Are virtual particles created spontaneously out of the vacuum?

[marlon]

Not if you accept the Higgs mechanism. Then a naked particle has NO MASS until it is "dressed" around a cloud of higgs bosons. In a condensed matter system, you NEVER measure the "naked" particles. All that you measure are the dressed particles. Based on this, an undressed particle is NOT real, since you never measure them, at least not directly.

Zz.

Are you implying here that you can measure a naked particle indirectly ?

Following QFT you will always have to incorporate the evolution of the self-energy due to the surrounding cloud of virtual particles...Can you tell me how you measure an undressed particle indirectly?

marlon

 

[ZapperZ]

Are you implying here that you can measure a naked particle indirectly ?

Following QFT you will always have to incorporate the evolution of the self-energy due to the surrounding cloud of virtual particles...Can you tell me how you measure an undressed particle indirectly?

marlon

I'm getting a bit annoyed here because you appear to double back and decide to address an old posting which you have already responded to, while still wanting to continue from the end of the thread. You have done this more than once here.

Note that NOWHERE did I ever said you CAN measure this so-called "naked" particles. So the whole question is moot.

Zz.

 

[Haelfix]

QFT is the subject in my mind that makes the least amount of effort to distinguish between something that is 'real' in the sense of say what Einstein wanted, and what is just useful to predict what an experiment's instrumentation will eventually read out.

For the life of me, I still have no idea what a 'particle' is.. In words or in thought.

And I am pretty sure no one knows either.

I do however know a bunch of equations that yield sensible predictions that are experimentally verifiable. But still, the sense of classicalism is still so entrenched in our thought processes, that at one point its best to just 'shutup and calculate'.

So in that sense, I guess I am off the mind to follow the equations. Virtual particles are purely perturbative phenomena (that probably describe at some level a nonperturbative effect that we call reality).

But hell, its not even clear what to call a real 'particle' in physics. Look at the example of neutral kaons, as they are only apparent in accelerators as some sort of weird superposition. Or seperately, some people like to think of such and such a 'particle' that is left handed, or right handed... Equally as wrong (or perhaps right).

 

[marlon]

I'm getting a bit annoyed here because you appear to double back and decide to address an old posting which you have already responded to, while still wanting to continue from the end of the thread. You have done this more than once here.

Note that NOWHERE did I ever said you CAN measure this so-called "naked" particles. So the whole question is moot.

Zz.

Indeed, you are not the only one who is annoyed here. You keep on asking the same questions as to what is real and you do not have an answer yourself. Besides from my second post on I stated that your first remark was going to trigger an useless discussion on "personal" tastes, now you see i am right. I don't have a problem with personal opinions, yet i DO have a problem with incorrect information...

Finally you make false statements on stuff like the Higgs-mechanism and that even is not relevant here.
I think i am donna drop this subject here since thanks to you it has evolved into an endless discussion on a very "relative" subject.

marlon

 

[marlon]

Note that NOWHERE did I ever said you CAN measure this so-called "naked" particles. So the whole question is moot.

Zz.

Well, er i could have sworn that you said that you cannot measure undressed particles directly.

NOTE that i did not ask whether you can measure undressed particles. I want to know what is indirectly means ? Yet it seems to me you don't know it yourself so let's just drop it.

marlon

 

[ZapperZ]

Indeed, you are not the only one who is annoyed here. You keep on asking the same questions as to what is real and you do not have an answer yourself. Besides from my second post on I stated that your first remark was going to trigger an useless discussion on "personal" tastes, now you see i am right. I don't have a problem with personal opinions, yet i DO have a problem with incorrect information...

Finally you make false statements on stuff like the Higgs-mechanism and that even is not relevant here.
I think i am donna drop this subject here since thanks to you it has evolved into an endless discussion on a very "relative" subject.

marlon

Look, from your reply on the Higgs stuff because it was obvious we were dealing with semantics here. What you meant as "dressed" is obviously different than what *I* meant as "dressed". You seem to make the distinction between dressed an "interaction, as in

Well, this is my whole point. A real particle is NOT dressed by the virtual particles surrounding it, it is dressed due to the INTERACTIONS of these virtual particles...

You can't tell if something is "dressed" UNTIL it interacts with the dressing! A particle surrounded by something it doesn't intract with is no different than a particle that is NOT surrounded by something at all! So it is meaningless to make a distintion between a particle surrounded by something it is not interacting with against a particle surrounded by something it IS interacting with. So when I say a particle is dressed around something, I mean it is INTERACTING with that something. It will, for example, affect the self-energy term in the perturbative expansion. Thus, when I said:

"Not if you accept the Higgs mechanism. Then a naked particle has NO MASS until it is "dressed" around a cloud of higgs bosons. In a condensed matter system, you NEVER measure the "naked" particles. All that you measure are the dressed particles."

.. and put it in the context of what I meant as something being "dressed", I was saying EXACTLY what you replied here:

Once again, the mass is NOT acquired because of this surrounding, it is acquired because of interactions between the elementary particles and the quanta of the Higgs-field. The Higgsbosons are absorbed by the massless elementary particles and this PROCESS yields the acquired and NAKED mass of a particle.In QFT mass is the coupling constant of the Higgs-mechanism expressing the strength of the interaction involved here...

Again, interaction with the surrounding IS what is meant as something being "dressed".[1,2] If it isn't interacting with the surrounding, when why even bother saying it is just surrounded with such garbage when it makes no difference whatsoever between that and no surrounding garbage? Now, knowing this, compare to what I said above when what you said.

I read this and immediately realized we were talking about the same thing but using different terminology, and so, I dropped it by not replying to this very specific comments that you made. But because of my non-reply, you appear to want to turn this into a "I'm right, you're wrong" type of argument.

I hope we are all happy with our "virtual wins". And we know just how "real" those virtual things are, don't we?

Zz.

[1] J.E. Hirsch, Phys. Rev. Lett., v.87, p.206402 (2001).
[2] C. Zhang et al., Phys. Rev. B v.60, p.14092 (1999).

 

[marlon]

I hope we are all happy with our "virtual wins". And we know just how "real" those virtual things are, don't we?

Indeed, we do, they do not exist
Glad to hear you are following me...


marlon

 

[marlon]

But because of my non-reply, you appear to want to turn this into a "I'm right, you're wrong" type of argument.

hehe easy man, there is enough war going on in the world...

Don't take this the wrong way but i just wanted to answer to your questions and i wanted to make clear the difference between mass-generation via the Higgs mechanism and mass-generation via the dressing of particles. Reading you previous post clearly showed that you were mixing them up.

But in the end you are totally right on the virtual victories that we all achieved here in this virtual place...

marlon

 

[reilly]

About virtual particles: they are an artifact of perturbation theory. And whether relativistic or non-relativistic, perturbation theory utilizes sums over states, convenient states at that. In QED, for example, an electron absorbs a photon, and remains an electron. But, restricting things to free particles, that's a non-physical transition -- if momentum is conserved, energy is not. That intermediate state decays into an electron and a photon(s) , giving Compton Scattering.

Are these virtual particles real? Why not? Like the resonances of strong interations, these QED virtual particles, like the intermediate electron above, they'll be known by their decay products.

In non-relativistic QM the perturbation expansion involves the term (1/(E-H0)) * V and all its powers. The 1/(E-H0) term is represented by a line in a Feynman diagram. This line is said, by common convention, to represent a virtual state/particle, -- unless it is associated with the initial or final states. It works exactly the same way for relativistic QM, with 1/(E-H0) replaced by the appropriate 4-D propegator. Again, the idea of a virtual particle is a convenient and universally (well, almost) accepted fiction. Makes talking about diagrams and perturbation theory much easier.

Usually a dressed particle/state refers to an eigenstate of the full Hamiltonian while the bare particle/state is an eigenstate of the free Hamiltonian. Perturbation theory then allows a cloud/dressed picture of a bare particle enveloped in a cloud of photons, and pairs, and,,,, A cloud of 100 photons, no pairs, would require 50th order or higher perturbations.

Virtual particles may or may not be real. but, they are very usful.
Regards,
Reilly Atkinson

 

[humanino]

Feynman diagrams are not a picture of what is happening in spacetime. They are an equivalence class of terms in a perturbative expasion, all those terms being summed over in an integral, then again one has to sum the different equivalence classes. Restricting the expansion to the first order is often sufficient, but even in the simple case of deep inelastic Compton scattering of an electron on a proton, my humble opinion is that we can only imagine that the complicated excitations of the EM field, are mainly similar to the exchange of a massive photon, which does not occur in reality. All the complicated next terms partly cancel out each other, letting a first order contribution artificially allowed to use the gauge-forbiden degrees of freedom. If one requires high accuracy, one takes into account the next terms, and those too receive an averaged contribution from higher order terms, letting them also allowed to be "virtual".

 

[reilly]

Indeed, Feynman diagrams do not necessarily picture reality. But, what's a poor physicist to do? If you go back and look at QED papers (see Schwinger's Dover collection of work up to the 1950s) of the 1930s, you will find them dense with algebra, with few if any places to use your intuition. People made mistakes -- working everything out in positron theory was a nightmare. As Schweber notes in his QED and the Men Who Made It, Schwinger is supposed to have said about Feynman, with a bit of derision, 'He made calculations available to the masses."

I'd say that Feynman's approach was was a jet plane, Schwinger's a WWII aircraft carrier , with an infinite fuel tank. Yes, diagrams, strictly speaking, represent equivalence classes, but that's part of the deal from the beginning. Recall that there is a strict procedure for converting a diagram into algebra, and vic versa. So the language of diagrams can be exactly transformed into algebra and vic versa. That means that the two expressions of perturbation theory are one and the same. The math of perturbation theory can be formidable; the math of the diagrams is simple and highly intuitive -- apart from the reality of spin factors, and difficult integrals.

Sad to say, however, the diagrammatic approach does not work well, or at all, with strong interactions, with non-perturbative solutions. So, nothing's perfect. Diagrams, virtual particles, are great aids to computation.
Regards,
Reilly

 

[humanino]

I fully agree with professor's Reilly posts. I want to illustrate my words with the following paper :
How to reconcile the Rosenbluth and the polarization transfer method in the measurement of the proton form factors
The two photons exchange had always been disregarded, partly because we needed not any contribution from it. We see here that, the contribution from the two photons exchange when disragarded could average to zero in the longitudinal momentum transfer, while the transverse contributions add. The energy transfer stays the same, so overall, when written as a one photon exchange, the total momentum cannot fully balance the energy part of the 4-vector, thus becoming massive.

This is a naive analogy, I am aware of that.

 

[Rhizomorph]

Thanks for all who've replied. Your responses have definitely shed some light on virtual particles. I have, however, a couple of followup questions:

Focusing less on whether or not they are "real" (which will keep us in a semantic debate forever) - what, if anything is supposed to explain how these particles come about? In other words, while there is no doubt that the presense of virtual particles plays a significant role in explaining certain QM effects, nothing I have read really address the mechanism through which these pairs come into... being... existence... whatever word you want to use?

Would it be accurate to say that the current popular QM understanding is simply that these pairs come in and out of existence randomly and we just accept that, but that we have no understanding of the mechanism which actually accounts for the behaviour of the "quantum foam".

I suppose if the popular view is that they "come from nowhere" then above would be accurate, no?

And lastly, is the density of virtual particle/anti-particle pairs conistent/identical throughout the universe, or is it higher in some places (e.g. massive bodies) and lower in others (empty space) - and whatever the answer how sure are we of it (empirically proven?)?

Thanks again everyone.

 

[vanesch]

Focusing less on whether or not they are "real" (which will keep us in a semantic debate forever) - what, if anything is supposed to explain how these particles come about? In other words, while there is no doubt that the presense of virtual particles plays a significant role in explaining certain QM effects, nothing I have read really address the mechanism through which these pairs come into... being... existence... whatever word you want to use?

In a way, they come about in the series development of an exponential, and your question is very close to:
where do the higher powers of x come from in:
exp(-x) = 1 - x + x^2/2 - x^3/6 + x^4/24 ...

cheers,
Patrick.

 

[Rhizomorph]

vanesch - well, I'm more interested in the physical interpretation than the mathmatical formalism. I'm looking for an answer that addresses the physical mechanism/processes that account for the quantum foam, rather than a mathmatical justification for it.

 

[vanesch]

vanesch - well, I'm more interested in the physical interpretation than the mathmatical formalism.

But that was exactly what I was trying to point out
As stated before here, virtual particles arise in a quite natural manner in a series development (perturbation theory). In my simplistic example, you wouldn't ask where the products of the x came from if you knew that the answer was an exponential. In about the same way, if somehow we knew how to solve directly for the total correlation functions without series devellopment, I think nobody would mention virtual particles. It is just because this seems to be our only tool to calculate the total correlation functions, that we have to sum over classes of terms which are described by "virtual particles".
However, it is true that in those cases where perturbation theory works very well (as in QED), there seems to be something physical to virtual particles - I think that that simply comes from the fact that the successive powers separate clearly the different diagram classes. I think that this can also be illustrated with another example: a fabry-perrot interferrometer.

---> (1)|///| (2) --->
If we are far from resonance, you can consider an incoming beam, a reflection on side 1, on side 2 and then you calculate what's left. You can correct this result by a reflection on side 1, on side 2, and a back-reflection of this last reflection on side 1 back forward again. So it is as if you have a "virtual back reflection on (1). You can correct this even further by considering also the backreflection on (2) of this backreflection on side (1)...

However, you can also consider the problem in all its generality, by solving the Maxwell equations with boundary conditions on (1) and (2). As such, you have an incoming and reflected wave before (1), a standing wave in the interferrometer and a forward going wave in (2). There are no "multiple reflections" anymore. I have the impression that "virtual particles" are like "multiple reflections" in this problem.

cheers,
Patrick.