What the term virtual particles referred to

In summary, "virtual particles" refer to internal lines in Feynman diagrams which do not have free ends and are a product of perturbation theory in quantum field theory. They are not observed and are a manifestation of the expansion, thus they are termed "virtual". They are considered to be messengers of interaction in static conservative force fields, and their existence allows for a violation of energy conservation before being re-established through mutual energy transfer. The term was first coined by Dirac and is a commonly-accepted term in quantum field theory.
  • #1
actionintegral
305
5
what the term "virtual particles" referred to

I wanted to know what the term "virtual particles" referred to. I found many "descriptions" of them on webpages. Fine.

Then I went to the index of many advanced QM books, and the term "virtual"
doesn't really show up as prominently as I thought it would.

Is virtual "slang"? If so, what is the proper term for a virtual object so that I may focus on those pages that describe them?
 
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  • #2
"Virtual particle" is a commonly-accepted term. However, it doesn't play a role in ordinary quantum mechanics (Schrödinger's equation, etc.). You don't really encounter virtual particles until you get to quantum field theory, e.g. quantum electrodynamics (QED).
 
  • #3
In QFT, when we do perturbative expansions, there are internal lines in the diagrams (Greens functions) which to not have free ends. At each vertex (where lines join), we integrate over momenta. Although in QFT, particles interact via exchange of other particles (gauge bosons), these particles are not observed and are a manifestation of the expansion. Thus they are termed virtual, as they don't 'exist' as such.
 
  • #4
Epicurus said:
In QFT, when we do perturbative expansions, there are internal lines in the diagrams (Greens functions) which to not have free ends. At each vertex (where lines join), we integrate over momenta. Although in QFT, particles interact via exchange of other particles (gauge bosons), these particles are not observed and are a manifestation of the expansion. Thus they are termed virtual, as they don't 'exist' as such.

Ok - that seems to fit what I have found by searching this forum. I was looking for a precise location where "virtual" was introduced. It sounds like they are "internal lines in the Feynman diagrams which do not have free ends."

I'll run with that!
 
  • #5
Now it is not to say that a virtual particle is a particle which does not exist. Now take for example QED. The particles are electrons, positrons and photons. Although all these particles are 'real' (we only infer existence), when say 2 electrons scatter via the exchange of a photon, this photon is virtual in that we do not observe it, but is a results of the perturbation expansion, and we integrate over all the possible value of momentum that the photon could have, i.e. -infinity to infinity.
 
  • #6
Any particle/line in a Feynman diagram which is not on its mass sheet is called "virtual particle".

Daniel.
 
  • #7
actionintegral said:
Ok - that seems to fit what I have found by searching this forum. I was looking for a precise location where "virtual" was introduced. It sounds like they are "internal lines in the Feynman diagrams which do not have free ends."

I'll run with that!
That is a good definition of the otherwise often abused and misunderstood notion of a "virtual particle".
 
  • #8
As I have read many times the virtual particle concept is just a product of the perturbation theory in quantum field theory. It’s another word for internal line or propagator, and so its ‘ontological status’ is questionable, that means without perturbation theory there were no virtual particles.

Also, it is always point out that real photons are internal lines in are larger system, too, so that the distinction between is not so clear.

OK. But isn’t there a need for them even without/ before QFT?

Here is my long-winded, probably nonsense idea of virtual particles:

Take a proton. The proton has a conservative force field, which means that it conserves the ability to do work. In order to do work, it does not need fuel like an engine, it does not have to decay like an atom and it does not have to be accelerated. In order to do work and to accelerate electrons again and again, it simply is a proton with its conservative force field.

When an atom decays a real photon is emitted and energy is transferred via a wave motion through a field. But this very field, when no waves are sent through it and it is static, does also transmit interaction, as above described. Now, the interaction of a static conservative field is mutual (electron and proton do work on each other), whereas the wave through the field sent off by the decaying atom has a direction.

But the mutuality of the conservative force is not immediate, no forces at distance at work.
Instead, static conservative force fields also send something that transports energy (which of course needs time to arrive, although being very fast since sent by light velocity). While being on its way, this something violates energy conservation, since the sender is now not in a lower energy state after sending away energy. Only after the mutual energy transfer has been completed and both senders have received their messages from one another energy conservation is re-established.

So that’s the need for virtual particles, for virtual energy messengers. By giving up forces at distance, and introducing messengers, we are confronted with a (momentary, also sometimes very long) violation of energy conservation. These messengers do not obey energy conservation, but fortunately energy-time uncertainty explains this violations and allows a non forces-at-distance description.
 
  • #9
actionintegral said:
Ok - that seems to fit what I have found by searching this forum. I was looking for a precise location where "virtual" was introduced. It sounds like they are "internal lines in the Feynman diagrams which do not have free ends."

I'll run with that!
But before Feynman it was Dirac who first coined the term.
No?
I think I heard he used to described them to a layman as those particles lurking from a big bad dark:smile:vacuum to pop up in the reality due to the nonzero vacuum "zero point" energy.
Something like that :rolleyes:
 
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  • #10
There is this view of QFD that it's a theory of Feynman diagrams. People have picked up and run with this and produced theories based on this satz. Basically they are combinatorial, looking at permutations and combinations of Feynman diagrams as the elements of reality (!), and of course the category people come and redefine all that and it's Katie bar the Door. Arxiv papers available on request, or people who are into this can respond with whatever they think is apposite.
 
  • #11
Ratzinger said:
Instead, static conservative force fields also send something that transports energy (which of course needs time to arrive, although being very fast since sent by light velocity). While being on its way, this something violates energy conservation, since the sender is now not in a lower energy state after sending away energy. Only after the mutual energy transfer has been completed and both senders have received their messages from one another energy conservation is re-established.

I know nothing of virtual particles and less about QED, but doesn't this you said makes you think a bit more on the fact that it's the interaction between charged particles, or between EM waves and charges, that seems "more real" than photons meant in the sense of traveling particles?
 
  • #12
lightarrow said:
I know nothing of virtual particles and less about QED, but doesn't this you said makes you think a bit more on the fact that it's the interaction between charged particles, or between EM waves and charges, that seems "more real" than photons meant in the sense of traveling particles?

Well, photons are certainly no traveling particles. They are quantized energy and momentum of the EM field. Photons are unlocalized/ "smeared out" due to quantum uncertainty, they have no rest frame, and also they are bosonic, i.e. they are all identical, for short they are very strange for us to picture, but they are real inhabitants of our physical universe.
 
  • #13
Ratzinger said:
Well, photons are certainly no traveling particles. They are quantized energy and momentum of the EM field. Photons are unlocalized/ "smeared out" due to quantum uncertainty, they have no rest frame, and also they are bosonic, i.e. they are all identical, for short they are very strange for us to picture, but they are real inhabitants of our physical universe.


And what is it that photon counters count?

Does anybody feel that this is just another of those semantic arguments that goes round and round, but can never ever reach a conclusion?
 
  • #14
selfAdjoint said:
Does anybody feel that this is just another of those semantic arguments that goes round and round, but can never ever reach a conclusion?
You mean something like a "circular argument"?
 
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  • #15
tehno said:
You mean something like a "circular argument"?

More like the old joke about the two guys arguing over the back fence between their properties: "They can never agree because they are arguing from different premises."
 
  • #16
The idea of virtual particles or states, transcends perturbation theory. And, the notion of virtual particles is best thought of as a metaphor, a very useful one indeed.

Look at a deuteron, a bound state of a proton and a neutron. Both nucleons can be called virtual. Why? Because they do not have all the properties of free nucleons, that is E*E -M*M -P*P=0 does not hold. Clearly to see either or both nucleons, one must probe with enough energy to dissociate the deuteron. We know the nucleons are there because when we bash the deuteron, we get a neutron and a proton. But, unless we bash we don't know the composition of the deuteron. Over time it's grown to be convenient to call bound particles, virtual particles.

With an important caveat, that convenience is most useful for resonances in particle physics. For example, there's the famous 3-3 resonance in pion-neucleon scattering, with both spin and isospin of the system equal to 3/2.. The resonance is an "almost bound state", unstable because energetics favor resonance decay -- the energy of the resonant state is some what higher than just a non-interacting state. One can say this resonance is composed of a virtual nucleon and a virtual pion, because the resonance decays into a nucleon-pion state. Whether one does depends on the problem at hand.

In ordinary QM, the tunneling particles are virtual when inside a high enough potential barrier -- they have the wrong energy-momentum relationship to be a free particle.

But, of most importance, the idea of a virtual particle is a simple linguistic convenience, a metaphor helpful, for some at least, in working with QM and QFT. The virtual particle notion is not meant to be precise nor rigorous, but merely helpful.

Regards,
Reilly Atkinson

Regards,
Reilly Atkinson
 
  • #17
The idea of virtual particles or states, transcends perturbation theory.

Right.

I'm just saying this for the last time, especially for those who compute one Feynman diagramm to many. There is something called conservative force field. So forget your Peskin and Schroeder, we talking first year physics course.

How do two unaccelerated charges (a concervative force field that is, no external forces acting on these charges) communicate? They can't produce photons, pions, whatever constantly out of nothing. There has to be messengers that violate conservation of energy but obey time-energy uncertainty. I call them virtual particles. Period.
 
  • #18
How do two unaccelerated charges (a concervative force field that is, no external forces acting on these charges) communicate? They can't produce photons, pions, whatever constantly out of nothing. There has to be messengers that violate conservation of energy but obey time-energy uncertainty. I call them virtual particles. Period.

I especially refer to the 'There has to be messengers'. In what sense does they have to exist. They are certainly never detected. Classical physics does not require quantised fields for charges to interact via a finite propagation speed of the interaction. Look at GR.
 
  • #19
Ratzinger said:
Right.

I'm just saying this for the last time, especially for those who compute one Feynman diagramm to many. There is something called conservative force field. So forget your Peskin and Schroeder, we talking first year physics course.

The holy church might want to learn that Maxwell's theory interacting with currents goes beyond conservative forces (see the Lorentz Dirac equation).

Ratzinger said:
How do two unaccelerated charges (a concervative force field that is, no external forces acting on these charges) communicate? They can't produce photons, pions, whatever constantly out of nothing.

Perhaps you have ever thought that these radiative losses are very very small, the coupling constant in front of the dissipation term in the lorentz Dirac equation equals (2 \mu q^2 )/(3 c^3) which is roughly around
10^{-54} in SI units. Moreover, the Lorentz Dirac equation comes from conservation of total energy momentum (and angular momentum if one includes magnetic dipole degrees of freedom). In the classical theory, there are no vacuum fluctuations, so that invalidates your argument below

Ratzinger said:
There has to be messengers that violate conservation of energy but obey time-energy uncertainty. I call them virtual particles. Period.

Now, about the word virtual as Reilly explains it. Come on guys, do we call planets in the solar system virtual ? There is no difference between those planets (regarding this aspect) and the nucleons building the deuteron, Einstein's theory is even predicting a dynamical mass for our Earth coming from the gravitational field of the sun (in the same way as the nucleons are dressed by gluons I guess).
We call them virtual because they are useless to include in the state description for the scattering matrix where we ideally measure and prepare free particles.

Careful
 
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  • #20
selfAdjoint said:
And what is it that photon counters count?

Just because something is non-localised and probabilistic doesn't mean it can't interact with something else. A photon has a probability of being detected by a detector... that doesn't make it anyless real imo. It certainly doesn't introduce a circular argument, it just makes it more difficult to model the photon's interactions.
 
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  • #21
reilly said:
In ordinary QM, the tunneling particles are virtual when inside a high enough potential barrier -- they have the wrong energy-momentum relationship to be a free particle.
Regards,
Reilly Atkinson

This "feels" right to my tiny brain. I would like to learn how to prove this. Does it follow from the schroedinger equation?
 
  • #22
Jheriko said:
Just because something is non-localised and probabilistic doesn't mean it can't interact with something else. A photon has a probability of being detected by a detector... that doesn't make it anyless real imo. It certainly doesn't introduce a circular argument, it just makes it more difficult to model the photon's interactions.

Or the photon just is something different than most of us imagine it to be. :smile:
 
  • #23
Careful said:
Or the photon just is something different than most of us imagine it to be. :smile:


Yeah; back when I was even more ignorant than I am now I used to say that the photon is like a beautiful woman of a certain age. We don't know, and it's not important, what she looks like at home, it only matters what she looks like when she comes to visit us.:cool:
 
  • #24
selfAdjoint said:
Yeah; back when I was even more ignorant than I am now I used to say that the photon is like a beautiful woman of a certain age. We don't know, and it's not important, what she looks like at home, it only matters what she looks like when she comes to visit us.:cool:
Haha, the thing is that we do not wish to remain that ignorant (at least not when you are a healthy man with some potential), we want to keep her during the night and inevitably will see her in the morning. :smile: :smile: The thing is : if she was good during her first night, we MUST learn to appreciate her at breakfast. :biggrin:
 
  • #25
Careful said:
Haha, the thing is that we do not wish to remain that ignorant (at least not when you are a healthy man with some potential), we want to keep her during the night and inevitably will see her in the morning. :smile: :smile: The thing is : if she was good during her first night, we MUST learn to appreciate her at breakfast. :biggrin:


The experienced and loving heart finds no problem with this. The morning can and should be a time of affection, not clinical inspecdtion!:approve:
 
  • #26
selfAdjoint said:
The experienced and loving heart finds no problem with this. The morning can and should be a time of affection, not clinical inspecdtion!:approve:
Exactly, and that is why you will appreciate the photon more when you find a natural, local (in the sense that there is no action at a distance) model which explains its appearances to macroscopic apparati. In this regard, the ideas spelled out by the group of people Zbyszek belongs to (with amongst others Bill Unruh), are very interesting.

Careful
 
  • #27
Careful said:
Exactly, and that is why you will appreciate the photon more when you find a natural, local (in the sense that there is no action at a distance) model which explains its appearances to macroscopic apparati. In this regard, the ideas spelled out by the group of people Zbyszek belongs to (with amongst others Bill Unruh), are very interesting.

Careful

I have no particular animus against action at a distance, recalling that Newton was right and Descartes was wrong in their day, and we are all, after all, in "a day", and not at the asymptotic end of time.

But if I was going out on a risky limb for the sake of local action, I'd go by way of Einstein's and Schroedinger's unsymmetrical theory: http://www.artsci.wustl.edu/~jashiffl/einstein-schrodinger.html , which is better than the rap the quantum consensus gives it. Especially look for Hlavaty's book Geometry of Einstein's Unified FIeld Theory. He gives a derivation of a spinor bundle over spacetime arising from the geometry. Reading this as a young man I was somewhat inoculated against quantum mysticism and its syntactic inverse, classical reductionism. A plague on both their houses.
 
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  • #28
selfAdjoint said:
I have no particular animus against action at a distance, recalling that Newton was right and Descartes was wrong in their day, and we are all, after all, in "a day", and not at the asymptotic end of time.

But if I was going out on a risky limb for the sake of local action, I'd go by way of Einstein's and Schroedinger's unsymmetrical theory: http://www.artsci.wustl.edu/~jashiffl/einstein-schrodinger.html , which is better than the rap the quantum consensus gives it.

You mean gravitation with an antisymmetric ``metric'' field, I guess. Might be interesting, but it is for sure better to start with understanding Maxwell theory properly (which is much ``easier'' to start with but still more than difficult enough).

Anyway, reading over this thread again, I noticed I should have said something about the Coulomb force. There is nothing which suggests that the latter needs to be of ``quantum mechanical origin'' and (therefore) neither needs to be associated to some time-energy uncertainty. Actually, the total ``mass'' of the (infinite) Coulomb field equals the rest mass of the particle assuming it has the classical radius R (at least when the particle is not accelerating and energy is measured in the local Lorentz frame associated to the particle); it seems clear to me that there is no such thing needed as constant particle production out of nothing. But it is certainly interesting to contemplate the inner workings of the Coulomb force.

Careful
 
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  • #29
I just noticed that I wrote in my previous 'Feynmann diagramm' which gives me five points on the crackpot list, I believe.

I have further realized that taking virtual particles too literal is also seen as crackpotterish by knowledgeable physicsts. But I like to point out that I was referring to conservative force fields and not those virtual particles coming from scattering. I admit that my billard ball thinking is pretty crappy, but still I like to know more about conservative force and its quantum treatment.


Anyway, reading over this thread again, I noticed I should have said something about the Coulomb force. There is nothing which suggests that the latter needs to be of ``quantum mechanical origin'' and (therefore) neither needs to be associated to some time-energy uncertainty. Actually, the total ``mass'' of the (infinite) Coulomb field equals the rest mass of the particle assuming it has the classical radius R (at least when the particle is not accelerating and energy is measured in the local Lorentz frame associated to the particle); it seems clear to me that there is no such thing needed as constant particle production out of nothing. But it is certainly interesting to contemplate the inner workings of the Coulomb force.


So there is none. That's intersting. Could you please expand on it?

By the way a great http://arnold-neumaier.at/physics-faq.txt" by a guy called Arnold Neumaier. Check S3, especially S3e in it.
 
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  • #30
Ratzinger said:
I just noticed that I wrote in my previous 'Feynmann diagramm' which gives me five points on the crackpot list, I believe.

Don't bother ...

Ratzinger said:
I have further realized that taking virtual particles too literal is also seen as crackpotterish by knowledgeable physicsts. But I like to point out that I was referring to conservative force fields and not those virtual particles coming from scattering. I admit that my billard ball thinking is pretty crappy, but still I like to know more about conservative force and its quantum treatment.

Well, I for sure believe off shell particles should be taken seriously, and finding the carriers of the Coulomb field is definately a deep problem which is IMO not properly solved at all in QED. What I do consider as a problematic aspect is that people believe this problem has a priori something to do with the second quantized theory as it stands now.

Ratzinger said:
So there is none. That's intersting. Could you please expand on it?

I assume you want to know more about my motivations for banning vacuum fluctuations or about my comments on time/energy uncertainty ? For the latter, I have given my opinion in a thread on the ``beyond the standard model'' forum (basically there is no time operator). Concerning the vacuum fluctuations, let me say that a deeper study of the classical theory can bring new insights. A very useful paper is ``Classical electrodynamics of retarded fields and point particles'' by Teitelboim, Villarroel and Van Weert : Rivista del nuovo cimento vol 3, n° 9, 1980. Their aim was to write a review paper containing contemporary insights, hoping that it might serve people who try to solve this problem.

I quickly read a part of Arnold Neumaier : pretty much the standard story, if he would apply the scattering matrix approach to the universe, he would speak of virtual planets and by no means would he be able to say whether the universe contains black holes or not (pretty much the Hawking story), the only information he considers real are (special relativistic) free field states (so he works with states on some conformal past and future boundary). Of course this is as unphysical as one can only get, you can expect this kind of nonsense when people forget that a very useful approximation (in the laboratory) is not to be taken as a fundamental axiom. He makes a valid point though when he says that the virtual particle content depends upon the quantization method (which is a pretty obvious thing), but I disagree that it would lead to a strange picture of reality. It merely tells me that the quantization method cannot give a complete description of reality as it stands. But yeah, if you take the latter as ``forbidden territory'' then you are pretty much forced to state what this (clever) man says.

Careful
 
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  • #31
actionintegral said:
This "feels" right to my tiny brain. I would like to learn how to prove this. Does it follow from the schroedinger equation?


Yes indeed. Follows from the standard solution.
Regards,
Reilly Atkinson
 

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