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

[kexue]

The key idea that is missing is that there is no calculation performed with virtual particles that cannot be performed some other way. They are convenient, but not necessary. As I said before, a good analogy is image charges. They make certain calculations much simpler, but you wouldn't go around trying to collect them with a battery.

When we collide an electron and a positron in an accelerator, you can calculate the scattering result without the corrections coming from virtual particles?

Virtual particles are nothing but quantum field excitations that do not obey energy-momentum relation. Though they never appear in the initial or final condition of the experiment, they are necessary, since they have important impact on the result of the experiment.

As for the perturbative vs. non-perturbative argument, note that there are problems in interpreting non-perturbative processes in terms of particles but this is due to the breakdown of the particle concept itself. For example in a proton, there are perturbatively three quarks. If one shoots high-energy electrons onto the proton on will find this idea quite well confirmed. But that is because the interaction is very small at those energies. If one take lower energies it will become less clear, how the proton is made up of quarks, or whether it is at all because the interaction between the quarks gets to strong. Being somewhat pedantic one could even say that the particle concept does not even work if there is any interaction at all, but to see this is a complicated matter.

Since the words of Frank Wilczek has been disregarded by some here, let me quote for your convenience again Gerad t'Hooft.

Virtual particles have little to do with perturbation expansion. They "are really out there" in the sense that their contribution certainly affects the amplitudes of particle transitions. But all of quantum mechanics is based on "states" that are not usually there in the classical sense. It's just like the two slit experiment. The particle goes through one slit or through the other, while*nevertheless*the behavior afterwards is determined by the fact that there were two slits. Similarly, virtual particles may have been present or absent.
Some scattering events may be entirely due to the exchange of a virtual particle; in that case, it is hard to deny that the particle was there. Sometimes, you don't know whether it was a particle going from A to B, or an antiparticle going from B to A, this happens for instance when charged particles attract or repel one another by the exchange of a photon.

Virtual particles are allowed by the uncertainty principle, though they are not directly observable, they have real and observable effects.

 

[lightarrow]

On-shell virtual particles, like epicycles, are described by the maths as existing in ordinary space-time, but, like epicycles, they're still not real.

Off-shell virtual particles are described by the maths as existing in "momentum space", so from that fact alone they're not real.

Sorry for the naive question but is there a definition of "real" particle?

 

[tiny-tim]

Not that I'm aware of.

 

[pallidin]

Sorry for the naive question but is there a definition of "real" particle?

This is not an answer, rather an opinion:

"Real" is what "virtual" is not.

 

[weejee]

Virtual particles are nothing but quantum field excitations that do not obey energy-momentum relation. Though they never appear in the initial or final condition of the experiment, they are necessary, since they have important impact on the result of the experiment.

No. They are not the excitations themselves. They are really perturbative corrections to 'excitations'.

Of course they have important impact on the result of the experiment.
Even for something as simple as electron-electron scattering, the lowest order contribution to the scattering amplitude is second order in the perturbation theory. We'd better include virtual particles(=perturbative corrections) in the calculation, since otherwise we get zero.

Virtual particles are allowed by the uncertainty principle, though they are not directly observable, they have real and observable effects.

Please tell me how in a free field theory, where both the uncertainty principle and the particle concept are valid and everything is described non-perturbatively, virtual particles can arise. (Actually you can't, since there is no virtual particle in free field theories.)

Furthermore, it is a very loose statement that virtual particles are allowed by the uncertainty principle. Can you give us any quantitative result out of that statement? What are the EQUATIONS corresponding to this statement? For example, can you calculate something like the lifetime of a virtual particle, provided that such concept ever exists?

Virtual particles have little to do with perturbation expansion. They "are really out there" in the sense that their contribution certainly affects the amplitudes of particle transitions. But all of quantum mechanics is based on "states" that are not usually there in the classical sense. It's just like the two slit experiment. The particle goes through one slit or through the other, while*nevertheless*the behavior afterwards is determined by the fact that there were two slits. Similarly, virtual particles may have been present or absent.
Some scattering events may be entirely due to the exchange of a virtual particle; in that case, it is hard to deny that the particle was there. Sometimes, you don't know whether it was a particle going from A to B, or an antiparticle going from B to A, this happens for instance when charged particles attract or repel one another by the exchange of a photon.

How would you reconcile this with Weinberg's statement that virtual particles are mere artifacts? Would you say that Weinberg is wrong?

It is pointless to defend yourself with a bunch of words, which simply aren't able to respond in anyways to what people ask. (e.g. How can virtual particles arise in non-perturbative ...)
I'm sure if t'Hooft were here, he would give reasonable answers to those questions, rather than playing around with some 'wordings'.

 

[PhanthomJay]

This hi-tech stuff is very interesting, but is meaningless to anyone like me unskilled in QM or QFT. Let me ask this...if virtual particles are not 'real' (although 'real' is undefined), are Vacuum Fluctuations (I think it's sometimes called Quantum Foam(?)), 'real' or 'virtual' or 'neither'?

 

[kexue]

Furthermore, it is a very loose statement that virtual particles are allowed by the uncertainty principle. Can you give us any quantitative result out of that statement?

Lamb shift, vacuum polarization, Casimir effect, any transition amplitude calculation in qed, anomalous magnetic moment of the electron, the asymptotic freedom behaviour of QCD

also, forces can be understood as the exchange of virtual particles and because of the spin of these virtual particles, we can beautifully explain why charges repel or attract

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

Because of this, I say they are "real" or "really out there".

(The argument that they do not appear in non-perturbation theory is misleading, they certainly DO appear in non-perturbative calculations, though perhaps in that context one doesn't usually think of them as ``off-shell particles''. In the Feynman path-integral approach, one is certainly instructed to sum over all histories, most of which are off-shell. Also, reread my explanation in post 31 that there are problems in interpreting non-perturbative processes in terms of particles but this is due to the breakdown of the particle concept itself.)

 

[kexue]

Again, my request to Vanadium 50 who claimed that there is no calculation performed with virtual particles that cannot be performed some other way: a collision of an electron and a positron in an accelerator, please calculate the scattering result without the corrections coming from virtual photons.

 

[A. Neumaier]

Sorry for the naive question but is there a definition of "real" particle?

It is generally taken as an elementary system (described by an irreducible representation
of the Poincare group) separated well enough from the environment to be tractable with
creation and annihilation operators (e.g., as an in or out state in scattering).

This separation makes it distinguishable enough from the environment to merit the designation ''particle''. Note that it is only an approximate concept, but a very useful one.
When the separation gets poorer (as during scattering or in many-body contexts),
the notion of a particle becomes less and less useful. In particular, in the solid state,
one has no longer identifiable particles but only so-called quasi-particles. Again their
characteristics is that they are described by (effective) creation and annihilation operators.

On the other hand, there are no creation and annihilation operators for virtual particles, not even in theory. This makes them unreal - they cannot be created or annihilated, not even in theory.
They can only be used to write down Feynman diagrams!

That calculations of perturbative effects involve integrals corresponding to internal lines of Feynman diagrams (which may be interpreted loosely as virtual particles) doesn't make
these virtual particle real. (Nowhere in physics is reality ascribed to diagrams related to mathematical techniques that help one evaluate the terms of a series.)

 

[lightarrow]

Thanks very much for your answer.
Lightarrow.

 

[Vanadium 50]

Again, my request to Vanadium 50 who claimed that there is no calculation performed with virtual particles that cannot be performed some other way

And again, my request to you - stop derailing this thread. As you pointed out, there is already a thread where your unorthodox viewpoint is discussed. And the answer to your question is "use the S-matrix".

PhanthomJay, you really have only two choices. One is to learn QFT (and QM before it), so you can understand what's really happening. The other is to stick with the popularizations, understanding that they are really "lies to children". You will never be able to create a coherent picture of the universe by stringing several of them together, and will have to live with a patchwork of things that are kinda sort of true. That's not the end of the world - we can't be experts in everything, and that's how I get through biology, auto mechanics, etc.

 

[PhanthomJay]

You will never be able to create a coherent picture of the universe by stringing several of them together, and will have to live with a patchwork of things that are kinda sort of true. That's not the end of the world - we can't be experts in everything, and that's how I get through biology, auto mechanics, etc.

Well, OK, I'm going to ask a question about the Origin of the Universe on the Cosmology forum, based on what I've read in Hawking's book, which was the reason for my original questions which were moved to this Forum by the moderators. If I can't get a layman's understanding, I'll give the book away. If ultimately there is an ultimate 'theory of everything' that cannot be explained simply and elegantly to a layman, then I might as well stop seeking answers and accept my Universe for what it is, and not why it is. And go back to believing in a god of creation.

 

[Vanadium 50]

Well, we don't have a "theory of everything" yet, much less one that can be explained.

Popularizations have their uses, but you need to recognize them as what they are: a watered-down, oversimplified collection of what I called "lies to children". It's like a picture of a turkey dinner - it gives you some idea of what all the fuss is about, but you can't eat one.

I took 8 classes in QM and QFT in school, and that just got me to the point where I was capable of it, but bad at it. (At least compared to the people who make their living doing these calculations) Compressing that to a single book is hard to imagine. Now compressing it to a single book that requires no real background. And that's why you can only get a photograph of the turkey dinner.

 

[Boy@n]

Don't Black Holes create particles out of virtual particles?

From Wikipedia:
"Physical insight on the process may be gained by imagining that particle - antiparticle radiation is emitted from just beyond the event horizon . This radiation does not come directly from the black hole itself, but rather is a result of virtual particles being " boosted " by the black hole ' s gravitation into becoming real particles . A slightly more precise , but still much simplified, view of the process is that vacuum fluctuations cause a particle - antiparticle pair to appear close to the event horizon of a black hole . One of the pair falls into the black hole whilst the other escapes . In order to preserve total energy , the particle that fell into the black hole must have had a negative energy ( with respect to an observer far away from the black hole ) . By this process, the black hole loses mass , and , to an outside observer , it would appear that the black hole has just emitted a particle . In another model, the process is a quantum tunneling effect , whereby particle - antiparticle pairs will form from the vacuum , and one will tunnel outside the event horizon."

 

[Boy@n]

Don't Black Holes create particles out of virtual particles?

From Wikipedia:
"Physical insight on the process may be gained by imagining that particle - antiparticle radiation is emitted from just beyond the event horizon . This radiation does not come directly from the black hole itself, but rather is a result of virtual particles being " boosted " by the black hole ' s gravitation into becoming real particles . A slightly more precise , but still much simplified, view of the process is that vacuum fluctuations cause a particle - antiparticle pair to appear close to the event horizon of a black hole . One of the pair falls into the black hole whilst the other escapes . In order to preserve total energy , the particle that fell into the black hole must have had a negative energy ( with respect to an observer far away from the black hole ) . By this process, the black hole loses mass , and , to an outside observer , it would appear that the black hole has just emitted a particle . In another model, the process is a quantum tunneling effect , whereby particle - antiparticle pairs will form from the vacuum , and one will tunnel outside the event horizon."

 

[weejee]

Lamb shift, vacuum polarization, Casimir effect, any transition amplitude calculation in qed, anomalous magnetic moment of the electron, the asymptotic freedom behaviour of QCD

also, forces can be understood as the exchange of virtual particles and because of the spin of these virtual particles, we can beautifully explain why charges repel or attract

You are talking about some results from the perturbation theory(except for Casimir effect), when I'm asking you how one can derive any quantitative result out of the statement that virtual particles arise due to the uncertainty principle. I hope you are not saying that the perturbation theory follows directly from the uncertainty principle.

(The argument that they do not appear in non-perturbation theory is misleading, they certainly DO appear in non-perturbative calculations, though perhaps in that context one doesn't usually think of them as ``off-shell particles''.

Let's set aside the discussion on the non-perturbative treatment of the interaction. I actually asked you on HOW VIRTUAL PARTICLES ARISE IN FREE FIELD THEORIES. What is your opinion?

In the Feynman path-integral approach, one is certainly instructed to sum over all histories, most of which are off-shell.

What exactly are you talking about? There is no notion of 'being on(off)-shell' for a path in the Feynman path-integral. I remember I told you that a 'non-classical path' and a 'off-shell particle' are two totally different concepts.

Most of all, I really want to know you opinion on how virtual particles arise in FREE FIELD THEORIES.

 

[Vanadium 50]

Let's avoid the kexue sidetrack in this thread. He now has two other threads to discuss that, so let's get back to PhanthomJay's question.

 

[weejee]

Let's avoid the kexue sidetrack in this thread. He now has two other threads to discuss that, so let's get back to PhanthomJay's question.

Sorry. I really stop.

 

[kexue]

Phanthomjay, did you find my posts unhelpful and do you feel I derailed "your" thread?

 

[PhysDrew]

... then I might as well stop seeking answers and accept my Universe for what it is, and not why it is. And go back to believing in a god of creation.

Oh no don't say that...

 

[kexue]

No more derailing, back to PhanthomJay's questions!

If virtual particles pop into and out fo existence in less than the blink of an eye,
1. How much less than a blink...less than 10^-43 seconds?
2. Where is this happening now as i speak..in front of my eyes, in my next door neighbor's house, or in places far far away..?
3. If some of these particles created matter, why is no matter being created now...or is it?
4. Is the creation and destruction of virtual particles the long sought perpetual motion machine?

Thanks.

1. every particle has a so-called Compton length (hbar/(Mc)), when the particle propagates over a distance much smaller than hbar/(Mc) (or when it lives for a time much smaller than hbar/(Mc^2) ) it is useful to call it a virtual particle, to stress that we cannot observe it directly as a track left in a detector. Still, it has other physical effects as I have pointed out in earlier posts.

2. it happens now as you speak, everywhere in the universe

3. one of the main underlying ideas of quantum field theory is that the number of particles is not fixed, that particles are constantly come in and of existence as you were correctly told by Steven Hawking book, but to make those 'virtual' particles 'real' we have to introduce some extra energy

4. no, due to their 'fleeting' existence just described

 

[Boy@n]

If virtual particles pop into and out fo existence in less than the blink of an eye,
1. How much less than a blink...less than 10^-43 seconds?
2. Where is this happening now as i speak..in front of my eyes, in my next door neighbor's house, or in places far far away..?
3. If some of these particles created matter, why is no matter being created now...or is it?
4. Is the creation and destruction of virtual particles the long sought perpetual motion machine?

Thanks.

No expert here, but would like to expose my views to potential 'beating' (corrections).

1. Popping in and out of existence has to take less than Plank's time, or else we might observe them in 'our' physical existence.

2. Eveywhere, all the time.

3. Since they always come in pairs of particle and anti-particle they annihilate each other and disappear from our existence in the moment they appear. The way one particle can 'survive' is near Black Hole horizon, where antiparticle enters BH and particle escapes and stays in Universe. Mass of Universe stays the same, since antiparticle which got sucked in BH annihilates with particle inside the BH (reducing BH by one particle). And if BH stays 'hungry' for long enuff it 'dies' from this process, called also Hawking's radiation.

4. There is no surplus of particles/energy, as explained above. Thus no perpetuum-mobile because of this.

 

[PhanthomJay]

Phanthomjay, did you find my posts unhelpful and do you feel I derailed "your" thread?

I found your first 2 posts helpful, then I got lost, through no fault of your own.

No more derailing, back to PhanthomJay's questions!



1. every particle has a so-called Compton length (hbar/(Mc)), when the particle propagates over a distance much smaller than hbar/(Mc) (or when it lives for a time much smaller than hbar/(Mc^2) ) it is useful to call it a virtual particle, to stress that we cannot observe it directly as a track left in a detector. Still, it has other physical effects as I have pointed out in earlier posts.

Thanks, and that value of hbar/Mc^2 is equal to 10^-43 seconds?

2. it happens now as you speak, everywhere in the universe

Thanks, confirming my understanding.

3. one of the main underlying ideas of quantum field theory is that the number of particles is not fixed, that particles are constantly come in and of existence as you were correctly told by Steven Hawking book, but to make those 'virtual' particles 'real' we have to introduce some extra energy

you mean like dark energy or vacuum energy?

4. no, due to their 'fleeting' existence just described

Thanks

No expert here, but would share my layman's views, which might be far from truth...

1. Popping in and out of existence has to take less than Plank's time, or else we might observe them in 'our' physical existence.

2. Eveywhere, all the time.

3. Since they always come in pairs of particle and anti-particle they annihilate each other and disappear from our existence in the moment they appear. The way one particle can 'survive' is near Black Hole horizon, where antiparticle enters BH and particle escapes and stays in Universe. Mass of Universe stays the same, since antiparticle which got sucked in BH annihilates with particle inside the BH (reducing BH by one particle). And if BH stays 'hungry' for long enuff it 'dies' from this process, called also Hawkin's radiation.

4. There is no surplus of particles/energy, as explained above. Thus no perpetuum-mobile because of this..

Thank you for the response

 

[weejee]

1. every particle has a so-called Compton length (hbar/(Mc)), when the particle propagates over a distance much smaller than hbar/(Mc) (or when it lives for a time much smaller than hbar/(Mc^2) ) it is useful to call it a virtual particle, to stress that we cannot observe it directly as a track left in a detector. Still, it has other physical effects as I have pointed out in earlier posts.

I don't think this is a correct view. Actually you've just claimed that all massless particles are virtual.

 

[Vanadium 50]

It's not and he did.

 

[kexue]

I don't think this is a correct view. Actually you've just claimed that all massless particles are virtual.

Well, I want to answer you, but by doing so I hope I do not derail this thread and as a consequence this post could get deleted and I could possibly receive infractation points.

Take an static electric charge. It is surrounded by static electric field which must be thought of as sea of virtual photons. When we move the electric charge (introduce extra energy!), we make more and more 'virtual' photons 'real'.

 

[Deric Boyle]

In this heated disscussion I would just like to say that as virtual particles are a mathematical trick same were photons at the time of Planck to explain blacbody radiations and same was Omega minus at the time of Gell-Mann.
Simply what PhantomJay wants to enquire is that do they(virtual particles) have lifetime enough so as to make physical measurements, this was his question at the first place. And the simple answer is a simple no in present tense,if you strictly follow uncertainity relation, but I don't know about future.

 

[PhanthomJay]

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.

 

[weejee]

Well, I want to answer you, but by doing so I hope I do not derail this thread and as a consequence this post could get deleted and I could possibly receive infractation points.

Take an static electric charge. It is surrounded by static electric field which must be thought of as sea of virtual photons. When we move the electric charge (introduce extra energy!), we make more and more 'virtual' photons 'real'.

Maybe I had to be clearer on what I was saying.

You claimed that any particle that lives shorter than \hbar/mc^2 is considered virtual. My point is that from your claim, the conclusion that any massless particle (m=0) which doesn't live forever is virtual, follows directly, which doesn't make sense.

Since I didn't want to derail this thread either, I wasn't requesting you to elaborate on your theory of virtual particles or trying to refute it as a whole. I was just pointing out one specific error in your answer to OP.

 

[kexue]

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.

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.