Insights Misconceptions about Virtual Particles - Comments

[vanhees71]

Well, all these quantities are measured with the corresponding measurement devices like spectrometers, particle detectors (for the Z-boson mass and width you measure dilepton spectra in various ways), etc. you find in the physics labs around the world. In physics in fact quantities are defined by giving appropriate (equivalence classes of) measurement protocols to quantitatively observe them. That's why they are called observables after all. Also there is nothing more needed concerning the application of the quantum-theoretical formalism (e.g., formulated as the representation of an observable algebra on Hilbert space, based on various symmetry principles which themselves are discovered by observation of conservation laws) than Born's rule, i.e., the minimal interpretation.

Where you need something like a "thermal interpretation" is when it comes to understand the overwhelming success of classical physics (including classical relativistic and non-relativistic mechanics, electrodynamics, and thermodynamics) to describe macroscopic systems. Here you need some coarse graining to describe macroscopic effective (relevant) degrees of freedom as (spatio-temporal) averages over many microscopic degrees of freedom.

 

[A. Neumaier]

Well, all these quantities are measured [...] as (spatio-temporal) averages over many microscopic degrees of freedom.

To avoid that this thread (about virtual particles) is polluted by a discussion of measurement issues I created a new thread answering this.

 

[Collin237]

However, one cannot go the other way with virtual particles - you cannot make them real by going to a higher resolution, since the calculations then lead to different predictions.

So? External lines become internal if the diagram is extended to show more interactions "surrounding" the scattering, but internal lines don't become external unless the diagram is changed by breaking a line. That's just basic topology. What does it have to do with "existence" vs "myth" or anything?

 

[A. Neumaier]

External lines become internal if the diagram is extended to show more interactions "surrounding" the scattering, but internal lines don't become external unless the diagram is changed by breaking a line.

In both case you get diagrams describing different situations, corresponding to calculations of different collision processes, compared to the original diagram.

 

[Collin237]

In both case you get diagrams describing different situations, corresponding to calculations of different collision processes, compared to the original diagram.

There are collision processes happening everywhere all the time. The vast majority of them occur without anyone situating or calculating them.

 

[A. Neumaier]

There are collision processes happening everywhere all the time. The vast majority of them occur without anyone situating or calculating them.

This doesn't change what I said. One can always ignore or coarse-grain things (unobserved things being very coarse-grained) without changing what happens. It is a big conceptual mistake to think of a sequence of collisions with particle worldlines in between as being a particular Feynman diagram, so that the worldlines would become internal lines of a diagram. Most paths in a complex Feynman diagram have a very different topology from those you'd get by a sequence of collisions, and their mathematics is completely different!

 

[Collin237]

What? The theory is non-extensible? Can someone corroborate this?

 

[A. Neumaier]

What? The theory is non-extensible? Can someone corroborate this?

My assertion was that the theory of properties of virtual particles (aka internal lines of Feynman diagrams aka integration variables of multivariate integrals) has very different properties from the theory of properties of systems of multiply colliding particles (aka systems with states changing upon collisions according to computable scattering statistics). Thus there is no way to mix them up.

 

[Collin237]

(aka systems with states changing upon collisions according to computable scattering statistics).

But wouldn't that be a collapse, i.e., an interpretational issue? Which, as I've seen oft repeated here, doesn't change the basic math?

I'm still waiting for someone else to corroborate.

 

[A. Neumaier]

But wouldn't that be a collapse, i.e., an interpretational issue? Which, as I've seen oft repeated here, doesn't change the basic math?

No. The Boltzmann equation is based on such a collision picture, and nothing ever collapses there. Collapse is related to the change of state of a few-particle system upon acknowledging a measurement as definite. But as you correctly observed, most collisions in nature are never observed. But they still happen, or the Boltzmann equation would not work.

Better learn the math rather than complaining about formal issues without understanding them!

 

[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.