# How Real Are Virtual Particles?

## Main Question or Discussion Point

How "Real" Are Virtual Particles?

Are they a help to particle interaction or are they physically real? How come they are not observable to us? what i want to find out is if they are an artefact or not.I also know that there is alot of debate to whether virtual particles exist in reality or not.

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## Answers and Replies

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sas3
Gold Member

From what I have read about virtual particles, the Lamb shift is a direct measurement of the effect of virtual particles on the hydrogen atom. So yes they do exist.
But I am by no means an expert on this subject. There are other interactions that require virtual particles to exist such as the Coulomb Force, Weak and Strong Nuclear Forces,
and the Casimir effect to name a few. The key to understanding virtual particles is that there existence is limited by an extremely small time and space but they do leave evidence of their existence.
The fact that they leave evidence of their existence has always bothered me because of the law of conservation but I guess the way they get around this is that they leave a +1 effect and a -1 effect that eventually cancel each other out.

Virtual particles come into play in perturbation theory, with an emphasis on Feynman diagrams. In quantum field theory it's all about the correlation function which we want to determine:

$$\langle \phi_1(x_1)\cdots\phi_n(x_n)\rangle$$

Unfortunately, in practically all interacting theories we cannot come up with a decent way of solving these correlation functions. What we can do is come up with methods to approximate them. This is called perturbation theory. This is where virtual particles come into play.

The reason is that, in some sense, a correlation function is calculated by summing over all possible ways in which the transition from the initial to final state can occur (note that a correlation function is nothing but that: we start out with an initial state, then something happens in between, and we end up with a final state). This sum is quite difficult to manage, but we can keep track of all different contributions through use of Feynman diagrams. For instance, the emition of a photon by an electron is the sum of diagrams as drawn in this picture: http://www.strings.ph.qmul.ac.uk/%7Ebigdraw/feynman/lots_diagrams-main.jpg [Broken]. Keep in mind that when we completely specify the initial and final states, then each diagram is in fact nothing but a number! It is the contribution of that specific progress to the overall correlation function.

What we notice in this "sum of possible transitions" is the creation and annihilation of virtual particles. They are virtual, because they can carry any combination of energy and momentum (real particles satisfy: $$E^2 + p^2 = m^2$$).

In the end, virtual particles come into play because we use them to keep track of our approximation techniques. Hence, they are part of our calculational approach to approximate the physical quantities we would like to calculate (which are always written in terms of correlation functions).

If we somehow can come up with a way to perform these calculations of the correlation functions without the need to use approximation techniques then we wouldn't even have to mention the virtual particles. They are mathematical constructions, nothing more.

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stevebd1
Gold Member

In the case of the Casimir effect, virtual particles popping in and out of existance can push two plates together which suggests that work is being done. If the virtual particles obey the Heisenberg principle, $\Delta E \Delta t \ge \hbar/2$ (the virtual particles being less than $\hbar/2$) how is it they can exert force while complying with the first law of thermodynamics?

IMO, virtual particles are about on par with the concept of pilot waves, and while they serve some purpose in modeling, they are fairly nonsensical in practical terms of actually understanding particle interaction - ie, they are so non-intuitive that they tend to confuse discussion more than assist it.

i believe that virtual particles are an artifact of the endless attempt to understand QM via particle interaction, when in reality (again, IMO) we should be working toward understanding QM via field interaction.

malawi_glenn
Homework Helper

As stated in the many threads elsewhere in this topic, the virtual particle are just a "tool" that comes up when one does calculate observables using perturbation theory in quantum field theories. xepma's answer is good on this.

One can not say that since lamb shift comes from higher order correction terms in QED calculations the virtual particles are real, that is to say that integrals exists for "real" etc.

tiny-tim
Homework Helper
Welcome to PF!

Are they a help to particle interaction or are they physically real? How come they are not observable to us? what i want to find out is if they are an artefact or not.I also know that there is alot of debate to whether virtual particles exist in reality or not.
Hi Super_Jew! Welcome to PF!

As xepma says …
They are mathematical constructions, nothing more.
And, as Fredrik said in another thread , which I recommend you look at …
I consider virtual particles to be nothing more than a graphic representation of the individual terms in a series expansion of a mathematical expression for a probability amplitude.
Virtual particles are maths, not physics … representation, not reality

1. QM has serious problems with REALITY and REALISM. So I would be more careful saying this.
2. math, not physics? Very likely physics IS mathematics.

George Jones
Staff Emeritus
Gold Member

From a quantum field theory that I recently ordered and that seems quite nice:
The correspondence between the integrals that make up the Dyson series and Feynman diagrams is perfectly precise and well-defined. However, it is customary to to further and think of the Feynman diagrams as schematic pictures of physical processes, and here the interpretation acquires a more imaginative character. ... They are, in short, the infamous virtual particles that are so ubiquitous in physicists' discourse. In the final analysis, the only existence they possess for certain is as picturesque ways of thinking about the ingredients of the integrals in the Dyson series.

2. math, not physics? Very likely physics IS mathematics.
Interesting...........I would think Physics is the study of the nature of things, perhaps properties of matter and energy more specifically.

Math,(but not the integers), is an invention of man. It is used as the inexact language of the sciences and engineering. (Calculus gives you the answer to nothing, but it often gets you infinitely close to the answer).

You could argue the flight of the ball is parabolic in nature, but I dont think the ball or the universe knows the formula.

Interesting...........I would think Physics is the study of the nature of things, perhaps properties of matter and energy more specifically.

Math,(but not the integers), is an invention of man. It is used as the inexact language of the sciences and engineering. (Calculus gives you the answer to nothing, but it often gets you infinitely close to the answer).

You could argue the flight of the ball is parabolic in nature, but I dont think the ball or the universe knows the formula.
It is not my idea, but I really like it:
http://arxiv.org/abs/0704.0646

so they are just as real as unicorns? thanks everybody for not judging me for my name.

so they are just as real as unicorns? thanks everybody for not judging me for my name.
Yes, in principle (virtual) unicorns will also make a contribution to the amplitude for some state |A> to evolve to some other state |B>.

jtbell
Mentor

so they are just as real as unicorns?
What processes can you calculate to ten decimal places, in agreement with experiment, using unicorns? :uhh:

Are they a help to particle interaction or are they physically real? How come they are not observable to us? what i want to find out is if they are an artefact or not.I also know that there is alot of debate to whether virtual particles exist in reality or not.
This is an argument that I love, and since you clearly ask in the question whether they or real or not, I can give an opinion.

Math is a representation of what happens in the universe. Laws are governed by mathematical descriptions. You can not explain the universe without math. Similarly, you can not explain the math without the universe. Virtual particles have to be real in some sense, whether they are even particles or not is debatable. Without virtual photons, there would be no law of electromagnetism.

Perturbation theory describes them in mathematical expansions, and then the answer gives you what you want to hear. They are a way of saving QM from inaccuracies. And then, physicists contradict their own math by saying that they can't exist.

If QM is right, then they have to exist in some manner, shape or form. That's my opinion, anyway.

tiny-tim
Homework Helper
What processes can you calculate to ten decimal places, in agreement with experiment, using unicorns? :uhh:
well, d'uh obviously you also need to use bicorns, tricorns, and and so on
… Without virtual photons, there would be no law of electromagnetism.

Perturbation theory describes them in mathematical expansions, and then the answer gives you what you want to hear. They are a way of saving QM from inaccuracies. …
No, without virtual photons, there would be no perturbation theory of the law of electromagnetism.

The law of electromagnetism itself would still make perfect sense.

There exist nonvanishing effects that are zero to all orders in perturbation theory. So, they cannot be described in terms of virtual particles. But you can still compute these effects by resumming the divergent tail of the perturbative expansion. If g is the coupling, you can then find that the effect is something like
exp(-c/g).

well, d'uh obviously you also need to use bicorns, tricorns, and and so on

No, without virtual photons, there would be no perturbation theory of the law of electromagnetism.

The law of electromagnetism itself would still make perfect sense.
What I meant was more along the lines of that we wouldn't have an accurate explanation as to why charged particles attract / repel. Electromagnetic attraction could still be given by coulumb's law and so forth, but the underlying cause of this would be unknown.

What I meant was more along the lines of that we wouldn't have an accurate explanation as to why charged particles attract / repel. Electromagnetic attraction could still be given by coulumb's law and so forth, but the underlying cause of this would be unknown.
Don't know if you are right or not about virtual particles, but why the "underlying cause" would be known, with vir. par.? Only because we were grown up with the sensorial experience that two distant objects cannot interact if they don't come in direct contact. But with particles we wouldn't have solved anything, IMO; then we should answer the question: "and why do these particles behave that way? Because of the presence of some fields which..." and we would never finish the cycle.

Don't know if you are right or not about virtual particles, but why the "underlying cause" would be known, with vir. par.? Only because we were grown up with the sensorial experience that two distant objects cannot interact if they don't come in direct contact. But with particles we wouldn't have solved anything, IMO; then we should answer the question: "and why do these particles behave that way? Because of the presence of some fields which..." and we would never finish the cycle.
Perturbation theory completes electromagnetism with an explanation involving probability amplitudes and virtual particles and so forth. Virtual particles are not the only thing at hand. If it was just virtual particles acting, we would not have a full theory, because of the presence of fields. You are right. The underlying cause I am referring to is a deeper sense. Einstein showed how space-time is the cause of gravity. There was no need to believe that two distant objects could interact magically. Similarly, virtual particles, while they are not the full story, are a step to giving you a similar picture. Without them, we are one step closer to magic.

You can also ask the question if perhaps real particles also do not exist. Suppose that that quantum mechanics is not a fundamental theory. Suppose there exists some fundamental theory in which you can make probabilistic computations. It could be that you have to do some complicated counting of fundamental states which can be done approximately in a clever way by introducing a path integral formalism and a fictitious Lagrangian that takes the form of the Standard Model Largrangian. The fictitious terms being necessary to make sure all configurations are counted once.

Then the correct interpretation of quantum mechanics would be the Copenhagen interpretation. If you do an experiment in which you think you are manipulating/measuring the spin of an electron, what you are really doing is different. But this can then be described as if you are manipulating a fictitious particle which we call the electron.

What processes can you calculate to ten decimal places, in agreement with experiment, using unicorns? :uhh:
you can't,they are not real.I've read that a future theory will or may replace virtual particles? i also thought they exist for like example 1/10,000 of a second physically.

Virtual particles normally exist only if they do not violate the Heisenberg uncertainty principle (delta E delta t < Plancks constant). They are certainly necessary in strong central Coulomb fields (vacuum polarization - - Uehling integral). What would happen if a virtual particle pair were created near a black hole event horizon, and only one escaped?