Debunking the Existence and Duration of Virtual Particles

[Polyrhythmic]

Regarding interpretational issues and comparisons of the virtual particle problem to interpretation of quantum mechanics:
The interpretation of the wavefunction is something entirely different than the interpretation of the virtual particle. The wave function is the central element of quantum mechanics and therefore obviously asks for an interpretation. The virtual particle however is neither central nor of any physical significance, it's just a mathematical curiosity.

 

[muppet]

The way I look at it is this.

We say that virtual particles mediate interactions; they're whatever physically corresponds to the internal lines in Feynman diagrams. As we can always draw lots of diagrams that contribute to the same process, it's impossible to say whether or not you have one, or two, or twenty, or any definite number of virtual particles involved in any given process. This isn't just because we can't count the virtual photons in two-electron scattering; it's because the amplitude for a particular process receives contributions from one- and two- and billion- photon exchange. So to my mind, it doesn't make a great deal of sense to say that particles existed as physical things in the process- if they did, you'd be able to say that there was a definite number of them.

The point is really that particles, from the QFT PoV, aren't really "things"; they're configurations of the underlying electron or photon or quark fields, that make sense only in the limit of negligible interactions. So it makes sense to say that "virtual electrons mediate light-by-light scattering" in the sense that it's only as a result of interaction with the electron field that such processes occur.

 

[byron178]

why can't virtual particles be detected? is it because they live very very short?

 

[Polyrhythmic]

why can't virtual particles be detected? is it because they live very very short?

What isn't there can't be detected.

 

[Vanadium 50]

Polyrhythmic has it right.

A good analogy is image charge in the Method of Images in electromagnetism. It's convenient and allows you to get a mental picture, but it can't be used in all cases, and it's not like you can put image charges in a box.

There is no problem in QFT that requires virtual particles. Anything that can be solved using them can be solved some other way.

 

[my_wan]

In order to detect them you have to supply them with the energy to become real, at which point they are no longer not real, hence can be measured. The question of how real they were before you supplied that energy is empirically moot, since you have to measure something in order to measure it. We can and do measure things by not measuring it in some cases, but that allows interpretations that does not require agreement on what constitutes real.

 

[Lapidus]

Polyrhythmic has it right.

A good analogy is image charge in the Method of Images in electromagnetism. It's convenient and allows you to get a mental picture, but it can't be used in all cases, and it's not like you can put image charges in a box.

There is no problem in QFT that requires virtual particles. Anything that can be solved using them can be solved some other way.

Virtual particles give contributions to the probality amplitudes of measurable events. These contributions are real. When you calculate in some other way, i.e. non-perturbativly you still got 'virtual' processes to account for. 'Virtual' is just another word for violating on-shell relation for the short-time allowed by the energy-time uncertainty relation.

So these processes are required and predicted by the laws of relativistic quantum physics.

By definition, virtual particles are not directly observable. So are all the infinite paths a quantum particle takes in the calculations or the superposed states between measurements. Very the same as we collapse the state vector when carrying out a measurement, a virtual particle becomes real when you supply enough energy to reveal it.

 

[Polyrhythmic]

Virtual particles give contributions to the probality amplitudes of measurable events.

Correct, but this doesn't mean that you have to interpret those mathematical objects as actual particles.

These contributions are real.

By which standards? Those "particles" are neither measured nor needed for the explanation of any measurements.

When you calculate in some other way, i.e. non-perturbativly you still got 'virtual' processes to account for.

For example?

'Virtual' is just another word for violating on-shell relation for the short-time allowed by the energy-time uncertainty relation.

That doesn't mean you have to introduce them into "reality".

So these processes are required and predicted by the laws of relativistic quantum physics.

No.

By definition, virtual particles are not directly observable. So are all the infinite paths a quantum particle takes in the calculations or the superposed states between measurements. Very the same as we collapse the state vector when carrying out a measurement, a virtual particle becomes real when you supply enough energy to reveal it.

There is no consensus on the interpretation of such quantum mechanical processes. It also seems that the interpretation isn't that important, since quantum mechanics works anyways.

 

[haushofer]

I regard it this way:

Already for real particles can you ask yourself the question "are they really there"? For that you have to measure them, and for that they have to be in an in- or outstate.

Virtual particles however don't appear in in- or outstates, because they are (as I see it) mathematical remnants of doing perturbation theory.

 

[Polyrhythmic]

I regard it this way:

Already for real particles can you ask yourself the question "are they really there"? For that you have to measure them, and for that they have to be in an in- or outstate.

Virtual particles however don't appear in in- or outstates, because they are (as I see it) mathematical remnants of doing perturbation theory.

This seems like an accurate description, I agree.

 

[Lapidus]

In and out states are mathematical fictions! Every particle that is emitted and absorbed in some finite time is somewhat off-shell.

Mass-shell relation plus energy-time uncertainty relation allow/ predict particles to be 'virtual'. 'Virtual' particles are not tied to perturbation calculations.

 

[Antiphon]

For what it's worth, the Wikipedia article on virtual particles is not good. It even claims incorrectly that the near radiation fields around an antenna are composed of virtual photons while the far-field radiation terms are real photons.

That this is false can be seen by noting that the near fields of an antenna contain fields that are no different than the photon in a box. They have energy but they are not in a propagating mode.

Virtual photons do not deliver energy to a charge; the near fields of an antenna do.

 

[byron178]

For what it's worth, the Wikipedia article on virtual particles is not good. It even claims incorrectly that the near radiation fields around an antenna are composed of virtual photons while the far-field radiation terms are real photons.

That this is false can be seen by noting that the near fields of an antenna contain fields that are no different than the photon in a box. They have energy but they are not in a propagating mode.

Virtual photons do not deliver energy to a charge; the near fields of an antenna do.

So virtual particles travel backwards through time?correct me if I am wrong but doesn't relativity say if something were to travel faster than light then in one frame it will travel backwards in time?

 

[Antiphon]

So virtual particles travel backwards through time?correct me if I am wrong but doesn't relativity say if something were to travel faster than light then in one frame it will travel backwards in time?

My post had nothing to say about going forward or backward in time. What I'm saying is all the photons around an antenna are non-virtual, even the ones in the non-radiative near field.

 

[Drakkith]

So virtual particles travel backwards through time?correct me if I am wrong but doesn't relativity say if something were to travel faster than light then in one frame it will travel backwards in time?

No. Again, virtual particles are not real. They inherently cannot be detected and, at least from some of the posts here, seem to be merely a mathematical tool in a hypothesis.
If something cannot be detected, not because we can't measure accurately enough but because of their very nature, then they do not exist as physical objects.

 

[byron178]

No. Again, virtual particles are not real. They inherently cannot be detected and, at least from some of the posts here, seem to be merely a mathematical tool in a hypothesis.
If something cannot be detected, not because we can't measure accurately enough but because of their very nature, then they do not exist as physical objects.

if they are not real,what are they?

 

[Drakkith]

if they are not real,what are they?

As I understand it, they are merely a mathematical construct to explain QFT.

From wikipedia:

The concept of virtual particles arises in the perturbation theory of quantum field theory, an approximation scheme in which interactions (essentially forces) between real particles are calculated in terms of exchanges of virtual particles. Any process involving virtual particles admits a schematic representation known as a Feynman diagram which facilitates the understanding of calculations.

Also:

They are "temporary" in the sense that they appear in calculations, but are not detected as single particles. Thus, in mathematical terms, they never appear as indices to the scattering matrix, which is to say, they never appear as the observable inputs and outputs of the physical process being modeled. In this sense, virtual particles are an artifact of perturbation theory, and do not appear in a non-perturbative treatment.

See the article on Virtual Particles for the full context of that.

 

[maverick_starstrider]

if they are not real,what are they?

Alright, it's basically like this. In the math of Quantum Field Theory we come across integrals (a special sort of math equation I guess) that look like this:

A = \int D \phi e^{\frac{1}{2}(\nabla \phi)^2 + a \phi + b \phi^2 + c \phi^4}

and if we could write down the solution to this guy on a piece of paper then we'd be done and you'd never have heard of "virtual particles". The problem is we can't, however, we can write down the solution to an equation like:B = \int D \phi e^{\frac{1}{2}(\nabla \phi)^2 + a \phi + b \phi^2}

the difference being the c \phi^4 term (which is commonly called, for various reasons, an "interaction" term. But that's not the same equation. However, it turns out that there's a mathematical way to get the answer to A given B. The way you do it involves taking the answer to B and continually adding new terms (equations), each term smaller than the next (for the sake of argument). We call this a PERTURBATION approach. And it turns out the math you solve to figure out which extra terms to add KINDA LOOKS (but not totally) like the interactions of certain particles (but it's not quite right). Richard Feynman took this realization and came up with a simplified way of figuring out what all these extra terms you have to add are by PICTURING an infinite number of interactions that could be drawn in simple diagrams and translated to elaborate, but usually solvable, bits of math. It is from this APPROXIMATION scheme of computing these bits of math that look similar to (but not exactly like) the propagation (goings on) of particles that we get the concept of virtual particles. To summarize, if we could write down the answer to A the concept wouldn't exist, but we can't and it turns out we can get infinitely close to it by applying math that looks similar but not fully like interaction integrals. Thus, as been said many times before, virtual particles aren't a real thing, they're a convenient mental image to figure out what integrals (math equations) you have to write down to fully figure out the math of a REAL particle. They're a mathematical slight of hand.

Does this help clear things up?

P.S. The word "tachyon" is a name RESERVED for a physics defying particle that moves faster than the speed of light, the mathematical repercussions of such a thing would be to bizarre to fathom (imaginary velocities... what?). There isn't actually a shred of evidence to say they exist however. It's just a name that gets tossed around episodes of Star Trek when they've over-used "subspace", "omega radiation", "graviton emitters", etc.

 

[Lapidus]

Thus, as been said many times before, virtual particles aren't a real thing, they're a convenient mental image to figure out what integrals (math equations) you have to write down to fully figure out the math of a REAL particle. They're a mathematical slight of hand.

Does this help clear things up?

No, not at all.

These integrals are physics, they represents contributions to probability amplitudes of measurable events.

In all quantum physics, prior measurement there are states that are not real in the classical sense. But they contribute to computations for correct probabilties of measurable outcomes.

That is enough for many physicists to consider them physical reality. Or enough, not to bother who calls what real or not, or who says that integral is mathematics and that is physics. Most physicists simply do not care, that's why you find no papers about the 'reality of virtual particles', but only endless discussions on internet forums.

Also, a particle which is on-shell is one which travels forever after interacting. So if you insist something is mathematical fiction, it clearly has to be 'real' particles!

And yes, virtual particles do not originate from perturbation theory, as people often claim,
it is just a particle that does not obey E^2-p^2.c^2=m^2.c^4 for a time allowed by the energy-time uncertainty relation.

 

[maverick_starstrider]

No, not at all.

These integrals are physics, they represents contributions to probability amplitudes of measurable events.

In all quantum physics, prior measurement there are states that are not real in the classical sense. But they contribute to computations for correct probabilties of measurable outcomes.

That is enough for many physicists to consider them physical reality. Or enough, not to bother who calls what real or not, or who says that integral is mathematics and that is physics. Most physicists simply do not care, that's why you find no papers about the 'reality of virtual particles', but only endless discussions on internet forums.

Also, a particle which is on-shell is one which travels forever after interacting. So if you insist something is mathematical fiction, it clearly has to be 'real' particles!

And yes, virtual particles do not originate from perturbation theory, as people often claim,
it is just a particle that does not obey E^2-p^2.c^2=m^2.c^4 for a time allowed by the energy-time uncertainty relation.

My QFT is pretty limited but the only place I've seen virtual particles is when you expand in a Dyson series. That's clearly just a mathematical crutch. Taking an integral you can't solve, rephrasing it as a green function, which you can't solve, and then rephrasing (again) as an iterative ground-state greens function plus interactions. If I remember correctly the propagators are most definitely off the light cone. It's like saying the sum over all E_n in a regular perturbation expansion represents the system simultaneously undergoing all possible transitions at once. If an experiment says that a value of a certain quantity is Q and theory predicts Q it's kinda pointless to say that "actually, the quantity is Q + J - J".

 

[Polyrhythmic]

No, not at all.
These integrals are physics, they represents contributions to probability amplitudes of measurable events.

So you claim that whenever you expand something into a Taylor series, each term has to have a physical interpretation?

In all quantum physics, prior measurement there are states that are not real in the classical sense. But they contribute to computations for correct probabilties of measurable outcomes.

What do you mean by real in the classical sense? In quantum mechanics, the concept of a state allows us to make physical predictions. The virtual particle however is just a handy visualization of something which allows us to make physical predictions. It is not needed to regard those integrals physical reality in order to arrive at measurable quantities.

That is enough for many physicists to consider them physical reality. Or enough, not to bother who calls what real or not, or who says that integral is mathematics and that is physics. Most physicists simply do not care, that's why you find no papers about the 'reality of virtual particles', but only endless discussions on internet forums.

Not caring is alright, but falsely stating that virtual particles are real is simply unscientific.

Also, a particle which is on-shell is one which travels forever after interacting. So if you insist something is mathematical fiction, it clearly has to be 'real' particles!

In physics, we have to deal with idealizations in order to describe concepts accurately. "Forever" is exactly such an idealization. That it is unlikely that in reality such a particle wouldn't have enough time to travel "forever" is quite clear.

And yes, virtual particles do not originate from perturbation theory, as people often claim,

it is just a particle that does not obey E^2-p^2.c^2=m^2.c^4 for a time allowed by the energy-time uncertainty relation.

I don't see the logic in your argument. How does the second line justify the first statement?

 

[Lapidus]

Alright, it's basically like this. In the math of Quantum Field Theory we come across integrals (a special sort of math equation I guess) that look like this:

A = \int D \phi e^{\frac{1}{2}(\nabla \phi)^2 + a \phi + b \phi^2 + c \phi^4}

and if we could write down the solution to this guy on a piece of paper then we'd be done and you'd never have heard of "virtual particles".

You do realize that in your integral given here, one is instructed to sum over all
histories, most of which are off-shell?

 

[my_wan]

You do realize that in your integral given here, one is instructed to sum over all
histories, most of which are off-shell?

In practical terms you do not have to sum over ALL of them. That would actually entail summing over every possible path through the Universe. Generally you can merely paths outside the experimental arrangement cancel before the calculation even starts. The path discontinuities imposed by the experiment is way more than sufficient in practical terms.

 

[Lapidus]

So you claim that whenever you expand something into a Taylor series, each term has to have a physical interpretation?

When the terms give/ contribute to quantities that are measurable, yes of course. If you would label every Taylor expansion, every linear approxiamation in physics as unphysical and just mathematical fiction, then there would be not much physics left by this definition.

What do you mean by real in the classical sense? In quantum mechanics, the concept of a state allows us to make physical predictions. The virtual particle however is just a handy visualization of something which allows us to make physical predictions. It is not needed to regard those integrals physical reality in order to arrive at measurable quantities.

My visualization is not that little balls are shooting back and forth. 'Virtual' particles, except being not directly measurable, are (relativistic) quantum particles as much as 'real' particles are. Prior measurement the only thing real about a quantum particle are the probability outcomes for measurements. The contribution coming from off-shell or virtual states are essential for computing these and thus (very) real.

Not caring is alright, but falsely stating that virtual particles are real is simply unscientific.

I gave my defintion of reality. You can go on and say only what we directly measure in quantum physics is real. I would assign reality also to computable, yet unmeasurable processes that interact with events that we can measure.

In physics, we have to deal with idealizations in order to describe concepts accurately. "Forever" is exactly such an idealization. That it is unlikely that in reality such a particle wouldn't have enough time to travel "forever" is quite clear.

But if it does not travel forever, then it is not on-mass shell. That's why it is much, much more coherent to adopt the picture that all particles are off-shell. The almost on-mass shell are those that we can measure. The others more transient, which we do not measure or only can measure if we supply enough energy, those nevertheless ineract with the almost on-shell, 'real' particles.

I don't see the logic in your argument. How does the second line justify the first statement?

What's the mechanism that forbids particles to be off-mass shell for the time the energy-time uncertainty allows it? What can happen in quantum physics, happens.

 

[maverick_starstrider]

When the terms give/ contribute to quantities that are measurable, yes of course. If you would label every Taylor expansion, every linear approxiamation in physics as unphysical and just mathematical fiction, then there would be not much physics left by this definition.



My visualization is not that little balls are shooting back and forth. 'Virtual' particles, except being not directly measurable, are (relativistic) quantum particles as much as 'real' particles are. Prior measurement the only thing real about a quantum particle are the probability outcomes for measurements. The contribution coming from off-shell or virtual states are essential for computing these and thus (very) real.



I gave my defintion of reality. You can go on and say only what we directly measure in quantum physics is real. I would assign reality also to computable, yet unmeasurable processes that interact with events that we can measure.



But if it does not travel forever, then it is not on-mass shell. That's why it is much, much more coherent to adopt the picture that all particles are off-shell. The almost on-mass shell are those that we can measure. The others more transient, which we do not measure or only can measure if we supply enough energy, those nevertheless ineract with the almost on-shell, 'real' particles.



What's the mechanism that forbids particles to be off-mass shell for the time the energy-time uncertainty allows it? What can happen in quantum physics, happens.

Going back the simplest case of expanding about "Q" vs. "Q+J-J" you will get different expansions with different terms which, if you say terms are reality, means entirely different prescriptions for what is allegedly "going on". If you're saying these things are real then how would you come to terms with that? Considering you can always add extra terms provided they cancel,, which randomly added term represents the going on of "reality" and which don't?

 

[Polyrhythmic]

When the terms give/ contribute to quantities that are measurable, yes of course. If you would label every Taylor expansion, every linear approxiamation in physics as unphysical and just mathematical fiction, then there would be not much physics left by this definition.

The approximation itself is not physical, it's a tool. And that's what the perturbation series and "virtual particle" terms in QFT are.

My visualization is not that little balls are shooting back and forth. 'Virtual' particles, except being not directly measurable, are (relativistic) quantum particles as much as 'real' particles are. Prior measurement the only thing real about a quantum particle are the probability outcomes for measurements. The contribution coming from off-shell or virtual states are essential for computing these and thus (very) real.

They are not even indirectly measurable. They are not needed for the explanation of any physical phenomenon, they are strictly mathematical. Of course the terms contribute, but the interpretation of those terms as virtual particles does not.

I gave my defintion of reality. You can go on and say only what we directly measure in quantum physics is real. I would assign reality also to computable, yet unmeasurable processes that interact with events that we can measure.

The virtual particle is not needed as an explanation for anything we measure, comparing this to quantum mechanical objects like the wave-function is far-fetched, since regardless of its interpretation, in the process of computing of computing measurable quantities, the wave-function is a fundamental entity. This is not true for virtual particles. A concept which is neither measured nor needed in any other way shouldn't be considered "real".

 

[tiny-tim]

And yes, virtual particles do not originate from perturbation theory, as people often claim,
it is just a particle that does not obey E^2-p^2.c^2=m^2.c^4 for a time allowed by the energy-time uncertainty relation.

virtual particles are talked about only in perturbation theory

"a particle that does not obey E² - p².c² = m².c⁴ for a time allowed by the energy-time uncertainty relation" is just science fiction writing, much like "a spaceship that goes faster than light", or "Heisenberg compensators" … you can't just string words together, you need to be able to insert the idea into a theory

the only theory that contains such particles is perturbation theory, and in particular the Dyson expansion …

and that theory places those particles in momentum space, not real space …

are you seriously claiming that momentum space exists? ​

 

[Lapidus]

virtual particles are talked about only in perturbation theory

Again, what about sum over all histories in the path integral?

"a particle that does not obey E2 - p2.c2 = m2.c4 for a time allowed by the energy-time uncertainty relation" is just science fiction writing, much like "a spaceship that goes faster than light", or "Heisenberg compensators" … you can't just string words together, you need to be able to insert the idea into a theory

That is just a standard fact found in countless physics books. I really can't see what you are driving at here.

Since the discussions gets a bit empty, I think we should end here and agree to disagree, or not.

thanks for your posts

 

[byron178]

virtual particles are talked about only in perturbation theory

"a particle that does not obey E² - p².c² = m².c⁴ for a time allowed by the energy-time uncertainty relation" is just science fiction writing, much like "a spaceship that goes faster than light", or "Heisenberg compensators" … you can't just string words together, you need to be able to insert the idea into a theory

the only theory that contains such particles is perturbation theory, and in particular the Dyson expansion …

and that theory places those particles in momentum space, not real space …

are you seriously claiming that momentum space exists? ​

How come some say that virtual particles exist and some say they don't exist?

 

[tiny-tim]

virtual particles are talked about only in perturbation theory

Again, what about sum over all histories in the path integral?

Well, let's look at the free online "INTRODUCTION TO QUANTUM FIELD THEORY' (142 pages, 2007, at http://www.phys.uu.nl/~bdewit/qft07.pdf"), which does use the path integral approach (unlike eg Tong, who uses the canonical approach), see page 5 …

A central role in these lectures is played by the path integral representation of quantum field theory, which we will derive and use for both bosonic and for fermionic fields. Another topic is the use of diagrammatic representations of the path integral.

the words "virtual" and "shell" do not appear in this book (except at page 87, in two problems quoted from a different book) …

so where in this book do you say is the confirmation that virtual particles exist because they appear in "sum over all histories in the path integral"? ​