Virtual Particles: Creation & Destruction in 10-43 Seconds?

[weejee]

For massive particles, just as I told you: hbar/Mc^2. M is the mass of the particle you have at hand. For massless particles (i.e. the photon or the gluon), this formula does not quite apply as weejee pointed out. Since a Coulomb field drops off with 1/r^2, I assume the lifetime of a virtual photon can be infered from that.

From where does it follow that the lifetime of a virtual particle is hbar/mc^2? The energy-time uncertainty relation? No. It just says, "If an initial quantum state has an uncertainty in its energy around DeltaE, it loses its original shape (i.e. the overlap with the initial state becomes very small) after some time around DeltaT ~ hbar/DeltaE."

For the claim that the lifetime of a virtual photon can be inferred from the 1/r^2 law, I'd say that you can't just assume something out of nowhere.

I didn't say that your claim has problems only for massless particles. It is incorrect for all cases. Please don't interpret what other people say thoroughly in your favor.

This is really muddying the water since non-specialists may actually believe in your claims and get misled.

“He who learns but does not think, is lost. He who thinks but does not learn is in great danger.” - Confucius

 

[Dynamic Sauce]

For massive particles, just as I told you: hbar/Mc^2. M is the mass of the particle you have at hand. For massless particles (i.e. the photon or the gluon), this formula does not quite apply as weejee pointed out. Since a Coulomb field drops off with 1/r^2, I assume the lifetime of a virtual photon can be infered from that. For the gluon field, things are trickier due to confinement. It is larger in both cases than 10^-43 seconds.

Note again PhanthomJay, we can not, due to the uncertainty principle, directly observe 'virtual' particles, hence the word virtual.
We can only observe their effects on 'real' particles. Some would say only the effects are real and 'virtual' particles are only convenient language. Others would attribute to them a part of reality. It is really more or less a matter of taste as in the Wilczek quote explained.

Unfortunately, I do not know anything about curled up higher dimensions of spacetime, so I can't answer the last part of your question.

Kexue,so it's a matter of taste whether they are real or not? are there people who think virtual particles aren't real at all?

 

[jtbell]

are there people who think virtual particles aren't real at all?

Yes. See this thread:

https://www.physicsforums.com/showthread.php?t=75307

 

[Born2bwire]

i just want to know what the lifetime is of a virtual particle, if such a lifetime can be calculated for a virtual particle or pair of particles. And I'd also like to know whether Vacuum Fluctuations, on the assumption that such fluctuations are 'real' and not 'mathematical' , occur in the curled up higher order dimensions of spacetime, within the confines of the Planck time and Planck length.

As to the physicality of vacuum fluctuations, I do not think that it has been proven. Vacuum fluctuations can be used to derive a number of phenomenon like the Lamb shift and Casimir force. However, like I stated previously, you can also derive these phenomena without the vacuum theory. So there are strong indicators that the vacuum fluctuations have physical consequences but I have not heard that it has been conclusively proven that they are the actual physical perpetuators.

If you want to learn more about the vacuum, Peter Milonni has a very good book called "The Quantum Vacuum" where he goes through and derives a large amount of QED from the quantum vacuum theory. It's a good book but keep in mind that the quantum vacuum is just one of many aspects to QED. Also take a look at R. L. Jaffe's paper called "Casimir effect and the quantum vacuum" where he makes a few comments about the question on the reality of the vacuum fluctuations (and he demonstrates a vacuum-less approach to the Casimir force). If you want to go the opposite direction from Milonni, take a look at Schwinger who tried to reformulate QED without a quantum vacuum in his effective action formulation. He put out a series of papers about his theory and a series of books called "Particles, Sources and Fields" but his work is rather inaccessible, in my opinion, if you do not already know QED (and my recollection is that his approach is not a complete theory). Milonni is a good book as long as you know non-relativistic quantum mechanics. He steps you through the basics of quantum field theory that is needed for his vacuum approach.

 

[kexue]

well, again, the point is what is meant by "real". We describe our world with the help of theoretical entites. Which ones should be called "real"?

To give a specific example: are quarks "real"? They enter the common theoretical description of the strong interactions in QCD, perturbatively and non-perturbatively. But they are not physical particle states in the Hilbert space of the theory. It is even possible to describe the theory of strong interactions without using quarks (integrating out the quarks fields, getting a non-local effective action for hadrons). So it depends on the theoretical scheme whether it contains quarks or not. So, are they real?

You can even formulate QED without photons, I somewhere heard. (Gossip, exuse me!) So, are photons "real"? What is 'real'? Read the Pollitzer quote in the thread jtbell two posts earlier provided a link to.

A more precise scientific question would be: are there measurable physical phenomena that we can explain quantitatively using the concept of virtual particles? The answer to this is a definite yes.

Also it can be asked: do many physicists would say it is 'real' as it is making a real effects in experiments? Or, would many physicists say they are more than a mathematical artifact and 'lies to children' and and a watered-down explanation in populariazation? The answer to this is a definite yes.

Unfortunately, PF with its mentors and science advisors fails repeatedly to point that out. This what I have a problem with.

 

[sheaf]

You can even formulate QED without photons, I somewhere heard. (Gossip, exuse me!) So, are photons "real"? What is 'real'? Read the Pollitzer quote in the thread jtbell two posts earlier provided a link to.

I suspect here you're referring to the suggestion of doing QED in the Coulomb gauge ? It's my understanding (maybe a QFT expert could confirm ?) that there is no implication from this that there can't be photons in the in and out states, merely that by using the Coulomb you can swap the picture where you describe, say, the Coulomb interaction in terms of virtual photon exchange for one where the virtual photons are not needed.

 

[weejee]

Also it can be asked: do many physicists would say it is 'real' as it is making a real effects in experiments? Or, would many physicists say they are more than a mathematical artifact and 'lies to children' and and a watered-down explanation in populariazation? The answer to this is a definite yes.
Unfortunately, PF with its mentors and science advisors fails repeatedly to point that out. This what I have a problem with.

Why did all of us have problems with you, but not quite with those physicists who appeared to share the same view as you?

The reason is very simple. It is that they said nothing really wrong, although possibly misleadingly phrased. Those physicists of course knew what they were talking about - how they are described in precise forms - terribly well. None of the physicists played around with some words plus their imaginations to draw blatantly wrong conclusions like your theory on the lifetime of virtual particles, while you repeatedy did so.

 

[tiny-tim]

kexue, are you saying that the on-shell virtual particles (in ordinary space-time) are real, or that the off-shell virtual particles (in "momentum space") are real?

(because they can't both be real, can they? there can't be two different "seas" of virtual particles which both participate in every interaction? )

 

[kexue]

All have a problem with me? Where and when did I play only with words? Pretty strong claims that you should better back up.

As far as my "theory of lifetime of virtual particles" is concerned, I got that from a Professor who wrote a leading book on quantum field theory.

Your question somewhere earlier, where do virtual particles arise in free field theory? Nowhere. A photon that never interacts, that is nowhere emited and absorbed is indeed a 'real' particle.
But every field (and it associated particle) we probe in an experiment is no free field anymore!

Also, which you won't believe me, 'virtual' particle do appear (implicitly) in non-perturbation theory. I learned that from many replies I received. One explanation, I gave to you in post 37.

My view is that of Frank Wilczek, no more no less, which will qoute again and for the last the time for your convenience.

It comes down to what you mean by "really there". * When we use a concept with great success and precision to describe empirical observations, I'm inclined to include that concept in my inventory of reality. * *By that standard, virtual particles qualify. * *On the other hand, the very meaning of "virtual" is that they (i.e., virtual particles) don't appear *directly* in experimental apparatus. * Of course, they do appear when you allow yourself a very little boldness in interpreting observations. * It comes down to a matter of taste how you express the objective situation in ordinary language, since ordinary language was not designed to deal with the surprising discoveries of modern physics.*

Weejee, Vanadium 50, Tiny Tim, Nismaratwork, whoelse, do you disagree with him?

Why is that view not pointed out to learners coming to PF? But instead they get told, it is a simple issue: they are only fiction, just artefact of mathematics, stories to children, like the theory of epicycles. Again, this is my problem with many threads in this forum.

 

[A. Neumaier]

i just want to know what the lifetime is of a virtual particle, if such a lifetime can be calculated for a virtual particle or pair of particles.

Lifetimes are defined in terms of the imaginary part of the sigenenergy E of an unstable particle, given by a solution to the Schroedinger equation H psi = E psi in rigged Hilbert space. Virtual particles don't have associated wave functions, hence cannot satisfy such an equation, and therefore have no life-time in any meaningful sense.

Numbers quoted at various places are figments of the imagination only, nowhere derived from proper definitions.

And I'd also like to know whether Vacuum Fluctuations, on the assumption that such fluctuations are 'real' and not 'mathematical' , occur in the curled up higher order dimensions of spacetime, within the confines of the Planck time and Planck length.

The vacuum fluctuations involved in calculations of the Casimir force are in the usual 4 dimensions of space-time.

 

[A. Neumaier]

I suspect here you're referring to the suggestion of doing QED in the Coulomb gauge ? It's my understanding (maybe a QFT expert could confirm ?) that there is no implication from this that there can't be photons in the in and out states, merely that by using the Coulomb you can swap the picture where you describe, say, the Coulomb interaction in terms of virtual photon exchange for one where the virtual photons are not needed.

Even QED in the Coulomb gauge (done, e.g., in the well-known book by Bjorken and Drell)
needs photons.

 

[Dav333]

what about the talk of VP in the documentary of BBCs Atom, the Illusion of Reality?

 

[tom.stoer]

It's my understanding (maybe a QFT expert could confirm ?) ... that ... you can swap the picture where you describe, say, the Coulomb interaction in terms of virtual photon exchange for one where the virtual photons are not needed.

Exactly!

 

[tom.stoer]

Even QED in the Coulomb gauge ... needs photons.

Agreed ...

... but this proves that virtual photons are gauge-dependend objects. Look at QCD a) in Coulomb gauge and b) in axial gauge. In (a) you have ghost (always virtual), whereas (b) is ghost-free. So how could ghosts "exist" if their existence depends on the gauge? As ghosts do exist to cancel certain contributions of virtual partucles how can these virtual particles be "real"?

 

[A. Neumaier]

Agreed ...

... but this proves that virtual photons are gauge-dependend objects. Look at QCD a) in Coulomb gauge and b) in axial gauge. In (a) you have ghost (always virtual), whereas (b) is ghost-free. So how could ghosts "exist" if their existence depends on the gauge? As ghosts do exist to cancel certain contributions of virtual partucles how can these virtual particles be "real"?

Yes. It is even worse. The virtual photons are completely different in the instant form (''old-fashioned perturbation theory''), the covariant form (the modern textbook formalism) and the front form (light cone formalism), and completely disappear from the picture in lattice QED.

But people like kexue have a very liberal view, in which reality looks very different depending on who describes it, and how. That's why it can be determined simply by asking people about their views via email and discarding dissenting answers (such as Weinberg's). Objective arguments don't count in such a view of reality.

This is not the reality physicists are studying, though.

 

[tom.stoer]

But people like kexue have a very liberal view, in which reality looks very different depending on who describes it, and how. That's why it can be determined simply by asking people about their views via email and discarding dissenting answers (such as Weinberg's). Objective arguments don't count in such a view of reality.

Agreed again!

If you like - and if you want to waste your time - you can check other threads where I made the same experience with him :-)

 

[PhanthomJay]

As to the physicality of vacuum fluctuations, I do not think that it has been proven. Vacuum fluctuations can be used to derive a number of phenomenon like the Lamb shift and Casimir force. However, like I stated previously, you can also derive these phenomena without the vacuum theory. So there are strong indicators that the vacuum fluctuations have physical consequences but I have not heard that it has been conclusively proven that they are the actual physical perpetuators.

Thank you for the response and references!

 

[PhanthomJay]

Lifetimes are defined in terms of the imaginary part of the sigenenergy E of an unstable particle, given by a solution to the Schroedinger equation H psi = E psi in rigged Hilbert space. Virtual particles don't have associated wave functions, hence cannot satisfy such an equation, and therefore have no life-time in any meaningful sense.

Numbers quoted at various places are figments of the imagination only, nowhere derived from proper definitions.



The vacuum fluctuations involved in calculations of the Casimir force are in the usual 4 dimensions of space-time.

Thanks for the response. I found this explanation which is a bit simpler to understand. Whether it is correct is something else:

http://www.angelfire.com/ga/dracodraconis/physics.html

But in that article, it talks about virtual electrons and positrons, the real counterparts of which have mass. It also also talks about the virtual photon, the real counterpart of which has no mass. The synopsis, however, is silent on the existence of other virtual particles like virtual gluons, virtual W and Z bosons, and the postulated virtual graviton. I assume that the mathematics of virtuality covers these particles as well?? Does it cover protons? atoms?

Now here's what I understand also...a photon can interact with the quantum field to produce the virtual electron and virtual positron, which annihilate each other in less than 10^-43 seconds. Then it goes on to say, that sometimes a photon does not have to interact with the Field, and virtual particles can be created spontaneously. That seems to make them real.

Am I being fed more lies?

 

[Born2bwire]

Thanks for the response. I found this explanation which is a bit simpler to understand. Whether it is correct is something else:

http://www.angelfire.com/ga/dracodraconis/physics.html

But in that article, it talks about virtual electrons and positrons, the real counterparts of which have mass. It also also talks about the virtual photon, the real counterpart of which has no mass. The synopsis, however, is silent on the existence of other virtual particles like virtual gluons, virtual W and Z bosons, and the postulated virtual graviton. I assume that the mathematics of virtuality covers these particles as well?? Does it cover protons? atoms?

Now here's what I understand also...a photon can interact with the quantum field to produce the virtual electron and virtual positron, which annihilate each other in less than 10^-43 seconds. Then it goes on to say, that sometimes a photon does not have to interact with the Field, and virtual particles can be created spontaneously. That seems to make them real.

Am I being fed more lies?

I have not heard of any lifetimes being associated with virtual particles. You can read about physical analogues that describe the process of a virtual particle. That is, from the Heisenberg Uncertainty Principle, over a very very small amount of time we can observe a large bandwidth of possible energies. If these energies are high enough, you can spontaneously pull out a particle or pair of particles (whatever be appropriate) from the vacuum but since the energy is just a fluctuation these particles must be quickly annihilated. So you can think of these particles popping in and out of the vacuum all the time and thus some may or may not play a role in the interactions. But the big caveat in all this is that these particles are never observed.

So take your vacuum fluctuations of the electromagnetic fields. If we are in the vacuum state, there are no photons but virtual photons pop in and out to create the fluctuations of the electromagnetic field. Still, we will never observe these photons under this theory. A real particle is different. We can detect individual photons, electrons, buckyballs and so forth. But we will not detect a virtual particle.

I don't think it is very useful to think of virtual particles having a lifetime. Because if we were to ascribe a lifetime for them, then we could define a bandwidth of energies to be observed over this lifetime via the Heisenberg Uncertainty Principle. But the theories of vacuum fluctuations ascribe the vacuum to have a zero point energy of infinite bandwidth. Now proof of the zero point energy is even less certain than the vacuum fluctuations themselves because, from the theories that I have read that have dealt with it, we can always renormalize the zero point energy without consequence. It is the change in the zero point energy that is of concern and which produces such effects like the Casimir effect.

You might be able to get away with the vacuum particles being produced only over a finite bandwidth. The vacuum phenonenon that I am familiar with assume that interactions at higher frequencies become negligible and can be ignored (thus we truncate the bandwidth of the problem). But I would think that the restrictions on the energy bandwidth by a given lifetime would cause the lower energies to be more favorable and these vacuum phenomena assume no such weighting to the vacuum energies. For the most part, like I said above, I have not heard of lifetimes being discussed when it comes to virtual particles.

 

[Deric Boyle]

weejee and kexue please don't derail the thread as it had put PhantomJay in trouble.
I think tom.stoer, A.Neumaier are the ones who can guide us; as I've heard from other PF member that they are QFT QCD QED giants.

 

[tom.stoer]

I certainly was a QCD expert; but NEVER a giant.

 

[A. Neumaier]

Thanks for the response. I found this explanation which is a bit simpler to understand. Whether it is correct is something else:

http://www.angelfire.com/ga/dracodraconis/physics.html

It starts off with ''The preceding paragraph was taken from a mock-science journal known as "THE MOCK SCIENCE JOURNAL OF UNTRUE PHYSICS" and as such is totally untrue.''
which should make you already suspicious...

Accounts of virtual particles for the laymen are almost always unreliable when you want to dig deeper. They are meaningful only in a very roundabout fashion, when you don't ask about more precision of what the concepts mean.

But in that article, it talks about virtual electrons and positrons, the real counterparts of which have mass. It also also talks about the virtual photon, the real counterpart of which has no mass. The synopsis, however, is silent on the existence of other virtual particles like virtual gluons, virtual W and Z bosons, and the postulated virtual graviton. I assume that the mathematics of virtuality covers these particles as well??

Yes, since these are the particles corresponding to elementary fields in the standard model and quantum gravity. But they are as unreal as virtual photons, so let us stick to the latter.

Does it cover protons? atoms?

No. These don't occur as internal lines of Feynman diagrams, hence are no virtual particles according to the standard definitions in QFT. (But layman's sources
associate the notion of virtual particles also with other things, such as unstable particles...)

Now here's what I understand also...a photon can interact with the quantum field to produce the virtual electron and virtual positron, which annihilate each other in less than 10^-43 seconds. Then it goes on to say, that sometimes a photon does not have to interact with the Field, and virtual particles can be created spontaneously. That seems to make them real.

Am I being fed more lies?

I wouldn't call it lies but superficial illustrations for lay people.

But the talk about annihilation in10^-43 seconds, or spontaneous creation is meaningless on a more formal level - one cannot translate it into formulas. This would require annihilation and creation operators for virtual particles, which don't exist.

Stuff that is real (according to high standards of reality) always can be modeled on a mathematical level - one can consistently translate all statements in ordinary language into meaningful statements on the level of formulas. This makes electrons, electromagnetic fields, black holes real. For virtual particles, however, you cannot even write down a wave function, let alone calculate a creation probability or a decay time
(things that are related to S-matrix properties, which relate only real particles).

 

[A. Neumaier]

Where and when did I play only with words?

That you only play with words, playing back without real understanding what you read in various sources, can be seen when one tries to give your words more substance. It is impossible, proving the emptiness of your talk.

Try to write down a state vector containing a virtual particle. It is impossible - a physical Hilbert space does not contain such states. But one can easily write down state vectors for the usual, real objects, such as quarks, nuclei, electrons, or photons.

Try to substantiate any of your other statements by backing them up with formulas that match the contents in a verifiable way. This gives you a powerful test for which of your words have substance and which ones are empty.

Virtual particles live only in the thin air created for those who cannot handle a more technical account.

For the experts, the only reality of virtual particles is as internal lines of Feynman diagrams. Here they stand for certain propagators to be integrated over - not for objects that, in real time, pop in and out of existence. That's only the visualization for making the subject sort of intelligible to the non-experts.

 

[tom.stoer]

the only reality of virtual particles is as internal lines of Feynman diagrams. Here they stand for certain propagators to be integrated over - not for objects that, in real time, pop in and out of existence. That's only the visualization for making the subject sort of intelligible to the non-experts.

Agreed! So one could say that the invention of diagrams and lines to draw virtual particles w/o the command to calculate something was the biggest mistake Feynman ever made :-)

 

[kexue]

A.Neumaier, this is still physics, not mathematics. As they like to say in the theoretical physics community, too much rigor soon leads to rigor mortis. Indeed, as Feynman would tell students, if this ain’t rigorous enough for you the math department is just one building over.

On reality in physics, read the Pollitzer quote in the other thread (where I wasted so much of Tom's time).

What a physicist cares about is whether the model matches observation. Quantum field theory, with its virtual particles, matches observation very very well.

Because of this, many, many physicists allow themselves a very little boldness and say that virtual particles are "really out there". "Really out there" in the sense that their contribution certainly affects the amplitudes of particle transitions. "Really out there" in the sense that they represent some real effects, namely a transfer of energy and momentum via fields. "Really out there" in the sense that they are part of a mathematical description of reality, and therefore must correspond to some feature of reality.

But enough of my word play without substance. I'm still waiting on your thoughts why Frank Wilczek would say in http://arxiv.org/PS_cache/hep-th/pdf/9803/9803075v2.pdf" on page 3 that the association of forces with the excange of 'virtual' particles is a general feature of quantum field theory.

Do you really think he is not an expert? That he can not handle a more technical account?

 

[PhanthomJay]

Stuff that is real (according to high standards of reality) always can be modeled on a mathematical level - one can consistently translate all statements in ordinary language into meaningful statements on the level of formulas. This makes electrons, electromagnetic fields, black holes real. For virtual particles, however, you cannot even write down a wave function, let alone calculate a creation probability or a decay time
(things that are related to S-matrix properties, which relate only real particles).

Thank you very much for the response.

I am becoming more and more convinced that the secret of the Universe may only be unlocked by discovering the secrets of the miniscule quantum dimensions and properties of spacetime. The secret of the Zero, I call it. Speculation, of course, but do you agree?

 

[weejee]

Where and when did I play only with words? Pretty strong claims that you should better back up.

Where and when did you 'not' play with words? Well, maybe I'm being a little bit unfair here, since for something like the 'realness of virtual particles', there is not much to do with quantitative stuff (and therefore it is a dangerous realm where it is hard to distinguish between physics and mere words).

I used strong words because when I didn't, you seemed to interpret what I said too much in your favor. I realize that using strong words doesn't help much either, since it seems to only make you angry.

As far as my "theory of lifetime of virtual particles" is concerned, I got that from a Professor who wrote a leading book on quantum field theory.

Whoever said this, it doesn't make sense. Maybe you are misinterpreting what the professor actually said.

Your question somewhere earlier, where do virtual particles arise in free field theory? Nowhere. A photon that never interacts, that is nowhere emited and absorbed is indeed a 'real' particle.
But every field (and it associated particle) we probe in an experiment is no free field anymore!

Right. However, my point was that according to your claim that virtual particles are allowed by the uncertainty principle, there is no reason for them not to appear in free field theories.

Also, which you won't believe me, 'virtual' particle do appear (implicitly) in non-perturbation theory. I learned that from many replies I received. One explanation, I gave to you in post 37.

Yes they do, as long as we adhere to start from free particles and treat the interaction as a correction to them. Yet this just means that we are pushing the limit of the perturbation theory into the regime where it doesn't really hold, although it isn't necessarily true that such attempt is totally meaningless.
Still, if we somehow exactly solve the problem, that is, without treating the interaction as a correction to free particles, no virtual particle should appear.

My view is that of Frank Wilczek, no more no less, which will qoute again and for the last the time for your convenience.

It comes down to what you mean by "really there". * When we use a concept with great success and precision to describe empirical observations, I'm inclined to include that concept in my inventory of reality. * *By that standard, virtual particles qualify. * *On the other hand, the very meaning of "virtual" is that they (i.e., virtual particles) don't appear *directly* in experimental apparatus. * Of course, they do appear when you allow yourself a very little boldness in interpreting observations. * It comes down to a matter of taste how you express the objective situation in ordinary language, since ordinary language was not designed to deal with the surprising discoveries of modern physics.*

He is just talking about how he would describe certain things in ordinary language, presumably because your original question is formulated in terms of ordinary language.

I think that what he said is acceptable, but I also think it might be misleading.

The thing is, he didn't talk about anything like the lifetime of virtual particles or mix up between 'being virtual' and 'being non-classical'. If you are really saying that what you've been claiming is no more or no less to what Frank Wilczek said, it is very confusing to me.Let me tell you just one more thing. I think that people use the term 'fluctuation' in the quantum field theory to describe two different things.

1. Fluctuation from the classical solution: This corresponds to non-classical paths in the path integral.

2. Virtual particles that "pop out from the vacuum": I would say that in a formal sense, this just means the difference between the interacting vacuum(=free vacuum + various particle states * small coefficients) and the free vacuum.

I highly suspect that you are mixing up these two.

p.s. No more derailing. I quit arguing with kexue.

 

[Dynamic Sauce]

What Does it mean when virtual particles travel "Backwards in time" does it mean literally?

 

[Dynamic Sauce]

I certainly was a QCD expert; but NEVER a giant.

I also see that virtual particles travel faster than c?

 

[A. Neumaier]

Agreed! So one could say that the invention of diagrams and lines to draw virtual particles w/o the command to calculate something was the biggest mistake Feynman ever made :-)

No. The diagrams were actually the device that made his approach intelligible and superior to the approaches by Tomonaga and by Schwinger.

The mistake was to sell the internal lines to the public as ''virtual particles''. This interpretation is not needed for the working of QFT and had done more damage than good.