Why don't virtual particles cause decoherence?

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This is a classic discussion, and there isn't really a right or wrong answer here. Just levels of approximation, and what you are or are not prepared to take as a true statement (always like this with why questions), as Demystifier correctly points out, there is definitely semantics here.
....
Still, in so far as it might be *useful* to picture the photon that you absorb as really being there, then you are allowed to make use of such an ontology, with the caveat that there are perfectly acceptable methods that make no use of perturbative methods at all and that you have to be careful ascribing reality to things that are strictly speaking mathematical fictions (for all the correct reasons that Tom pointed out).
We can clear up the semantic problem by having a better definition of 'real' and 'virtual' photons. And there is one. Davies began grappling with this issue in his QED treatment of the Wheeler-Feynman theory (Davies 1971, 1972, email me if you want refs) but I don't think he quite solved it. The problem is solved unambiguously in the transactional picture as follows: virtual particles are unconfirmed, nascent offer waves; whereas real photons are confirmed offer waves resulting in actualized transactions. Virtual photons do not transmit real energy, while real photons do. But in TI, a 'real photon' is just an actualized transaction.
More quantitatively, in terms of the Davies theory, a virtual photon is just the time-symmetric propagator while a real photon corresponds to the pole in the Feynman propagator. (Davies considers the difficulty of a 'real' photon being an internal line -- but never quite solves it. It is solved by defining the real photon as an actualized transaction.)

As Davies noted, it is misleading to try to define the real vs virtual distinction in terms of 'off-shell' vs 'on-shell', since any emitted and detected photon has a finite lifetime. But the photon discussed in the example above is certainly a real photon since it transferred real energy to your eyeball. However virtual photons need not be regarded as mere 'fictions' -- yes, they are sub-empirical, but that doesn't mean they don't exist. People sometimes want to throw them out as mere artifacts of a purely mathematical process (perturbation theory), but arguably they play a genuine physical role, for example, in the Kondo effect. The mistake is to equate 'real' with 'empirical' i.e. to say if something is not detected or does not transfer real energy it cannot be 'real'. This is just a metaphysical presupposition. Remember that Ernst Mach thought atoms were fictitious, and he turned out to be wrong.

I argue in my book that the fundamental message of quantum theory is that there is a level of physical possibility beneath physical actuality. That is, as Heisenberg said, "Standing in the middle between the idea of an event and the actual event, a strange kind of physical reality just in the middle between possibility and reality.” He wasn't just speaking figuratively here. He was onto something.
 

JK423

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@rkastner
Whether internal lines in Feynman diagrams of perturbation theory (=virtual particles) are described by a quantum state or not, during the time Δt of their supposed existence, is irrelevant to any interpretation of quantum mechanics.
 
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@rkastner
Whether internal lines in Feynman diagrams of perturbation theory (=virtual particles) are described by a quantum state or not, during the time Δt of their supposed existence, is irrelevant to any interpretation of quantum mechanics.
Why?
 

JK423

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Interpretations are about interpreting the quantum formalism. The question whether something is described by a quantum state or not, is irrelevant to how this quantum state is interpreted. Am i wrong?
 
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Interpretations are about interpreting the quantum formalism. The question whether something is described by a quantum state or not, is irrelevant to how this quantum state is interpreted. Am i wrong?
Propagators are certainly part of the quantum formalism at the relativistic level. They describe how the quantum field is propagated. 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 -- yet they are the very basis of quantum field theory. It makes no sense to say that only excited states of the field exist but the fields themselves do not. If they don't exist, then there is nothing there to get excited.

And if one is trying to understand the physical meaning of quantum states (which is what theory interpretation is about), then it's relevant to consider whether a given entity is or is not described by a quantum state, isn't it?
 

JK423

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Propagators are certainly part of the quantum formalism at the relativistic level. They describe how the quantum field is propagated. 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 -- yet they are the very basis of quantum field theory. It makes no sense to say that only excited states of the field exist but the fields themselves do not. If they don't exist, then there is nothing there to get excited.

And if one is trying to understand the physical meaning of quantum states (which is what theory interpretation is about), then it's relevant to consider whether a given entity is or is not described by a quantum state, isn't it?
I am not talking about propagators! A propagator is defined only when you have two events, that take place e.g. at times t1 and t2. What i am talking about is what happens inbetween these two times. A real excitation has a quantum state during that time. Does a virtual particle (defined as an internal line in Feynman diagrams of perturbation theory) acquires a quantum state during this time of its supposed existence?

EDIT:
Are you implying that there (quantum?) entities that are not described by a quantum state? The only quantum entities are quantum states! I am not aware of any other entity. Can you give me an example?
 
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tom.stoer

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Propagators are certainly part of the quantum formalism at the relativistic level.
yes, for a limited range of applicability.

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 ...
in which sense do propagators exist? and in which sense do quantum fields exist?

It makes no sense to say that only excited states of the field exist but the fields themselves do not. /QUOTE]nobody is saying that.

suppose you have N radioactive atoms with decay constant k; in which sense do the atoms exist? and in which sense do the numbers (-k)^n / n! exist? do you understand why I think that you reasoning is not very plausible?
 

Haelfix

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Yes, your enviroment in this case is the sea of virtual particles (=internal lines). If you say that these are described by a quantum state, then they can cause decoherence to other quantum systems. So, please state your position precisely,
During the time Δt of the supposed existence of an internal line (in a Feynman diagram of perturbation theory), is it described by a quantum state or not?
It's a yes/no answer.. We don't have to complicate things.
It is precisely the fact that you don't know the details of the environment (including what its quantum state is) that allows you to organize your Hilbert space in such a way as to allow for decoherence. If you did know what was in the environment, you would be forced to treat the problem differently. I keep harping on this point, b/c it makes the distinction between real and virtual completely irrelevant. At the level of detail that would allow you to discern the difference between the two (say by probing the system), would in fact also necessarily impy that you no longer have an 'environment'.

Anyway, once more. As to your other point. We agree. Virtual particles do not have a state. They do not even have energy-time uncertainty relationships full stop, period. At the level of the mathematics, this is completely unambigous. The problem is the model we use of the real world is not isomorphic to the above model, it is an approximation. The above model describes an idealized world where you measure particles off at infinity (and it makes certain assumptions about clustering, the asymptotic boundary conditions must be specified, and strictly speaking is only valid for nonabelian gauge fields, and really requires a lattice cutoff as well as perfect physically impossible detectors etc etc).

We'd like to use the same mathematics to describe particle interactions in a laboratory, which assuredly does not satisfy the above conditions. And indeed this works. However the price you pay for this, is that the distinction between real and virtual can be made arbitrarily small. So while it's true that the virtual particles don't have a state in this picture, NEITHER DO THE REAL 'realworld' particles.
 

JK423

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Virtual particles do not have a state. They do not even have energy-time uncertainty relationships full stop, period. At the level of the mathematics, this is completely unambigous.
It's good that we agree on this.

The problem is the model we use of the real world is not isomorphic to the above model, it is an approximation. The above model describes an idealized world where you measure particles off at infinity (and it makes certain assumptions about clustering, the asymptotic boundary conditions must be specified, and strictly speaking is only valid for nonabelian gauge fields, and really requires a lattice cutoff as well as perfect physically impossible detectors etc etc).

We'd like to use the same mathematics to describe particle interactions in a laboratory, which assuredly does not satisfy the above conditions. And indeed this works. However the price you pay for this, is that the distinction between real and virtual can be made arbitrarily small. So while it's true that the virtual particles don't have a state in this picture, NEITHER DO THE REAL 'realworld' particles.
I agree that these mathematical issues are present, i read about Haag's theorem and its implications that you told me in an earlier post. But noone i think is completely certain as to what this theorem actually means since QFT works perfectly well in practice. But is this thing relevant to our discussion? All these problems have to do with the particular QFT model that we are using! Noone said that quantum theory is wrong due to these problems. For example, if string theory turns out to be correct then all these problems would dissappear. But string theory is based on quantum mechanics!

My point is the following:
According to quantum mechanics, there are only quantum systems and quantum systems are described by quantum states. There is nothing else! If something is not described by a quantum state then it does not exist(!) since it's not a quantum system, thus it can't be anything else. And that is independent of any model of quantum mechanics that you will use.

Do you agree?


Edit: The fact that virtual particles (defined as internal lines in Feynman diagrams of perturbation series (sigh..)) do not acquire a quantum state has nothing to do with the mathematical difficulties that you mention. That's why i think that these mathematical difficulties are irrelevant to our discussion about the "reality" of virtual particles.
 
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"According to quantum mechanics, there are only quantum systems and quantum systems are described by quantum states. There is nothing else! If something is not described by a quantum state then it does not exist(!) since it's not a quantum system, thus it can't be anything else. And that is independent of any model of quantum mechanics that you will use.
Do you agree?"


These are all metaphysical presuppositions stated dogmatically. You're 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).

And Tom, yes I think people are denying the existence of fields when they deny the existence of the vacuum expectation value of the field (which is what a propagator is).

For further clarification of my proposed ontology of fields in the context of TI, please see Chapter 6 of my book or this paper: http://arxiv.org/abs/1204.5227
 
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JK423

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"According to quantum mechanics, there are only quantum systems and quantum systems are described by quantum states. There is nothing else! If something is not described by a quantum state then it does not exist(!) since it's not a quantum system, thus it can't be anything else. And that is independent of any model of quantum mechanics that you will use.
Do you agree?"


These are all metaphysical presuppositions stated dogmatically. You're 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).
First of all, what i said in the post you quoted is absolutely correct and standard. If you know any other systems other than quantum systems, and you know what physics describes these other systems, write a paper and publish it.

As for the explanation that you seek:
There is always an "excitation" present, the vacuum state |vac> of the field. This state can interact with other excitations and exchange energy so that other excitations can be formed {|n>}. All these things are described by a quantum state. Virtual particles are not. Period.
 
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There is always an "excitation" present, the vacuum state |vac> of the field. This state can interact with other excitations and exchange energy so that other excitations can be formed {|n>}. All these things are described by a quantum state. Virtual particles are not. Period.
Yes, there is energy in the vacuum state. What is it that has the energy? This actually supports my point: it is the field that has the energy. If you deny the existence of the field then there is nothing to which the real energy can be attributed.

Remember also that Feynman described the coupling constant for QED as the amplitude for a fermion to emit a virtual photon. There is a lot of really interesting physics going on in the area to which you want to summarily deny eligibility in the realm of physical systems.
 

JK423

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Yes, there is energy in the vacuum state. What is it that has the energy? This actually supports my point: it is the field that has the energy. If you deny the existence of the field then there is nothing to which the real energy can be attributed.

Remember also that Feynman described the coupling constant for QED as the amplitude for a fermion to emit a virtual photon. There is a lot of really interesting physics going on in the area to which you want to summarily deny eligibility in the realm of physical systems.
First of all, please quote the post where i said that

(1) "fields do not exist".

I didn't say such a thing. What I said was :

(2) "Virtual particles (defined as internal lines in Feynman diagrams of perturbation theory) are not described by a quantum state, hence they are not quantum systems, thus they do not exist".

If you think that these two statements, (1) and (2), are equivalent, then it's you that have a great misunderstanding of the theory, not me. The vacuum state, is a quantum state, hence it's a quantum system. This state always exists, hence the field always exists. The virtual particles (defined as internal lines in Feynman diagrams of perturbation theory) are NOT described by the vacuum state, they are not described by ANY quantum state! Hence they are not quantum systems! If they are not quantum systems, then they cannot be anything else since only quantum systems exist, thus virtual particles (defined as internal lines in Feynman diagrams of perturbation theory) do not exist! Why is it so difficult to accept such a thing?
 
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tom.stoer

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And Tom, yes I think people are denying the existence of fields when they deny the existence of the vacuum expectation value of the field (which is what a propagator is).
Translated to my example you are saying that I deny the existence of the radioactive atoms b/c I deny the existence of the coefficients of a Taylor series. The atoms (and their mathematical) reprentations do exist on a different ontological level; there is no reason why the coefficients of the Taylor series which arise in an approximation shall be 'real' in the same way as the atoms.

A quantum state (like a free particle Fock state) is not identical with reality, but it represents something which has attributes observed in the real world (momentum, spin, physical polarization) whereas a (gauge boson) propagator has no such attributes (no 4-momentum, no spin, partial unphysical polarization, it may be a ghost d.o.f., it is gauge dependent whereas the attributes aren't, ...). Therefore it does not represent reality (in the sense we can define it when we want to have agreement with observations).

In your paper
For further clarification of my proposed ontology of fields in the context of TI, please see Chapter 6 of my book or this paper: http://arxiv.org/abs/1204.5227
I cannot see how you address and resolve gauge issues, especially in non-abelian gauge theories. In QCD there is no single phenomenon which can be understood using only perturbation theory. Therefore any interpretation focussing strictly on perturbation theory misses all key insights for QCD (it's like trying to understand logarithms, cuts and Riemann sheets using Taylor series; it's undefined or wrong)

Please refer to my post #35
Neither QED nor QCD require perturbation theory or virtual particles ... In QCD nearly everything requires non-perturbative methods (even in DIS - using perturbation theory - one probes non-perturbative structure functions)

There is one problem, namely that QED is ill-defined in the UV (Landau pole), in contrast to QCD which is UV complete.

Anyway, most perturbation series (QED, QCD, phi^4 theory, ...) are ill-defined and divergent, so perturbation theory does not make sense to arbitrary high order; its radius of convergence is zero.
I don't see how you can derive a correct interpretation (of a formalism) based on an undefined formalism producing incorrect results or no results at all.
 
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I always understood "virtual particles"-internal Feynman propagator diagram lines- as the "dress" of those physical particles that don't have physical reality as bare particles(bare particles too only have mathematical existence actually). In that sense they exist as mathematical entities that are necessary to give a complete picture of certain physical particles, and the discussion about their existence is quite meaningless. Both the bare particle and its "dress" are necessary to describe what we observe as physical particles (for those that need it, quarks for instance don't follow this scheme but then again they are not directly observable, only indirectly thru the traces left by other particles).
What is misleading and really problematic in view of the repeated discussions is the name "virtual particles", because it seems to imply the autonomous existence of certain particles that for sure don't exist. But then again the same thing could be said about pointlike bare particles, they need to be dressed to count as real or existing physically.
 

JK423

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@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.
 
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@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"?

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.


"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

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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.

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.
 
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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.
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

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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! ;)
 
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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.

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.


I learned lots of things from this thread to be honest. I hadn't realized all these things before. Thanks PhysicsForums! ;)
Great.
 

JK423

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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

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The 'explantation' from Peskin is unacceptable.
 
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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

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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.
 
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