I What are the Misconceptions Surrounding Virtual Particles?

[Ranku]

Are the typical energies of virtual particles lower than real particles?

 

[PeroK]

Are the typical energies of virtual particles lower than real particles?

That question makes no sense.

 

[Ranku]

That question makes no sense.

Let me re-frame it then: are the masses of virtual particles comparable to real particles?

 

[PeroK]

Let me re-frame it then: are the masses of virtual particles comparable to real particles?

Search for "virtual particles mass shell".

 

[PeterDonis]

are the masses of virtual particles comparable to real particles?

If by "mass" you mean the appropriate factor in the Lagrangian, then yes. (Though even here there are complications, since the Lagrangian has to be the "bare" Lagrangian, not the renormalized one.)

If you mean something else by "mass", the question probably is not well-defined.

 

[Ranku]

If by "mass" you mean the appropriate factor in the Lagrangian, then yes. (Though even here there are complications, since the Lagrangian has to be the "bare" Lagrangian, not the renormalized one.)

If you mean something else by "mass", the question probably is not well-defined.

I mean mass in the sense of magnitude. Since virtual particles can become real upon addition of energy, how big is the gap in energy?

 

[dextercioby]

[...]Since virtual particles can become real upon addition of energy, [...]

Where do you get this from?

 

[PeterDonis]

I mean mass in the sense of magnitude.

"Magnitude" is much too vague. Magnitude of what?

Since virtual particles can become real upon addition of energy, how big is the gap in energy?

This is not a valid way of thinking about how virtual particles work.

 

[Ranku]

Where do you get this from?

This is not a valid way of thinking about how virtual particles work.

There are such mentions in these two papers:

1. "Due to the bosonic nature of the photon, increasing the peak intensity through a combination of raising the pulse energy and decreasing the pulse duration will pile up more and more photons within the same finite region of space. In the absence of material, this continues until the vacuum is stressed to the point of breakdown and virtual particles become real."
2. "A quite direct evidence of the existence of such virtual particles is provided by the dynamical Casimir effect (DCE). It predicts that rapid modulations of the boundary conditions of a quantum field induce vacuum amplification effects that result in the creation of real particles out of vacuum fluctuations."

 

[vanhees71]

These seem to be quite strange papers. Can you give the reference? One has to now the context to judge whether the statements make some sense when translated to more careful formulations.

"Virtual particles" are not observable physical objects but abstractions in the perturbative evaluation of S-matrix elements for scattering processes. Depicted by Feynman diagrams they are symbolized by internal lines and stand for propagators of (free) fields. As such virtual particles don't have physical properties, because they are never observables.

 

[Motore]

Can you give the reference?

Click on the numbers (1. and 2.), they're hyperlinks.

 

[vanhees71]

Well yes. As expected it's just the typical lingo some people think they have to use to make their papers more exciting. All what's written there is just standard QED. What's described are attempts to demostrate the Schwinger mechanism (creation of electron-positron pairs due to a strong electromagnetic field created with a high-intensity laser aka a coherent state aka a classical electroamgnetic wave; that's far from being vacuum) and Delbrück scattering (elastic scattering of two photons, which is a fourth-order effect of QED).

In paper 2 they use "polaritons", i.e., an in-medium (!) collective state to emit real photons. That's also not something "popping out of the vacuum" but just an in-medium photon-production process. As Einstein said (about theoretical physicists but it seems to apply nowadays as well to experimentalists): "Don't listen to their words but look at their deeds."

 

[Jarek 31]

Are the typical energies of virtual particles lower than real particles?

Neutron decay uses ~80x heavier W boson: https://en.wikipedia.org/wiki/Free_neutron_decay

 

[Cthugha]

In paper 2 they use "polaritons", i.e., an in-medium (!) collective state to emit real photons. That's also not something "popping out of the vacuum" but just an in-medium photon-production process. As Einstein said (about theoretical physicists but it seems to apply nowadays as well to experimentalists): "Don't listen to their words but look at their deeds."

To be honest, I disagree. I agree that this is a lingo problem, but I think the problem is not with the authors.

The quantum vacuum is commonly considered as the lowest energy state of whatever your system at hand is. It is quite common that researchers in cosmology, astrophysics or particle physics interpret this to mean the QED vacuum of empty space, but this is usually never the case in any publication outside of particle physics. In many other fields and most prominently in all branches of optics, people commonly have a look at the vacuum of cavity QED which is a significant difference. Most importantly, the system considered in cavity QED is an open system and (in contrast to bare empty space) its properties may be changed externally, e.g. by changing the properties of the cavity investigated. And usually all of the vacuum emission stuff in cavity QED can be traced back to exactly that: people change the system so that the properties of the vacuum state itself become time-dependent. Of course stuff "popping out of the vacuum" is not necessarily too exciting if you are allowed to change the vacuum state (and the system in the vacuum state is of course an open system coupled to another system so the total system consisting of all subsystems is not in the vacuum state), but the authors are quite clear about what they do.

However, of course I agree that such results are somewhat hyped and receive too much attention because usually laymen do not understand cavity QED well enough to realize what kind of vacuum people are talking about.

 

[vanhees71]

Well, but that's really a lingo problem then. In my field of research (relativistic HI collisions) nobody would call a many-body ground state "vacuum". As I said, as soon as you read the physics paper (not some popular-science covering of its contents) it's clear what's meant.

 

[Cthugha]

Sure, I see that and fully agree that it would be very unintuitive lingo in high-energy physics.

Taking the historical point of view, one can see quite easily where the language separation took place. The 1946 landmark paper by Purcell (well, actually it was rather a brief contribution to conference proceedings and it is reference 1 in the next link I will post) was the first of many papers on how to modify spontaneous emission rates by embedding atoms in a non-standard environment - an effect which has become known as the Purcell effect.

For a description of the modified spontaneous emission one does not have too many knobs to turn. In the simplest picture using Fermi's golden rule, we actually have the transition matrix elements and the density of final states. And it turns out that considering a local density of states of the unoccupied light field modes modified by the environment (cavity, crystal, whatever) works well.

This is obviously highly interesting for other emitters as well ( https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.58.2059 ). Enhanced spontaneous emission is useful for building emitters. Inhibited spontaneous emission is good for building optical memories or other applications that require long lifetimes. Accordingly, it has become quite standard to consider effects that rely on playing around with the local density of states of unoccupied light field modes as "something with vacuum".

 

[Prishon]

These seem to be quite strange papers. Can you give the reference? One has to now the context to judge whether the statements make some sense when translated to more careful formulations.

"Virtual particles" are not observable physical objects but abstractions in the perturbative evaluation of S-matrix elements for scattering processes. Depicted by Feynman diagrams they are symbolized by internal lines and stand for propagators of (free) fields. As such virtual particles don't have physical properties, because they are never observables.

They are not directly visible but their effects are. A freely propagating particle is not virtual.

 

[PeterDonis]

They are not directly visible but their effects are.

Please review PF's series of Insights articles on virtual particles, in particular this one:

https://www.physicsforums.com/insights/misconceptions-virtual-particles/