Debunking the Existence and Duration of Virtual Particles

[Polyrhythmic]

[...], then it's replacement uses the same dynamics, virtual particles in a zero-point energy field.

What? This doesn't make sense.

 

[Polyrhythmic]

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.

There is no such thing as a theory of virtual particles. Virtual particles may make sense in the half-educated way you view them, but that doesn't change the fact that they are not of physical significance.

 

[dm4b]

What does String Theory say as far as the "mechanism" behind a force? Does it shed any light on the whole virtual particle as a force-mediator topic?

Here's a crazy idea (totally made up in my head just now,so obviously I'm not giving it much credence)

Although not a perturbation series, a Fourier Series is another infinite series whose terms DO have a type of physical significance when applied to say a violin string. They represent different harmonics.

Well, they say each particle in nature is a certain frequency of vibration or energy state of strings. The Universe is a musical symphomy of sorts.

Wouldn't it be cool if virtual particles turned out to represent the "harmonics" of a string?

 

[Polyrhythmic]

What does String Theory say as far as the "mechanism" behind a force? Does it shed any light on the whole virtual particle as a force-mediator topic?

In string theory, there are also graphs comparable to Feynman diagrams coming from a perturbation series expansion. The difference is that those lines are now sheets, since particles are no longer zero-dimensional objects.

Although not a perturbation series, a Fourier Series is another infinite series whose terms DO have a type of physical significance when applied to say a violin string. They represent different harmonics.

That comes from the very nature of the Fourier expansion, its terms represent harmonic function, which in the case of a violin string have an actual physical interpretation.

Well, they say each particle in nature is a certain frequency of vibration or energy state of strings. The Universe is a musical symphomy of sorts.

Wouldn't it be cool if virtual particles turned out to represent the "harmonics" of a string?

That doesn't make sense. The different excitations of the string actually tell us which particle the string represents. This is even before we attempt to calculate interactions by perturbation series expansion.

 

[Goldstone1]

This has become a horrible argument among many posters.

I would like to say that there is nothing wrong with the idea of virtual particles, and if they are real, then the energy lamb shift certainly gives them something real to measure. If you don't want to call them virtual particles, then so be it. But most scientists will agree virtuals particles are a very good tool, even if they are not physically real.

 

[Polyrhythmic]

I would like to say that there is nothing wrong with the idea of virtual particles, and if they are real, then the energy lamb shift certainly gives them something real to measure.

In principle this is true, but unfortunately there is nothing which indicates that they are real.

But most scientists will agree virtuals particles are a very good tool, even if they are not physically real.

You have just captured the essence: they are tool!

 

[Goldstone1]

In principle this is true, but unfortunately there is nothing which indicates that they are real.



You have just captured the essence: they are tool!

You say it is a mathematical tool. How does singling out the virtual particle as a mathematical tool, any different to the very fact that the entire of QM is a mathematical tool. You do realize that we have never observed a particle directly. At this level, it's all about probabilities. And when you come to understand how important probability theory is for a wave function for instance, you will find that the wave function gives virtual particles a very possible existence - in fact, quantum theory predicts them.

So not believing they are there is a matter of choice. There can be no absolutes here.

 

[Polyrhythmic]

You say it is a mathematical tool. How does singling out the virtual particle as a mathematical tool, any different to the very fact that the entire of QM is a mathematical tool. You do realize that we have never observed a particle directly. At this level, it's all about probabilities. And when you come to understand how important probability theory is for a wave function for instance, you will find that the wave function gives virtual particles a very possible existence - in fact, quantum theory predicts them.

So not believing they are there is a matter of choice. There can be no absolutes here.

True, you can also view the wave function as a tool. There is much room for interpretation, but one thing is certain: the wave function plays a central role and we couldn't do any physics without it. This is not true for virtual particles. They arise as a (false) interpretation of a certain tool (perturbation theory), they are in no way a central element of the theory, they are simply not required.

Also your statement that quantum theory predicts them is plainly wrong. Could you give any reference on this?

 

[Goldstone1]

True, you can also view the wave function as a tool. There is much room for interpretation, but one thing is certain: the wave function plays a central role and we couldn't do any physics without it. This is not true for virtual particles. They arise as a (false) interpretation of a certain tool (perturbation theory), they are in no way a central element of the theory, they are simply not required.

Also your statement that quantum theory predicts them is plainly wrong. Could you give any reference on this?

As far as I am aware, so please correct me, when Casimir was in talks with his tutor, they discussed that as you enclose the plates, the wave lengths of photons become more compact... so I am basing this on history, if I can cite this, I will... So basically Casimir knew that quantum theory predicted this phenomenon. And low and behold, it did predict their existence.

 

[mattt]

You can use perturbations techniques in Classical Field Theory if you want, with its own kind of "virtual particles" there. Are you going to say then that the classical dynamics of fields are "mediated" by "its own kind of virtual particles"?

 

[Lapidus]

In quantum mechanics the uncertainty principle tells us that the energy can fluctuate wildly over a small interval of time. According to special relativity, energy can be converted into mass and vice versa. With quantum mechanics and special relativity, the wildly fluctuating energy can metamorphose into mass, that is, into new particles not previously present.

That is the core story of relativistic quantum field theory.

It has not one bit to do with any calculation scheme.

Please, and I ask probably for the tenth time, what forbids 'virtual' states from happening?

And yes, while we at it, how can we do without them, i.e. how are interactions mediated in a relativistic quantum physics theory?

 

[Polyrhythmic]

As far as I am aware, so please correct me, when Casimir was in talks with his tutor, they discussed that as you enclose the plates, the wave lengths of photons become more compact... so I am basing this on history, if I can cite this, I will... So basically Casimir knew that quantum theory predicted this phenomenon. And low and behold, it did predict their existence.

All you have to do in order to calculate the Casimir force is to take a look at the ground state energy in second quantization, both between the plates and free. Between the plates, the modes have to obey different boundary conditions than in the free case. From this energy difference you can directly derive the force. There, no virtual particles needed.

 

[Goldstone1]

You can use perturbations techniques in Classical Field Theory if you want, with its own kind of "virtual particles" there. Are you going to say then that the classical dynamics of fields are "mediated" by "its own kind of virtual particles"?

I think your queery is a matter of interpretation rather than empiracal fact.

 

[Goldstone1]

All you have to do in order to calculate the Casimir force is to take a look at the ground state energy in second quantization, both between the plates and free. Between the plates, the modes have to obey different boundary conditions than in the free case. From this energy difference you can directly derive the force. There, no virtual particles needed.

I think I am failing to show you that the particles where predicted first before their verification in a casimir force. Casimir had a thought-experiment which involves vacuum energy, and it required virtual particles. Beside, Lapidus says it best for an arguement.

 

[mattt]

For example:

$$A=\int_0^1 e^{-x^2}dx$$

$$A$$ is a real number perfectly defined (because every continuous function is Riemann-integrable in a compact interval).

We may get to know the first n decimal digits of this number (using one approximation method or another) or we may not know it, but THAT only tells about our knowledge (of the decimal digits of that number). That real number is perfectly defined whether we may know its digits or not.

Imagine we can prove that the following infinite sum $$1+a-ab+abc-abcd+...$$ is a good approximation to the number $$A$$.

It would be very weird to say "the real number $$A$$ is what it is because of the numbers $$a,b,c,d...$$".


NO, the real number $$A=\int_0^1 e^{-x^2}dx$$ is perfectly defined, whether we may approximate its value one way or another, or not.


Now, a QFT define a S-Matrix. Its elements (numbers) are perfectly defined (if that QFT can be defined to begin with). Those numbers are the theoretical predictions that that QFT produce (whether we may get to know them in one way or another, or not).

Using perturbation techniques to approximate those numbers is one way for us to get to know them (at least its first decimal digits), but there are other approximation (non-perturbative) techniques to obtain the same goal. But again, that only speak about our knowledge of those numbers, whereas those numbers are perfectly defined (by the mathematical structure of the theory) with or without our knowledge of its decimal digits.

 

[alxm]

I think I am failing to show you that the particles where predicted first before their verification in a casimir force. Casimir had a thought-experiment which involves vacuum energy, and it required virtual particles.

You're making stuff up. And not for the first time in this thread.

First off, Casimir was not first to predict the Casimir effect. Wheeler did, in 1941 (meeting abstract in Phys Rev, v59, p928). Casimir was the first to calculate that effect. He did so without using Feynman diagrams or making any references whatsoever to virtual particles.

Second, calculating the Casimir effect does not require perturbation theory, much less interpreting that perturbation expansion as a representation of a real process.

Third, there's no 'vacuum energy' involved here. The Casimir effect is the same thing as the London dispersion force. It's defined to be the deviation from the ordinary London force once the quantized and relativistic field effects are taken into account. It's a QED effect by definition. It 'proves' that the electromagnetic field is indeed quantized. It does not prove, in any way, that the virtual excitations of that field used in certain QED calculations are any more real than the fictional excitations used, for instance, to calculate a many-body system.

 

[Goldstone1]

You're making stuff up. And not for the first time in this thread.

First off, Casimir was not first to predict the Casimir effect. Wheeler did, in 1941 (meeting abstract in Phys Rev, v59, p928). Casimir was the first to calculate that effect. He did so without using Feynman diagrams or making any references whatsoever to virtual particles.

Second, calculating the Casimir effect does not require perturbation theory, much less interpreting that perturbation expansion as a representation of a real process.

Third, there's no 'vacuum energy' involved here. The Casimir effect is the same thing as the London dispersion force. It's defined to be the deviation from the ordinary London force once the quantized and relativistic field effects are taken into account. It's a QED effect by definition. It 'proves' that the electromagnetic field is indeed quantized. It does not prove, in any way, that the virtual excitations of that field used in certain QED calculations are any more real than the fictional excitations used, for instance, to calculate a many-body system.

If my information at the beginning was incorrect, I apologize.

However, there is vacuum energy involved in the Casimir Effect. It directly involves the zero-point energy field, so I don't know how one could not talk of a vacuum energy.

 

[Goldstone1]

And I don't think I mentioned perturbation theory as such... did I? My memory is not great.

 

[maverick_starstrider]

And I don't think I mentioned perturbation theory as such... did I? My memory is not great.

Virtual particles are perturbation theory. That's what they are. I feel like you have no ground on to discuss such a thing if you don't even know what a Wick Contraction is. Virtual particles and QFT perturbation theory are the same thing. Virtual particles is not a concept separate from perturbation they ARE the perturbations. They exist solely in the scheme of drawing stick figures to figure out the next largest perturbative correction to a field theory integral.

 

[Goldstone1]

Virtual particles are perturbation theory. That's what they are. I feel like you have no ground on to discuss such a thing if you don't even know what a Wick Contraction is. Virtual particles and QFT perturbation theory are the same thing. Virtual particles is not a concept separate from perturbation they ARE the perturbations. They exist solely in the scheme of drawing stick figures to figure out the next largest perturbative correction to a field theory integral.

Yes, I suppose virtual particles are perturbations of energy.

 

[dm4b]

Virtual particles are perturbation theory. That's what they are. I feel like you have no ground on to discuss such a thing if you don't even know what a Wick Contraction is. Virtual particles and QFT perturbation theory are the same thing. Virtual particles is not a concept separate from perturbation they ARE the perturbations. They exist solely in the scheme of drawing stick figures to figure out the next largest perturbative correction to a field theory integral.

"they ARE the perturbations"

That's not really correct, if you meant that literally.

In perturbation theory, it's the Interaction Hamiltonian that is treated AS a perturbation.

Virtual particles are just internal lines on a diagram that act as mnemonic for each term in the perturbation series.

To say, the virtual particles ARE the perturbation under consideration, would give them a certain level of reality.

Also, you can learn about virual particles through simple toy theories without ever resorting to Wick Contractions. That alone should be enough to demonstrate that they may just be mathematical artifacts.

 

[maverick_starstrider]

"they ARE the perturbations"

That's not really correct, if you meant that literally.

In perturbation theory, it's the Interaction Hamiltonian that is treated AS a perturbation.

Virtual particles are just internal lines on a diagram that act as mnemonic for each term in the perturbation series.

To say, the virtual particles ARE the perturbation under consideration, would give them a certain level of reality.

Also, you can learn about virual particles through simple toy theories without ever resorting to Wick Contractions. That alone should be enough to demonstrate that they may just be mathematical artifacts.

Well I simply mean that there entire existence is on those squigly little lines that we use to keep track of our orders of perturbation.

 

[Vanadium 50]

We don't want you to apologize. We want you to stop posting rubbish. "I don't know how one could not..." is an argument from incredulity.

There is an excellent paper by Bob Jaffe (Physical Review D 72 (2): 021301) where he calculates the Casimir attraction without resort to zero-point energies or virtual particles. And lest you think this is some sort of esoteric paper that nobody could be expected to find, let me point out that it is referenced in the Wikipedia article on the Casimir Effect.

 

[Goldstone1]

The paper you cite is also a theory as well. This has been my point all along. It is by choice of the observer to either accept they exist, or do not. Wavings papers in my face saying it doesn't need them does not, unfortunately seal the deal for me as my explanation of the Casimir force is by virtual particles. Either way, it's all a matter of interpretation.

 

[dm4b]

Well I simply mean that there entire existence is on those squigly little lines that we use to keep track of our orders of perturbation.

Ahh, I get ya. Agreed.

Hey, here's another argument for virtual particles I never had a good answer for. Maybe somebody else on here does.

Take two electrons moving toward each other. Ultimately, they will repel each other without ever actually coming into contact. Their momentum has changed as a result.

Well, when you do the whole Feynman diagram thing, the virtual particles, or internal lines, are pretty important in keeping track of, and making sure momentum is conserved.

So, you often hear folks say that the virtual particles provide "momentum transfer" between the two electrons, or some sort of wording along those lines. And, that alone has to establish some reality to them.

What's a good argument against this line of reasoning?

 

[dm4b]

We don't want you to apologize. We want you to stop posting rubbish. "I don't know how one could not..." is an argument from incredulity.

There is an excellent paper by Bob Jaffe (Physical Review D 72 (2): 021301) where he calculates the Casimir attraction without resort to zero-point energies or virtual particles. And lest you think this is some sort of esoteric paper that nobody could be expected to find, let me point out that it is referenced in the Wikipedia article on the Casimir Effect.

Hi Vanadium 50,

I was under the impression that Jaffe's work only applies to the static Casimir Effect. And, it is very decisive in showing that the Casimir Effect does not require the existence of virtual particles, or the ZPE, to work, imho.

But, recently there was a paper about the dynamic Casimir Effect and, of course, the authors start talking about virtual particles, as if they are real again.

Here is my thread: https://www.physicsforums.com/showthread.php?t=503456

Here is the paper: http://arxiv.org/abs/hep-th/0503158

Can the DCE be explained in other terms, as well?

Any insight would be much appreciated.

 

[Vanadium 50]

The paper you cite is also a theory as well. This has been my point all along. It is by choice of the observer to either accept they exist, or do not...Either way, it's all a matter of interpretation.

That would have been easier to accept had you not made the following direct - and wrong - statement:

However, there is vacuum energy involved in the Casimir Effect. It directly involves the zero-point energy field...

Now, you can complain that being corrected involves "waving papers in your face", but I think that says more about your willingness to learn than anything else.

 

[Goldstone1]

That would have been easier to accept had you not made the following direct - and wrong - statement:



Now, you can complain that being corrected involves "waving papers in your face", but I think that says more about your willingness to learn than anything else.

Unless you are saying the vacuum does not contain an energy density, then I don't understand this sudden sanctimonious nature. It's already been cleared there are contending theories, but physicists use and do believe they exist, so I said it was a matter of interpretation, yet again.

 

[Vanadium 50]

Maybe maxverywell was not quoting popular books, but things learned from Jackson "Electrodynamics", perhaps from page 3 of this very book where can be read "The concept of E and B are classical notions. It can be thought of as the classical limit of quantum mechanical description in terms of real and virtual photons."

True, he said that. But, if you read what he wrote, he's not making the point you are claiming he's making. Indeed, he's not talking about the reality of virtual photons at all. He's discussing the validity of classical electrodynamics in a world we know is fundamentally quantum mechanical, and the point he is making is that for large numbers of particles, the continuum approximation of classical electrodynamics is good enough.

Pulling quotes out of context may be considered acceptable for internet debate elsewhere, but it is not something that we like to see here. The point of science is to understand what is true, not to win points with cheap debating tricks.

Or maybe he was quoting from the Nobel laureate and one of the most respected man in field theory, Frank Wilczek:

Did you read the whole article? Nowhere does he say that these virtual particles have some sort of reality beyond the computational, and he even suggests on page 9 that the description in terms of fields is better. (Which it is) This is also a paper intended for a non-specialist audience.

Also, it is perhaps worth noting that his Nobel prize winning work discusses Feynman graphs and perturbation expansions, but never once mentions virtual particles.

Are electrons real? I would say yes, because there are some phenomena that cannot be explained any other way. Is there anything in QFT that can only be explained with virtual particles? The answer is "no".

 

[Lapidus]

True, he said that. But, if you read what he wrote, he's not making the point you are claiming he's making. Indeed, he's not talking about the reality of virtual photons at all. He's discussing the validity of classical electrodynamics in a world we know is fundamentally quantum mechanical, and the point he is making is that for large numbers of particles, the continuum approximation of classical electrodynamics is good enough.

Vanadium, how are interactions mediated in a relativistic and quantum physical theory? When you have a negative charge sitting here and a positive sitting over there, how is the attraction mediated? Don't tell me by the electromagnetic field, since I ask for a quantum realtivistic explanation.

Pulling quotes out of context may be considered acceptable for internet debate elsewhere, but it is not something that we like to see here. The point of science is to understand what is true, not to win points with cheap debating tricks.

It was claimed that only popular books say virtual particles are mediators of forces. To respond to that false claim, I gave quotes from Jackson's "Electrodynamics" and a survey article from Wilczek, with refrence where I found it and even a link where to read it.

Did you read the whole article? Nowhere does he say that these virtual particles have some sort of reality beyond the computational,

He says "...the generation of forces by intermediary fields corresponds to the exchange of virtual photons. The association of forces (or, more generally, interactions) with exchange of particles is a general feature of quantum field theory."

This is also a paper intended for a non-specialist audience.

It's a survey paper for physicists.My question (again) to you: What physical mechanism actually forbids 'virtual' particles/ processes from happening, processes which are perfectly allowed by the laws of quantum physics?

At the Compton wavelength when quantum relativistic theory is required, the concept of a single particle breaks down. When we don't look/ measure there is not one real particle flying around, but a superposition of (infinite) many particles. Are they 'really' there when we not measure?

So it's the same as at the Broglie wavelenght when we could ask at the double slit experiment if the intermediate states are there and real, or not.