Are virtual particles real or just math filler

[A. Neumaier]

What would those properties be? Do the virtual particles have all the properties of a real particle

They have precisely the properties of the Feynman integrals they represent; thus they have mass and spin. But no states; in particular no spin up/down, no polarization, no position; they lack all properties that would make contact with the real world. They are just a figure of speech; using them correctly means using the formal perturbation formalism correcly, for which they are an abbreviation.

Everything about them is virtual - unreal. They live in a different world from the world of real particles, namely in the platonic world of formulas. There they stitch together symbolic calculations that can be barely expressed in words, except by making gross simplifications that convey more magic than reality. But they sell well to the general public!

 

[vanhees71]

I stand corrected. I was thinking of another video where Susskind talked about spreading waves of entanglement with more and more particles, as an alternative to wave function collapse.

Thank you for linking that video. In the video, he does indeed seem to say what you said. Here's my transcript of what he said in that clip.
Someone else interjects. Susskind replies,

But in the strictest sense, he did not say the virtual particles are entangled, he said that the vacuum is entangled because of those virtual particles. Does that distinction have meaning? I can't say.

Hm, does he say what he really means? I mean, without a minimum of math and a clear definition of what is entangled these are just empty words.

 

[vanhees71]

Well, "virtual particles" are what's represented by internal lines of Feynman diagrams, and these stand for free propagators (in the most simple sort of Feynman diagrams used in calculations order by order in perturbation theory, and I'd keep the discussion to these most basic application). Each free propagator reflects the mathematical properties of the quantum field the particle describe. There is no other meaning to them than that. Feynman diagrams are very suggestive in making pictures on "what's going on in a collision", but that's misleading. The observables refer to counting "real particles", i.e., something that hits a detector that can write information to a storage device, and this information refers to observable "real particles", represented by the external legs of Feynman diagrams. These are defined in terms of free-particle states, and even this is problematic for electrically charged particles, because the usually used naive free-particle states are not the correct asymptotic free states due to the long-ranged nature of the electromagnetic interaction. The true asymptotic states in this case are characterized by a coherent state, which usually is taken into account by appropriate soft-photon resummation techniques as explained, e.g., in Weinberg, Quantum Theory of Fields, vol. 1, to cure the associated IR and collinear divergences.

The most simple example is tree-level bremsstrahlung in the scattering of a charged particle on a classical Coulomb field representing a very heavy charged particle (e.g., electron scattering on a heavy nucleus). There you need to take into account at least also the elastic scattering + the one-loop radiative correction a la Bloch and Nordsieck.

To make a long story short: Feynman diagrams only look simple and intuitive. In fact they are highly efficient symbols to express complicated mathematical manipulations of the Feynman diagrams occurring in perturbative evaluations of S-matrix elements in QFT (including the organization of renormalization of UV divergences and resummations to cure IR and collinear divergences).

 

[friend]

Can we say that virtual particles are only mathematical entities that have no reality? These "mathematical" artifacts (Feynman diagrams = virtual particles) are necessary in the calculation of physical events. They are just as "real" as the wave function itself. Consider an electron propagating through space. There are virtual particles (a.k.a. mathematical entities, Feynman diagrams) at various places around the bare particle that contribute to its overall properties. Now if another electron comes close to the first, then to which of the electrons does a virtual particle (Feynman diagram) at a particular point belong? Can you have two different virtual particles (Feynman diagrams), one for each real electron, at the exact same location at the exact same time? If not, then to which electron does the virtual particle belong? Is there some sense in which the virtual particle at a point belongs to both electrons? And if it contributes positively to the calculations of the properties of both electrons, is there an attractive force?

 

[jtbell]

These "mathematical" artifacts (Feynman diagrams = virtual particles) are necessary in the calculation of physical events.

No, they are not necessary. See the discussion of lattice field theory in this thread which has been going on parallel to this one. (in particular, post #6 onwards)

 

[vanhees71]

Well, you don't need to use Feynman diagrams but just the mathematical formalism. Famously Schwinger apparently never used Feynman diagrams but got the same results as Feynman. With Feynman diagrams it's of course tremendously more easy to get the calculations. I guess that the full understanding of perturbative renormalization theory (BPHZ) would have been also very much more complicated without the use of Feynman diagrams. Zimmermann's forest formula is even formulated in terms of Feynman diagrams. Ironically, the corresponding paper, where it's proven doesn't draw a single Feynman diagram ;-)).

 

[friend]

No, they are not necessary. See the discussion of lattice field theory in this thread which has been going on parallel to this one. (in particular, post #6 onwards)

Which particles are never used in a Feynman diagram? If they can possibly be used in virtual processes, then why should it be wrong to develop a theory using them?

 

[BertMorrien]

The name "virtual particle" suggests that there is something like "real particle", but we know that the name "particle" in quantum physics means something else than a classical particle. In this sense a virtual particle is as real as a 'real' particle, but it cannot be observed directly. However, their effect can be measured and this must be taken into account in theoretical models.
See also http://www.scientificamerican.com/article/are-virtual-particles-rea/

 

[friend]

See also http://www.scientificamerican.com/article/are-virtual-particles-rea/

There's something wrong with your link. The text only appears at the bottom and keep moving around.

 

[fresh_42]

There's something wrong with your link. The text only appears at the bottom and keep moving around.

Here it works fine. Plus I'm looking forward for another explanation of the Lamb effect.

 

[BertMorrien]

There's something wrong with your link. The text only appears at the bottom and keep moving around.

I don't have problems with the link, but the content may depend on your particular browser.
SA's way of advertising is a bit anoying. Try to get rid of ads by clicking on the X.

 

[vanhees71]

Here it works fine. Plus I'm looking forward for another explanation of the Lamb effect.

Well, I could read it too, and I'm shocked that someone like Kane could write it, who wrote a good textbook on introductory particle physics, including QFT.

 

[bob012345]

Welcome to PhysicsForums, J-eastwood!

The generally accepted answer is: Virtual particles are artifacts of the math of Quantum Field Theory. Many find them convenient for discussion purposes. Whether they are "real" or not is something of a matter of philosophy. There is no known physical test that would further answer this question.

I would only add that the entire QFT approach is unphysical or unreal if you like.

 

[Orodruin]

I would only add that the entire QFT approach is unphysical or unreal if you like.

Unphysical and unreal are not the same thing. QFTs are definitely physical theories. They have made several astonishing predictions which have later been verified, which is what a physical theory is all about. Something being "real" or not is more of a philosophy issue than a science one.

 

[bhobba]

Something being "real" or not is more of a philosophy issue than a science one.

Very true.

In this sense a virtual particle is as real as a 'real' particle, but it cannot be observed directly. However, their effect can be measured and this must be taken into account in theoretical models.

They do not appear in Lattice theory so obviously do not have to be taken into account.

They are simply the pictorial representation of terms that appear in a Dysen series, which is what a Feynman diagram is.

Real particles are responsible for things like clicks in a particle detector - virtual particles are not. That's pretty common-sense, but as Orodruin says its a philosophical minefield. Scientists generally don't worry about such things, the consequences of which can be seen by the progress each field has made.

There is thread after thread about this issue on this forum, and its all exactly the same - they get no-where because some simply do not want to accept the obvious. Anything said outside an actual QFT textbook is very suspect and must be taken with a grain of salt. Study the real deal and this semantic quibbling never comes up. Its a much better use of an enquiring minds time. Recently some very good books have started to appear that can, with effort, be studied after a first course, or the study of a good text, in QM:
https://www.amazon.com/dp/019969933X/?tag=pfamazon01-20

Having got that book and studied it myself I think, again with effort, it can be studied after reading Susskinds text:
https://www.amazon.com/dp/0465062903/?tag=pfamazon01-20

Thanks
Bill

 

[sheaf]

I've always taken the view:
In-State = physics
Stuff in between = mathematics to get the right transition amplitudes in<->out
Out-State = physics

Statements like
"Quantum theory predicts that every particle spends some time as a combination of other particles in all possible ways. These predictions are very well understood and tested"
strike me as misleading. If you've done a QFT course you know what he's alluding to, but to phrase it like this is a bit sloppy. But then he's a professional physicist and I'm just a guy on the internet!

 

[vanhees71]

I've always taken the view:
In-State = physics
Stuff in between = mathematics to get the right transition amplitudes in<->out
Out-State = physics

Yes! An the correct adiabatic switching a la Gell-Mann and Low is crucial. See

F. Michler, H. van Hees, D. D. Dietrich, S. Leupold, C. Greiner, Non-equilibrium photon production arising from the chiral mass shift
Ann. Phys. 336, 331 (2013)
http://dx.doi.org/10.1016/j.aop.2013.05.021
http://arxiv.org/abs/1208.6565

for an example.

 

[friend]

I'm a little surprised that I've not seen any math equations in this thread showing where virtual particles appear in the equations. Are virtual particles a part of QFT? Or do they exist in QM as well?

 

[A. Neumaier]

Are virtual particles a part of QFT? Or do they exist in QM as well?

One can find them in both, and even in classical field theory (as explained in the link given)!
But the corresponding Feynman diagrams are heavily used primarily in QFT.

Giving formulas is not really useful since their whole purpose is to substitute imagery for formulas. You can read the Feynman rules relating the diagrams to integrals in any QFT book.

 

[vanhees71]

I'm a little surprised that I've not seen any math equations in this thread showing where virtual particles appear in the equations. Are virtual particles a part of QFT? Or do they exist in QM as well?

Well, that's easy. What's called "virtual particle" in popular science books is symbolized by internal lines of Feynman diagrams, and they stand for free-particle Green's functions (in usual perturbation theory; sometimes they can have a different meaning, e.g., in the context of resummation schemes like the $\Phi$-derivable approximation or the functional RG methods), but that doesn't matter too much on the level of this discussion.

In the Standard Model you have only scalars, (Dirac-)spinors, and vectors (gauge fields). Thus the "virtual particles" stand for the corresponding propagators
$$\Delta(k)=\frac{1}{k^2-m^2+\mathrm{i} 0^+},$$
$$G(p)=\frac{p_{\mu} \gamma^{\mu}+m}{p^2-m^2+\mathrm{i} 0^+},$$
$$D_{\mu \nu}(k)=-\frac{g_{\mu \nu}}{k^2+\mathrm{i} 0^+},$$
the latter in the Feynman gauge.

 

[friend]

Well, that's easy. What's called "virtual particle" in popular science books is symbolized by internal lines of Feynman diagrams, and they stand for free-particle Green's functions (in usual perturbation theory; sometimes they can have a different meaning, e.g., in the context of resummation schemes like the $\Phi$-derivable approximation or the functional RG methods), but that doesn't matter too much on the level of this discussion.

What you've shown here is for perturbation theory. What about the virtual particles that are supposed to be everywhere, the ones that get ripped apart by black hole horizons or Unruh acceleration, or that are supposed to be involved in the Casimir effect, etc? What is the math for these?

 

[bhobba]

What about the virtual particles that are supposed to be everywhere, the ones that get ripped apart by black hole horizons or Unruh acceleration, or that are supposed to be involved in the Casimir effect, etc? What is the math for these?

As has been explained in many many threads they don't exist. They are simply representations of integrals. Everything you cite above can be explained without them.

This has been explained to you many times - the following simply being the latest:
https://www.physicsforums.com/threads/can-particles-be-absorbed-into-a-field.855789/#post-5368838

I gave you a link to John Baez's paper before:
https://www.physicsforums.com/insights/struggles-continuum-part-5/
'Each of these diagrams is actually a notation for an integral! There are systematic rules for writing down the integral starting from the Feynman diagram.'

Please please read it and post with any queries you have so it can be put to rest once and for all.

Thanks
Bill

 

[friend]

I'm sorry, but I hear every Professor that gives a lecture invoking them to explain things. Perhaps that is just a tool, but I have seen them show equations where they sum up all the zero point frequency modes and give this as the reason that the calculated vacuum energy is so many orders of magnitude greater than what is measured. I've heard professionals teach about virtual particles, virtual paths, and even virtual geometries. From what I can gather, virtual objects are the differential parts of the path integral that are being summed up in superposition. They aren't observable in and of themselves, but they are the basis of the integral. We sum up differential parts in other integrals of physics, and those differential parts can't be observed either.

What I think is going on is that this aversion to virtual particles is being fueled by the faith that quantum mechanics cannot be explained. And any attempt to do so is misguided. For if there were to be any explanation of QM, it would have to necessarily involve virtual particles just as classical physics is explained in terms of differential parts. So I think that your attempt to definitively rule out virtual particles is misguided.

 

[Orodruin]

They aren't observable in and of themselves, but they are the basis of the integral. We sum up differential parts in other integrals of physics, and those differential parts can't be observed either.

Already the "are not observable" should ring the warning bells. They are also not the basis of the path integral, but only a very convenient way of computing it (approximately). Much like you might use partial integration or the Leibniz rule to perform computations in calculus.

That being said, they are a very useful tool and I know many people like to think in terms of them.

 

[bhobba]

That being said, they are a very useful tool and I know many people like to think in terms of them.

Exactly.

To Friend - did you read what John Baez wrote? He stated it clearly - they are representations of integrals. Why exactly won't you accept it? Why do you chose instead to worry about what others say? Here you get the real deal - but for it to be of value you must take it on board. You will get nowhere constantly saying others say different. They are not being careful. We are. It's that simple.

For if there were to be any explanation of QM, it would have to necessarily involve virtual particles just as classical physics is explained in terms of differential parts. So I think that your attempt to definitively rule out virtual particles is misguided.

You pretty well admit you haven't studied an actual QFT textbook. Until you do you will not have the background necessary to reach conclusions like the above. It's wrong - but since you seem to doubt what we say here I don't know what to say. I tell you its wrong - but because you won't accept it it won't make any difference. You come here seeking to learn - but won't accept what those you have chosen to learn from say. In science you have two choices. Either you believe what those that have studied it tell you or you read the textbooks yourself. There is no middle path of reading what others say then using that as ammunition to challenge those that tell you different. That leads nowhere.

The challenge I have for you is to study an actual text:
https://www.amazon.com/dp/019969933X/?tag=pfamazon01-20

When you have done that then we can discuss if you still think they are real.

Thanks
Bill

 

[A. Neumaier]

I hear every Professor that gives a lecture invoking them to explain things.

This is because in explaining things to people without a thorough grounding in math you cannot explain much without using gross simplifications and imagery in place of the real thing. But if you come to this forum to learn you are expected to realize that the views created for the general public are different from the views physicists have when doing real work.

One talks informally as if virtual particles were real since it is a quick way of conveying superficial information. But the word ''virtual'' (which is opposite to ''real'') is added everywhere to signal that this is only a figure of speech. Once one tries to substantiate in which way the virtual particles could be thought of as real, the whole concepts dissolves into nothing but a metaphor for multivariate integrals. Please read https://www.physicsforums.com/posts/5334791/bookmark and the link posted there, where this is carefully explained.

 

[cremor]

Any commentary on direct sampling of electric-field vacuum fluctuations?

http://science.sciencemag.org/content/350/6259/420

 

[vanhees71]

Without having read the paper, I can only say that for sure they don't measure the vacuum. The very fact that they measure something means that there is a measurement apparatus present, and that's not vacuum. There are quantum fluctuations of the electromagnetic field, but they manifest themselves always only at the presence of charges, because we cannot detect anything without having the interaction of the electromagnetic field with matter consituting a measurement device, and this matter consists of electrically charged particles.

 

[friend]

I feel like I'm arguing the existence of God, something that is the cause of everything else but not in and of itself observable. What is it, then, that we are integrating in the path integral? The integrand in the path integral is the exponential of the complex Action integral. And this exponential of the Action can be broken up into exponentials of differential Actions. What do these exponentials of differential Actions mean if not virtual particles?

I think the problem comes in because the path integral involves an infinite number of integrations. In the development of classical physics the integrals are along a path or throughout a space which seem more intuitive. So we don't question what the integrand means in those classical integrals, and we feel that the integrand in those classical integrals do have intuitive physical meaning. They are differential objects described in terms of force, velocity, and acceleration on infinitesimal bits of matter and charge that we then have no trouble integrating to get overall energies and distances, etc. But these differential bits are not any more observable than anything in the path integral. Nobody observes these bit of mass or charge or these differential displacements. But nobody argues that they are not real because it seems more intuitive to integrate them to get observables.

We still have differential bits in the path integral; these are called virtual to stress that they are not observable, but that's not something new. The difference here is that we are using an infinite number of integrations to take into account every possible combination of the differential, virtual effects from one point to another in a continuum. That together with the fact that we are summing up complex numbers to get a superposition of all these virtual effects makes the path integral less intuitive. But if we are going to understand what's going on in the math, we're going to have to get a better idea of what these differential, virtual processes are just as we do in the classical picture. Then we can take every combination of them in superposition to get the observables that we can measure. It seems we are doing some of that when we describe the Lagrangian in terms of interacting terms of quantum fields and coupling constants and the like, that exist in the Action integral inside the path integral. And then these quantum fields are described by particle number at each point. Some of these particles are real and others cancel out in superposition and are described as virtual.

 

[Orodruin]

I feel like I'm arguing the existence of God, something that is the cause of everything else but not in and of itself observable. What is it, then, that we are integrating in the path integral? The integrand in the path integral is the exponential of the complex Action integral. And this exponential of the Action can be broken up into exponentials of differential Actions. What do these exponentials of differential Actions mean if not virtual particles?

I do not understand your desire to interpret more into the virtual particles than there is to it. The path integral is perfectly well defined without the introduction of virtual particles as an integral over all possible field configurations (with a given appropriate measure). Expanding the exponential in an asymptotic series is essentially only a trick we use to compute this integral because it is generally very difficult to compute it analytically in other ways. Once you have made the asymptotic expansion, the Feynman rules, including the virtual particles, are only a means of keeping track of the terms in this series. This does not change the fact that the path integral itself is well defined without virtual particles.

The path integral itself is the same type of integral which appears in normal quantum mechanics (where you integrate over actual paths and not field configurations). The situation is similar for ghost fields which do appear in Feynman diagrams due to what is essentially a mathematical trick for rewriting the path integral in a way which handles gauge invariance in a pleasant manner.