Insights Misconceptions about Virtual Particles - Comments

[A. Neumaier]

Isn't this getting out of hand?

It has been out of hand for many years now. It is a very bad state of affairs, and difficult to reverse since fantasy sells much better than science. But physics goes on as always, and the textbooks on QFT are much better than their popular science counterpart.

Perhaps someone reading this here has the motivation, time, and patience to edit the wikipedia articles (anyone can!) and fight the changes through (which is likely to be hard and time-consuming - I won't do it). It would be best if the most infected pages would get split into two, as suggested here.

 

[A. Neumaier]

From the point of view of QFT, a constant background energy is unobservable, so there is no reason not to define things so that the vacuum state has zero energy. However, when people eventually start dealing with quantum gravity, they might want to reassess this, because a constant background energy would contribute to spacetime curvature.

In quantum gravity, there is no distinguished vacuum; this is what the Unruh effect demonstrates. Thus there is also no vacuum energy. In full quantum gravity, there is also no background, as the metric is generated dynamically.

There are of course contributions of the various fields to the energy density, but these are everywhere exactly canceled by the gravitational energy. This happens classically as a consequence of what remains from Noether's theorem, and it would be strange if the quantum version wouldn't show the same feature.

The specific distribution of the various forms of energy are properties of the state realized by Nature, not of the quantum gravity theory itself. The latter is about all possible states, while Nature realizes only one of these. Our existence and what we observe proves that this state is neither a vacuum state, nor one obtained from such a state by considering it in the coordinates of a different observer.

Thus a reassessment of the question can only render it meaningless. That it is a question now is only because people are working on small trial fragments and try from these to make guesses about the whole thing without taking into account all constraints. This is legitimate as long as the final theory is not yet clear (since one doesn't know in advance where the relevant changes are needed) but must be a temporary feature that goes away when the final word can be spoken.

 

[stevendaryl]

This is the answer I feared. I don't think it's akin to Feynman's slight oversimplification in the beginning of the QFT story. It seems like a terrible outcome, in which a very slight imprecision is met with such a widespread enthusiasm it is insisted upon a little more. Then pieces are added little by little until you've created a big fraud that runs in parallel with actual science. It's what psychologists call "entrapment", instead of admitting a loss you keep investing upon it because it's become too big to count as a loss, you need to put more and more on stake to make it salvageable.

I know physicists who work at CERN who believe in virtual particle's existence. I've seen you reply on one nature's article saying virtual particles are real because of QCD, so you know what I'm talking about, it's not just a problem of popularization but it's feeding misconceptions of the physicists themselves (except those who specialize in QFT technicalities, I suppose). Isn't this getting out of hand?

On the other hand, is it so important that laymen have misconceptions about fundamental physics? What experts hope is that the layman's understanding of a technical topic is a subset of the expert's understanding. That is, the layman will of course understand less than the expert, but shouldn't believe false things. That may be desirable, but unrealistic, given the way that human minds work. Nobody is satisfied with an arbitrary collection of facts. They try to piece the facts together into something that seems like a coherent picture. People perform some kind of mental closure operation. Even if the layman is given only true facts, the closure will likely include some false statements. It might be inevitable.

So I don't think that it does any good for the expert to try to correct misconceptions by handing out individual nuggets of truth. You have to try to nudge the layman into a slightly more accurate closure. You can't just tell people: Your intuitive picture is wrong. You have to suggest a different, better intuitive picture. Or at least, that's my experience.

 

[friend]

Only a sickness of naive QFT with bare particles. In any sensible treatment the (renormalized = physical) vacuum energy is exactly zero by definition - this is the very starting point! And all physical quantities come out finite.

I heard one professor explain that this infinite vacuum energy is fixed by the fact that nearby vibrational modes propagate to that point and tend to cancel out what would otherwise be an infinite energy, or some other words to that effect. Is this the renormaization that you are referring to? Thanks.

 

[A. Neumaier]

I heard one professor explain that this infinite vacuum energy is fixed by the fact that nearby vibrational modes propagate to that point and tend to cancel out what would otherwise be an infinite energy, or some other words to that effect. Is this the renormaization that you are referring to? Thanks.

Nothing cancels; the vacuum energy is exactly zero by construction.

Renormalization of the vacuum energy means that you normally order the expression for the energy. As a result, each harmonic oscillator has ground state energy zero, hence the vacuum energy is zero, too. Normal ordering is the simplest of a number of renormalization steps that are necessary to arrive at a physically acceptable quantum field theory.

 

[Vanadium 50]

Perhaps someone reading this here has the motivation, time, and patience to edit the wikipedia articles

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.

 

[stevendaryl]

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.

I say don't worry about it so much. Instead, people who do know what they're talking about should work on two things: (1) Reminding people that popular accounts always are misleading in small or large ways, so you should take what you read with a grain of salt. (2) Work on coming up with more accurate intuitive pictures of physics to replace the less accurate intuitive pictures. Getting rid of all misconceptions is a fools errand.

 

[friend]

You have been told that's not what is going on. Yet you ignore it, simply say seem as if it makes it true, and continue on regardless.

What I have been told by Leonard Susskind in his ER=EPR lecture to his peers is that space itself is defined in terms of the entanglement of the "virtual particles" that fill all of space. 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.

 

[Vanadium 50]

What I have been told by Leonard Susskind in his ER=EPR lecture to his peers is that space itself is defined in terms of the entanglement of the "virtual particles" that fill all of space. How do I argue with that?

1. You could educate yourself.
2. A good start would be to read his paper with Juan Maldacena where the words "virtual particle" never appear.

 

[friend]

1. You could educate yourself.
2. A good start would be to read his paper with Juan Maldacena where the words "virtual particle" never appear.

There does seem to be different views on that.

 

[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?