Talk:Virtual particle

Badly written

First paragraph is incomprehensible jibberish. There is subject-verb disagreement in the first sentence (to what "they" does "their" refer)? Write to introduce subject to layperson, not to summarize all the physics in the fewest possible words. —Preceding unsigned comment added by 70.41.156.202 (talk) 21:29, 10 January 2008 (UTC)

Style: "large and complicated integrals over a large number of variables". Aside from being unsourced POV or OR, I'm not sure what this means or contributes beyond being condescending. I also noted lots of apparently popular sources, SciAm maybe being the most technical. It is not hard to find online full texts of many classic works ( these tend to be early 1900's in physics) and a few mondern papers dealing with open issues or utility may be nice too. Nerdseeksblonde (talk) 21:09, 25 August 2009 (UTC)

Agreed

Someone who understands this should delete the intro and start new. This is the worst I've seen for a subject of this type. Dave3457 (talk) 05:48, 23 June 2009 (UTC)

Real vs. virtual

I've got a question for the physicists here. I've always wondered about so-called "virtual photons." Is there REALLY any difference between real photons and virtual photons? What I mean is this: when we detect a photon with our eye or some instrument, the photon coming in excites some electrons into some higher energy states, or causes induced relaxation of some excited state, or whatever...but basically all the photon did before it "died" was move a charged particle (the electron). This is the same as what a virtual photon does! So, the only difference that I can see is that with a virtual photon, we consider that we observed a charged particle change its trajectory, whereas with a "real" photon, we claim that we observed a photon hitting our detector. But, really, they're the same thing? Comments? Ed Sanville 04:32, 23 Dec 2004 (UTC)

As described in the article, virtual particles occur strictly as intermediate steps in interactions. If your photodetector was triggered, something other than the virtual particle provided the initial energy for the triggering. The collision of two electrons is a good example for this: the force that the two electrons experience as they approach each other is carried by disturbances in a sea of virtual photons, but the change in trajectory of one electron is caused by exchange of energy and momentum with the other electron. The virtual photons acted solely as intermediaries. (ObCaveat: I'm still hazy on where the energy recovered from the work done by the pressure produced by the Casimir effect between plates whose separation distance is being changed comes from.)--Christopher Thomas 06:53, 12 Jun 2005 (UTC)

"virtual particles occur strictly as intermediate steps in interactions" But doesn't that depend on what you characterize as "the interaction?" For example, in my above example of a photon hitting a detector, you could consider the interaction to be between the particle that emitted the photon, and the particle in the detector that absorbed it. If you considered that to be the interaction, then it would seem that the photon has now become a virtual photon... is there some other requirement for a photon to be "virtual?" Ed Sanville 21:39, 17 April 2006 (UTC)

There are formal definitions of real vs. virtual photons. If you're using Feynman diagrams, virtual particles are ones that don't enter or leave the diagram (or alternatively, particles that do enter or leave the diagram are the real ones). If I recall correctly it was User:Linas who gave another description that treated virtual particles as the mathematical artifacts you get when you quantize behavior of forces in a certain way (via perturbation theory), with alternate ways of treating the problem removing the virtual particles, but leaving the real ones. My best advice for telling which is which is to treat any interaction over a distance as being mediated by real particles, as the range of interaction for virtual particles is very, very short (the simplified explanation which I was given was that they couldn't exist long enough for it to be _possible_ to measure them, due to uncertainty principle effects providing the loophole in conservation laws that the simplified explanation uses to handwave their existence in the first place). This is mostly-true down to scales approaching the size scale at which the Casimir effect becomes important, for photons, or scales comparable to the size of a nucleon, for things like mesons and gluons. One of the lurking physicists can give you a much clearer answer than I can. --Christopher Thomas 23:39, 17 April 2006 (UTC)

Ed's reasoning is correct: in a certain philosophical sense, there is no distinction between real and virtual photons. Chris Thomas' answer is correct as well: in practical matters, there's a clear distinction, and one has to cook up intriguing and confusing thought experiments to blur the distinction. linas 04:39, 20 April 2006 (UTC)

OK, thank you Chris Thomas and linas, this is one of those things I've always wondered about, and I wanted to get the consensus from the people who actually study these things. Ed Sanville 12:27, 20 April 2006 (UTC)

Me, I don't think electrons are real. Electrons are just perturbation theory to explain how cathode ray tubes work. They're just a fiction needed to make the calculations agree with reality. :-)WolfKeeper 01:25, 6 October 2007 (UTC)

I would just like to thank the person who wrote this: "Furthermore, in the photon's frame of reference, no time elapses between emission and absorption. This statement illustrates the difficulty of trying to distinguish between "real" and "virtual" particles as mathematically they are the same objects and it is only our definition of "reality" which is weak here." Excellent quote! Itistoday 06:52, 19 June 2006 (UTC)

More info

Where can I get more info on this (like where are the references for this article?) ChadThomson 1 July 2005 07:15 (UTC)

Google for "physics FAQ", or take a look at a first-year physics text of the same type I suggested at Talk:Big Bang. An introductory quantum mechanics text should have a more detailed description (virtual particles show up a lot when dealing with Feynman diagrams), but be warned that QM texts tend to have daunting amounts of math, so be prepared to brush up on relevant areas before such a text will make much sense.--Christopher Thomas 1 July 2005 15:18 (UTC)

Casimir effect dispute

See Talk:Casimir_effect -- Intangir 08:13, 26 December 2005 (UTC)

This page denies the validity of the common VP explanation of the Casimir Effect without citing a source for this. This is the same dispute as the one at that page- please discuss there instead of here.

Text under dispute:

Note that it is often believed that the Casimir effect has to do something with virtual particles. This is not directly true since the Casimir-effect results from an external perturbation of the zero point oscillations of the electromagnetic quantum field through two plates placed in the vacuum. -- Intangir 21:40, 26 December 2005 (UTC)

Yeah, this was based on a misunderstanding of the Casimir effect. Now fixed. I re-wrote the article entirely. linas 02:15, 28 December 2005 (UTC)

Ontological status

It doesn't seem to me to be terribly difficult to fix any perceived ontological problems with virtual particles. They are, of course, off shell because they can violate special relativity's speed limit and more. However they are still considered compatible with relativity because they violate this kind of stuff too briefly to be directly observed(naturally as they themselves exist too briefly to be observed). As they last longer(I mean word line interval) they become more probable to obey the rules. In essence, this is just saying that these rules are only approximate- they apply on all but the most incredibly short scales. Moreover, there need not be a binary classification between virtual particles and real particles where the particle is either one or the other. There is a continuum, as the worldlines get longer virtual particles smoothly become more 'real'. We don't need to postulate some new kind of unobservable thing. All we need do is postulate that some of the rules which we see to be followed on larger scales might not be followed on scales smaller than we have verified these rules.

As for the perturbative stuff, it may be the pertubation is only an approximation. However, it seems perfectly possible that there could be a maximum perturbation. Of course, the general failure of perturbative QCD casts doubt on this. --Intangir 00:21, 30 December 2005 (UTC)

I'm not sure what to say. Most practicing physicists understand (fairly clearly) that virtual particles are "fictitious", they're the side-effect of perturbation theory. The article tries to report this common, accepted understanding. One can try to "fix the ontological problem", but there's lots of dangerous reefs on that sea: one must steer towards philosphy, the interpretation of quantum mechanics, and "original research". Trying to somehow "clarify" this concept won't bring you new insight, nor will it bring you new experimetal predictions, and so the concept kind of gets put aside; one moves on to more promising ideas. linas 01:11, 30 December 2005 (UTC)

Interesting. Clearly "fictitious" is virtually in their very name. Hehe. However, it occurs to me that they are the insight. They seem to me to be an attempt to restore the various phenomena that we call 'particles' to the status of the fundamental building blocks of reality(atoms if you will). Through this "fixing of the ontological problem" behind particles, Feynman visualized the formulation of QED, ultimately enabling him to make new predictions. However, you can't really push this kind of issue aside. Whether you take waves, particles, or fields as fundamental, the issue is always which to choose and is always there. The insight is that it really does matter which.

I know that the philosophers of physics still coherently debate whether or not particles are fundamental. I would find it tragic if what you suggest is true, that physicists themselves have almost invariably given up on interpreting particles as potentially fundamental.

Well, actually, yes. For starters, there are many different interpretation of quantum mechanics, many of which are fraught with technical and philosophical difficulties. There is a small group of active workers who try to design experiments that distinguish between the different interpretations, but the work is hard, slogging and confusing. Most practicing physicists are "instrumentalists" (instrumentalist interpretation Gack!. Well, there is such a thing, but probably not what this article describes. I'm exhausted trying to keep WP honest about quantum.) in that they believe only in what the formulas predict, and don't ask "but what does the formula mean?". Its a very safe approach, keeps you on solid ground, keeps you from making errors.

As to "particles" ... good lord, left that ballpark long ago. For starters, particle-wave duality discarded the notion of a "particle" 80 years ago, and recognized the demolition of the particle with a nobel prize for the demolisher (de Broglie). You may argue that QED and QFT and Feynman resurrected the particle ... but good lord, the world has moved on since then. Take a look at Kaluza-Klein theory, supergravity, loop quantum gravity, string theory, 't Hooft loops, Wilson loops, BRST ... one measures "particles" in particle colliders, but that's not what the theories talk about.

You were thinking of instrumentalism, which is very close to my personal philosophy of science. From my point of view a 'particle' (these days) is a metaphor for a set of phenomena where events are discretely quantized but their probability distributions exhibits interference even 'self-interference'. The 'particle' in this case is a self-interacting one where the lack of independance between possibilities forces the abandonment of simple probability. Without localization, normal probability can't be used. What I mean is just this 'event' kind of phenomena. It makes good sense to directly reason from it, as Feynman essentially did. Especially since it can build up such fabulously predictive models without making any crazy assumptions. I know of the first four of those theories. None of them seem to have actually 'succeeded', only inspired. But, come on, extra dimensions, entire families of missing particles, and stringy super difficult math be damned- are we really that desperate? Has any of these paths achieved some new level of precision? Surely there are still plenty of more plausible paths to consider? Did U(4) and SU(4) just fail spectacularly or something? Are we fresh out of 'simple ideas', or have physicists forgot to induct over theories and not just experimental evidence? The fact that the world has moved on is what is precisely troubling. With so many caught up in a desire to create, who is trying to fix the old stuff? Oh right, thats below, hehe. --Intangir 05:35, 31 December 2005 (UTC)

Certainly virtual particles have bothersome, unresolved technical issues. But at this time you really can't argue that virtual particles are mere artifacts of a perturbative approach to the more fundamental field theory. As I see it, quantum field theory seems to have inherited the very same issues. Haag's theorem shows that the standard approach to QFT is just as "dippy" as renormalizing perturbative QED in the sense that QED couldn't rigorously handle the interaction picture, and so neither can QFT. QFT is just better at sweeping this same old inconsistancy deeper under the carpet(This is the same kind of success that Feynman said he earned the Nobel Prize for, hehe).

A lot of excitement and effort is going into non-perturbative models these days, which don't have "feynman diagrams" in them, because they are exactly solvable. Basically, over the last five decades, there were a few outstanding non-perturbative solutions (see Category:exactly solvable models) that have shed light on how to go about solving QFT "correctly". These include the Potts model, the KdV equation, the non-linear Shroedinger equation, Toda field theory, and to a lesser degree, the Skyrmion, the WZW model: all of these models have many aspects that are a lot like QFT ... much of the math in statistical mechanics resembles QFT, and so there is now mounting hope that these methods can be extended and pushed much farther. Modern work thus focuses on Quantum groups, Kac-Moody algebras, Virasoro algebras, Loop algebras. A lot of the excitement is coming from the fact that rapid strides are being made, with many disparate branches of mathematics yielding surprising, astounding, fore-head slapping, who-ever-would-have-thought-that-this-is-related-to-that insights.

Here's a real simple example: did you know that SL(2,C) is the Lorentz group, is the Mobius group? Did you know that the subroup SL(2,Z) aka the modular group is central to number theory and modular forms? Why are the symmetries of space-time (the lorentz group) so closely related to number theory? Its astounding and forehad-slapping! Some of this wildness, truth-is-stranger-than-fiction stuff got named monstrous moonshine because of its wildness. linas 03:47, 31 December 2005 (UTC)

Yeah, I don't know much of anything about these guys, but doing it 'correctly' does seem like the likely way to make real progress. Thanks for mentioniong this stuff- now I know some good nooks to look into. Also, understanding the symmetry stuff seems to me to be a great endeavor. --Intangir 05:35, 31 December 2005 (UTC)

To bring up a point of contention between us, this kind of issue is why I find Stochastic Electrodynamics fascinating. By taking both fields and particles as fundamental, it bypasses the mathematical inconsistancy which confounds QED/QFT. Particles are used to describe what they describe best- the old 'grains of sand' notion. While a field is added to model the quantum behavior. The advantage derived is an even more coherent handling of the old self-energy problem, one of the most important advantages that QED had over the old wave-based approaches. Of course, I find it unlikely that both particles and fields are truely seperate and fundamental since each seems to be able to mimic the other. However, SED shows promise in someday making the kind of predictive progress that QED derived theory cannot seem to do till the old "dippy" problem is resolved. --Intangir 12:55, 30 December 2005 (UTC)

My allergy to SED is multi-fold. First, it makes such bold claims that it don't sound believable. When I look into it, I seem to find only some fairly simple-minded math, which seems to be getting misused, and doesn't seem to be terribly powerful or insightful. I don't walk away slapping my forehead "but of course" with some grand new realization or insight. It seems to lack any sort of deep connections to anything that is well-known or well-understood; it doesn't touch base. What I've read just comes off as being shallow, unconvincing, not particularly bright, and smells "not even wrong" at times. The fact that some of it is associated with infamous cranks certainly doesn't help. My advice: before you get hooked on SED, crawl through some of the links I've posted above. linas 04:09, 31 December 2005 (UTC)

SED may very well be another predictive dead-end. However, it doesn't seem to postulate any a priori outrageousness. Its rather simplistic formulation makes it another easy study for non-physicists like myself. Part of the reason why I like virtual particles so much is that they are the only formulation I have been able to 'get a handle on' so far. It is icing on the cake that they end up working so well. The crazy SED claims about gravity and mass are just that- crazy. Yet another symptom of GUT-lust. The formulation itself seems to make good, mathematical model-style sense; what the researchers do with it is another story. --Intangir 05:35, 31 December 2005 (UTC)

BTW, I liked your antigravity of anti-matter article. I'd show more support of it, except that recently I've rubbed User:CH the wrong way, and am trying to keep his stress levels down by mostly avoiding things he gets tangled in. But I did like your article. linas 04:09, 31 December 2005 (UTC)

Thanks! I also admire your recent work on this article and for the Casimir Effect. You've gone above and beyond merely supporting my original dispute and have done much to improve these pages. --Intangir 05:35, 31 December 2005 (UTC)

Fermion virtual particles

I don't think this statement:

However, in order to preserve quantum numbers, most simple diagrams involving fermion exchange are prohibited.

is really appropriate. You can, of course, have a simple diagram involving fermions: electron-photon scattering (Compton scattering). In a sense, this is a good diagram to mention virtual particles with - you can think of it as an electron absorbing a photon, and going off-mass shell (becoming virtual). That virtual particle is then, in a sense, unstable (with the timescale determined by Heisenberg) and reradiates a photon, pushing the electron back on mass shell. The confusion with "what is real, and what is virtual" is obvious here, because as the energy transfered becomes very, very small, the timescale becomes infinitely long, which means any electron could be virtual, so long as it's sufficiently close to mass-shell for the purposes of the discussion.

I think the key word here may be exchange. $e\gamma \to e^{*}\to e\gamma$ isn't the exchange of a fermion, although it does proceed by way of a virtual fermion. Does that make sense? -- SCZenz 19:47, 25 January 2006 (UTC)

True, true. But exchange diagrams aren't the best way to explain virtual particles, in my mind, because virtual particles are just off-shell particles. Exchange diagrams have the "creation" of a virtual particle, which somewhat implies the particles could be "different". They're not, and I think that's made explicitly clear by a Compton scattering diagram, in which a normal particle "becomes" virtual, and then real again.

I'm not sure I understand your point. In electron-positron scattering, the diagram with $e^{+}e^{-}\to \gamma \to e^{+}e^{-}$ is fundamentally different from the one where they exchange a photon. In the former, the virtual photon is timelike. In the latter, the photon is spacelike; thus where it started and ended depends on your reference frame, so we just draw the thing horizontally (if time goes up). But the point is, I think it's reasonable to call the latter an exchange diagram, and I believe that's the terminology. Does that help, or can you clarify the issue? -- SCZenz 21:04, 30 January 2006 (UTC)

Oneloop diagram

Off-topic, but we really need a one-loop diagram with two legs, not three. Know where any of these are lying round? linas 01:11, 31 January 2006 (UTC)

I could make one. What exactly do you want? -- SCZenz 01:18, 31 January 2006 (UTC)

I had one, but it seems to have disappeared spontaneously. David (talk) 20:58, 16 August 2008 (UTC)

not so good

I've reverted recent additions by Enormousdude. The additions do not seem to be correct. -lethe ^{talk +} 02:38, 21 March 2006 (UTC)

todo

The article should be expanded to cover the topics that Enormusdude is trying to get at, with his recent edits:

• Spontaneous emission is an interaction between the "virtual particles" of the electromagnetic vacuum and the excited state of an atom. (I just fixed that article, but a discussion should be made here as well).

• The static electric field is an infinite ocean of "virtual photons" in the super-duper-deep infrared. (I cut the discussion from that article, it should be more precisely defined here).

These two, together with Casimir energy and pair-production (spontaneous decay of the vacuum) are the complete list of QFT/QED effects which require integration over the full phase space of the fields. Am I forgetting any other effects?

These four effects should be summarized by one or two sentences in the introduction. linas 15:27, 28 April 2006 (UTC)

Wait, what? Complete list? If you're looking for a complete list of field-theoretic physical phenomena modelled using virtual particles, you're missing hundreds of stuffs. Every interaction in nature comes exchange of virtual particles, so every interaction has virtual particles at the heart of its description. On the other hand, it's not clear to me what the Casimir effect or what a static field has to do with virtual particles. But I think maybe the Lamb shift should be mentioned along side the vacuum polarization. -lethe ^{talk +} 19:51, 28 April 2006 (UTC)

Thanks, yes. I dunno. In the back of my mind, I was thinking of "the usual field-theory type calculations", of which there are hundreds and I didn't want to try to describe, and the "unusual" effects, which would get reviewed. But maybe there's a big grey area in the middle, as you point out with Lamb shift. As to Casimir effect, read the article. Its essentially a one-loop effect, its just that its "unusual" in that instead of being an integral over momentum with propagators in the integrand, its a sum. As to "const electric field", you will occasionally find QFT textbooks that state that a const electric field can be written as a coherent state of zero energy photons, or some such (take a Glauber state for a laser, and run the laser frequency down to zero). A synonym is infrared catastrophe, referring to the infinite number of photons that arise. Clearly Enormusdude has heard of the idea; he tried to add it to the article on the electric field; I removed it just today mostly because it was confusingly presented, and is more appropriately detailed in this article instead of that. I see what he was trying to get at, and was hoping to just say it more elegantly and accurately. linas 03:10, 29 April 2006 (UTC)

Hmm, I think I see your point. Ah, we could understand it this way: every interaction in field theory requires virtual particles for its full nonperturbative description, but most can be understood already at the tree level, which is usually similar to the classical description (like how the tree level e-μ scattering coincides with classical Rutherford), and therefore is not a purely virtual particle description. Is that what you have in mind? I could get on board the list, in that case. There could just be a single item as the last entry of the list which said something like "additionally, every physical interaction requires the use of virtual particles for its full description, though simplified pictures without virtual particles are possible" or something like that.

Edit':uh, I'm not sure what I was thinking above. Even tree level diagrams have virtual particles. Maybe the tree level virtual particles are "less weird" in some sense, since there's only a single particle exchange, rather than a superposition of virtual states at each momentum? -lethe ^{talk +} 03:34, 29 April 2006 (UTC)

As for static fields and Casimir. Well, I'm familiar with coherent states. They're eigenvalues of the annihilation operator, and usually look like e^a†|0>. Because of the exponential, you can view this as a superposition of 0, 1, 2, 3 ... particle states. It's not exactly right to say it has an infinite number of particles, better to say it's just not an eigenstate of the number operator N=a†a. Anyway, those states are purely kinematic, they have nothing to do with interaction, and you can construct them even for free fields or the harmonic oscillator, so there isn't (or at least need not be) any perturbation involved. In short: I fail to see what virtual particles have to do with it. I am dubious about virtual particles being at all related. As for the Casimir effect, I was under the impression that this was a vacuum energy effect. But maybe vacuum energy is understood as a 1-loop effect? I admit I'm not crystal clear on that, so maybe you're right, it is a purely virtual particle effect. And the Lamb shift, I guess that's just a manifestation of vacuum polarization, but it's an important one, so it ought to get mentioned. -lethe ^{talk +} 03:29, 29 April 2006 (UTC)

Yes, Lamb shift should be added. You're right about the coherent states, they're not really "virtual", they're rather unusual real "particles", and illustrate the treachery of the concept of a "particle" in QFT. The vacuum contains all loop diagrams of arbitrary order. The vacuum, in perturbation theory, is a strange beast. I like to think of the vacuum as being sort-of like the partition function in dynamical systems: dull, flat, empty, boring, until you realize that it holds everything and everything else can be teased out of it with the appropriate derivative, coupling, limit, etc.

Re your earlier comment: my take on what this article should contain is this: if some pop lit book or technical article ascribes some effect to "virtual particles", using possibly loose, non-standard or layman's language, we should acknowledge that kind of a description here, in some way. So I was fishing for a "complete list" of the types things a Popular Science /Scientific American /New Scientist etc article might ascribe to "virtual particles". linas 05:14, 30 April 2006 (UTC)

I've removed static fields from the list. Looking at the list, I see spontaneous emmission, Casimir, pair production, van der Waals, and Hawking radiation. It seems to me that all of these phenomena are manifestations of vacuum polarization. I wonder therefore if the distinction you want to draw, between normal interactions which involve virtual particles and SciAm virtual particles, may just be the distinction between disconnected diagrams (virtual particles in a vacuum) and connected diagrams (virtual particles involved in an interaction). -lethe ^{talk +} 17:16, 30 April 2006 (UTC)

Wow, reading this article more closely, it seems to me that its intro is not very good. Seems to me that since the mass shell condition is a perfectly precise concept for a particle to satisfy, the definition of virtual particles is fairly precise. I guess the author was trying to get at the following fact: no interacting particle can be exactly on its mass shell (since it is just an internal leg between interactions). Therefore, no particle is exactly a real particle, and the distinction between real particles is arbitrary and not precisely defined. This does not change the fact the definition is precise, however, and I wonder how many times the intro has to say "loose" or "vague". -lethe ^{talk +} 17:23, 30 April 2006 (UTC)

new intro

I've written a new intro for the article. Comments welcome. I think some of the subsequent paragraphs need considerable help as well. -lethe ^{talk +} 18:21, 30 April 2006 (UTC)

I've reverted Enormousdude's revisions to the new intro. Some of my complaints are:

1. "for which product of mathematically entangled quantities is less than required by the uncertainty principle". The phrase "mathematically entangled" is bad, the uncertainty principle says nothing about products of quantities, and violating the HUP is not a good way to think.
2. "strictly speaking can not be required to comply with known laws - say, conservation laws". Since Feynman, no one claims that virtual particles violate conservation laws. This is an old-fashioned view that is still found in pop science books, but pop science books are no place to learn particle physics.
3. this bit about "origin of all known forces" leaves a bad taste in my mouth. Virtual particles don't explain the origin of anything. They are nothing more than a calculational tool for understanding terms in perturbation theory. -lethe ^{talk +} 00:37, 2 May 2006 (UTC)

Proposed addition

I would really like to add something like:

"Real" particles represent so called energy eigenstates of the quantum fields, which means time-independent, unchanging situations. Anything that cannot be described as a superposition of such unchanging energy eigenstates must be described in terms of virtual particles (if one insists on imposing a particle description at all). Therefore, in a sense, whenever "anything interesting" happens – anything interacts with anything else – it by necessity involves virtual particles.

but I would like to hear comments from other physicists first. - Mglg 03:21, 16 June 2006 (UTC)

I think this is rather misleading. A virtual particle isn't a state vector, it's a matrix element. Virtual particles are made up of particle states (which are eigenstates of the number operator/energy operator of the free field) just as much as real particles are. The difference is the kinematics of the matrix element (real particles have momenta fixed by observation and the Euler-Lagrange equation, virtual particles are integrated over all momenta, even off shell). -lethe ^{talk +} 03:34, 16 June 2006 (UTC)

Virtual Particles Gravity Theory

A man wrote me this: Gravity is not a pulling force, it is a pushing force, it is the virtual particles surrounding a planet, pushing down the objects. Matter is mostly space but some objects have farther between the smallest particles, and therefore they are lighter - there is less resistance, less particles to be hit by the virtual particles. Say a piece of lead for example, is very heavy simply because it is dense!. So what you think of that? /Minoya 12:17, 14 December 2006 (UTC)

I think your friend was pulling that out of his ass. Ed Sanville 22:46, 14 December 2006 (UTC)

Why is that? Have you really thought about it? Have you thought about in relation to the Dirac Sea? /Minoya 23:49, 15 December 2006 (UTC)

Because the statement does not accurately represent the concept of a virtual particle is (see main article!), contradicts the current (highly predictive) theories of gravity, and does not reflect ideas found anywhere in the literature of current physics research. -- SCZenz 23:53, 15 December 2006 (UTC)

There are ongoing efforts to write down a theory of gravity mediated by virtual particles. See graviton. -- SCZenz 23:54, 15 December 2006 (UTC)

Have you (Minoya) "really thought about it"? Why does not the pressure equalize and then the gravity would stop? Can you derive the form of the empirical gravity formula from the Casimir effect (which is effectively what you are suggesting)? This idea is just born of abysmal ignorance or an attempt to con people. JRSpriggs 09:32, 16 December 2006 (UTC)

Sorry im not into formulas, im into visualizing and perception so i cant provide you with any formula. Why would the pressure equalize, and what pressure? There are infinite virtual particles and sure they exert a pressure, which is gravity, its like we are the fish and the virtual particles is the sea. All of our universe is dwelling in a sea of infinite virtual particles, and theese particles are also energy and light, they have the potential to be anything and you can focus them with your mind, then their image is called a thought. The physical world of matter is in fact stagnated virtual particles. This is my direct perception and visual interpretation and understanding. Im just providing my truth, cheers /Minoya 19:21, 16 December 2006 (UTC)

Minoya, perhaps you shouldn't use the term "virtual particle," when you obviously haven't got a clue what they are. I suggest fairy dust for your concept. Ed Sanville 10:54, 17 December 2006 (UTC)

Actually, Minoya, I think the "burden of proof" is on you to provide any reason why I, or anyone else, shouldn't trust the findings of mainstream science that have been painstakingly obtained from centuries of scientific experimentation and theory. By the way, making vague, mystical comments, and attempting to legitimize that nonsense by throwing in a few unrelated scientific catchphrases like Dirac Sea, is not actual science. Ed Sanville 12:09, 16 December 2006 (UTC)

There is no need to be rude and elitist when dealing with those whose principal interest is the paranormal (reference: User:Minoya). I think the problem with Minoya's approach can be explained very simply: he or she is envisioning virtual particles as being some sort of substance that can be understood by using commonsense physics. The problem with Minoya's approach is that he or she isn't aware that commonsense (Newtownian) physics actually doesn't hold at very tiny dimensions. This tiny scale is where we find virtual particles. They are not what we normally call particles, which are clumps of atoms. Our commonsense reasoning doesn't work when applied to the very small (the level of the quantum, quarks, etc.) or the very fast (the level of special relativity). And, in fairness, if gravity (which is already strange enough due to its equivalence to the acceleration generated by the distortion of spacetime by mass) can be explained by hypothetical particles pushing objects toward the gravity well, then why not? If it works mathematically, then it's just as valid an explanation (although not sanctioned by usage). David (talk) 21:22, 16 August 2008 (UTC)

Quantum field theory

Quantum field theory simply says that the electromagnetic field in all of its manifestations, from static electric to static magnetic (what you see when a static electric field is in motion) to electromagnetic radiation, are all made of particles. We call these photons. Static EM fields are said to be made of virtual photons. This is presently mentioned for static E fields in the article, but it's just as true of static B fields (how could it not be? What OTHER particle would a magnetic field be made of, if it must be seen to be made of particles?? This is taken for granted by physicists. [1]. But my comment that the magnetic component of nonradiative electromagnetism is made of virtual photons also (as it must be) has been reverted. Enormousdude, let's see your references. SBHarris 00:23, 17 April 2007 (UTC)

HHGTTG

I read the first four paragraphs and couldn't stop ROFL: "A single virtual particle cannot be detected because that would result in its becoming real." ! and " The term is rather loose and vaguely defined, in the sense that it clings to a rather incorrect view that the world is somehow made up of "real particles". Douglas Adams must be spinning in his grave having missed these jewels. I suggest the article is linked to the Whole_Sort_of_General_Mish_Mash in his memory, especially as none of it can be understood by anyone under the rank of Advanced God. ;) --Miikka Raninen 23:09, 1 May 2007 (UTC)

Well, WP:SOFIXIT SBHarris 23:49, 1 May 2007 (UTC)

Virtual photons and electromagnetism

Since the electric and magnetic forces are due to virtual photons hitting other particles within the force field, and virtual photons have velocity greater than that of light, are magnetic fields instantaneous? Can anybody answer in terms of quantum field theory or quantum electrodynamics? -- kanzure 03:43, 26 May 2007 (UTC)

Some more information can be found in Arnold Neumaier's response to Eugene Shubert re: whether or not going faster or slower than c in QED applies to virtual/real photons. Also, it is important to point out that the velocity of some virtual photon could be greater than c, but the probability of this is notoriously small. So, my question could be slightly revised: when the virtual photons are more likely to travel faster than c, this does in fact make the magnetic field propagate faster than c, right? It is odd that some other websites (amateurish in style) claim that magnetic fields are limited by the speed of light, even though the field itself is not made out of physical photons. -- kanzure 05:34, 26 May 2007 (UTC)

Information, i.e. unpredictable changes, cannot propagate faster than the speed of light in a vacuum. Any particles which move superluminally do not carry information. JRSpriggs 08:13, 26 May 2007 (UTC)

Let me try this out. The uncertainty principle states that these virtual photons are going to have velocity, and we do not know their position, so they could appear anywhere (though this is a very small probability, Feynman claims), and therefore any electric force or magnetic force acting on particles past the light-sphere of some propagating change can only be observed as 'noise' rather than anything else. What about in a system where we know there are only two objects: the magnet and the particle past the light-sphere? Would we still claim that the distant particle is only observing random quantum bombardments? Thank you. -- kanzure 14:30, 26 May 2007 (UTC)

Fields transmitting non-variable forces actually do act as though they had no transmission delay-- they point right at the non-retarded position of their sources-- but of course this component of static fields can't be used to transmit information (like Morse code). "I'm here" is not information. Anything more than that --any disturbance in the field which carries information (aka, anything which has entropy or is unexpected, like Morse) has a "real photon" component that carries it, and must move at the maximal rate of information travel, which is the speed of light, c. SBHarris 14:45, 26 May 2007 (UTC)

Steve, how is "I'm here" not information and how can that not be used as binary signaling? The oscillation from on to off of a distant magnetic field looks like an interesting question to pursue, especially in the case of distant cosmic magnetic fields, which we currently analyze in astrophysics using some photooptical techniques. Also, what do you mean by, "they point right at the non-retarded position of their sources," ? -- kanzure 19:08, 26 May 2007 (UTC)

I mean, if a charged particle goes by you on a smooth path (with nobody wiggling it), your gravity detector and your charged particle detector point right at its real-time position, not its position delayed by the speed-of-light transit time between your point and where it's at. But that can't be used to send information, because it's just one bit-- a nice smooth perdictable path. Or like a light always being on. But any wiggles or flickers or jiggles induced in the thing's path, which could be used to send information, cross at the speed of light, and the direction THEY come from is the retarded one. Thus, the Earth orbits the sun's real position (that's where the gravity eminates), not its retarded aberrated one. But we SEE the retarded aberated light from the sun's retarded position. If the sun had a simple charge, presumably that would not be true-- we'd see the simple charge at the non-retarded position, just as we do the "mass-charge." If you look at Feynman volume II there's a discussion of the way the first order effects of charges project into the future and basically correct for the retardation effects-- these are the interactions mediated by virtual particles. Real particles that transmit information are not so lucky. SBHarris 00:34, 29 July 2007 (UTC)

Pauli exclusion principle

Do virtual fermions obey the Pauli exclusion principle? If so, is spacetime packed solid with them? Mike Serfas 02:32, 27 July 2007 (UTC)

Yes, they do, but no spacetime isn't packed solid with them. The reason is that, unlike particles confined to some simple quantized state like an atom, in quantum field theory a free particle is allowed any momentum rather than only integer multiples of some number. Thus there's more than enough "room" for an infinite number of virtual fermions. -- SCZenz 14:49, 27 July 2007 (UTC)

This makes some sense, but I still don't have an intuitive picture of how virtual electron/positron pairs can come into existence in the middle of a white dwarf, for example. Are they immune to electron degeneracy pressure because the positron's interactions cancel out those of the electron? Also, do virtual positrons in such an environment usually interact with "real" electrons? Mike Serfas 23:31, 28 July 2007 (UTC)

These are good questions. The thing about virtual particles is that they exist only from the time they appear until the time they interact with something. In between (for a very brief period of time) they are freely-propogating, and so can have arbitrary momentum, and my argument above applies. Electron degeneracy pressure arises because there are only a finite number of states available to bound particles, and virtual particles (as far as I'm aware) never exist long enough to become bound with non-virtual particles in any meaningful way.

As for what virtual particles interact with inside a white dwarf, the answer is that they're interacting with the same stuff as always. Electrons and nuclei exchange virtual photons to maintain the electromagnetic force. Virtual electron-positron pairs emerge from photons, and then disappear. And so on.

The bottom line is virtual particles don't really appear and "look for something to do." Instead, they are a bookkeeping method to mathematically calculate the details of other particles interacting, or the corrections to how a "real" particle moves. Since the actors are already known, and the interaction occurs nearly instantaneously, the environment can't really interfere. -- SCZenz 07:42, 29 July 2007 (UTC)

Virtual particles and space

Im no physicist, but I read about these virtual particles, and it appears that they come out of nothing. But couldnt it be that the fabric of space-time continuum or the fabric of the space is transforming into the virtual particle and vice-versa? Nikusor665 15:03, 1 August 2007 (UTC)

Sad state of physics

If Einstein lived today he'd have become an alcoholic at the ridiculous state of physics. I was the person who added the last paragraph of the introduction which called into question the foundation for the origin of virtual particles. I put it there based on my own understanding of physics, and though I didn't cite a reference I thought it was an innocuous commentary that just gave more context to the subject. After all, a virtual particle has never been observed, and by its own clever conception it cannot even exist if its observed. If that's not a catch 22 situation I guess they don't exist. I now see that the writing is "literally" on the wall that a citation is required. Well, I don't have one other than my own published writing and I refuse to cite my own writing based on a general cringing at self promotion.

So Wolfkeeper, remove that paragraph if you must and we can all chalk it up as a further example of physics moving in lockstep over the cliff. Physics has now become so absurd that as long as you cite an important person as ignorant as oneself you can pontificate on anything, including multiverses which by definition, being separate universes, have no causality with our own. What a sad situation where a science beloved by many of us has been taken over by accountants without the slightest shred of common sense or genuine intuition.75.7.4.16 09:28, 4 December 2007 (UTC)

I was going to leave it, or just remove some of the weasel words, but based on the evidence presented above, I'm taking it out entirely.- (User) WolfKeeper (Talk) 10:10, 4 December 2007 (UTC)

Oh and BTW, in physics we don't play 'count the universes' any more than we play 'count the atoms'. We play 'count the assumptions'. That's how physics works, that's how it has always worked.- (User) WolfKeeper (Talk) 10:26, 4 December 2007 (UTC)

Virtual particles – logical fallacy

I notice there is no citation for the initial summary or for virtual particles at all.

One citation (raymond) is already dead. I hope that wasn’t the one this article depended upon. The powerpoint has no direct or source references. The Gilman cite has no direct or source references. The references on phonons and photon are not about particles with mass.

This means the article has zero direct or source references to its definition.

Yet somehow this wiki article has wrongly reversed the Burden Of Proof to the skeptic.

That’s a logical fallacy called Burden Of Proof reversal. http://freethoughtpedia.com/wiki/Master_list_of_logical_fallacies#Burden_Of_Proof

The proper burden of proof is on the proposer – not the skeptic.

I agree with whoever “added the last paragraph of the introduction which called into question the foundation for the origin of virtual particles.”

I think that until a direct or source reference to virtual particles can be found – that introduction should be revived (thought I’ve never read it and can’t find it in history -- Dec 2007).

"but based on the evidence presented above, I'm taking it out entirely." In other words, Wolfkeeper, you are mad so you are you using that as an excuse to take out needed contextual counterbalance on the subject of virtual particles. Another sad commentary on the state of discourse in physics today. Your actions follow your emotion of the moment rather than logical thought processes. 75.7.4.16 19:19, 4 December 2007 (UTC)

No, it was mainly the fact that you admitted you were never, ever going to cite it, together with your admission that it was OR; but if you give us a link here to any published paper we can still evaluate it for notability, but otherwise it's staying gone because it's unverifiable.- (User) WolfKeeper (Talk) 19:57, 4 December 2007 (UTC)

Wolfkeeper, you are not thinking logically, but emotionally. Otherwise you'd realize the absurdity of talking about the "verifiableness" of my comments. We are talking about a subject, virtual particles, whose main definition is that they aren't verifiable. Haven't you figured out yet that all you are protecting on this subject is the idea of "precedence", in which a cockamamy idea which by definition couldn't be proved became accepted without proof. If you had any sense at all you'd realize the burden of proof is on you and the people who have accepted this idea. Virtual particles are a bookkeeping mechanism required because of our ignorance of what is really going on in this area of physics. Why are you pretending that it isn't, and instead acting like the burden of proof in denying the "bookkeeping" reality isn't on you and not me. You are just compounding ignorance with stubornness.75.7.4.16 20:37, 4 December 2007 (UTC)

Hey, the main point of quarks is that they aren't verifiable, either. Does that make them a mere bookkeeping device? For that matter, you've never seen an isolated real photon, let alone a virtual one. Are "real" photons mere bookkeeping devices to "explain" that energy is absorbed out of the EM field only in chunks? Why not just leave it at that, instead of making the "chunks" into clunky particles? And now, let's start on electrons. Ever seen one? How do you know they aren't just a bookkeeping device? And even more importantly, why doesn't we agree to let bookkeeping have two p's, so we can have four sets of double letters in a row, instead of just three? SBHarris 18:03, 6 February 2008 (UTC)

To take a more everyday argument, when you unevenly cool a lump of iron, internal forces are set up which don't have any external effects, you never have internal forces entering or leaving the lump of iron, and internal forces never appear in a non perturbative calculation, so it's questionable whether internal forces exist at all, right? It's just a bookkeeping thing; but you can measure the distortions due to them. And lumps of iron frequently crack open due to them.- (User) WolfKeeper (Talk) 18:25, 6 February 2008 (UTC)

The argument against virtual particles in the article is of this same form, and is completely unreferenced.- (User) WolfKeeper (Talk) 18:25, 6 February 2008 (UTC)

SB,Being a physician it seems strange to me that you would try to correct a physicist's arguments about physics. I don't try to correct your ideas about medicine. I'm referring to your comments on the discussion page about "virtual particles". What you are doing in that discussion is taking ideas that have been superceded and taken them as gospel. I'm sure there are things in medicine that I could quote from a 40 year old book, and do it in the most literal way and pretty much offend you by telling you that you are a quack because you don't agree with me. Well, that's what you and many other people are doing on the subject of virtual particles. To get a better idea of what is really going on in physics read the wiki article "zero-point field" which I've contributed about 90 percent of.75.7.27.34 (talk) 09:30, 31 January 2009 (UTC)

Being a physician I'd be glad to be taught by a real physicist. Are you one? What are your credentials? My name is on my userpage. All I see from you is a mix of IP-hopping like I'd expect from a banned sock or a vandal. Surely if you're an authority on physics you'd be proud of that? And people wouldn't laugh at you, but would take you seriously, like new-age guru Hal Puthoff? Wups, no, strike that: they do laugh at Puthoff, even though he is a physicist. Free energy suppression consipracy and New Age. My, my. Is this where physics is, today? From reading all the unreferenced stuff in the zero-point field article I'd guess perhaps you are Puthoff. If so, good luck, but the community of respectable physicists (not to mention physicians) is not backing you, and you well know it. PS: Where's your free energy, baby? We need it. SBHarris 21:14, 31 January 2009 (UTC)

Fermion number

Under the subject pair production, it states "In order to conserve the total fermion number of the universe..." with a link to a non-existent page on fermion number being produced. I myself have heard (fairly vaguely I must admit) of this concept however it seems almost impossible to find sources on the internet to create the article. Most of what google produced was not something that a layman or even someone of a bit more expertise could understand, and none of it was explicitly on its definition. I did find a site that had copied from this very article to describe virtual particles but that obviously doesn't help. This website[[2]] is the best I could come up with for this. It says that electrons have a fermion number of +1 and for positrons it's -1. It doesn't say if the other particles mentoned on the page have fermion numbers or not. So, what should be done about this? Anyone know anything about fermion numbers? Deamon138 (talk) 08:49, 3 May 2008 (UTC)

The fermion number F is just the sum of the baryon and lepton numbers F = B + L. See [3]. If these are separately conserved, so is F. Even if baryons sometimes change to leptons, as for example the postulated supersymmetrical decay of protons to positrons, F might or might not be conserved, depending on what else is made. But I can't see why it's necessary to discuss this in this article. Conservation of B and L should be enough. SBHarris 19:33, 3 May 2008 (UTC)

The Loop Interpretation of Virtual Particle Pairs

David/mindlapse - while we did talk about this on Friday, you should now that this was my own interpretation and view on the subject and not necessarily widely (or at all) accepted (I simply don't know and haven't tried to check it). As such, it may not be appropriate to have it on Wikipedia - at least not until we find some more references. --Aleksandar Šušnjar (talk) 16:17, 22 July 2008 (UTC)

Antenna Near Field

The article states that the near field of an antenna is a manifestation of virtual photons. I do not think this is correct. I am no expert in modern physics, but I can say that the near field of an antenna is completely predicted by classical electricity and magnetism. The E and H fields always have a certain relationship to each other when they are time-varying, but that relationship changes depending upon the geometry of the sources and boundary conditions. For example, an electromagnet can produce a large H field and very little E field. Conversely, a capacitor can produce a large E field with little H field. The common relationship between E and H fields (that the ratio of their magnitudes is equal to the impedance of free space) holds only in free space, not near conductors or dielectrics.

There are two senses to the near field of an antenna: the region containing the storage fields and the region where the radiation pattern depends upon the radius. In both cases, the E and H fields being spoken of are very real and measurable. It is simply the case that, in general, an antenna will have E and H fields in its immediate vicinity which do not approximate a plane wave. They roughly represent the capacitance and inductance of the antenna itself. Also, on a more abstract level, the radiation flux as a function of angle (radiation pattern) will be dependent upon the distance from the antenna when you are very close to the antenna. One simple example is the pattern of a parabolic dish antenna. When you are so close to the dish that the simple conical beam model would predict a beamwidth smaller than the dish diameter, you are in the near field and cannot use that model.

None of this has anything that I can see to do with virtual particles. Someone with more knowledge of virtual particles please comment. If I do not see anything to the contrary, I will remove the mention of antenna near field in this article.

Sbreheny (talk) 05:43, 31 August 2008 (UTC)

The two are not mutually exclusive. QED underpins Electromagnetism, and QED relies on virtual particles.- (User) WolfKeeper (Talk) 15:56, 31 August 2008 (UTC)

Hi Wolfkeeper. Of course, I'm not saying that there is no QED involved in antenna near fields - only that it is involved only to the degree that it is involved in ANY E&M example. The page as presently written seems to indicate that the near field of an antenna is itself somehow virtual because it does not extend out with the radiating wave. It does not extend out because the propagating plane wave is what the E and H field equations reduce to when the distance from the antenna is large compared to a wavelength. There is no mystery here, and no reference needed to QED to predict the presence of two fairly distinct regions within the EM field around an antenna. One might as well list transformers, capacitors, inductors, resistors, etc., anything electrical, as an example of virtual particles. Yes, presently we need the concept of virtual particles to make sense, at the deepest levels, of the phenomena going on in these situations. However, no physicist would claim that the easily measurable E and H fields around these devices is virtual. Sbreheny (talk) 19:57, 31 August 2008 (UTC)

Indeed they would! The energy is passed from one coil of tranformer to the other by virtual particles, which is what these fields are "made" of. They are virtual photons, not real photons! In the quantization of static fields, and fields which do not have the correct ratio of E and B (the one you see in radiated photons) virtual particles are the field quanta. You can say: "Look, it's just a simple static electric or magnetic field, no need for quanta." But there are need for regarding it in quantum terms when th field becomes very powerful. This happens near nuclei, where the field doesn't act in a pure smooth way, but requires other techniques to sort out its behavior. For very low energy work, as around antennas, the quantum description isn't needed. However, something of the virtual character of these fields is still seen in the fact that they don't radiate as well, and seem to be in some sense "stuck" near their source, unless the energy is extracted from them by some nearby object. SBHarris 02:22, 1 September 2008 (UTC)

An Inconsistency With Certain Particles

I have noticed that in the manifestations section of this article, the particles responsible for the interactions of the strong and weak forces are written to be virtual. In those pages dedicated to the description of the particles (The Gluons and the W, Z Bosons), experimental evidence have been presented to suggest that they are can no longer be considered as virtual. This inconsistency must be given attention to, for such things are one the most unwanted occurrences in the sciences.

• See the Wikipedia articles for Gluons and W, Z Bosons

I have the proposition to remove those with the inconsistencies from the section. The reason for this comment is to confirm the problem with others who have gained a considerable amount of understanding on the subject. If indeed the mediating particle must be considered as virtual in the interaction involved, then a discerning description between the experimentally observed particle and that of the virtual particle must be presented in the article. Baiald (talk) 15:18, 31 August 2008 (UTC)

I see this from the other direction

Virtual W particles were used to understand weak decay prior to the prediction and discovery of real W particles in accellerators, and their proposed past real existence in the big bang. It is not an inconsistency, it is evidence that virtual particles have real counterparts. Spope3 (talk) 05:55, 1 August 2009 (UTC)

Conceptual Explanation?

As a non-physicist, I find this article very confusing. It even seems contradictory, because in places it talks about virtual particles as if they are a purely mathematical device used in perturbation theory or Feynmann diagrams, while elsewhere it is implied that they have a real existence, and the external article starts "virtual particles are indeed real particles". I assume a true understanding of virtual particles requires knowing the math, but I would like to have a clear conceptual summary that is as accurate as possible.

Also, the part about time-energy uncertainty is confusing. I think it corresponds to some aspect of the math, but if I understand it right, it's a very loose correspondence and likely to give readers incorrect ideas.

After reading this article a few times along with some other sources (esp. QM: Myths and Facts and Virtual Particles FAQ), I came up with a conceptual summary. Could someone say if this is accurate, and worth inclusion in the introduction:

In quantum mechanics, virtual particles are particle-like mathematical terms that arise in perturbation expansions in quantum field theory. Perturbation expansions are approximations for quantum mechanics, i.e., approximate solutions to fundamental QM equations. Perturbation expansions contain terms that mathematically behave like particles, but do not correspond to any observed particles. The calculations are often represented with Feynmann diagrams, in which the lines that do not appear at the start or end of the process are virtual particles.

Whether virtual particles really exist is not a question of physics, but of the philosophical interpretation of QM. In orthodox interpretations, QM is a computation device for predicting the observed results of experiments. In these interpretations, virtual particles are strictly mathematical devices and it does even make sense to ask if they are "real" or not. In a nonstandard interpretation where the nonobservable quantum field is the primary reality (rather than particles or the wave function), both virtual particles and real particles are actually excitations of a particle field. In such an interpretation, virtual particles are simply short-lived excitations that are observed only as forces. Dmandelin (talk) 05:34, 2 September 2008 (UTC)

There is a whole article on quantum field theory, and even one on the history of quantum field theory. Basically, if you believe in the principles of QM, then every field is associated with a particle and vice versa. You can't have one without the other. The "field quanta" particles used in the quantum representation of static fields, and any fields which don't behave as perfect streams of proper particles, are called virtual particles. Quite often the particles are massive and don't last long-- such as the Higgs particle. It's so heavy it hasn't been experimentally created yet, but the weaker scalar Higgs field, composed of virtual Higgs particles, is rather important, because it is postulated to be the "sticky" stuff that gives every massive particle mass and inertia, when they move through it! When the intensity of fields grows very strong, there is enough energy in any volume of space to make "real" particles appear for infinite amounts of time. When the fields are weaker, there is enough energy to make "real" particles appear for only shorter times, after which they must disappear again, like money borrowed from a bank which must soon be returned. You might think all this would have no effect on the behavior of the field, but it does, since the "borrowed" energy of the particles is available to do things in the time it is borrowed, and this has experimental effects which can be verified. It was one such effect, the tiny change in spectral lines from a pure field theory called the Lamb shift, which proved the first quantum feild theory, the one for the electromagnetic field called quantum electrodynamics. So, assumptions of these particles, or something like them, is necessary for a complete theory of nature. SBHarris 23:38, 4 September 2008 (UTC)

It would be nice that you cited a reputable source, considering the confusion around this topic, since the above comment to me is nothing more than your personal and maybe wrong understanding of the subject. Besides, i´ve heard about virtual particles after the Lamb shift...and I´ll rather stick to tha previously mentioned FAQ rather than to...no other source at all.--190.190.87.136 (talk) 03:03, 19 August 2009 (UTC)

?

"The amplitude that a virtual particle exists interferes with the amplitude for its non-existence" I don't get it..is this just a typo? Wormcast (talk) 16:18, 26 July 2009 (UTC)

About what virtual particles really are

The article states that if a virtual particle is detected, then it is not virtual any longer and becomes just a particle. So in other words, virtual particles can´t be detected...so the play no role in physics. Virtual particles may help one get some physical intuition on how things work, but they can be omitted in any discussion. This should be emphasized in the article. Virtual particles are an interpretation of drawings (Feynman diagrams), but they can´t be detected...so they are not "real". At least, their existence cannot be proven and I doubt any physicist thinks that two electrons repel because they each kind of emit photons constantly or something of the sort, and that these photons interact with the other electron.--190.190.87.136 (talk) 01:19, 19 August 2009 (UTC)

begging the question?

There is a part in the article where it says photons are excitations of underlying quantum fields, and the same with the virtual photons. Now correct me if i'm wrong but the virtual photons were used as a physical explanation of fields in the first place, yet and underlying quantum field would suggest some other particles mediating whatever this underlying field is. what is the force mediator for that field then and what is its relation to the photon? it would seem to me that the term "field" was misused or the person was unsure of how to describe what is actually going on. —Preceding unsigned comment added by 24.121.104.225 (talk) 05:28, 25 August 2009 (UTC)

I'm no quantum physicist (this needs review by an expert who knows QED) but as I understand it, the real problem is that there ARE NO "field free" spaces in quantum mechanics. In a classical "zero field" space, virtual particles still pop in and out of the vacuum in pairs, borrowing their energy E for a time t given by the uncertainty principle E*t < h/4pi, before disappearing again. This process has small effects in confined spaces (the Casimir effect) due to selection of some excitations (particles) but not others, by physical or potential constraints. In regions where there is classical energy available in the space (a static E or B field) the fields may still be described in terms of pair of particles showing up and then disappearing, but now one kind predominates a bit. Still, the effect of the virtual particles and anti-particle gives a tiny correction effect on the effect of any field, so that the Maxwell equations must be modified and are no longer QUITE linear, the way they are written classically. All this can be thought of EITHER entirely in terms of modification to certain modes of the fields at higher energies, OR in terms of virtual particles which contribute more and more to the effects (ala Feynman). I have the distinct feeling that some physicists think only the fields are "real," and others, that only the particles are "real" (in terms of being the actual underlying "thing"). But this duality, and the fact that both views give the same answers, is part of the oddness of quantum mechanics. SBHarris 23:35, 25 August 2009 (UTC)

Quantized?

What properties of virtual pairs are quantized? Specifically, are any of the following quantized...?

• The length of the loop in a one-loop diagram
  • Specifically, can an electron-positron virtual pair be created and destroyed in a loop that is both smaller than about the Bohr radius (0.53 Angstroms) and shorter than about the equivalent time (0.18 attoseconds?) Would the particle not interfere with itself like an electron bound to an atom?
• The time that the pair exists
• The distance they travel apart
• The ratio of time to distance
• The area of the circle (assuming one-dimensional spatial separation + time for 2D)
  • Also, is the temporal "angular momentum" of a particle conserved as it moves around the circle? i.e. do particles that move further apart annihilate at a greater distance from one another?
    • While I'm at it, are the spacelike parts of the circle in the diagram an accurate depiction? They actually annihilate one another at a distance greater than they could reach if they moved at each other at lightspeed?
      • And if so, is it also possible for a "virtual pair" to appear in the spacelike plane perpendicular to time, so that a ring of transluminal particle appears and vanishes in one instant (as seen from some particular frame of reference) or consists of two antiparticles that never slow beneath the speed of light from others?

Wnt (talk) 05:19, 27 August 2009 (UTC)

Mesons and bosons

Virtual particles need not be mesons or bosons, as in the example above

Mesons are bosons, right? 68.239.116.212 (talk) 18:13, 23 January 2010 (UTC)

Yes. The article probably mean to say vector bosons instead of just bosons. These are the elementary force carriers. SBHarris 03:02, 24 January 2010 (UTC)

Span

In the first paragraph, what is "span" referring to? I would link to it if I knew, but I don't! —Preceding unsigned comment added by 24.19.135.46 (talk) 04:16, 21 May 2010 (UTC)

This borrow argument

... is wrong.

In the Wikipedia article on the energy-time uncertainty principle it is clearly stated that virtual particles shouldn't be thought of as particles that borrow enough energy to exist as long as they return it in enough short time to satisfy the Heisenberg relation. Instead, a state with uncertain energy can't exist for an indefinite amount of time and viceversa (this is the right interpretation of the principle), so to calculate a transition amplitude one has to include the contributions of all the various cases and these comprise higher energy states too, i.e. the loops.

I suggest to fix this soon, because it's really misleading. —Preceding unsigned comment added by Ziofil (talk • contribs) 13:00, 8 November 2010 (UTC)

I'm not so sure of that. This re-states the argument that the Heisenberg principle itself (whether in momentum-distance form, or energy-time form) is about "measurement." Which finally reduces to Einstein's comment to Bohr about whether he really thought the moon was "there" when nobody was looking at it. If you don't want to cast the laws of wave-mechanics in terms of measurement, you end up with the simple fact that energy is not conserved over short periods of time (for whatever reason-- the deep underlying REASON is not understood; anymore than why wave equations apply to objects, is understood). This is what you get when wave-equations (periodic equations) are used to describe objects: waves are funny things that aren't MEANT to apply to objects as we know "objects." Waves don't have "normal" object behavior. It isn't a matter of measurement, it's a matter of the nature of "waves"! That's the whole quantum problem in a nutshell-- the single problem that is at the base of quantum mechanics, which nobody really understands. SBHarris 18:04, 8 November 2010 (UTC)

Lost me here

This one was one of the weirdest articles I've seen so far. A lot of it doesn't make sense to me. Where do you find a virtual photon has mass? Anyone ever measured one? As far as I know it's a mathematical proposal explaining some really weird properties, and not anything measurable. Which makes all of it pure theory, I don't question virtual particles as such. HUP allows them, and so they should be there at a QM plane, but giving them all kind of properties and presenting it as if it is 'commonly known' doesn't strike me as being especially 'scientific'. Yoron. 83.185.220.55 (talk) 03:17, 22 October 2011 (UTC)

You'll just have to read more QED theory, in which the fact that virtual photons have mass is what DEFINES them as virtual. Real photons are all massless. But virtual photons are those for which E^2-p^2 = m^2 is not equal to zero. If you need a basic reason why virtual photons must have some mass, note that they are "emitted" from charged particles, and represent the quanta of the static field that charged particles interact with other particles with. Problem: a single elementary charged particle cannot emit a single zero-mass particle like a photon, and still conserve energy and momentum. Which is why you never see it happen for real photons. (Only excited atoms and nuclei and collections of two or more passive particles emit photons). But in QED it does happen for virtual photons, which is how one elementary charged particle "pushes" or "pulls" another one. SBHarris 03:38, 22 October 2011 (UTC)

A layman explanation can be found here: http://profmattstrassler.com/articles-and-posts/particle-physics-basics/virtual-particles-what-are-they/ Pavel Vozenilek (talk) 00:48, 1 February 2012 (UTC)

ordinary language versus quantum field theoretical language

The lead of this article starts: "In physics, a virtual particle is a particle that exists for a limited time and space."

This sentence might seem to be a sentence of the ordinary language as is common to many Wikipedia articles. But it does not use the notion 'exists in time and space' in the most usual sense of the ordinary language. This departure from the most usual sense of the ordinary language is not explicitly and clearly signalled in the article, nor is it supported directly by reliable sources in the article.

This sentence uses the word 'exists' in a special technical sense belonging to quantum field theory. It can therefore be called a usage as a term of art.

The word 'virtual' here means that the existence is not real or concrete as supposed in the most usual sense of the ordinary language. It could fairly be said in the ordinary language that a virtual particle does not really exist. Indeed in the article it is eventually admitted that a virtual particle cannot be observed. An ordinary language meaning of the words 'such and such exists' is 'one can find such and such'. But one cannot find virtual particles.

Virtual particles exist only in an abstract sense, just as numbers exist only in an abstract sense. Virtual particles have more direct reference to real physical or natural processes than do numbers, in that numbers can refer to a logically vast diversity of collections of abstract and concrete entities, while virtual particles refer only to specific models of specific kinds of concrete physical process.

This article is very light on for reliable sourcing for most of its statements. This is partly because reliable sources are not easy to find that give explicit ordinary language accounts of these matters, which are largely in the province of theoretical physics.Chjoaygame (talk) 07:39, 2 April 2012 (UTC)

In what sense are virtual particles less real than (say) quarks? Or gluons? Or, for that matter, even electrons? We see even electrons only from their effects-- we never see them "directly." Feynman once asked a philosophy teacher if the inside of a brick is "real." You can't see it. An X-ray is indirect and shows you only X-ray scattering, not the thing itself. Break the brick in half and now you only have two half-bricks, and what you thought was previously the inside, is now the outside, so you (again) can't ever see the inside. Every bit you look at, is some kind of an "outside." So is the "inside" of a brick real? Maybe it's virtual? SBHarris 19:00, 2 April 2012 (UTC)

Are you saying that the article, when it says "virtual photons, not real photons", is talking nonsense? Surely not, but what you write seems to say so.Chjoaygame (talk) 19:45, 2 April 2012 (UTC)

The QFT people will tell you that the distinction is a bit artificial, or a matter of taste. For massless fields it's used to distinguish fields that are on mass shell, or off it, but most fields are mixes of both. A good example is the EM field, which can be said to be "decomposed" into near field and far field "parts," or sets of terms (ultimately coming from the different terms in Maxwell's equations). But is this separation "real," or just in your mind? It's not as though the near field stops and the far field begins at some point. It's just that one fades out faster than the other, but both are there, everywhere. In quantum language the near field component is quantized as "virtual photons" and the far field is quantized as "real photons." But that suggests that induction effects, static E and B effects, and all other near field effects, are not mediated by quanta that are "real". Well, are they quanta, yes or no? If the fields are real, and the effects of them are real, and they really are quantized, how can those quanta NOT be real? You see the problem? What the hell do we mean by "real" here? Are static fields real? Are quanta of static fields real? At some point, this is a game of semantics, where we start to get into philosophical naive realism vs antirealism vs idealism, blah, blah. How much of the world out there is real, vs. your mental image made by your language? Is ultraviolet real, and a different "thing" than X-rays? Are gamma-rays a real thing as opposed to X-rays? We name many things for the utility, not because of any natural boundaries that would be obvious to any intelligent creature that looked at them. SBHarris 20:12, 2 April 2012 (UTC)

Real particles are detectable by particle detectors. Virtual particles are not. The scattering matrix refers to far-field inputs and outputs. It refers to input quantum fields that if taken separately, not interacted with other fields, would be detected by particle detectors suitably placed. It refers to output quantum fields that are actually detected by particle detectors suitably placed. Virtual particles are abstract calculational conceptual constructs, of indefinite calculational depth, that are calculated as explanatory of the interactions of the input fields but in principle cannot be detected by particle detectors because they are not from or in the far field where the particle detectors are placed. The distinction is based on the premise that one cannot actually observe or actually know what is going on inside a quantum field interaction process, a premise that is accepted by Bohr and Dyson. The distinction is based on whether the particles are in the far field and are therefore detectable, or are in the near field and therefore are not. The distinction between real and virtual particles is therefore not based on the lifetime of the particles.Chjoaygame (talk) 01:12, 3 April 2012 (UTC)

What is a "particle detector"? When a process absorbs a quantum of energy, is that not the same as detecting a quantum of the field? In an MRI machine, when a hydrogen proton switches spin and emits a radiowave signal when it flips back, it is emitting a real photon, yes? This signal certainly generates a weak far-field. But first, that proton had to absorb a virtual photon from the RF induction field of the exciting coil to get the energy. That's a near-field process-- only the magnetic field of the coil is involved and no far-field RF at all. But why doesn't the initial flip of such a proton "count" as the detection of a quantum of field energy, if the later flip counts as emission of one? A quantum of energy certainly disappeared. And it certainly reappeared when the proton emitted a radio wave. Or, actually was prodded into emitting into (what will become) the RF field in an induced-emission sort of process, much like electrons in a radio antenna. You see, this is not such an easy distinction to make. SBHarris 01:36, 3 April 2012 (UTC)

You create logical problems out of the logical vacuum. You write: "But why doesn't the initial flip of such a proton "count" as the detection of a quantum of field energy, if the later flip counts as emission of one?" The later flip does not count as a photon detection, not even if the photon that it generates it is actually detected by a photon detector. An emission is an emission, and a detection is an absorption-linked process of a particular character but not simply an emission. An emission of that kind is not a detection. You write as if it were so. That creates a logical problem out of nothing. You write "When a process absorbs a quantum of energy, is that not the same as detecting a quantum of the field?" No, it's not the same. A detector involves a specific kind of absorption, not just any arbitrary absorption. A thing is what it is and not something else. You are half-aware of this when you put the word count in inverted commas. That two processes share in part a common mechanism is important and worth saying explicitly, but it does not justify saying that the two processes are the same thing. It might seem very sophisticated to do so, but it does not save time and is not quick and efficient, to leave out expressions such as that the later flip generates a photon that might be detected by a photon detector. Rather than saving time and being quick and efficient, ellipsis like that creates confusion and wastes time. To judge from what you have written above, it seems that you like to create confusion for its own sake.Chjoaygame (talk) 07:15, 3 April 2012 (UTC)Chjoaygame (talk) 09:22, 3 April 2012 (UTC)

To be explicit, in response to your comment: A particle detector is a laboratory device into which a beam of a quantum field can be directed, which can be switched on and later switched off, at times determined by the laboratory clock, and which will give a macroscopic registration that can be interpreted as a definite integer number of particles of the quantum field that are considered to be detected during the time the detector was on. An example would be a Geiger counter. Usually particle detectors have a quantum efficiency and a dark current.Chjoaygame (talk) 03:30, 4 April 2012 (UTC)

Well, then, you've just conveniently and I think very artificially defined a "real particle" in terms of what a human artifact of high complexity can interact with. As in no true Scotsman, you'll never be proven wrong. But that means that the difference between real and virtual particles did not exist until this century, when quantum detectors were first built as laboratory devices. That's an instrumentalist view of nature, and I suppose you can make a phiolsophy out of it. However, I don't have to agree with you. It's not a matter of logic, which is what you say, but rather a matter of philosophy. It's a fundamental matter of believing that things exist in nature even if you can't detect them with a lab device, or before anybody could detect them, like faint stars before there were telescopes to see them. Examples in particle physics are quarks and gluons, which you failed to address. Are these quantum things "real"? No lab device has detected them, or even in present theory could. When Gell-Mann proposed and named quarks, in fact, he defined them as mathematical entities and speculated that it would be interesting if they were "real." His colleagues have been mocking him for cowardice ever since, saying he was hedging his bets in wanting to have it both ways, which ever way the experimentalists ultimately decided. But ultimately, the experimentalists decided it didn't make a difference if these things could not be detected by quantum detectors. The math says they are there in some sense, and so they are. SBHarris 02:17, 5 April 2012 (UTC)

Thank you for this response.

Indeed, the way you go, it is a matter of empirically untestable speculative verbigeration, as I see your position.

You attribute to me various views that you would, I think, call philosophy. Indeed it is true that I have philosophical views, but they are not as you seem to impute to me. My position in physics is that I rely on experiment most of all, and that I would analyze the results of experiments, along with an element of creative thinking, to generate theories, that I would use to predict the results of experiments not yet done, and judge the theories accordingly when they are done; then I would do it again. Of course I believe that things exist in nature even if no one is likely to detect them with a laboratory device; what reasonable person wouldn't? But I don't regard conceptual entities of kinds that in principle no one could possibly detect with a laboratory device as physically real as things of kinds that have often been immediately and directly detected with laboratory devices. So, yes, I have a strong empiricist leaning. That's partly why I am interested in physics. I would regard the Pax Romana as real in another sense, though I wouldn't try to detect it immediately and directly with a laboratory device. But the question of whether virtual particles can be detected with a laboratory device calls on a different approach to reality, one that I would call physical as distinct from socio-historical.

Discussions of this sort are philosophical, and I see little point in pursuing them here. I think this article should be about physics.

I have often heard it urged that Galileo was more of a physicist than Aristotle, on the alleged ground that Galileo had much more regard for experiments in physics than did Aristotle. I am inclined to agree with that view, though with reservation. Aristotle was to a significant extent an empiricist, even if perhaps not as much so as Galileo. Aristotle went to some trouble to collect a large museum of natural history specimens and to examine them and think about them carefully, and report his findings. It is true that he did not reach the conclusions that Charles Darwin eventually reached. But, with this in mind, I would still say that Aristotle had a scientific position nearer my own than did Plato, more empirical. Plato is said to have held that his ideal forms were more real than their particular instantiations, and that is the origin of the older (now obsolete) reading of the term 'realist', contrary to the usual present day reading, which is loosely speaking that the real is more or less at least potentially directly empirically detectable. On this spectrum, judging from your comments above, I read you as a Platonist, much less empirically tempered than Aristotle.Chjoaygame (talk) 07:13, 5 April 2012 (UTC)

Well, damnit, do you believe in the "physical reality" of quarks and gluons, or not? ARE THEY PHYSICALLY REAL THINGS? If you think so, then you too are a Platonist sometimes. Just one who won't admit it. I see such hypocrisy in Randroids. Perhaps you lean in this direction? SBHarris 17:30, 5 April 2012 (UTC)

Quarks and nuclear gluons are virtual particles. They are not real in the present sense. If you want to call a photon a variety of gluon then I would say that a photon has more reality than a nuclear gluon. But somewhere here I seem to recall that someone is saying that all photons are virtual particles. This is a different sense of the word virtual. It is true that photons are not particles in the same sense as are electrons; Einstein was right when he said he was taking a heuristic point of view.Chjoaygame (talk) 19:41, 5 April 2012 (UTC)

Note that guarks are massive fermions, like electrons are. They're just undetectable massive fermions. Gluons are more like photons. SBHarris 21:07, 5 April 2012 (UTC)

compare old and new leads

The first paragraph of the lead of 19:06, 21 Feb 2006 read:

"In physics, a virtual particle is a particle-like abstraction used in some models of quantum field theory. Virtual particles exhibit some of the phenomena that real particles do such as conservation of charge. Virtual particles cannot be directly detected, and they do not necessarily respect some of the most fundamental laws associated with physical particles. The concept of virtual particles necessarily arises in the perturbation theory of quantum field theory where interactions between real particles are described in terms of exchanges of virtual particles. Any process involving virtual particles admits a schematic representation known as a Feynman diagram which facilitates understanding of calculations."

This was an informative and useful and accurate and scientifically stated paragraph.

The present first paragraph of the lead reads:

"In physics, a virtual particle is a particle that exists for a limited time and space. The energy and momentum of a virtual particle are uncertain according to the uncertainty principle. The degree of uncertainty of each is inversely proportional to time duration (for energy) or to position span (for momentum)."

This is uninformative or misleading or meaningless waffle.Chjoaygame (talk) 07:48, 4 April 2012 (UTC)

much of this article is unsourced unscientific waffle

Much of this article is unscientific waffle. Much of it is half-baked homespun pseudo-scientific pseudo-philosophy. Most of it is unsourced.

A popularist article or book or internet page is not a reliable source for a relatively difficult area of physics. It is difficult to find reliable sources for scientifically accurate ordinary language accounts of the physics of quantum field theory. This is not a reason for Wikipedia policy violation by allowing unsourced material. It is a reason for deleting unscientific and unsourced material and spending time carefully making scientifically valid and reliably sourced entries in ordinary language.Chjoaygame (talk) 01:26, 5 April 2012 (UTC)

Why no mention of this being related to the source of the big bang?

the new idea of the vacuum, we can speculate on the origin of the biggest thing we know—the universe. Maybe the universe itself sprang into existence out of nothingness—a gigantic vacuum fluctuation which we know today as the big bang. - Heinz Pagels

I myself have thought about this for quite a while, the size of the universe at the point following the big bang was on a comparable scale to the virtual particles we are talking about. Isn't it a well established theory/explanation of the origin of the universe that such a vacuum fluctuation could have caused to universe to 'pop' into existence? I would have thought that including such a paragraph would be extremely important as the origin of the universe is often searched and debated about?

--Allcarwiki (talk) 02:07, 27 April 2012 (UTC)