Debunking the Existence and Duration of Virtual Particles

[Goldstone1]

No quantum field theory relies on virtual particles. And regarding the Dirac equation, virtual particles have got nothing to do with it.

Decouple the dirac equation into left movers and right movers, and then the theory asks where the positron comes from when it is created. It is effectively a hole in the sea, once a virtual particle. The Dirac Equation does involve the ''mathematical'' concept of virtual particles.

 

[Polyrhythmic]

Decouple the dirac equation into left movers and right movers, and then the theory asks where the positron comes from when it is created. It is effectively a hole in the sea, once a virtual particle. The Dirac Equation does involve the ''mathematical'' concept of virtual particles.

This is just not true. The Dirac equation describes fermionic particles and their anti-particles. The hole interpretation is outdated, and virtual particles have got nothing to do with all this.

 

[mattt]

You can compute the lamb shift and anomalous magnetic moment non-perturbatively. It has nothing to do with virtual particles.

 

[Lapidus]

It's you who should rewatch it. What he shows is that even if they existed, they were not detectable. But since they are only a mathematical construct anyways, this is just additional info.

Poly, they are by definition not directly detectable. Hence the name virtual.

But, as Prof. Susskind explains, or Prof. Randall, or anybody else with some very basic understanding of quantum physics will tell you that the time-uncertainty relation allows undetectable states/ particles to exist for short times.

And no perturbation theory mentioned.

In addition, they are necessary to turn up in the calculations to make the probabilty amplitudes for the measurable states come out right.

And yes, again my questions, if someone cares to answer in this very busy thread:

How do we know that 'virtual' states/ processes do not appear in non-perturbative qft?
(For example, what about all the off-shell histories in the path integral)

What physical mechanism actually forbids 'virtual' particles/ processes from happening, processes which are perfectly allowed by the laws of quantum physics?

How do you explain quantum physical static forces, such as the Coulomb force, without virtual particles?

 

[maverick_starstrider]

Decouple the dirac equation into left movers and right movers, and then the theory asks where the positron comes from when it is created. It is effectively a hole in the sea, once a virtual particle. The Dirac Equation does involve the ''mathematical'' concept of virtual particles.

Virtual particles only appear in QFT when you essentially expand in a taylor series. If you DON'T need to expand in a taylor series they never show up. If we could solve all these integrals directly... they would never show up. They are solely an artifact of a specific APPROXIMATION scheme, of which there are known phenomena that cannot be described within this scheme. As I said, look at the equation for the first-order correction in non-relativistic (i.e. regular quantum) perturbation. It looks like this

$$\sum_{n \neq 0} \frac{\langle n \vert V \vert 0 \rangle}{E_n - E_0}$$

the

$$\langle n \vert V \vert 0 \rangle$$

COULD be interpreted as a kind of PROPAGATOR or TRANSITION AMPLITUDE. Saying that virtual particles are real is EXACTLY like saying that in reality this system ACTUALLY IS undergoing every possible transition to every higher-state through this funny, not quite right, propagation. No one thinks this way of course, and if it's not true in QM then why should it be true in QFT?

Let's be absolutely clear here. An expansion in Feynman Diagrams (i.e. using virtual particles) IS a Taylor series expansion of a non-gaussian integral. That's what it is. The little pictures help you keep the ordering of your Wick's Expansion straight. It's a visual tool to help you order your terms right.

 

[Goldstone1]

This is just not true. The Dirac equation describes fermionic particles and their anti-particles. The hole interpretation is outdated, and virtual particles have got nothing to do with all this.

Well I beg to differ... the Dirac sea is a reseviour of negative virtual energy states... have you done any work on this side of the theory?

 

[Goldstone1]

Virtual particles only appear in QFT when you essentially expand in a taylor series. If you DON'T need to expand in a taylor series they never show up. If we could solve all these integrals directly... they would never show up. They are solely an artifact of a specific APPROXIMATION scheme, of which there are known phenomena that cannot be described within this scheme. As I said, look at the equation for the first-order correction in non-relativistic (i.e. regular quantum) perturbation. It looks like this

$$\sum_{n \neq 0} \frac{\langle n \vert V \vert 0 \rangle}{E_n - E_0}$$

the

$$\langle n \vert V \vert 0 \rangle$$

COULD be interpreted as a kind of PROPAGATOR or TRANSITION AMPLITUDE. Saying that virtual particles are real is EXACTLY like saying that in reality this system ACTUALLY IS undergoing every possible transition to every higher-state through this funny, not quite right, propagation. No one thinks this way of course, and if it's not true in QM then why should it be true in QFT?

If the energy shift is measured, it is real. End of. Virtual particles are objects which have taken this identity.

 

[Goldstone1]

As Lapidus explains:

''But, as Prof. Susskind explains, or Prof. Randall, or anybody else with some very basic understanding of quantum physics will tell you that the time-uncertainty relation allows undetectable states/ particles to exist for short times. ''

This is what this energy shift is. It is a detectable difference in energy by a quantum which cannot be detected. It is is real though, because it leaves real physical effects in the world.

 

[tiny-tim]

Decouple the dirac equation into left movers and right movers, and then the theory asks where the positron comes from when it is created. It is effectively a hole in the sea, once a virtual particle. The Dirac Equation does involve the ''mathematical'' concept of virtual particles.

Goldstone1, this is complete rubbish

Even Dirac himself declared in print that the "sea" and "hole" idea should be abandoned.

(and holes never had anything to do with virtual particles, anyway … holes came in with the Dirac equation in 1928(?), and went out again when virtual particles came in)

hmmm … I'll guess that you've been looking at http://en.wikipedia.org/wiki/Virtual_particles#History" …

Paul Dirac was the first to propose that empty space (a vacuum) can be visualized as consisting of a sea of virtual electron-positron pairs, known as the Dirac sea.​

… this is simply historically wrong …

the Dirac sea was a "sea" of infinitely many electrons, and real positrons were holes in the sea: there were no virtual positrons, and no electron-positron pairs at all

And (for what it's worth) the historical article http://en.wikipedia.org/wiki/Dirac_sea" makes no mention of virtual particles! ​

 

[Lapidus]

Virtual particles only appear in QFT when you essentially expand in a taylor series.

Since I assume you are not an expert on non-perturbative quantum field theory that must just be a wild claim.

Why should there be no virtual particles or processes in non-perturbative qft? How do you know?

 

[Polyrhythmic]

Poly, they are by definition not directly detectable. Hence the name virtual.

And something which is neither directly detectable nor needed for any explanations shouldn't be considered real.

And no perturbation theory mentioned.

Because that was implicitly assumed in the lectures. Feynman diagrams or virtual particles can only be talked about in the context of perturbation theory.

In addition, they are necessary to turn up in the calculations to make the probabilty amplitudes for the measurable states come out right.

This is plain and simply incorrect. The terms required for the correct calculations are there, whether you interpret them as virtual particles is an entirely different question.

 

[Polyrhythmic]

have you done any work on this side of the theory?

I have studied the Dirac equation and and never came across any virtual particles.

 

[Polyrhythmic]

If the energy shift is measured, it is real. End of. Virtual particles are objects which have taken this identity.

That just makes no sense.

 

[Goldstone1]

Goldstone1, this is complete rubbish

Even Dirac himself declared in print that the "sea" and "hole" idea should be abandoned.

(and holes never had anything to do with virtual particles, anyway … holes came in with the Dirac equation in 1928(?), and went out again when virtual particles came in)

hmmm … I'll guess that you've been looking at http://en.wikipedia.org/wiki/Virtual_particles#History" …

Paul Dirac was the first to propose that empty space (a vacuum) can be visualized as consisting of a sea of virtual electron-positron pairs, known as the Dirac sea.​

… this is simply historically wrong …

the Dirac sea was a "sea" of infinitely many electrons, and real positrons were holes in the sea: there were no virtual positrons, and no electron-positron pairs at all

And (for what it's worth) the historical article http://en.wikipedia.org/wiki/Dirac_sea" makes no mention of virtual particles! ​

Well, no... I've not been reading anything of lately. I just recall the class which I took which was an introductory course in QFT and it spoke of the Dirac Sea - it has been replaced, but the general idea of particles being ''embedded'' within the vacuum is still preserved - QFT calls them Virtual particles. I don't know how much into the history you seem to understand, all I know is that QFT predicts virtual particles to be throthing at the subatomic level, and that is all I really wish to know. As for Diracs Equation, I can say that in particle creation, a positron must be created from a virtual entangled particle in the vacuum to the corresponding electron.

 

[Goldstone1]

That just makes no sense.

As Lapidus explains:

''But, as Prof. Susskind explains, or Prof. Randall, or anybody else with some very basic understanding of quantum physics will tell you that the time-uncertainty relation allows undetectable states/ particles to exist for short times. ''

This is what this energy shift is. It is a detectable difference in energy by a quantum which cannot be detected. It is is real though, because it leaves real physical effects in the world.

 

[maverick_starstrider]

Since I assume you are not an expert on non-perturbative quantum field theory that must just be a wild claim.

Why should there be no virtual particles or processes in non-perturbative qft? How do you know?

I don't understand what you're saying. Am I crazy? Virtual particles as a concept and as a calculational tool have absolutely no existence outside of perturbation of quantum field theories in feynman diagrams. In non-perturbative qft the concept of virtual particles simply doesn't exist.

 

[Polyrhythmic]

Well, no... I've not been reading anything of lately. I just recall the class which I took which was an introductory course in QFT and it spoke of the Dirac Sea - it has been replaced, but the general idea of particles being ''embedded'' within the vacuum is still preserved - QFT calls them Virtual particles. I don't know how much into the history you seem to understand, all I know is that QFT predicts virtual particles to be throthing at the subatomic level, and that is all I really wish to know. As for Diracs Equation, I can say that in particle creation, a positron must be created from a virtual entangled particle in the vacuum to the corresponding electron.

If that's what you heard in your QFT course, I can only recommend you to retake it somewhere else.

 

[maverick_starstrider]

As Lapidus explains:

''But, as Prof. Susskind explains, or Prof. Randall, or anybody else with some very basic understanding of quantum physics will tell you that the time-uncertainty relation allows undetectable states/ particles to exist for short times. ''

This is what this energy shift is. It is a detectable difference in energy by a quantum which cannot be detected. It is is real though, because it leaves real physical effects in the world.

There is a world of difference between USING Feynman Diagrams to CALCULATE what the magnitude of the shift and saying that the shift is DUE to the Feynman Diagrams. Once again I keep bringing it back to perturbation in regular QM and you consistently ignore the point. Perturbation is not seen as real QM, why should it be real in QFT?

 

[Polyrhythmic]

As Lapidus explains:

''But, as Prof. Susskind explains, or Prof. Randall, or anybody else with some very basic understanding of quantum physics will tell you that the time-uncertainty relation allows undetectable states/ particles to exist for short times. ''

This is what this energy shift is. It is a detectable difference in energy by a quantum which cannot be detected. It is is real though, because it leaves real physical effects in the world.

No. All physical effects you mention can be calculated without assuming that virtual particles have any physical significance.

 

[mattt]

Virtual particles are gauge-dependent: different perturbation schemes use different kind of virtual particles that have nothing to do one to another.

In Lattice QED (or in general, Lattice QFT) you can compute everything (in principle, assuming enough time and superfast computers).

The S-Matrix is what gives the answers of the Theory. The different ways of computing its elements (perturbatively, non-perturbatively) by means of different kind of approximations, are just that, calculations techniques.

 

[dm4b]

I'd love to see a good explanation of this still too.

More generally, what is the mechanism behind the electromagnetic force.

The popular view is two electrons (or some other charged particle) exchanging virtual photons, which mediate the message. But, what is the correct way to view this, without the use of fictitious entities such as virtual particles?

I've never seen this explained well.

Bump. Anybody? ;-)

 

[Polyrhythmic]

Bump. Anybody? ;-)

You can view those interactions as caused by an intrinsic link of the fields, realized by the coupling terms in the lagrangian.

 

[tiny-tim]

… an introductory course in QFT and it spoke of the Dirac Sea - it has been replaced

exactly!

so why are you (still) talking about it?? …

Well I beg to differ... the Dirac sea is a reseviour of negative virtual energy states... have you done any work on this side of the theory?

Decouple the dirac equation into left movers and right movers, and then the theory asks where the positron comes from when it is created. It is effectively a hole in the sea, once a virtual particle. The Dirac Equation does involve the ''mathematical'' concept of virtual particles.

Goldstone1, you've not done any research yourself, so you have to rely on established theories as quoted by others …

the Dirac sea is not an established theory, it is discredited, or in your own words replaced …

do you not see that you cannot use it to support any argument? ​

 

[dm4b]

You can view those interactions as caused by an intrinsic link of the fields, realized by the coupling terms in the lagrangian.

Thanks for the reply Polyrhythmic.

Are you talking about the higher-order terms in a Lagrangian for an Interacting field, as opposed to the Lagrangian for a Free field, which is missing those terms?

Sure, mathematically speaking, "forces" (interactions) arise from terms like that. But, I still don't see how that is a "mechanism". That's math.

Nor, does it give a good visualization of the mechanism behind the force, as does the virtual particle picture.

 

[Polyrhythmic]

Are you talking about the higher-order terms in a Lagrangian for an Interacting field, as opposed to the Lagrangian for a Free field, which is missing those terms?

Yes.

Sure, mathematically speaking, "forces" (interactions) arise from terms like that. But, I still don't see how that is a "mechanism". That's math.

I thought that this answer would be unsatisfying, but I don't have a better one. Maybe somebody does, but as far as I see it, that coupling is the fundamental core of the interaction.
When you think about it, does the Coulomb law provide a better mechanism? It just a mathematical formula, but it predicts the experiment quite well. It also doesn't explain "why" opposite charges attract each other, or in other words, what causes those objects to move.

Nor, does it give a good visualization of the mechanism behind the force, as does the virtual particle picture.

That's not a criterion. Try to visualize curved spacetime. You'll have a hard time, yet it gives great testable predictions. I admit that the virtual particle picture looks nice and plausible at first glance, but once you dig deeper into the theory you see that it is false. A correct explanation that is hard to visualize should be preferred, not a false visualizable one.

 

[maxverywell]

I'm not an expert (I'm still undergrad physics student) but I think that the fact that in non-perturbative QFT there are no virtual particles doesn't mean that they don't exist. As in Coulomb law, there are no virtual particles, but we need them to describe the mechanism of interactions, not just to compute the results of interactions as in non-perturbation QFT, Coulomb law etc. So they are indeed real in the sense that they are responsible for the interactions. This is a matter of how you define the reality and the particles.

 

[Polyrhythmic]

I'm not an expert (I'm still undergrad physics student) but I think that the fact that in non-perturbative QFT there are no virtual particles doesn't mean that they don't exist.

There is no evidence in other theories because they just arise as a mathematical construct, namely perturbation series expansion.

As in Coulomb law, there are no virtual particles, but we need them to describe the mechanism of interactions, not just to compute the result of interactions as in non-perturbation QFT, Coulomb law etc.

No, we don't.

So they are indeed real in the sense that they are responsible for the interactions.

Your logic is flawed.

This is a matter of how you define the reality and a particles.

Something which has neither experimental evidence nor theoretical backing should not be considered real.

 

[dm4b]

I thought that this answer would be unsatisfying, but I don't have a better one. Maybe somebody does, but as far as I see it, that coupling is the fundamental core of the interaction.

When you think about it, does the Coulomb law provide a better mechanism? It just a mathematical formula, but it predicts the experiment quite well. It also doesn't explain "why" opposite charges attract each other, or in other words, what causes those objects to move.


That's not a criterion. Try to visualize curved spacetime. You'll have a hard time, yet it gives great testable predictions. I admit that the virtual particle picture looks nice and plausible at first glance, but once you dig deeper into the theory you see that it is false. A correct explanation that is hard to visualize should be preferred, not a false visualizable one.

Coulomb's Law in basic EnM (no SR, or QFT, considerations) is almost like magic. Instant and no mechanism. So, I agree, no better. But, the virtual particle picture does make it more appealing.

I actually really like the spacetime curvature picture, which is why I didn't bring it up.

It would be nice if there was something like that for EnM (that's not fictitious, that is)

It could very well be that as we advance further and further in physics, the nice visualizations just aren't available anymore. But, I always had my doubts about that

 

[Vanadium 50]

There seems to be two sides to this debate - on one side are people who have done the actual calculations. On the other are people who are using quotes from popularizations in support of their position. That alone should tell you something.

You can't use the Lamb shift as evidence for virtual particles, since it was calculated a year before anyone had every heard of virtual particles.

 

[maxverywell]

No, we don't.

Why? How do you explain the Coulomb interactions between charged particles or interactions on non-perturbation QFT?

 

[Polyrhythmic]

Why? How do you explain the Coulomb interactions between charged particles or interactions on non-perturbation QFT?

Read one of my last posts, #165.

Besides, virtual particles don't even explain anything in perturbative quantum field theory.

 

[maxverywell]

Read one of my last posts, #165.

So you don't have an answer. Why not to adopt the notion of virtual particles which explains the interactions?

Read one of my last posts, #165.
Besides, virtual particles don't even explain anything in perturbative quantum field theory.

They are the mediators of fundamental interactions.

 

[Polyrhythmic]

So you don't have an answer.

I didn't say that I don't have one. It's just not the answer you would like to hear.

Why not to adopt the notion of virtual particles which explains the interactions?

For reasons that have been discussed to death in this thread.

They are the mediators of fundamental interactions.

Please don't randomly quote popular phrases as if they gave any meaning to your argument. Do some research, study some quantum field theory. Then we can continue ;)

 

[Goldstone1]

exactly!

so why are you (still) talking about it?? …



Goldstone1, you've not done any research yourself, so you have to rely on established theories as quoted by others …

the Dirac sea is not an established theory, it is discredited, or in your own words replaced …

do you not see that you cannot use it to support any argument? ​

I think it is premature to say I have not investigated these claims.

Very well, if you don't want me to use the Dirac Sea, then it's replacement uses the same dynamics, virtual particles in a zero-point energy field.

 

[Goldstone1]

No. All physical effects you mention can be calculated without assuming that virtual particles have any physical significance.

No, I'd say a theory tries to make sense of the physical effects. They are real. And if the theory of virtual particles are correct, then they will almost certainly have a physical significance, again, as they attempt to explain many physical characteristics of many field theories borne from QM.