Could High Energy Virtual Particles Destroy a Starship?

[Ken G]

OK, the triple jets are well described by gluons that ferry energy around, but it doesn't seem like the gluons end up as gluons, so their "reality" is not quite as clear as if we ever actually "detect a gluon." In the scale of "levels of abstraction", it's not obvious where the gluon resides, but they do seem to carry real energy so in principle they should be detectable directly during one of those interactions. I'm wondering if that's ever been done.

As for the interaction between charged particles, I'm not talking about a scattering calculation. This is the point I made originally-- a scattering calculation is set up to talk about an incoming and an outgoing state, where what happens in between is something of a black box. Of course virtual particles don't show up directly in a setup like that, they aren't allowed to exist in either the incoming or outgoing states. That's why I asked for a description of two stationary charges that are experiencing a force, framed classically. Framed quantum mechanically, I'm asking for a description of "what happens next" to the charges-- on timescales during which virtual particles can still survive (in the limited sense that they ever survive, and within the picture that they do anything in the first place). It seems to me that it is on these timescales where virtual particles would reveal their "true claim to reality", where we must recognize that the very concept of "what is real" on such short times becomes rather nebulous-- which is very much the point.

 

[FrediFizzx]

Even people who should know better can be mistaken about some things. There is no evidence that virtual particles are real in any way, that's an empirical fact.

What do you mean by explained? Muon decay rates can be calculated without the need to invoke the reality of any virtual particles. It's a standard textbook exercise, see for example "Quantum Field Theory" by Srednicki.

Did you even read the article by Kane? There is *empirical* evidence that virtual particles are real. I think I will take what Gordon Kane has to say over you. Sorry.

How does a muon decay to a muon neutrino, electron and electron anti-neutrino without the involvement of a real "off mass shell" W boson? Does the muon decay directly to those three outgoing particles? I don't think that is hardly possible.

I don't have that reference you quote; perhaps you could point it out in one of the following that I do have since I can't find an example that doesn't use a real "off mass shell" W boson. Or perhaps you could quote something from your reference that would indicate what you are describing?

"Quantum Theory of Fields" all three volumes - Weinberg
"Quantum Field Theory in a Nutshell" - Zee
"Leptons and Quarks" - Halzen & Martin
"Modern Elementary Particle Physics" - Kane
"Into. to Elementary Particles" - Griffiths

Fred

 

[Vanadium 50]

It's very difficult to have a discussion about virtual particles. There are 100x as many people whose understanding of virtual particles comes only from popularizations as those who have actually studied them. Unfortunately, when someone from the second group says something in conflict with the popularizations, people from the first group inevitably jump all over them telling them they are wrong.

And once again the people who haven't studied QFT in detail have managed to overwhelm those who have by sheer force of numbers. Heck of a way to win a scientific argument.

 

[FrediFizzx]

Standard Model description of muon^- decay; muon^- decays to a muon neutrino and a real "off mass shell" W^- boson. The W boson then decays to an electron and an electron anti-neutrino. Of course it is difficult to have discussion about that since there is no other way to describe the decay than by a virtual W boson.

I can repeat myself also. Yes, "virtual" is probably a poor name for what virtual particles are. There are only two differences between real particles and what a virtual particle is in particle physics. A virtual particle is the same as a real particle except that it is *off mass shell* and therefore not detectable. Because once you have detected it it becomes a real particle. *All other properties are the same.*

 

[Ken G]

And once again the people who haven't studied QFT in detail have managed to overwhelm those who have by sheer force of numbers. Heck of a way to win a scientific argument.

It isn't a democracy, no one thinks numbers of people means anything. All I would personally like to know is whether there is a way to understand electrostatic interaction (not scattering) at the quantum level without the virtual photon concept being useful. If the claim is made they are not, that calls for some justification, and that is how scientific argumentation should go.

 

[Lapidus]

It isn't a democracy, no one thinks numbers of people means anything. All I would personally like to know is whether there is a way to understand electrostatic interaction (not scattering) at the quantum level without the virtual photon concept being useful. If the claim is made they are not, that calls for some justification, and that is how scientific argumentation should go.

Ken G, I was curious about that, too! How can electrostatic interaction at the quantum level be explained without the virtual photon concept?

I asked that many, many times here at this forum before, but sadly never got an answer to this.

 

[dm4b]

It isn't a democracy, no one thinks numbers of people means anything. All I would personally like to know is whether there is a way to understand electrostatic interaction (not scattering) at the quantum level without the virtual photon concept being useful. If the claim is made they are not, that calls for some justification, and that is how scientific argumentation should go.

KenG,

Just want to say you have made some good points in your posts here. The same points I have been trying to make for some time, but you did so rather eloquently.

Your post above is something I have been asking for some time. All you get in return is abstract mathematics (which I have studied some). This is not a physical description. I think you are probably right that there may not be a better physical description for a "force" than the virtual particle depiction, at this point in time anyhow. If there is one, I would love to hear it too.

 

[jtbell]

All I would personally like to know is whether there is a way to understand electrostatic interaction (not scattering) at the quantum level without the virtual photon concept being useful.

Ken G (and others): See tom.stoer's posts in this thread:

https://www.physicsforums.com/showthread.php?t=445730

where he refers to a paper that he further describes here:

https://www.physicsforums.com/showthread.php?p=3017397#post3017397

with some further comments in this post:

https://www.physicsforums.com/showthread.php?p=3020424#post3020424

 

[Lapidus]

Ken G (and others): See tom.stoer's posts in this thread:

https://www.physicsforums.com/showthread.php?t=445730

where he refers to a paper that he further describes here:

https://www.physicsforums.com/showthread.php?p=3017397#post3017397

with some further comments in this post:

https://www.physicsforums.com/showthread.php?p=3020424#post3020424

Sorry, but I can't follow it. Maybe Tom Stoer could exlain it a bit further...

Jtbell, in the meantime, we ask for a quantum physical explanation of static fields between two charges without virtual exchanges between the charges. Do you have one? It's been said everything that 'virtual' particles explain, can be explained some other way. Then I liked to have answered:

1. Two electron heading at each other, then they repel. The have changed both direction, thus momentum. But energy for both is still the same. What transmitted that momentum change?

2. How do we explain attractive forces?

3. And last but not least, what are all these particle experimenters in CERN doing?

They are searching for particles, very heavy particles, particles they believe exist 'virtually' and if they supply enough energy they will make them become real, i.e. directly measurable. At least, that is the standard story, which every particle physics book and QFT book tells us. No dumbed down popularizations, but the way how the majority of working high energy physicists thinks.

'Virtual' particles have nothing to do with some calculation scheme, it is only time-energy uncertainty and Einstein relation together combined from which they follow, which makes them necessary to exist. They physically explain how in a local quantum relativistic theory interactions are transmitted.

And that's all.

 

[Polyrhythmic]

You should also read this:

http://arnold-neumaier.at/physfaq/topics/virtual3

 

[dm4b]

You should also read this:

http://arnold-neumaier.at/physfaq/topics/virtual3

In his other virtual particle FAQ (http://arnold-neumaier.at/physfaq/topics/virtual) I found this interesting:

-----

" ... it allows one in the simplest (H-like) exchange
diagram between two real particles to relate the possible momenta
of the virtual particle to the measurable ingoing and outgoing momenta.
If the ingoing momenta are p and p' then the outgoing momenta are
p+q and p'-q, where q is the momentum exchanged, i..e, the momentum
transported by the virtual particle. In particular, one can determine
q from measurements.

That's all; this makes up virtual particles and their alleged influence
on real (observable) particles. Everything else is superstition."

-----

How can something that "carries" an experimentally determined momentum not be physically real? In addition, once you allow the virtual particles enough of an existence to do even that, it sort of let's the cat out of the bag, so to speak, and all the "superstitions" come along for the ride, whether we like it or not.

So, as related to the above questions asked by multiple people, how can one describe the above particle interaction w/o the use of virtual particles? What is the "mechanism" behind the force between the two particles? How is the momentum exchanged?

 

[Polyrhythmic]

How can something that "carries" an experimentally determined momentum not be physically real? In addition, once you allow the virtual particles enough of an existence to do even that, it sort of let's the cat out of the bag, so to speak, and all the "superstitions" come along for the ride, whether we like it or not.

If you read the other paragraphs as well, you will see that he doesn't want to say that this is actually happening.

So, as related to the above questions asked by multiple people, how can one describe the above particle interaction w/o the use of virtual particles? What is the "mechanism" behind the force between the two particles? How is the momentum exchanged?

Well, the best description is in the reference to tom.stoer's post. Simply speaking, quantum field theory doesn't change the fact that there is still an electrostatic potential that the field excitations (particles) react to. In this sense, they still feel the presence of an electromagnetic field, which they react to.

 

[dm4b]

If you read the other paragraphs as well, you will see that he doesn't want to say that this is actually happening.

I did read the rest. Whether he wanted to say it, or not, virtual particles are the only, as KenG put it, "useful" way to describe this process, as I believe Numier is saying. But, if at the same time, you ascribe any "reality" to them for that purpose, what I am saying is all the "superstitions" he notes come along for the ride.

As far as I know, there are no other "useful" ways to describe the momentum exchange that takes place. Would love to hear one, though.

Well, the best description is in the reference to tom.stoer's post. Simply speaking, quantum field theory doesn't change the fact that there is still an electrostatic potential that the field excitations (particles) react to. In this sense, they still feel the presence of an electromagnetic field, which they react to.

Yeah, of course they feel a presence. How? How is that "presence" transmitted to the particle? What is the clear-cut physical mechanism? Why do they react?

 

[Polyrhythmic]

I did read the rest. Whether he wanted to say it, or not, virtual particles are the only, as KenG put it, "useful" way to describe this process, as I believe Numier is saying. But, if at the same time, you ascribe any "reality" to them for that purpose, what I am saying is all the "superstitions" he notes come along for the ride.

Then you clearly don't understand it. It has been explained over and over in clear english language in this thread as well.

Yeah, of course they feel a presence. How? How is that "presence" transmitted to the particle? What is the clear-cut physical mechanism? Why do they react?

Well, that question is trickier, but it applies to field theory in general: why does a particle react to the presence of a field? I think it goes more into philosophy than science. Quantum field theory does not provide an answer to this question, it just does what a physical theory is supposed to do: it provides a theoretical framework within which experimental results can be predicted. Virtual particles are certainly not the answer to this question, for the many reasons that have already been discussed.

 

[dm4b]

Really, when you think about it even the virtual particle description is not a very "useful" description, or heruistic, for describing and explaining how forces work. It also seems rather magical. In addition, when you realize the perturbation series is an infinite serires and therefore any process involves an infinite number of virtual paritcles, it gets even more magical.

I personally think all this shows we still have a lot to learn, regardless of how much we think we know.

Lots of folks feel that all the divergences and infinities within QFT indicate an underlying problem with the theory. I can't help but wonder if somehow that was fixed, would we get a new view of how forces "work", which does not include virtual particles.

 

[dm4b]

Then you clearly don't understand it. It has been explained over and over in clear english language in this thread as well.

I'm sorry, what has? I don't think YOU understood what I said. Either that, you are reading too much into it.

Quantum field theory does not provide an answer to this question.

BINGO!

 

[Polyrhythmic]

Really, when you think about it even the virtual particle description is not a very "useful" description, or heruistic, for describing and explianing how forces work. It also seems rather magical. In addition, when you realize the perturbation series is an infinite serires and therefore any process involves an infinite number of virtual paritcles, it gets even more magical.

I personally think all this shows we still have a lot to learn, regardless of how much we think we know.

Lots of folks feel that all the divergences and infinities within QFT indicate an underlying problem with the theory. I can't help but wonder if somehow that was fixed, would we get a new view of how forces "work", which does not include virtual particles.

I agree. Even though perturbative quantum field theory has proven to be extremely successful, the lack of a proper non-perturbative theory is kind of unsatisfying.

 

[dm4b]

it just does what a physical theory is supposed to do: it provides a theoretical framework within which experimental results can be predicted.

When did our physical theories stop having the requirement of actually describing physical processes, and just become a calculational tool.

And, more importantly, why?

 

[Polyrhythmic]

I'm sorry, what has? I don't think YOU understood what I said. Either that, you are reading too much into it.

Sorry, maybe I was misinterpreting you! I thought that you interpreted the text in such a way that virtual particles actually were physical objects, sorry!

BINGO!

Then why are so many people here claiming that virtual particles are the answer?

 

[Polyrhythmic]

When did our physical theories stop having the requirement of actually describing physical processes, and just become a calculational tool.

And, more importantly, why?

I never said that they did. My point is that a physical theory can only answer "why"-questions to a certain degree. At some point, you have to assume fundamental entities within such a theory. In our case the fundamental aspect would be the influence of fields on charges. One could always ask further "why"-questions which the theory can't answer.

 

[dm4b]

Sorry, maybe I was misinterpreting you! I thought that you interpreted the text in such a way that virtual particles actually were physical objects, sorry!

No worries ;-)

Then why are so many people here claiming that virtual particles are the answer?

Well, maybe some are. I think others like myself are saying they're currently the only useful answer, or way of talking about these things, even if a potentially incorrect way at that.

 

[dm4b]

My point is that a physical theory can only answer "why"-questions to a certain degree. At some point, you have to assume fundamental entities within such a theory. In our case the fundamental aspect would be the influence of fields on charges. One could always ask further "why"-questions which the theory can't answer.

Agreed with that. I think the whole "clear-cut mechanism behind forces w/o virtual particles" is a "why" question that QFT seems somewhat ill-equipped to answer.

Regardless, you do see a trend for folks not requiring physical theories to actually describe physical reality. The old "shut up and calculate" mantra in QM is one example.

 

[Polyrhythmic]

Well, maybe some are. I think others like myself are saying they're currently the only useful answer, or way of talking about these things, even if a potentially incorrect way at that.

I understand that point, but I also think that people should be more understanding when people who have actually studied QFT and know details about the story tell them that they a wrong in certain aspects. It's like people are stuck with a false concept despite all the evidence against it. I personally believe that it is better to have no explanation for something than employ a wrong explanation. It leads to too many misconceptions and misunderstandings.

 

[Polyrhythmic]

Regardless, you do see a trend for folks not requiring physical theories to actually describe physical reality. The old "shut up and calculate" mantra in QM is one example.

Well, this basically goes down to the question whether something like an absolute reality exists, or not. All we can rely on is experimental data, and theories can only be judged by whether they fit the data or not. "Shut up and calculate" might seem like a narrow-minded approach, but it makes sense in most cases.

 

[dm4b]

I understand that point, but I also think that people should be more understanding when people who have actually studied QFT and know details about the story tell them that they a wrong in certain aspects. It's like people are stuck with a false concept despite all the evidence against it. I personally believe that it is better to have no explanation for something than employ a wrong explanation. It leads to too many misconceptions and misunderstandings.

I think part of the problem is there is not a clear message coming from the professionals on the issue. You can find publications on arxIv, and such, that talk about VPs as if they are "real", you got Lisa Randall talking about them on the Science channel as if they are real, and you got that one thread from on here a ways back where that one dude emailed many big name physicists on the reality of virtual particles, on got back a whole host of answers with more disagreement between them than agreement.

And, like, who is Joe Schmoe going to believe? Me, you, or Lisa Randall?

I also think the Joe Schmoes of the world are smart enough to sense, that even the professionals don't totally know what is going on, even if they don't technically know the details, as to why.

I think it all falls back to that issue that there are really no other clear-cut, simple and concise ways to communicate what is physically going on. So, when the "scientists" try to convey results to the general public, they can fall into the same trap.

It's just one big vicious cycle, lol

 

[Polyrhythmic]

I think part of the problem is there is not a clear message coming from the professionals on the issue. You can find publications on arxIv, and such, that talk about VPs as if they are "real", you got Lisa Randall talking about them on the Science channel as if they are real, and you got that one thread from on here a ways back where that one dude emailed many big name physicists on the reality of virtual particles, on got back a whole host of answers with more disagreement between them than agreement.

And, like, who is Joe Schmoe going to believe? Me, you, or Lisa Randall?

I also think the Joe Schmoes of the world are smart enough to sense, that even the professionals don't totally know what is going on, even if they don't technically know the details, as to why.

I think it all falls back to that issue that there are really no other clear-cut, simple and concise ways to communicate what is physically going on. So, when the "scientists" try to convey results to the general public, they can fall into the same trap.

It's just one big vicious cycle, lol

I agree. When you break down something complex so that the general public can understand it, there is always the danger of losing or mixing up important information, so that the outcome is something completely different than what you originally wanted to convey. One has to be careful!

 

[FrediFizzx]

You should also read this:

http://arnold-neumaier.at/physfaq/topics/virtual3

Arnold Neumaier thinks that the Coulomb field is fundamental and not quantized. IMHO, he is very wrong. But it seems to have become un-necessarily contraversial but you have to take that stance if you don't think virtual particles are real. I don't think it was this way 20 years ago. But all this posturing is silly for the simple reason that you can't properly describe muon decay without a real "off mass shell" W boson being involved. That really should be the end of the discussion about this. "Virtual" is really a bad name as they really aren't virtual. They have all the properties of "real" particles except that they are "off mass shell".

Fred
moderator sci.physics.foundations

 

[Mordred]

Part of the problem is when professional physicists use terms such a popping in and out of existence, extra undescribable dimensions, etc. The public loses faith in the research. I know when I first started hearing those terms used my first gut reaction was one of scorn. This scorn was so strong that it took me another year or two to even consider any validity in the whole field of quantum mechanics in general.
After finally accepting that quantum mechanics does have very accurate predictions and only after realizing that did I finally look deeper than that initial reaction.
This isn't an uncommon problem talk to any non physicist about these concepts and many would think your crazy lol.

 

[Polyrhythmic]

Arnold Neumaier thinks that the Coulomb field is fundamental and not quantized. IMHO, he is very wrong.

By stating this, you are stating that QED is "wrong".

But all this posturing is silly for the simple reason that you can't properly describe muon decay without a real "off mass shell" W boson being involved.

You keep claiming this, but you have nowhere shown something to back you up. What you say is simply not true.

 

[Polyrhythmic]

Part of the problem is when professional physicists use terms such a popping in and out of existence, extra undescribable dimensions, etc. The public loses faith in the research. I know when I first started hearing those terms used my first gut reaction was one of scorn. This scorn was so strong that it took me another year or two to even consider any validity in the whole field of quantum mechanics in general.
After finally accepting that quantum mechanics does have very accurate predictions and only after realizing that did I finally look deeper than that initial reaction.
This isn't an uncommon problem talk to any non physicist about these concepts and many would think your crazy lol.

This is why one should stick to facts, not to intuition.

 

[FrediFizzx]

By stating this, you are stating that QED is "wrong".

You keep claiming this, but you have nowhere shown something to back you up. What you are saying is simply not true.

How would that make QED wrong? There are many examples of quantization of the Coulomb Field in textbooks.

I have given a very simple description of muon decay. Here it is again,

Standard Model description of muon^- decay; muon^- decays to a muon neutrino and a real "off mass shell" W^- boson. The W boson then decays to an electron and an electron anti-neutrino. Of course it is difficult to have discussion about that since there is no other way to describe the decay than by a virtual W boson.

Fred

 

[qsa]

Really, when you think about it even the virtual particle description is not a very "useful" description, or heruistic, for describing and explaining how forces work. It also seems rather magical. In addition, when you realize the perturbation series is an infinite serires and therefore any process involves an infinite number of virtual paritcles, it gets even more magical.

I personally think all this shows we still have a lot to learn, regardless of how much we think we know.

Lots of folks feel that all the divergences and infinities within QFT indicate an underlying problem with the theory. I can't help but wonder if somehow that was fixed, would we get a new view of how forces "work", which does not include virtual particles.

let say we believe that virtuals are real and "physical" . So what is charge anyway, and what is mass, it is equivelant to energy, oh , what is energy. and what is a photon made of, and how it leaves an electron and how an electron jumps to another level after that. What is a particle ,is it field (of what) or is it litte ball (of what). So can we say we know something but we don't know where and how it became! What is "physical" about these little things and gostly forces which we have nothing in the macroscopic world to compare to. These are models , just models . until a full theory is in hand that explains the why of all these things. The theories remain just approximate models.

 

[Polyrhythmic]

How would that make QED wrong? There are many examples of quantization of the Coulomb Field in textbooks.

I think I got you wrong, do you mean the quantization of the vector potential A_{\mu}? Anyways.

Standard Model description of muon^- decay; muon^- decays to a muon neutrino and a real "off mass shell" W^- boson. The W boson then decays to an electron and an electron anti-neutrino. Of course it is difficult to have discussion about that since there is no other way to describe the decay than by a virtual W boson.

What you describe here is one distinct Feynman diagram. It is no picture of physical reality...

 

[FrediFizzx]

Yes, quantization in the Lorentz gauge of A_{\mu}.

Please get Feynman diagrams out of your head. Now, can you please describe muon decay any other way from the way I have described it? You can't. There has to be a W boson involved in the description or quantum numbers will not add up correctly.

 

[bbbeard]

Wow, this thread has an angels-on-pinheads quality to it, doesn't it? A lot of people tossing around terms they aren't entirely comfortable with, trying to describe a reality that is stubbornly complicated.

All I would personally like to know is whether there is a way to understand electrostatic interaction (not scattering) at the quantum level without the virtual photon concept being useful.

Surely you know this already, don't you? There are answers at every level. At the simplest level, we have the Schrodinger equation in 3 dimensions. How do we describe the interaction of the negative electron with the positive nucleus? Simple. We insert the Coulomb potential into the equation, V = -K/r. Then we solve for the stationary eigenstates, which gives us the familiar spherical harmonics times radial eigenfunctions (exponential times Laguerre polynomials). We also have to make an outer product with the electron spin state, of course. This gives a wonderfully accurate description of hydrogenic (i.e. single-electron) atoms. [My best reference on this is an obscure book by Sol Wieder, The Foundations of Quantum Theory, but this is really basic stuff.] The same machinery would apply to the electrostatic interaction between any two slowly-moving charged particles -- however, if there are no bound states (e.g. electron-electron scattering) then you get a different set of eigenfunctions.

If we want to understand the dynamic interaction of an charged particle with an electromagnetic field, then we have to write the electromagnetic vector potential as an operator. The operator is a sum of creation and annihilation operators, with appropriate doo-dads in front. Then we make a canonical substitution in the Schrodinger equation p -> p - e*A/c. You also have to add a term for the spin-magnetic interaction. [My best resource is Sakurai, Advanced Quantum Mechanics]

Beyond that, we have learned a lot from perturbation theory. And that's where virtual photons come in...

 

[Mordred]

Re: High energy virtual particles

--------------------------------------------------------------------------------

Originally Posted by Mordred
Part of the problem is when professional physicists use terms such a popping in and out of existence, extra undescribable dimensions, etc. The public loses faith in the research. I know when I first started hearing those terms used my first gut reaction was one of scorn. This scorn was so strong that it took me another year or two to even consider any validity in the whole field of quantum mechanics in general.
After finally accepting that quantum mechanics does have very accurate predictions and only after realizing that did I finally look deeper than that initial reaction.
This isn't an uncommon problem talk to any non physicist about these concepts and many would think your crazy lol.



This is why one should stick to facts, not to intuition.



I fully agree Polyrhythmic one should stick to facts not fiction, sounds easy doesn't it until you account to all the fictionous articles commonly published on the internet, broadcasted by news articles etc. Even highly reputable magazines doesn't always publish articles based on facts alone. When you don't work everyday with dealing with these facts, or in a different profession seperating fact from fiction is rather tricky lol.

Makes me glad my job is merely making robots and plant machinery simply work to within an acceptable degree of accuracy.

 

[dm4b]

Part of the problem is when professional physicists use terms such a popping in and out of existence, extra undescribable dimensions, etc. The public loses faith in the research.

Speak for yourself. The weirder physics gets the more I like it, lol.

 

[Mordred]

I have to agree with that in a way it does keep things interesting lol

 

[bbbeard]

Can you describe for me an experiment that detects a real gluon? That would make the discussion more concrete. Also, what is a quantized description of the interaction between two stationary charges that admits to no reasonable interpretation involving virtual photons?

A better question is whether any experiment ever really detects "real" particles. Here is an interesting quote from David Griffiths, Introduction to Elementary Particles:

Actually, the physical distinction between real and virtual particles is not quite as sharp as I have implied. If a photon is emitted on Alpha Centauri, and absorbed in your eye, it is technically a virtual photon, I suppose. However, in general, the farther a virtual particle is from its mass shell, the shorter it lives, so a photon from a distant star would have to be extremely close to its "correct" mass; it would have to be very close to "real." As a calculational matter, you would get essentially the same answer if you treated the process as two separate events (emission of a real photon by star, followed by absorption of a real photon by eye). You might say that a real particle is a virtual particle which lasts long enough that we don't care to inquire how is was produced, or how it is eventually absorbed.

When we glance at the sun, do we see "real" photons? Griffiths suggests: not exactly, but they are darn close.

The question is even trickier with uncharged particles, because we humans lack direct sensors for the weak and strong nuclear forces. Do we ever detect "real" neutrinos? Every now and then the photomultiplier tubes at Kamiokande go crazy, and the pattern of Cerenkov radiation indicates a solar neutrino has struck an atom in the detector. But those neutrinos are ostensibly "virtual" in exactly the sense Griffiths describes -- but darn close to "real".

So far the jet experiments are the closest we have to detecting "real" gluons; the jet gluons are close enough to being on-shell that they form collimated jets when they decay. But, like the neutrinos and photons, they are really only "almost real".

Given all the other approximations and contortions associated with quantum field theory, I wouldn't get too wrapped around the pole thinking that "real" particles (in the QFT sense) have more than a passing correspondence to "real" particles (in the everyday sense). In QFT, when we say a particle is "real", we simply mean that it is represented by an external line in a collection of Feynman diagrams. The patter that connects the collection of Feynman diagrams to an experimental result is just an imperfect way to express the relation of mathematics to reality.

 

[bbbeard]

Well, I would like to see how *anyone* here disputing that virtual particles aren't real would model decay of charged pions and muons without virtual W bosons. This is the Standard Model of particle physics, folks. Well established even by the LHC now.

You are using "real" in a sense that is not used in quantum field theory.

In QFT, internal lines in Feynman diagrams represent "virtual" particles. The external lines in a collection of Feynman diagrams represent "real" particles. So "real" particles are not "virtual", at least in the sense that QFT uses the words.

If all you are trying to say is that there is such a thing as a W boson, no one is disputing that. If you are trying to say that the internal line (that represents a W- in muon decay) is an external line also, then no, that is not true.

However, I sense that you have a misunderstanding of the meaning of the tree-level muon-decay-via-W diagram. Individual Feynman diagrams do not correspond to physical processes. Only the sum of the infinite number of Feynman diagrams that satisfy the boundary conditions (i.e. momenta on the external lines) represents a physical process.

 

[Ken G]

OK, a lot of good points are being made. A particularly important one would seem to be that electric forces are only mediated by virtual photons in some gauges-- this was interpreted above as indications they are "not real." There are several problems with using this to dismiss taking virtual particles seriously as ontological constructs:
1) virtual particles are not claimed to be real, they are claimed to be virtual. This means they seek a new ontological status that normal standards of "realness" cannot be applied to. That seems obvious, but is getting overlooked.
2) virtual particles that mediate forces like the one corresponding to the Coulomb potential are gauge bosons, so it is perhaps not surprising they are not present in all gauges. The purpose of a gauge is not to make a claim on reality, it is to make a calculation simpler. It might well be argued the same can be said about physics in its entirety (echoing qsa above). So although it is useful to establish that virtual photons can be done away with in certain gauges, the question that remains is whether or not we gain access to certain forms of computational convenience by choosing a gauge that supports such gauge boson activity, at the conceptual level. Is anything in physics theory not at the conceptual level, after all?

So anyway, I'm not advocating for the reality of virtual particles, we shouldn't even call them virtual were that the attitude. I'm merely questioning whether or not it really makes consistent logical sense to imagine that physics supports a clear line between what is real and what is just a mathematical trick in some theory. We use any foothold we can get when trying to interpret and understand reality, and we cannot be burdened by the need to make the case that any ontological entity we invoke is "actually real."

 

[Ken G]

When we glance at the sun, do we see "real" photons? Griffiths suggests: not exactly, but they are darn close.

Yes, I think I am very much on Griffiths' page here, that's a nice quote. The point is, when we do physics, we make all kinds of choices about what we care about and what we don't. Those choices will have as much an influence on what we regard as real as what is "actually real." In particular, the former has a clear-cut meaning, and the latter does not.

So far the jet experiments are the closest we have to detecting "real" gluons; the jet gluons are close enough to being on-shell that they form collimated jets when they decay. But, like the neutrinos and photons, they are really only "almost real".

I'd say you've really set the discussion on its ear-- we were debating whether there is actually any such thing as virtual particles, and you are making a strong case that we should really be asking whether there is actually any such thing as real particles. Coup de grace.

The patter that connects the collection of Feynman diagrams to an experimental result is just an imperfect way to express the relation of mathematics to reality.

Spoken like a true empiricist. I think it's times like this that the gap between empiricist views and rationalist views really creates communication barriers, it's just as well that physics proceeds without reaching resolution of this kind of issue.

 

[Mordred]

Spoken like a true empiricist. I think it's times like this that the gap between empiricist views and rationalist views really creates communication barriers, it's just as well that physics proceeds without reaching resolution of this kind of issue.

LOL I had a math professor once state that mathematics is the only universal lanquage, that may be true but it certainly has many dialects and interpretations.

I like this site as an excellent and simple explanation of virtual particles for the novice
It doesn't swamp one with formulas or complex lanquage. I personally think that the term "intermediate particle " or something similar would have led to greater overall acceptance than the usage of the word virtual. Granted this site doesn't present the full picture but it often is useful in explaining the concept to the novice.

http://www2.slac.stanford.edu/vvc/theory/virtual.html
http://www2.slac.stanford.edu/vvc/theory/quantum.html

edit: I also found this article was informative and useful in the past

http://math.ucr.edu/home/baez/physics/Quantum/virtual_particles.html

I posted these to aid understanding to the beginners that frequent this site tidbits like tthese have proven useful in the past so I gathered a collection of em currently around 4 giga bytes worth lol

 

[bbbeard]

Even though perturbative quantum field theory has proven to be extremely successful, the lack of a proper non-perturbative theory is kind of unsatisfying.

You should look into what's going on in lattice gauge theory. For example, check out http://pdg.lbl.gov/2011/reviews/rpp2011-rev-ckm-matrix.pdf" . Scroll through and see how lattice gauge theory is used to produce estimates of important physical quantities.

While there is no doubt that lattice methods are not as well-developed as perturbative QFT, they don't suffer the same limitations in the strong-coupling regime...

 

[bbbeard]

I'd say you've really set the discussion on its ear-- we were debating whether there is actually any such thing as virtual particles, and you are making a strong case that we should really be asking whether there is actually any such thing as real particles. Coup de grace.

It seems to me there are a number of situations like this in physics. Are you familiar with the "thermodynamic limit"? The theory of phase transitions in thermodynamics, it turns out, really only works for infinite systems, or more precisely, in that limit. So we can reasonably ask whether the water boiling on the stove is "really boiling" or just "virtually boiling"...

 

[Mordred]

bbbreard do you happen to know of a good site that teaches the Cabibbo-Kobayashi-Maskawa (CKM) matrix? If so I would be highly interested I use standard matrix calculations regularly however the standard 3 degrees of motion matrixes used in engineering which are limitted in that they do not include the time dimension if you happen to know of a means to describe a 4-d matrix I would be highly interested in how its derived and used. I would also be interested in virtual boiling and the phase transitions in thermodynamics my first impulse is the transitory point from one state to another where additional energy is needed to change from solid to liquid etc but these are new terms to me so am probably wrong on that score lol.

 

[bbbeard]

bbbreard do you happen to know of a good site that teaches the Cabibbo-Kobayashi-Maskawa (CKM) matrix? If so I would be highly interested I use standard matrix calculations regularly however the standard 3 degrees of motion matrixes used in engineering which are limitted in that they do not include the time dimension if you happen to know of a means to describe a 4-d matrix I would be highly interested in how its derived and used. I would also be interested in virtual boiling and the phase transitions in thermodynamics my first impulse is the transitory point from one state to another where additional energy is needed to change from solid to liquid etc but these are new terms to me so am probably wrong on that score lol.

The CKM matrix is a 3x3 matrix that relates the strong (i.e. QCD) eigenstates of the down, strange, and bottom quarks to the weak eigenstates of the same quarks.

One of the little-publicized quirks of the Standard Model is that the down particle in QCD is slightly different from the down particle in the weak interactions. Ditto strange and bottom. The free parameters in the CKM matrix (three Cabibbo angles and a phase factor) are some of the plethora of free parameters in the Standard Model.

If you want to learn more, just google "CKM matrix"... ;-)

 

[Vanadium 50]

CKM matrix? This has gone way far afield of the original question.