all these misconceptions that are taught even in Universities and fool so many students, with "virtual particles" popping out from the vacuum etc. THERE IS a clear cut in this case, things that are described by states exist (well not always, depends on the model, see ghosts) and things that are not described by quantum states do not exist (in any model). This is such a clear definition of existence, which allows you to throw the notion of virtual particles as "something that exists" out of the window forever. They are only mathematical artifacts.
This is not an extreme position. If you think that it is, please elaborate so that i can understand what you have in mind.
No serious physicists thinks about internal lines in Feynman diagrams as "particles popping in and out" , that's pop-sci stuff, I don't think people like Bill-k, mfb or Healfix take that nonsense seriously, you seem to be fighting a straw man.
JK423
#95
Feb24-13, 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! ;)
TrickyDicky
#96
Feb24-13, 11:04 AM
P: 3,012
Quote Quote by JK423 View Post
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".
Now, whether you can or cannot understand the significance of this fact is another matter
Fine, sci-popping in and out particles are not described by quantum states (in fact they are only described by pop-sci writers and some confused professors because they are just a silly picture for the math), that was agreed by me a few posts ago.

Quote Quote by JK423 View Post
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.
If the ontology is what had you worried, rest assured "virtual particles" as autonomous entities have no ontological significance whatsoever. Note however that in general physicists are more interested in mathematical models that reflect as accurately as possible the measurement of observables, and ontological consideration are quite secondary.


Quote Quote by JK423 View Post
I learned lots of things from this thread to be honest. I hadn't realized all these things before. Thanks PhysicsForums! ;)
Great.
JK423
#97
Feb24-13, 11:32 AM
P: 381
Quote Quote by TrickyDicky View Post
Fine, sci-popping in and out particles are not described by quantum states (in fact they are only described by pop-sci writers and some confused professors because they are just a silly picture for the math), that was agreed by me a few posts ago.


If the ontology is what had you worried, rest assured "virtual particles" as autonomous entities have no ontological significance whatsoever. Note however that in general physicists are more interested in mathematical models that reflect as accurately as possible the measurement of observables, and ontological consideration are quite secondary.
I am happy that we agree (and stopped playing with words!)!

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 second-order 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 energy-time 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.
tom.stoer
#98
Feb24-13, 11:58 AM
Sci Advisor
P: 5,366
The 'explantation' from Peskin is unacceptable.
TrickyDicky
#99
Feb24-13, 12:02 PM
P: 3,012
Quote Quote by JK423 View Post

Example from Peskin (p. 13):
Even when there is not enough energy for pair creation, multiparticle states appear, for example, as intermediate states in second-order 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 energy-time uncertainty principle, when we said that this is not the case.
I wouldn't give so much significance to that introductory paragraph, when he writes "multiparticle states" he is simply justifying the necessity of dealing with more than a single particle in relativistic QM. The example is admittedly not very fortunate.
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.
tom.stoer
#100
Feb24-13, 12:27 PM
Sci Advisor
P: 5,366
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.
JK423
#101
Feb24-13, 12:50 PM
P: 381
Quote Quote by tom.stoer View Post
The 'Explantations' from Peskin is unacceptable.
Yep! And by the way, i don't say that Feynman had such intentions, but unwillingly caused a lot of trouble (when at the same time made perturbation theory more approachable).
Quote Quote by TrickyDicky View Post
I wouldn't give so much significance to that introductory paragraph, when he writes "multiparticle states" he is simply justifying the necessity of dealing with more than a single particle in relativistic QM. The example is admittedly not very fortunate.
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.
Yeah, multiparticle states do appear! You want to talk about them, teach them and explain them? Begin with the electromagnetic vacuum, put an interaction and
[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)
maxverywell
#102
Feb24-13, 02:27 PM
P: 161
Quote Quote by JK423 View Post
Yep! And by the way, i don't say that Feynman had such intentions, but unwillingly caused a lot of trouble (when at the same time made perturbation theory more approachable).
Yep, but that's why he was saying "shut up and calculate". Feynman's diagrams can be used for calculations, but when you try to interpret them ontologically you get into troubles.
maxverywell
#103
Feb24-13, 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).
rkastner
#104
Feb24-13, 02:47 PM
P: 161
Quote Quote by maxverywell View Post
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.
JK423
#105
Feb24-13, 02:48 PM
P: 381
Quote Quote by maxverywell View Post
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!
JK423
#106
Feb24-13, 02:58 PM
P: 381
Quote Quote by rkastner View Post
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.
rkastner
#107
Feb24-13, 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 2-slit 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 non-classical 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
maxverywell
#108
Feb24-13, 03:48 PM
P: 161
Quote Quote by JK423 View Post
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 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.


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