Debunking the Existence and Duration of Virtual Particles

[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"? ​

 

[tiny-tim]

hi byron178!

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

some people have a strange idea of the meaning of "exist"

here's a more psychological reason from Hrvoje Nikolic, at page 33 of "Quantum mechanics: Myths and facts" (51 pages, 2007, at http://arxiv.org/abs/quant-ph/0609163" ) …

9.3 Virtual particles?
The calculational tool represented by Feynman diagrams suggests an often abused picture according to which “real particles interact by exchanging virtual particles”. Many physicists, especially nonexperts, take this picture literally, as something that really and objectively happens in nature. In fact, I have never seen a popular text on particle physics in which this picture was not presented as something that really happens. Therefore, this picture of quantum interactions as processes in which virtual particles exchange is one of the most abused myths, not only in quantum physics, but in physics in general. Indeed, there is a consensus among experts for foundations of QFT that such a picture should not be taken literally. The fundamental principles of quantum theory do not even contain a notion of a “virtual” state. The notion of a “virtual particle” originates only from a specific mathematical method of calculation, called perturbative expansion. In fact, perturbative expansion represented by Feynman diagrams can be introduced even in classical physics [52, 53], but nobody attempts to verbalize these classical Feynman diagrams in terms of classical “virtual” processes.

So why such a verbalization is tolerated in quantum physics? The main reason is the fact that the standard interpretation of quantum theory does not offer a clear “canonical” ontological picture of the actual processes in nature, but only provides the probabilities for the final results of measurement outcomes.

In the absence of such a “canonical” picture, physicists take the liberty to introduce various auxiliary intuitive pictures that sometimes help them think about otherwise abstract quantum formalism. Such auxiliary pictures, by themselves, are not a sin. However, a potential problem occurs when one forgets why such a picture has been introduced in the first place and starts to think on it too literally.​

 

[danR]

hi byron178! some people have a strange idea of the meaning of "exist"

here's a more psychological reason from Hrvoje Nikolic, at page 33 of "Quantum mechanics: Myths and facts" (51 pages, 2007, at http://arxiv.org/abs/quant-ph/0609163" ) …

9.3 Virtual particles?
The calculational tool represented by Feynman diagrams suggests an often abused picture according to which “real particles interact by exchanging virtual particles”. Many physicists, especially nonexperts, take this picture literally, as something that really and objectively happens in nature. In fact, I have never seen a popular text on particle physics in which this picture was not presented as something that really happens. Therefore, this picture of quantum interactions as processes in which virtual particles exchange is one of the most abused myths, not only in quantum physics, but in physics in general. Indeed, there is a consensus among experts for foundations of QFT that such a picture should not be taken literally. The fundamental principles of quantum theory do not even contain a notion of a “virtual” state. The notion of a “virtual particle” originates only from a specific mathematical method of calculation, called perturbative expansion. In fact, perturbative expansion represented by Feynman diagrams can be introduced even in classical physics [52, 53], but nobody attempts to verbalize these classical Feynman diagrams in terms of classical “virtual” processes.

So why such a verbalization is tolerated in quantum physics? The main reason is the fact that the standard interpretation of quantum theory does not offer a clear “canonical” ontological picture of the actual processes in nature, but only provides the probabilities for the final results of measurement outcomes.

In the absence of such a “canonical” picture, physicists take the liberty to introduce various auxiliary intuitive pictures that sometimes help them think about otherwise abstract quantum formalism. Such auxiliary pictures, by themselves, are not a sin. However, a potential problem occurs when one forgets why such a picture has been introduced in the first place and starts to think on it too literally.​

The problem for laypersons reading is the comparisons of authors' CV, and where they are attacking or defending virtual particles in print.

Physics Forums is strongly associated with Scientific American, wherein a highly qualified expert says (2006) that virtual particles are flat-out real, and he doesn't mince words:

http://www.scientificamerican.com/article.cfm?id=are-virtual-particles-rea
['rea' sic. That's a valid URL]

The layperson has to compare that with Hrvoje Nikolic's credentials. The latter may indeed be right, but only in a debate before peers is any definitive answer going to emerge. If the only outcome among peers is that everyone agrees to disagree, or there are arguments that the other side doesn't understand each other, that they are arguing at cross-purposes, etc. then the layperson will tend to side with the guy with the C.V.

Edit: I have noted the comment by neum on the article at the bottom of the page in the link above. This is the sort of comment that commends itself as the beginning of a debate the VP-real defenders must answer to.

 

[Lapidus]

The problem for laypersons reading is the comparisons of authors' CV, and where they are attacking or defending virtual particles in print.

Physics Forums is strongly associated with Scientific American, wherein a highly qualified expert says (2006) that virtual particles are flat-out real, and he doesn't mince words:

http://www.scientificamerican.com/article.cfm?id=are-virtual-particles-rea
['rea' sic. That's a valid URL]

The layperson has to compare that with Hrvoje Nikolic's credentials. The latter may indeed be right, but only in a debate before peers is any definitive answer going to emerge. If the only outcome among peers is that everyone agrees to disagree, or there are arguments that the other side doesn't understand each other, that they are arguing at cross-purposes, etc. then the layperson will tend to side with the guy with the C.V.

Edit: I have noted the comment by neum on the article at the bottom of the page in the link above. This is the sort of comment that commends itself as the beginning of a debate the VP-real defenders must answer to.

After some googling, I hit upon this thread right here at PF
https://www.physicsforums.com/showthread.php?t=75307&highlight=virtual+particles&page=3

Well, I don't know about Hrvoje Nikolic's credentials but I take sides with Wilczek, t'Hooft, Callan, Peskin, Pollitzer and Susskind!

Here is Wilzcek view from https://www.physicsforums.com/showpost.php?p=3016157&postcount=36

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

 

[Lapidus]

Or Lisa Randall, as I pointed out a few weeks ago!

https://www.physicsforums.com/showpost.php?p=3240965&postcount=86

Again, I do not know about credentials of all the pf posters, Hrvoje Nikolic or A. Neumaier , but as a humble layperson I go with the just mentioned.

 

[Polyrhythmic]

Physics Forums is strongly associated with Scientific American, wherein a highly qualified expert says (2006) that virtual particles are flat-out real, and he doesn't mince words:

http://www.scientificamerican.com/article.cfm?id=are-virtual-particles-rea
['rea' sic. That's a valid URL]

This article is flawed, for reasons given for example in comment #10.

 

[Polyrhythmic]

Or Lisa Randall, as I pointed out a few weeks ago!

https://www.physicsforums.com/showpost.php?p=3240965&postcount=86

Again, I do not know about credentials of all the pf posters, Hrvoje Nikolic or A. Neumaier , but as a humble layperson I go with the just mentioned.

As a humble layperson, you should either start studying the theory before arguing false opinions or simply admit that you are not in the position to judge. "xy said it" is no reason for something to be true.

 

[danR]

Or Lisa Randall, as I pointed out a few weeks ago!

https://www.physicsforums.com/showpost.php?p=3240965&postcount=86

Again, I do not know about credentials of all the pf posters, Hrvoje Nikolic or A. Neumaier , but as a humble layperson I go with the just mentioned.

Lisa Randall seems well-qualified to have an expert opinion on the matter, and she knows how to gear it down to popular prose, and she appears to side with the 'real' camp.

Perhaps it's better to draw up a weighted checklist of properties, interactions, states, lifetimes, wavelengths, spaces, taxonomies, etc. for all particles and check off what gets a check, the weight of the check and add it all up for all particles.

A lump of sugar would probably get a high 'real' rating, a gold nucleus would do pretty good, an unmeasured electron OK, a 1 metre photon a passing grade, a virtual electron, nyaah...


Perhaps some particles are more real than others. Perhaps 'realness' is a continuum.

 

[maverick_starstrider]

Lisa Randall seems well-qualified to have an expert opinion on the matter, and she knows how to gear it down to popular prose, and she appears to side with the 'real' camp.

Perhaps it's better to draw up a weighted checklist of properties, interactions, states, lifetimes, wavelengths, spaces, taxonomies, etc. for all particles and check off what gets a check, the weight of the check and add it all up for all particles.

A lump of sugar would probably get a high 'real' rating, a gold nucleus would do pretty good, an unmeasured electron OK, a 1 metre photon a passing grade, a virtual electron, nyaah...


Perhaps some particles are more real than others. Perhaps 'realness' is a continuum.

I don't see how there could be such a "real" following. If tomorrow somebody figured out how to solve functional integrals of non-gaussian functions there'd really be no room for debate, the entire concept would simply go away would it not? The reality of something shouldn't depend on the mathematical approach taken. To put it out there again, if something calculates and experiments as "Q" then how could you say it's actually "Q+A-A+C-C+3D-D-2D"? It's just a meaningless contrivance.

 

[danR]

I don't see how there could be such a "real" following. If tomorrow somebody figured out how to solve functional integrals of non-gaussian functions there'd really be no room for debate, the entire concept would simply go away would it not? The reality of something shouldn't depend on the mathematical approach taken. To put it out there again, if something calculates and experiments as "Q" then how could you say it's actually "Q+A-A+C-C+3D-D-2D"? It's just a meaningless contrivance.

Now it's over my head. The way Randall puts it sounds similar to Asimov's popular description 40 years ago, and ran something like: virtual particles shouldn't exist because they are contrary to the laws of physics, but because of Heisenberg, they can get away with breaking the law because they are so short-lived no one is around soon enough to enforce it.

But that's no evidence they do exist. I find it odd that two physics heavyweights find them parsimonious enough to let them live, though the SA (2006) argument sounds deplorably pop-sciencey, something like: 'Well, the Casimir effect shows they exist.'

Here's another layperson-way of weighting the evidence: do the proponents depend on internally consistent, well accepted, standard theories for VP's 'reality' (loosely defined)? How similar is their definition of 'real' space that virtual particles inhabit to that of educated average people's realist views of space? Or is it a momentum-space that is so familiar to physicists that it seems real enough, like mathematician's imaginary number 'i' is to mathematicians? (This was an educated argument against acceptance, in lay terms, of the 'proof' of Fermat's last theorem.)

Do critics depend on the gratuitousness of VP by cherry-picking theories that specifically invoke alternative mechanisms, which theories contradict each other, or mainstream theories? Or are less parsimonious than the offending VP themselves? That's not something I would be able to figure out.

Naturally, scientists aren't required to satisfy public understanding of esoteric things, however. In my own area, there is something called 'Optimality Theory' that has gained wide acceptance for explaining the abstract representations of sound in the mind, but it is hopelessly hard to explain. Students spend the first few weeks scratching their heads over it.

I have a course starting soon, and can't do much more on PF for a while, regrettably.

 

[maverick_starstrider]

Now it's over my head. The way Randall puts it sounds similar to Asimov's popular description 40 years ago, and ran something like: virtual particles shouldn't exist because they are contrary to the laws of physics, but because of Heisenberg, they can get away with breaking the law because they are so short-lived no one is around soon enough to enforce it.

But that's no evidence they do exist. I find it odd that two physics heavyweights find them parsimonious enough to let them live, though the SA (2006) argument sounds deplorably pop-sciencey, something like: 'Well, the Casimir effect shows they exist.'

Here's another layperson-way of weighting the evidence: do the proponents depend on internally consistent, well accepted, standard theories for VP's 'reality' (loosely defined)? How similar is their definition of 'real' space that virtual particles inhabit to that of educated average people's realist views of space? Or is it a momentum-space that is so familiar to physicists that it seems real enough, like mathematician's imaginary number 'i' is to mathematicians? (This was an educated argument against acceptance, in lay terms, of the 'proof' of Fermat's last theorem.)

Do critics depend on the gratuitousness of VP by cherry-picking theories that specifically invoke alternative mechanisms, which theories contradict each other, or mainstream theories? Or are less parsimonious than the offending VP themselves? That's not something I would be able to figure out.

Naturally, scientists aren't required to satisfy public understanding of esoteric things, however. In my own area, there is something called 'Optimality Theory' that has gained wide acceptance for explaining the abstract representations of sound in the mind, but it is hopelessly hard to explain. Students spend the first few weeks scratching their heads over it.

I have a course starting soon, and can't do much more on PF for a while, regrettably.

Well first of all Isaac Asimov certainly was not a "heavy-weight" of physics, he wasn't even a physicist. He was a bio-chemist and a science fiction writer. Also, virtual particles are not needed to explain the Casimir effect. Essentially the issue is something like this. Imagine the function

$$e^x$$

the exponential of x, it'll be on any scientific calculator you can just pick various values of x to see what its value is. Now what if I told you that for any value of x the following will give exactly the same answer:

$$1 + x + \frac{x^2}{2} + \frac{x^3}{6} + \frac{x^4}{24} + \ldots$$

where the ellipsis means you keep adding terms, to make things easier only consider values of x that are less than 1, then a number less than 1 raised to some power becomes an even smaller number. Thus we see that each of these terms (the next would be

$$\frac{x^5}{120}$$

then

$$\frac{x^6}{720}$$

and so on) is going to be a smaller number (try it on a calculator if you like, try something like 0.3 so you'd type in exp(0.3) and get the answer and then you try the infinite sum and you'd get

$$1 + 0.3 + \frac{0.3^2}{2} + \frac{0.3^3}{6} + \ldots$$

and you'll see as you add more and more terms you'll get closer and closer to the answer exp(0.3) gave. This is called a TAYLOR SERIES or TAYLOR APPROXIMATION. If you actually include and infinite number of terms (i.e. $$x^5,x^6,x^7,x^8,\ldots$$) it is EXACTLY $$e^x.$$ However, if you only care about, say only the first 5 decimal points, then you can get away with only having to calculate a dozen or so terms, the terms after that are so small they only effect decimal positions further away.

This is basically the situation, no dumbing down, we have an equation like

$$e^x$$

and we want to do something to it (what's called a functional integral) but we can't. We don't know how. However, we CAN do it to each of the terms in the series form:

$$1 + x + \frac{x^2}{2} + \ldots$$

and the more terms we do it to the more accurate we are (obviously we could never ACTUALLY sum all the infinite terms). Now, imagine the math of $$x$$ looked kinda but not really like the math of a particle moving in a vacuum and the math of $$\frac{x^2}{2}$$ looked kinda but not really like that of two particles moving with opposite momentum or some such and so it goes for the next term and the next term.

Now, saying that virtual particcles are "real" is basically saying that these terms represent real-life goings on of magical new particles. The thing is, if we could just integrate $$e^x$$ in the first place then there'd be nothing to debate and we'd never have even heard of the concept of virtual particles much less insist that they're part of reality. However, since we CAN'T write down the answer to that guy and have to move to an approximation scheme than, allegedly, the very nature of reality magically changes. Now, let me ask you Does it make sense to say that reality changes because your math skills aren't good enough to get the direct answer you want so you're forced to move to approximations? I would say no. Virtual particles are a calculational tool, not reality, they're a way we can mathematically get the answer we want to arbitrary accuracy, they're not new physics.