Misconceptions about Virtual Particles - Comments

[nikkkom]

No, definitely not. Virtualness is a matter of where something appears in a Feynman diagram, hence none-or-all. It is meaningless to say that a particle is described to 90% by an external line and to 10% by an internal line. Virtual particles are by definition (in any textbook where they are defined) terminology for internal lines of a diagram.

I think everyone agrees with the above.

The thing is, you don't just state the above statement, you proceed with other statements such as:

"The word virtual is an antonym to real – unlike the general readership of popular literature on particle physics, the creators of the terminology were well aware that virtual particles are not real in any observable sense".
and
"They cannot cause anything or interact with anything".

Since we established that all unstable particles can be seen as virtual, this means that muons from cosmic ray showers are virtual too.

It's hard to agree that these muons "are not real in any observable sense" and "cannot cause anything or interact with anything", when physicists have to hide under several kilometers of rock (!) to decrease muon-induced background in their DM detection experiments. Clearly, muons do "interact", "cause" events to happen and thus are "real" and "observable".

 

[nikkkom]

>> But when they come close, the semiclassical description breaks down and one needs full quantum field theory to describe what happens.

> Define "close".
> Two electrons repelling each other (in term of Feynman diagrams, "by exchanging virtual photons") over a separation of, say, one kilometer, still involves virtual photons.

This doesn't change the status of the electrons from real to virtual.

My point is that virtual *photons* do not need electrons to be close. Virtual photons work across any distances, even quite macroscopic. There is no cutoff when QFT should be abandoned and semiclassical description should be used.

 

[A. Neumaier]

Since we established that all unstable particles can be seen as virtual, this means that muons from cosmic ray showers are virtual too.

I didn't establish this. There is a difference between ''particle'' and ''particle'' dependent on the context; you are mixing context to ''establish'' this.

A particle is virtual in the context of a Feynman diagram with internal lines, and real in the context of something requiring a state. This makes the difference. A given occurrence of the word particle must be interpreted in its context according to this rule. Then things are unambiguous.

 

[A. Neumaier]

Since we established that all unstable particles can be seen as virtual, this means that muons from cosmic ray showers are virtual too.

Muons from cosmic rays are not virtual at all since they are located in space and time, which is only possible if one can assign states to them. A virtual muon cannot ''arrive'' since that notion is meaningless for an internal line in a Feynman diagram.

It's hard to agree that these muons "are not real in any observable sense" and "cannot cause anything or interact with anything", when physicists have to hide under several kilometers of rock (!) to decrease muon-induced background in their DM detection experiments. Clearly, muons do "interact", "cause" events to happen and thus are "real" and "observable".

All these muons are real since to talk about interaction, cause, real, and observable all requires that they have a state.

 

[RockyMarciano]

The assigning of states you keep bringing up is again referring back to free states of the free quantum field theories that are known not to be valid in the interacting qft's in the presence of observable interactions, so why do you use it as an argument?

 

[A. Neumaier]

The assigning of states you keep bringing up is again referring back to free states of the free quantum field theories that are known not to be valid in the interacting qft's in the presence of observable interactions, so why do you use it as an argument?

Particles on external lines in Feynman diagrams always belong to asymptotic states, and these are free and have well-defined states. The particle picture is valid anyway only when this asymptotic description is sufficiently accurate.

 

[RockyMarciano]

Particles on external lines in Feynman diagrams always belong to asymptotic states, and these are free and have well-defined states. The particle picture is valid anyway only when this asymptotic description is sufficiently accurate.

Ok but then you seem to be mixing back and forth the Feynman diagrams, which are limited to a graphical description of terms in the Dyson-Wick expansion, with a clear enough distinction between external and internal lines meanings within this graphic representation that shouldn't be confused with the actual physics in renormalized interacting qft's, with your own ontological views about quantum field theory.

 

[A. Neumaier]

Ok but then you seem to be mixing back and forth the Feynman diagrams, which are limited to a graphical description of terms in the Dyson-Wick expansion, with a clear enough distinction between external and internal lines meanings within this graphic representation that shouldn't be confused with the actual physics in renormalized interacting qft's, with your own ontological views about quantum field theory.

In doing real physics it is often unavoidable to switch between representations featuring different levels of detail. If one uses the particle picture at all (and in particular always when one has to interpret what people using the particle language say), one acknowledges that one works in a semiclassical picture where a particle description makes approximate sense except during collisions.

Thus between collisions the particles are described by asymptotic states, hence we have real particles, while during collisions, a black box view featuring the S-matrix is used. To calculate the S-matrix one may work in renormalized perturbation theory using quantum field theory. In this case one utilizes for the computation integrals represented by Feynman diagrams, which are then described pictorially. here the same, free real particles show as external lines, while the interaction is represented in terms of internal lines, figuratively called virtual particles. This is the only mixing that is actually used, and there is nothing ambiguous about it.

 

[RockyMarciano]

In doing real physics it is often unavoidable to switch between representations featuring different levels of detail. If one uses the particle picture at all (and in particular always when one has to interpret what people using the particle language say), one acknowledges that one works in a semiclassical picture where a particle description makes approximate sense except during collisions.

Thus between collisions the particles are described by asymptotic states, hence we have real particles, while during collisions, a black box view featuring the S-matrix is used. To calculate the S-matrix one may work in renormalized perturbation theory using quantum field theory. In this case one utilizes for the computation integrals represented by Feynman diagrams, which are then described pictorially. here the same, free real particles show as external lines, while the interaction is represented in terms of internal lines, figuratively called virtual particles. This is the only mixing that is actually used, and there is nothing ambiguous about it.

Well, here you have taken the work of spelling out the mixing, if you don't spell it out this clearly it might be confusing for many. I would then ask you, because this is the impression I get(correct me otherwise) why you consider the asymptotic states that nobody can actually observe to have existence in detriment of the actual interactions that can be observed.

 

[Mohd Abdullah]

The former is what quantum field theory says (and hence what I say): The vacuum is the state containing exactly zero particles anywhere in space and at all times. Since it is an eigenstate of the number operator, there is no uncertainty at all about this.

So, Arnold, vacuum is absolutely nothing according to you? If I imagine a universe with a lone object, for example a lone quark, is there any external influence outside this lone quark?

 

[Nugatory]

So, Arnold, vacuum is absolutely nothing according to you?

No, he is saying (and this is not a "according to you" thing) that the the vacuum is an eigenstate of the number operator.

If I imagine a universe with a lone object, for example a lone quark, is there any external influence outside this lone quark?

That question doesn't make any sense in the context of this discussion.

 

[A. Neumaier]

So, Arnold, vacuum is absolutely nothing according to you?

Already in the noninteracting case, a system in the vacuum state contains exactly zero particles (since we have an eigenstate of the number operator) without any fluctuation. You can easily check this yourself. States in the interacting case are less well understood, but still the vacuum state has zero 4-momentum since it belongs to a trivial representation of the Poincare group. Hence it has zero energy, again exactly without any fluctuation. But the creation of a particle pair requires a positive energy since states containing particles have positive energy. Thus no particles can ever be created.

If I imagine a universe with a lone object, for example a lone quark, is there any external influence outside this lone quark?

A lone quark is impossible, since physical systems cannot be colored in the sense of quark colors. But already a lone photon in an otherwise empty universe would no longer be a vacuum. On the other hand, there is nothing outside the universe, and by assumption nothing in it except the photon - so a lone photon would not interact with anything and behave freely.

 

[Mohd Abdullah]

Already in the noninteracting case, a system in the vacuum state contains exactly zero particles (since we have an eigenstate of the number operator) without any fluctuation. You can easily check this yourself. States in the interacting case are less well understood, but still the vacuum state has zero 4-momentum since it belongs to a trivial representation of the Poincare group, and hence zero energy, again exactly without any fluctuation, while the creation of a particle pair requires a positive energy. Thus no particles are ever created.A lone quark is impossible, since physical systems cannot be colored in the sense of quark colors. But already a lone photon in an otherwise empty universe would no longer be a vacuum. On the other hand, there is nothing outside the universe, and by assumption nothing in it except the photon - so a lone photon would not interact with anything and behave freely.

Thank you for the response, Arnold.

Btw, if someone imagine the photon as a tiny ball, then there is space that gives the photon its shape. But in the context of quantum physics, a photon has no shape (I'm sorry if this is a mistake) as it is massless. So, in an imaginary scenario where there is a lone photon with no other existing objects, the lone photon is actually the whole Universe itself as it has no shape. Thoughts?

 

[weirdoguy]

if someone imagine the photon as a tiny ball

You can't imagine photons as tiny balls.

 

[A. Neumaier]

in the context of quantum physics, a photon has no shape (I'm sorry if this is a mistake) as it is massless. So, in an imaginary scenario where there is a lone photon with no other existing objects, the lone photon is actually the whole Universe itself as it has no shape.

Yes, but this is not our universe, so I don't care.

 

[PeterDonis]

in the context of quantum physics, a photon has no shape (I'm sorry if this is a mistake) as it is massless

What do you mean by "shape"? What in the math does this correspond to?

in an imaginary scenario where there is a lone photon with no other existing objects, the lone photon is actually the whole Universe itself

No, it isn't. In this imaginary situation, at least if you are imagining it according to the laws of quantum field theory (if you're not then you're imagining something that is out of bounds for discussion here), you have a quantum field in a particular state in a background spacetime. You don't have just the field, which is what "the lone photon is the whole universe" would mean.

 

[Karolus]

I am very interested in the virtual particles.
For what I understand their existence follows from the principle of indeterminacy in the form

$\Delta E \Delta T \geq \hbar$

When $\Delta T \rightarrow 0$ , $\Delta E \rightarrow \infty$

For instants of very small time then the energy fluctuation can become so great as to be compatible with the existence of masses. These particles are called "virtual" because their existence times tend to virtually 0 and therefore are not directly observable.
The first point is that I do not see where and how the gravity enters into the matter.
The second point is, if I understand it, that the virtual particles are only the lines that result in calculations of processes but who have no physical reality.
I do not understand how do you reconcile these versions, adding that the fluctuations of the vacuum and all the virtual processes are widely accepted by physics, just think Casimir effect or the Hawking radiation

 

[mfb]

For what I understand their existence follows from

I think you should read the article. Your reasoning is flawed, and the article explains why.

There is no proper energy/time uncertainty principle. Time is not even an operator.

The Casimir effect can be described without virtual particles, and the derivation of Hawking radiation only works without virtual particles. You'll only see virtual particles in pop-science descriptions.

 

[Karolus]

I think you should read the article. Your reasoning is flawed, and the article explains why.

There is no proper energy/time uncertainty principle. Time is not even an operator.

The Casimir effect can be described without virtual particles, and the derivation of Hawking radiation only works without virtual particles. You'll only see virtual particles in pop-science descriptions.

Then the particles "virtual" does not exist?

 

[mfb]

Then the particles "virtual" does not exist?

They certainly do not exist as objects that exist and move around. Every "existence" lower than that is up to interpretation.

and the quantum fluctuations of the vacuum is all a hoax invented by Feynman Hawking, taught in universities, in the more advanced courses in theoretical physics, and so much of pop-science?

Nothing is a hoax. It is a different way to describe things. You can use virtual particles - just keep in mind that they are mainly a mathematical tool in perturbation theory. Their existence is about as real as the existence of an integral sign, another mathematical tool used in perturbation theory.

 

[vanhees71]

Well, in my QFT lecture nobody taught "vacuum fluctuations", nor do I in my lectures ;-). There are radiative corrections, symbolized by Feynman diagrams containing loops, and these Feynman diagrams are just a very clever method to write down complicated formulae in a very efficient way. The "real" thing, i.e., what's measurable calculated in this way are transition-probability matrix elements $|S_{fi}|^2$, which usually are expressed in terms of cross sections and which are measured by the experimentalists with there detectors. There's not a single observation of "vacuum fluctuations", and there never can be, because to measure something you have to use a detector, and introducing a detector makes the situation not to be vacuum anymore.

The Casimir effect is indeed, as discussed at length in this thread and elsewhere here on the forums, indeed pretty analogous to the van der Waals force. It's not due to fluctuations of the vacuum but due to quantum fluctuations of real (not virtual!) charges and fields.

 

[Karolus]

Nothing is a hoax. It is a different way to describe things. You can use virtual particles - just keep in mind that they are mainly a mathematical tool in perturbation theory. Their existence is about as real as the existence of an integral sign, another mathematical tool used in perturbation theory.

So even an electron is as real as the psi schrodinger, as real as a Greek letter or mass is just as real a Latin letter ...
in short, all of physics is as real as a series of mathematical signs...

 

[Karolus]

Well, in my QFT lecture nobody taught "vacuum fluctuations", nor do I in my lectures ;-). There are radiative corrections, symbolized by Feynman diagrams containing loops, and these Feynman diagrams are just a very clever method to write down complicated formulae in a very efficient way. The "real" thing, i.e., what's measurable calculated in this way are transition-probability matrix elements $|S_{fi}|^2$, which usually are expressed in terms of cross sections and which are measured by the experimentalists with there detectors. There's not a single observation of "vacuum fluctuations", and there never can be, because to measure something you have to use a detector, and introducing a detector makes the situation not to be vacuum anymore.

The Casimir effect is indeed, as discussed at length in this thread and elsewhere here on the forums, indeed pretty analogous to the van der Waals force. It's not due to fluctuations of the vacuum but due to quantum fluctuations of real (not virtual!) charges and fields.

So Feynman lost a lot of time to identify arrows as fermions, wavy lines as photons and so on .. a picturesque symbolism just to make a tattoo ... who knows, maybe his car, though as I read, had drawn them on his car ...

 

[mfb]

So even an electron is as real as the psi schrodinger, as real as a Greek letter or mass is just as real a Latin letter ...
in short, all of physics is as real as a series of mathematical signs...

You can detect electrons in a detector. You cannot detect virtual particles in a detector.

So Feynman lost a lot of time to identify arrows as fermions, wavy lines as photons and so on .. a picturesque symbolism just to make a tattoo ... who knows, maybe his car, though as I read, had drawn them on his car ...

Your sarcasm doesn't help. Feynman invented calculation tools. Those tools are useful. He did not discover new particles, he just found a much better way to describe existing particles and do the calculations.

 

[vanhees71]

So Feynman lost a lot of time to identify arrows as fermions, wavy lines as photons and so on .. a picturesque symbolism just to make a tattoo ... who knows, maybe his car, though as I read, had drawn them on his car ...

How do you come to this conclusion? Among all the ingenious ideas by Feynman his diagrams are among his most important. Without Feynman diagrams, I'm pretty sure we'd not have been able to use QFT to describe all known elementary particles in such a detailed way as is now represented by the Standard Model, let alone the solution of the difficult problem of renormalization. Last but not least the diagram technique is nowadays used not only in particle physics but also in any other field using QFT methods as in condensed-matter physics etc.

 

[weirdoguy]

So Feynman lost a lot of time to identify arrows as fermions, wavy lines as photons and so on

Your attitude is like "I know better then working physicists do!" but most of the things that you say are just common misconceptions and it's really quite easy to check their fallacy. How many academic courses on QM and QFT have you taken? How many advanced book have you read? It seems to me that you base your knowledge only on pop-sci books and stuff, and that is not a good basis for your sarcastic attitude. Show us some math that justifies it. Or tell us in which textbook we'll find information that virtual particles are as real as electrons. If you really did take any serious course on QM/QFT you'd already know what virtual particles are (Feynman certainly did), or that particle-wave duality is not part of modern QT.

 

[ftr]

I think you all misunderstood what Karolus was saying. He is certainly not against Feynman since his picture(playing Samba, whatever) is his Avatar.

English can be tricky when it comes to indirect talk. He was trying to say ( I think) that the term is and has been widely used and there is probably good reason, although the math is the same for QED, but Casimir controversy and multiple definitions of vacuum do make things confusing.

 

[rkastner]

Hello, I appreciate your disambiguation of the issue of stability from that of virtual quanta. However, I have to point out that your argument against the ontological reality of virtual quanta doesn't work. You say:
"That virtual particles transmit the fundamental forces proves the ”existence” of virtual particles in the eyes of their afficionados. But since they lack states (multiparticle states are always composed of on-shell particles only), they lack reality in any meaningful sense. States involving virtual particles cannot be created for lack of corresponding creation operators in the theory. Thus they cannot cause anything or interact with anything."
Now, I am not an 'aficionado' of virtual quanta. I have no particular need for them to be real. Maybe they're not. But it seems to me that dismissing the reality of an entity simply because there is not a Hilbert (or Fock) space state attribution for that entity is a misplaced dependence on a particular sort of theoretical description to dictate what can be considered real. You acknowledge that forces are mediated by off-shell quanta, and there certainly is a theoretical description of such objects (the propagator)--it's just not a Hilbert Space description. So how can it make sense to say these things don't exist, when they convey forces that can be empirically observed,? Why should a Hilbert space state attribution be a requirement for reality that trumps the fact that these entities mediate empirically observable forces?

 

[PeterDonis]

these entities mediate empirically observable forces

We don't know that virtual particles mediate empirically observable forces. We know that those forces exist because they are empirically observable, but our empirical observations don't tell us that virtual particles mediate the forces.

 

[rkastner]

No, there is no disagreement around whether virtual particles mediate the forces. This is part of the theory--forces are mediated by the off-shell propagators. The original poster acknowledged this when he said "That virtual particles transmit the fundamental forces.." So this is not under dispute. What he's disputing is that we should take virtual particles as real, not whether they mediate forces. He already admits that they do. If he were doubting the theory, he would have said so. Instead, he points to an aspect of the theory --its on-shell states--to argue that virtual particles are not real. So this is not about whether we can know that they exist based on empirical issues, as you express. It's about disqualifying virtual particles for reality based on the kind of theoretical description applicable to them.
I should add that disqualifying an entity's reality based on the fact that it is not directly observable is a mistake. This is what Ernst Mach did with Ludwig Boltzmann's atoms. Clearly, Boltzmann was right. We indirectly observe the results of the mediation of forces by virtual quanta. So it makes no sense to claim that they are not real just because they aren't on-shell. We would have no measurable forces without them!

 

[PeterDonis]

there is no disagreement around whether virtual particles mediate the forces. This is part of the theory--forces are mediated by the off-shell propagators

In one particular theoretical model (perturbation theory), yes. But that is not the only theoretical model for these forces. As the Insights article that is the subject of this comment thread points out, in non-perturbative QFT virtual particles do not appear at all. And perturbation theory is only an approximation to the non-perturbative theory anyway; we use it in cases where we can't compute the non-perturbative theory exactly so we have to fall back on an approximation.

The original poster acknowledged this when he said "That virtual particles transmit the fundamental forces.."

Where did he say that? Quoting out of context is not helpful to the discussion.

 

[rkastner]

I included a more lengthy quote in my original post. It's in his insight article "Misconceptions About Virtual Particles." Here's the quote I originally included: "That virtual particles transmit the fundamental forces proves the ”existence” of virtual particles in the eyes of their afficionados. But since they lack states (multiparticle states are always composed of on-shell particles only), they lack reality in any meaningful sense. States involving virtual particles cannot be created for lack of corresponding creation operators in the theory. Thus they cannot cause anything or interact with anything."
His only argument here against the physical reality of virtual particles (really, whatever is described by propagators) is that they are off-shell. All I'm saying here is that if he accepts that a sound theory describes forces as mediated by propagators, and that one can detect the effects of forces, it makes no sense to say that what the propagators describe is not physically real.

 

[PeterDonis]

His only argument here against the physical reality of virtual particles (really, whatever is described by propagators) is that they are off-shell.

That's the argument he makes in that particular quote. But in the article, he also makes the argument that non-perturbative QFT does not include virtual particles at all.

Also, when he says "that virtual particles transmit the fundamental forces proves the 'existence' of virtual particles in the eyes of their aficionados", he is describing the aficionados' point of view; he is not unequivocally agreeing with it. Much of the rest of the article, in fact, is devoted to showing how the phrase "virtual particles transmit the fundamental forces" is not actually a description of "real physics", but just a convenient heuristic story that should not be taken too seriously. So you should be very careful about laying too much stress on that particular phrase.

 

[A. Neumaier]

It's in his insight article "Misconceptions About Virtual Particles." Here's the quote I originally included: "That virtual particles transmit the fundamental forces proves the ”existence” of virtual particles in the eyes of their afficionados. But since they lack states (multiparticle states are always composed of on-shell particles only), they lack reality in any meaningful sense. States involving virtual particles cannot be created for lack of corresponding creation operators in the theory. Thus they cannot cause anything or interact with anything."
His only argument here against the physical reality of virtual particles (really, whatever is described by propagators) is that they are off-shell. All I'm saying here is that if he accepts that a sound theory describes forces as mediated by propagators, and that one can detect the effects of forces, it makes no sense to say that what the propagators describe is not physically real.

when he says "that virtual particles transmit the fundamental forces proves the 'existence' of virtual particles in the eyes of their aficionados", he is describing the aficionados' point of view; he is not unequivocally agreeing with it.

Indeed, I am not at all agreeing with it, and say in this very quote why. Everything we can say about how a particular physical system behaves in time is derived from its state. Thus properties not encoded in the state can in principle not be observed or even theoretically checked for their validity. The main article on Misconceptions about Virtual Particles explains this in much more detail.

To avoid in the future your kind of misunderstanding, I rephrased in the quoted Insight article the first sentence of the quoted statement to

That Feynman diagrams display virtual particles ''transmitting'' the fundamental forces proves the ''existence'' of virtual particles in the eyes of their afficionados.

 

[Tendex]

It appears to me that here the classical stance about particles instead of the quantum field view is being used to create an artificial debate about the "existence" of particles whether "real" or "virtual". In relativistic quantum field theory the concept of quantum field is stressed instead since there is no particle probability conserved in the same sense as there was for wave functions in non-relativistic quantum mechanics and clicks of detectors refer to excitations of the field and don't have a clear cut identification with "particles" even though the term has unfortunately stuck. The number of particles is not conserved and detections and measurements are not equivalent to "particles" in the classical sense, whether one calls them "real" or "virtual".
Detections' relativistic causality is not preserved using "particles" states but commutators(that vanish for operators at spacelike separation) according to quantum canonical commutation relations, so the argument using states is quite irrelevant here. And all this refers to free quantum fields which are the fundamental bricks with which any QFT is built so it doesn't have anything to do with perturbative versus non-perturbative math(notwithstanding that all this is of course very important to the perturbative S-matrix construction). By the way as of now there exists no non-perturbative relativistic QFT in physical 4-space so I'm not sure why is it being mentioned at all.
Summarizing, this debate is addressing a non-issue and doing it introducing wrong information as it insists on the "existence" of "real particles" suspiciously similar to classical "billiard" particles(which ignores concepts that appear in all QFT texts) when the fundamental concept in RQFT is that of the quantum field that uses relativistic concepts of measurements/detections rather than that of single particles(whose number is not even conserved already for free fields).