Misconceptions about Virtual Particles - Comments

[A. Neumaier]

The problem is (if history is any indication) that they will immediately be reverted by someone who thinks he knows what he is doing, but doesn't.

If it is done poorly, yes. To be successful in the enterprise of someone else one must of course respect the rules of the enterprise. Reversions are mainly made when these are not respected. Thus the first thing to do before making the first changes is to read the rules for making good contributions. (Together with the dependent pages it is a lot, but not an endless amount. Knowing and respecting it better than the previous editors of the page is a big plus.)

One must fight them with their own weapons, not against them. If you can argue that all you do is according to your rules - and with higher standards than what was there before - you can re-revert any attempted reversion. (One of my brothers working on the interface of mathematics and music had this experience.) It is not easy but it would be worth doing - it just needs enough time and commitment, good preparation, a perceptive communication style, and a way of writing that accommodates alternative views without compromising correctness.

Thus one needs to make sure that whatever is claimed is justified by an explicit link to a textbook, and whatever is called in question must be done by pointing out the lack of proper sources. Then one can replace it by equivalent but proper text, with proper citations, or add qualifying remarks that this is the popular science version (since only a popular science book or an article in the Scientific American, etc. is cited). Whenever your text fits the rules significantly better than the previous version of the text, your text will stay or be further improved. Each page also has a talk page associated with it where controversies about the content can be discussed prior to corrections made or after corrections have been reverted. This talk page should also be consulted before changing the main page.

Finally, the fitting advice of someone whose students changed the world within a few generations: ''Unless your sense of truth and your knowledge of the rules surpasses that of the scribes and pharisees, you will never reach the goal.'' (Matthew 5:20, my paraphrase)

 

[friend]

One must fight them with their own weapons, not against them.

Can you create your own wikipedia.org article "Non-existence of virtual particles"?

I think the problem is that virtual particles by themselves (not as part of a perturbation series) have not been fully justified. If they could be explained on first principles and shown how they are used to explain physical properties, then I think this argument would end. This would be equivalent to explaining the necessity of quantum theory to begin with. And I think the present paradigm is to "shut up and calculate". So it will take someone outside that community to get the ball rolling.

 

[A. Neumaier]

virtual particles by themselves (not as part of a perturbation series) have not been fully justified.

They have never been given a meaning; so there is nothing to justify them. Fantasizing something based on words copied from popular lectures and essays is not a way to make science.

 

[A. Neumaier]

Can you create your own wikipedia.org article "Non-existence of virtual particles"? So it will take someone outside that community to get the ball rolling.

Good luck! With the knowledge of quantum mechanics you demonstrated on PF, you'd make things much worse. You didn't heed the final advice of my post #71.

 

[ddd123]

My experience with Wikipedia is that, in practice, the most crucial content is decided by a tightly knit community of editors that band wih each other to keep a certain narrative in place.

In any case, the example of Susskind is another one. Does anyone want to answer that? Here it's even advanced seminars where the above tale as explained by Neumaier is taken entirely seriously. I still feel justified in asking why is that, even if I end up annoying bhobba.

 

[A. Neumaier]

the example of Susskind is another one

In a book or survey article for physicists? Please give references.

 

[mfb]

I don't think it's possible to derive the justification of quantum theory from the axioms we now have. Those are just given to us a priori. And why nature operates according to these rules is not apparent yet. What we have so far are just reverse engineering equations that we have converged on through trial and error. So they are just curve fitting equations that happen to work - so far.

Every physical theory is ultimately made to describe experiments. That is the whole point of physics. If you don't like it, go to philosophy. Making up theories that have no connection to our universe is not physics.
If you can find a function that gets fit to 25 data points and then predicts thousands of others, you have a really successful "curve fitting".

 

[ddd123]

In a book or survey article for physicists? Please give references.

Friend should give it, but I believe him because I hear it all the time. The next time I'll hear it I'll try to remember to let you know.

 

[A. Neumaier]

In a book or survey article for physicists? Please give references.

Friend should give it, but I believe him because I hear it all the time.

I found a reference on p.13 of Susskind's TASI lectures where he mentions virtual processes on p.13, without accompanying mathematics. He describes the laymen version of the Hawking effect in similar terms as Carlip - and on a similar level of superficiality. I wonder how he can substantiate his claim there ''the quark spends part of the time in a virtual state with the wrong baryon number even in empty flat space. What percentage of the time is the baryon number wrong? One might think the answer is incredibly small given the stability of the proton. But it is not. An explicit calculation gives a probability [...]''. The calculation would give an indicator of what he means with his talk. But he doesn't even outline it nor give a reference. The same holds for his other statements on the same page, such as ''The baryon number is constantly undergoing very rapid quantum fluctuations''. No substantiation or reference is given; so it is mere rhetorics, not credible physics.

I found another reference on p.7 of his paper on ER=EPR, perhaps related to what friend mentioned. Again only pictures without formulas or references - mere illustrative rhethorics. Indeed, one can easily see that the virtual particles play no role in the remainder of the paper; they are just a casual remark. And the whole paper is very philosophical anyway with very few formulas - not hardcore quantum field theory. He makes other statements that are careless and wrong if taken literally, e.g., on p.2, ''The universe is filled with subsystems, anyone of which can play the role of observer.''

In both cases, Susskind ostensibly applies very low technical standards to his presentation. The papers appear to be transscripts of the actual lectures (in the second case this is explicity stated at the end of p.2), where one often takes ad hoc liberties to keep the audience alive.

The real point is that (to my knowledge) never ever has anyone substantiated the talk about the properties of virtual particles by a mathematical derivation of these properties from the principles of QFT; they appear (with various degrees of sloppiness) exclusively in informal overviews, and thus are nothing but didactical gadgets aimed at capturing the attention of the audience.

 

[ddd123]

"the quark spends part of the time in a virtual state with the wrong baryon number even in empty flat space. What percentage of the time is the baryon number wrong? One might think the answer is incredibly small given the stability of the proton. But it is not. An explicit calculation gives a probability [...]''. The calculation would give an indicator of what he means with his talk. But he doesn't even outline it nor give a reference.

So there are mystery calculations which we cannot know about that contain the secret of the virtual particles, will we get to the bottom of this? Shouldn't this be in some published article? Or does Susskind keep his work private?

 

[Vanadium 50]

Re: Just Curve Fitting

It always amazes me to see how people who aren't able to do the science themselves belittle those who can. The electron magnetic moment (technically the gyromagnetic ratio) is known to thirteen decimal places. That's certainly more than you'd expect from your dismissive "just curve fitting".

 

[bhobba]

How do I argue with that? Here Prof Susskind is not referring to virtual particles defined in terms of some perturbation series in the calculation of some observable. He says that these virtual particles are those virtual pairs that pop in and out of existence as in the popular accounts.

By doing a post pointing to the lecture and asking the experts to explain what he said. It's obvious that's how to proceed - why you don't do it and instead keep arguing about it has me beat. I strongly suspect you are misinterpreting what he said, and if its the lectures I am thinking of they are not to his peers - its for a semi lay audience.

Another course is to study the theory for yourself.

Thanks
Bill

 

[bhobba]

There does seem to be different views on that.

There isnt. You simply want to read that into it because you have already made your mind up about it. If you think otherwise post a link to a paper that says it and ask the experts here to explain. Unfortunately many papers shouldn't really have passed peer review - but somehow did. The actual scientists that post here are in the best position to judge that.

Thanks
Bill

 

[ddd123]

and if its the lectures I am thinking of they are not to his peers - its for a semi lay audience.

The culpit lectures at TASI linked by Neumaier? Theoretical Advanced Study Institute in Elementary Particle Physics (TASI).

Refusing to acknowledge the existence of misconceptions among experts won't make them go away. Maybe send Susskind an email.

 

[bhobba]

Refusing to acknowledge the existence of misconceptions among experts won't make them go away. Maybe send Susskind an email.

First we need to know what was said and then we need to know he was not speaking heuristically. I highly doubt both those conditions are meet.

But even aside from that I know I am on firm ground. If those arguing about it can point to a QFT textbook that says it then it will provide evidence - without that - well the implication is obvious.

Thanks
Bill

 

[ddd123]

I'm not doubting that you're right, I'm discussing the problem that extremely influential theorists really do seem to believe in the existence of virtual states or at least discuss them as if they existed with non-lay audiences (the arxiv paper above with the TASI lectures says that minimal string theory knowledge is required - I take that to imply that non-minimal knowledge of QFT is). Neumaier answered me in the affirmative, it is a problem. Also Neumaier seems interested in this sociological conundrum.

 

[bhobba]

I'm discussing the problem that extremely influential theorists really do seem to believe in the existence of virtual states

And I am saying they don't believe that - they are simply being loose. This can go around and around. Since you can't get into their head you can't know. But having studied it I am certain that's what's happening - as anyone would be if they studied any QFT textbook.

Thanks
Bill

 

[ddd123]

Then what is Susskind being loose about when he says, in the TASI lecture linked above, that "An explicit calculation gives a probability for a quark spending a non-infinitesimal time in a virtual state"? I have studied Sterman and I can't answer myself this question. Being dismissive won't help others to dispel the myth.

 

[A. Neumaier]

Then what is Susskind being loose about when he says, in the TASI lecture linked above, that "An explicit calculation gives a probability for a quark spending a non-infinitesimal time in a virtual state"? I have studied Sterman and I can't answer myself this question. Being dismissive won't help others to dispel the myth.

I don't think a calculation showing this exists. Moreover, even if this calculation exists and shows what is claimed it doesn't make his point since virtual states are not states of virtual particles. (See the definitions in the Insight article ”The Physics of Virtual Particles”)

Some related calculation probably exists, but it most likely says something else than what he takes it to mean unless the meaning is taken with many grains of salt (as statements about virtual particles always should - and his (probably grad student) audience should already be aware of this. (He still sets a bad example by being too sloppy.)

Such blunders (of claiming occasionally a bit more in an informal statement than what really holds) are not rare in lectures. I am slightly prone to them myself, even in math, where everything is fully checkable. Note that both sources are not refereed papers but transscripts of lectures placed on the arXiv. I made a search on google scholar and found only these two sources by Susskind involving the word ''virtual particle''. Thus it seems that in his formal publications he is much more careful.

In general, using sloppy language doesn't mean that the user believes in its literal truth. It is used like common currency. Does everyone using a 1-dollar bill express that ''in God we trust''? The majority of users probably mean ''in Gold we trust''.

 

[ddd123]

But if even you can't guess what Susskind actually meant, how are his students supposed to understand it?

 

[A. Neumaier]

But if even you can't guess what Susskind actually meant, how are his students supposed to understand it?

I ask this myself about every lecture or paper about many worlds.

In the TASI lectures I have no difficulty guessing the intended meaning. It is clear that Susskind meant to say (and illustrate in a visually impressive way) that Hawking radiation implies that there is a complex ''interplay between gravity and quantum mechanics'' (p.14). The details didn't matter since they were not needed for what follows. (It is usually in such situations that inaccuracies creep into a description.) Thus intelligent students lose nothing by being mystified about his remarks on p.13.

Only the dumb ones that take for gospel everything uttered by a famous physicist have problems. Rightly so. It is the standard payoff of credulosity.

 

[Nugatory]

Closed for a bit of moderation

 

[Nugatory]

This thread has been reopened. However, I have to remind everyone that the point of this thread is to discuss the article by @A. Neumaier. Arguments and disagreements with its content should be based on experience with the computations that he describes, not non-specialist and popular presentations.

 

[A. Neumaier]

any misconceptions related to Hawking radiation and virtual particles? As I recall, it was in the 1970s when I attended a presentation at MIT by Hawking describing his concept of black hole radiation based on the creation of particle pairs [...]

I just learned from a discussion on http://chat.stackexchange.com/rooms/71/the-h-bar that [quoting ACuriousMind, bold is his]

Hawking's original article contains the "virtual particle analogy" with an explicit warning that that is not the reason! It says: "One might picture this negative energy flux in the following way. [virtual particles, blah, blah]. It should be emphasized that these pictures of the mechanism responsible for the thermal emission and area decrease are heuristic only and should not be taken too literally."

Indeed, Hawking's original article gives on p.4 the following version of the fairy tale, including the caveat at the end:

One might picture this negative energy flux in the following way. Just outside the event horizon there will be virtual pairs of particles, one with negative energy and one with positive energy. The negative particle is in a region which is classically forbidden but it can tunnel through the event horizon to the region inside the black hole where the Killing vector which represents time translations is spacelike. In this region the particle can exist as a real particle with a timelike momentum vector even though its energy relative to infinity as measured by the time translation Killing vector is negative. The other particle of the pair, having a positive energy, can escape to infinity where it constitutes a part of the thermal emission described above. The probability of the negative energy particle tunnelling through the horizon is governed by the surface gravity K since this quantity measures the gradient of the magnitude of the Killing vector or, in other words, how fast the Killing vector is becoming spacelike. Instead of thinking of negative energy particles tunnelling through the horizon in the positive sense of time one could regard them as positive energy particles crossing the horizon on pastdirected world-lines and then being scattered on to future-directed world-lines by the gravitational field. It should be emphasized that these pictures of the mechanism responsible for the thermal emission and area decrease are heuristic only and should not be taken too literally.

In the discussion mentioned above, yuggib also mentioned an (idealized, but within the idealization fully rigorous) derivation by Fredenhagen and Haag.

 

[Haelfix]

At the heart of this is a little bit of philosophy. Do you prefer an ontology based on the concept of states, or do you prefer an ontology based on Feynman diagrams.

The real answer is that neither quite works in QFT, the Feynman diagram ontology for all the reasons listed here, the state ontology b/c no Hilbert space has ever been constructed for interacting quantum fields in 3 + 1 dimensions (bound states and states in a confining phase are also mathematically difficult to deal with).

So I disagree a little with the thrust of this thread. I would say one uses the concept that is useful to solve problems with. Practicing physicists have absolutely no problem talking about the Dirac sea for instance, even though it's clear the concept has limited validity. In particle physics, it is often useful to visualize things with the Feynman diagram ontology, although again it depends the details of the circumstance. It works great for an Abelian theory like QED, less useful for something like QCD.

 

[A. Neumaier]

the state ontology b/c no Hilbert space has ever been constructed for interacting quantum fields in 3 + 1 dimensions

This doesn't mean that it doesn't exist, only that the mathematical tools to prove its existence with full rigor are not yet strong enough. The concept of an S-matrix would be completely meaningless if the Hilbert space wouldn't exist. Most physics is not mathematically rigorous, but nevertheless believed to be correct.

it is often useful to visualize things with the Feynman diagram ontology

I never disputed that. Diagrams are there to illustrate, not to provide causal agents (as virtual particles are considered in the view for lay people). In the insight article under discussion I had stated explicitly:

The only way the usual dynamical language for virtual particles is justified by the theory is as purely figurative analogy in ”virtual reality”, useful for informal talk about complicated formulas and for superficial summaries in lectures capturing the imagination of the audience. This has to be kept in mind when reading in professional scientific publications statements involving virtual particles. Otherwise many statements become completely misleading, inviting a magical view of microphysics and weird speculation, without the slightest support in theory or experiment.

 

[vanhees71]

At the heart of this is a little bit of philosophy. Do you prefer an ontology based on the concept of states, or do you prefer an ontology based on Feynman diagrams.

The real answer is that neither quite works in QFT, the Feynman diagram ontology for all the reasons listed here, the state ontology b/c no Hilbert space has ever been constructed for interacting quantum fields in 3 + 1 dimensions (bound states and states in a confining phase are also mathematically difficult to deal with).

So I disagree a little with the thrust of this thread. I would say one uses the concept that is useful to solve problems with. Practicing physicists have absolutely no problem talking about the Dirac sea for instance, even though it's clear the concept has limited validity. In particle physics, it is often useful to visualize things with the Feynman diagram ontology, although again it depends the details of the circumstance. It works great for an Abelian theory like QED, less useful for something like QCD.

What's the difference? Feynman diagrams are just a mathematical notation for the perturbation series for S-matrix elements (in the original version applied to "vacuum QFT", i.e., for treating the scattering of two particles (or decays of one particles) into a few other particles). Underlying is just the formalism of QFT, as has been demonstrated by Dyson in 1948ff.

As a practitioner of QFT, including equilibrium and non-equilibrium relativistic many-body QFT, I've never ever used nor had the desire to use the Dirac sea, which doesn't exist but is renormalized away at the very first steps in building up the formalism employing "normal ordering" to define local observables of (asymptotic) free fields.

I also don't know, what you mean by "Feynman diagram ontology". Is there in ontology implied by Leibniz's vs. Newton's notation of calculus or any other mathematical notation used in physics? Imho this is an example for philosophical mumbo-jambo that discredits philosophy in the opinion of many scientists!

 

[Haelfix]

Hi Vanhees,

I've never ever used nor had the desire to use the Dirac sea, which doesn't exist but is renormalized away at the very first steps in building up the formalism employing "normal ordering" to define local observables of (asymptotic) free fields!

What you say is true, nevertheless, the concept is still utilized all the time by colleagues in solid state physics as a sort of effective description. Indeed it is even utilized more broadly as a cursory google scholar search shows. The point is convenient fictions are ubiquitous in physics.

Feynman diagrams are just a mathematical notation for the perturbation series for S-matrix elements (in the original version applied to "vacuum QFT", i.e., for treating the scattering of two particles (or decays of one particles) into a few other particles). Underlying is just the formalism of QFT, as has been demonstrated by Dyson in 1948ff

I certainly never suggested the contrary.

I also don't know, what you mean by "Feynman diagram ontology". Is there in ontology implied by Leibniz's vs. Newton's notation of calculus or any other mathematical notation used in physics? Imho this is an example for philosophical mumbo-jambo that discredits philosophy in the opinion of many scientists!

Perhaps philosophy is a poor word choice and an example would make the point. When we talk about a background like Higgs to WW(star), what we might have in mind is a decay that first produces a W and a virtual W, and then is completed to a final state which might be something like l l v v. The intermediate state is just going to influence the final amplitude much like an extra slit does in the interpretation of the final result of a double slit experiment. However if you are completely dogmatic about the state interpretation, the first part of the sentence is nonsense as one of the W's is offshell and carries no interpretation as a particle state. Despite this, hundreds of papers analyzing backgrounds has been written about this exact thing. Ok?

So now if you follow this convenient fiction down the rabbit hole, you might ask, well what we measure is actually not even those final leptons (and missing energy). What we measure is a voltage drop after some long chain of indirect emissions, absorptions and inferences. So the curious student would then say.. Wait a second, since what we measure is not those leptons but they in fact have a finite lifetime within the detector, why couldn't I write the whole thing as a larger diagram where the leptons are in fact internal lines of a bigger diagram? What's the difference between doing something like this and talking about virtual W's?

That is the sort of chain of reasoning (and ontology) that Profesor Neumaier I think would reject, for reasons given in this thread and countless others, but my point was that it is sometimes useful to talk about decays like W Wstar.

At the end of the day, philosophy enters into this b/c things we measure don't exactly correspond mathematically to idealized Von Neuman measurements (with perfect response functions) of S Matrix elements in an infinitely large box off in the infinite future. The mathematics is unambigous, but how you apply the math to the physics does correspond to implicit choices.

 

[Haelfix]

This doesn't mean that it doesn't exist, only that the mathematical tools to prove its existence with full rigor are not yet strong enough. The concept of an S-matrix would be completely meaningless if the Hilbert space wouldn't exist. Most physics is not mathematically rigorous, but nevertheless believed to be correct.

Yes, I agree although I do prefer not to prejudice myself too much. The failure of things like AQFT likely means the tools we use are wrong, but it could also mean that the theory actually doesn't exist or even alternatively a qft in 3+1 might exist but doesn't correspond to anything physical (much like trying to make the Navier-Stokes equation is essentially an academic exercise, as atomic structure enters into the picture at a certain scale).

 

[A. Neumaier]

that it is sometimes useful to talk about decays like W Wstar.

If it is sometimes useful to talk about a decay like this in terms of virtual mythology, could you please be more specific about what its usefulness consists of?

Unstable particles (with complex mass) are very real - in the present case observable as a resonance. I cannot see what's the use of treating them as virtual particles (with real mass). One trades a clear physical picture with a clear mathematical representation (as complex pole of certain cross sections ) for a fuzzy picture in virtual reality without any substance .

After the trade, there is no longer a way of talking about half-life (an essential property an unstable particle) except in terms of a vague reference to an alleged uncertainty principle that would allow particles to pop in and out existence for a split fraction of a picosecond.

So where is the usefulness?

[Added May 2: Part of the subsequent discussion in posts #101-#152, partly based on a misunderstanding on my part, is resolved in post #153.]

 

[A. Neumaier]

W Wstar

The physical way of handling $W^*$ is not to represent it as a virtual particle but to treat it as a transition state.
This is a well-developed science in the case of chemical and nuclear reactions, and it applies in nprinciple down to the smallest scales. See, e,g,,
Hänggi, P., Talkner, P., & Borkovec, M. (1990). Reaction-rate theory: fifty years after Kramers. Reviews of modern physics, 62(2), 251.

There is no sound reason at all that would justify confusing transition states with virtual particles.

 

[vanhees71]

Hi Vanhees,

What you say is true, nevertheless, the concept is still utilized all the time by colleagues in solid state physics as a sort of effective description. Indeed it is even utilized more broadly as a cursory google scholar search shows. The point is convenient fictions are ubiquitous in physics.

In solid-state physics you usually have a Fermi sea, and the advantage of the Fermi's sea over Dirac's is that the former really exists ;-)).

 

[vanhees71]

Unstable particles (with complex mass) are very real - in the present case observable as a resonance. I cannot see what's the use of treating them as virtual particles (with real mass). One trades a clear physical picture with a clear mathematical representation (as complex pole of certain cross sections ) for a fuzzy picture in virtual reality without any substance .

Yes, but it is important to keep in mind that resonances are, strictly speaking, not asymptotic free states, and it is sometimes even important to tell, how you define there properties in terms of the cross sections, where they appear as "resonance peaks". An example is the $\rho$ meson, which in the particle data book is defined (!) as the resonance appearing in $\mathrm{e}^+ \mathrm{e}^{-} \rightarrow \pi \pi$ or in $\tau \rightarrow \pi\pi \nu$ in the invariant-mass region of the $\pi \pi$ around 770 MeV.

If you work in my field of relativistic heavy-ion collisions, the light vector mesons also occur in dilepton emission rates in terms of effective hadronic models, describing the electromagnetic transition form factors of hadrons (most importantly baryon resonances), the socalled vector-meson dominance model. In other words, here the $\rho$ appears as an intermediate state in the Dalitz decay of hadrons, and there its shape of course looks completely different. To take a not too narrow resonance as a kind of "particle" can lead to many misunderstandings and a lot of discussion. The $\rho$ meson, however, is not that narrow with a mass of around 770 MeV and a width of about 150 MeV. So one should keep the meaning of resonances as compared to "particles" in mind to avoid such misunderstandings! See, e.g., an informal presentation I've given some time ago for discussing right this:

http://th.physik.uni-frankfurt.de/~hees/publ/tud-dileps15.pdf

 

[A. Neumaier]

it is sometimes even important to tell, how you define there properties in terms of the cross sections, where they appear as "resonance peaks".

This is covered by the notion of a transition state - see post #101. They are asymptotic states in a complex deformation of the Hilbert space, e.g., by what is called complex scaling. I covered this in the companion Insight article to that under discussion. I updated the discussion there to include the references in posts #101 and #103.

 

[mfb]

The physical way of handling $W^*$ is not to represent it as a virtual particle but to treat it as a transition state.

So where is the border? Is the W in a pion decay still a transition state? What about the Ws in neutral meson mixing? What about gluons in a NLO Feynman diagram?