Misconceptions about Virtual Particles - Comments

[A. Neumaier]

They are constructs.

But (in principle) measurable constructs: One can check whether or not people are in the room, buckyballs are present, or the magnetic field is nonzero. Presence = being significantly nonzero.

 

[ddd123]

I'm not sure I'm satisfied with these answers about pop science. You guys say:

the description of the force as arising from virtual particle interactions is just a heuristic.

But a "heuristic" is something like a crude calculation with inconsistent premises (like old quantum theory). Here it seems different, since there is no such crude calculation with virtual pairs, but instead something like a mythology, if I assume your view is correct. And:

You will find calculations that do not involve virtual particles at all. But those calculations are impossible to describe to laymen accurately, so the description with the virtual particles was invented. It is not true, but it sounds nice - if you don't understand the actual physics.

But it seems actually possible, simply by saying:

Nothing virtual happens. The dry facts are that two real particles are created from gravitational energy (from two gravitons or from an external gravitational field), not from the vacuum. One particle escapes, the other is absorbed.

Then why don't popularizers (extremely respected scientists) just say this? Again, assuming your view is correct, it seems like they made some story up for no reason.

 

[friend]

Nothing virtual happens.

Then what is the cosmological constant if not the vacuum energy that is doing something - accelerating the universe? This is not the result of real particle interaction. So there must be something going on in the world of the virtual that is having a real effect, right?

 

[mfb]

Then what is the cosmological constant if not the vacuum energy that is doing something - accelerating the universe?

Maybe it is just a term in general relativity. This is by far the easiest option.
Maybe it is some undiscovered field.
Maybe it is our poor understanding of quantum gravity. But even then it is not from virtual particles.How many scientists and posts do you need to tell you "virtual particles do not exist" until you stop asking the same questions in 100 different ways, while always getting the same answer? Do you really expect a different answer in post 101?

 

[bhobba]

There are such things as real particles, right?

Of course

And there was a time before there were these real particles, during inflation, for example, right?

https://en.wikipedia.org/wiki/Inflaton

'Random quantum fluctuations triggered a phase transition whereby the inflaton field released its potential energy as matter and radiation as it settled to its lowest-energy state.'

This is similar to spontaneous emission - nothing to do with virtual particles
https://en.wikipedia.org/wiki/Spontaneous_emission

It does seem that real particles do pop into existence from the vacuum when acceleration is involved

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.

Instead of arguing the point your time would be better spent studying the theory so you understand why you are mistaken.

Thanks
Bill

 

[bhobba]

Isn't it true that virtual particles are just another name for quantum fluctuations from which these real particle come when there's acceleration

You have been told time and time again, so many times I have lost count they are just pictorial representations of terms in a Dyson series
http://rutracker.org/forum/tracker.php
'In scattering theory, a part of mathematical physics, the Dyson series, formulated by Freeman Dyson, is a perturbative series, and each term is represented by Feynman diagrams.'

It isn't just us that says it - its Wikipedia the general lay source on such things. Yet you still want to argue it. It simply makes no sense.

Now if you want to chat about something actually interesting, that article mentions the series is asymptotically divergent. How can a divergent series predict anything? That is a much more interesting issue, but requires a new thread.

Thanks
Bill

 

[bhobba]

I'm not endorsing these uses of "virtual particles", I'm just saying that it's not surprising that laymen believe these things about them.

100% agree.

But this particular poster has been given the facts innumerable times yet still wants to argue it.

It's perfectly understandable a person reading popularisations, and even some professional literature and textbooks where the authors are being 'loose', gets that impression. They come here and we tell them the truth. There may be a bit of tooing and frowing but after they go away enlightened. But in this case the poster just refuses to let the issue go. I don't know why.

Thanks
Bill

 

[bhobba]

Then why don't popularizers (extremely respected scientists) just say this? Again, assuming your view is correct, it seems like they made some story up for no reason.

Its the same as the wave particle duality. The truth is impossible to convey without math, so they resort to half truths that can be conveyed with pictorial vividness.

You don't have to take our word for it. The truth is there in standard textbooks eg
https://www.amazon.com/dp/019969933X/?tag=pfamazon01-20

You can check it for yourself.

I have mentioned that many many times, yet no one wants to take up my offer. I suggest they look into themselves as to why.

Thanks
Bill

 

[bhobba]

Then what is the cosmological constant if not the vacuum energy that is doing something - accelerating the universe?

Vacuum energy and virtual particles are different things. Vacuum energy in QFT is actually infinite and one of the first indications of a sickness in QFT and the need for renormalisation, although it can be eliminated by what's called normal ordering.

Thanks
Bill

 

[A. Neumaier]

Vacuum energy in QFT is actually infinite and one of the first indications of a sickness in QFT

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.

 

[A. Neumaier]

real particle interaction

Whatever happens in Nature (as described by quantum field theory) is the result of real field interaction, not necessarily of real particle interaction. Real particles are only semiclassical, approximate talk about the quantum fields with limited validity. Whenever the particle description is appropriate (and only then) it gives a simple abbreviated summary of what really happens on the field level, essentially by representing the field processes in space and time as sequences of collisions with random outcomes.

what is the cosmological constant if not the vacuum energy

The cosmological constant is a term in the action of classical general relativity. It is not the vacuum energy. Neither the concept of vacuum nor the concept of a total energy does exist in general relativity. There cannot be a vacuum in the strict sense since there is always a gravitational field. (Except in a completely empty and flat universe, which is physically meaningless in general relativity as one may interpret everything in terms of accelerated observers seeing a different gravitational field.)

In quantum gravity the cosmological constant is like any constant in the action - it is a meaningless bare label that must be renormalized to get its proper (renormalized = physical) meaning. Since there is so far no consistent theory of quantum gravity every talk about the cosmological constant is pure speculation - intutitive subjective attempts to relate poorly understood fragments of information. None of the major approaches to quantum gravity can tell anything definite about it.

Virtual particles are unconnected to all this, except in the very simplified accounts created for the entertainment of the general public. Virtual particles live in a completely different world, a virtual reality created by humans, where they are used to illustrate complicated mathematical formulas in terms that give an illusion of understanding. The reason why this is done is that people want to get an idea of what is going on in the microscopic world. But without a good command of mathematics it is impossible to give more than an illusion of what actually happens.

On the other hand, physics is about what really happens, in the real world, not in virtual reality. Thus whoever wants to learn about real physics on the microscopic level first needs to be thoroughly disillusioned. PhysicsFoums is a place for discussing real physics, and this is incompatible with the illusions that make up the simplified lay view.

In this forum you have the opportunity to grow up in your understanding of physics. If you want to keep your illusions you are here in the wrong place. As there is no way of reconciling the stork bringing babies with genetic reproduction there is no way of reconciling virtual particles populating the vacuum with real physics. Growing up includes saying good bye to seemingly valid myths that stand in the way of real understanding.

 

[bhobba]

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.

That is indeed the modern view.

Thanks
Bill

 

[vanhees71]

Vacuum energy and virtual particles are different things. Vacuum energy in QFT is actually infinite and one of the first indications of a sickness in QFT and the need for renormalisation, although it can be eliminated by what's called normal ordering.

Thanks
Bill

Ironically "vacuum energy" (i.e., closed bubbles without external legs in terms of Feynman diagrams) are just what's canceled when calculated connected scattering matrix elements which are encoding what's really observable, namely transition rates and cross sections. Have a look in any textbook on QFT (the keyword is the Lehmann-Symanzik-Zimmermann (LSZ) reduction formula), which is mathematically a tricky business.

What's often paraphrased as a vacuum "filled with virtual particles" is indeed just some popular-science myth. What's behind it are indeed quantum fluctuations of quantum fields which become however not observable just in vacuo but you need to probe the vacuum with something, e.g., you need an electron (and some equipment) to measure its anomalous magnetic moment leading to a deviation from the leading-order ("tree level" in the language of Feynman diagrams) prediction, these deviations are due to quantum fluctuations ("radiation corrections", i.e., "loop diagrams").

Another example is the deviation of the electric field of a charge from the classical (tree level) Coulomb field, which is also due to quantum fluctuations, called "vacuum polarization", and this puts it in much better terms than "virtual particles". Indeed due to the quantum fluctuations of the fields there's a kind of polarization of the vacuum, but this polarization is not due to the vacuum but the reaction of the vacuum to the point charge making up the electrostatic field.

Last but not least there are pure quantum effects like the Casimir effect. Also the Casimir effect is due to the presence of charges within an overall neutral material. The usual calculation in the first pages of QFT books, where a boundary-value problem is solved and two zero-point energies (both infinite by the way) are subtracted is in fact an idealization in the limit of infinite coupling constant (when ideal-conductor boundary conditions are empolyed as usual in this very simplified treatment). I think the famous paper by Jaffe has been cited alread in this thread.

You can go on and on with such examples: Whenever something is argued with "virtual particles", in fact it's something induced due to the presence of real particles and/or fields. Whenever you read about "virtual particles" in the real physics book or paper it's a paraphrase for "internal lines in Feynman diagrams", i.e., the asymptotic formal power series in the coupling (number of vertices in the Feynman diagrams) or $\hbar$ (number of loops in the Feynman diagrams), i.e., some formal expressions in terms of propagators, vertices and (often divergent) integrals!

 

[ddd123]

Its the same as the wave particle duality. The truth is impossible to convey without math, so they resort to half truths that can be conveyed with pictorial vividness.

I'm afraid I've actually fallen into the opposite extreme here, by what you say it just seems downright false to me, not a half truth. Where is the other half?

Wave particle duality is a half truth because of Hilbert space (as atyy has explained a few times), for example. But if Hawking radiation is due to the gravitational field, why do they say it is due to the vacuum? They're just different propositions, they don't intersect...

 

[vanhees71]

There is no wave-particle duality. That's it and that's known since 1925!

 

[ddd123]

There is no wave-particle duality. That's it and that's known since 1925!

I meant something like this:

Yes, the language is not standard, but I hope to convince you it can be correct. The idea is that "wave-particle duality" which is a vague heuristic in old quantum theory is still worth teaching, because there are several things in the proper theory which can be seen as formalizations of the heuristic.

Here it isn't the case. Maybe I haven't explained myself well enough.

 

[vanhees71]

I'm of the completely opposite opinion. Teaching wave-particle duality is misleading students with concepts overcome more than 90 years ago. It is hard enough to get used to the way to think in terms of modern quantum theory. You must not overcomplicate things with teaching concepts to students that you then tell them they must forget again, learning the modern theory. We also don't teach Aristotelian physics anymore but start with Newtonian mechanics for the very same reason.

Of course, history of science is another thing. It's very interesting to learn about how the modern concepts were found in centuries of hard experimental and theoretical work, and it can also help to understand the meaning of the modern concepts better. So there should be some introduction to the history of science/physics for any physics student, but it should not be mixed up within the lectures on physics itself!

 

[ddd123]

Okay but that's a matter for a different thread maybe. What I'm asking is: some are of the opinion that the traces of such concepts found in the modern view still justify such a didactical approach. Where does exactly this vacuum fluctuations causing the Hawking radiation heuristic come from? It's not even in an old theory. Was it just made up? Why? Bhobba says it's a half-truth, others say it's a heuristic, but what's the other half, how does it work as a heuristic?

 

[bhobba]

Where does exactly this vacuum fluctuations causing the Hawking radiation heuristic come from?

Being loose with concepts. Is that simple.

As I said if you don't believe the experts that post here go and study it yourself - its that easy.

The following would have to be the silliest dialectical imaginable. You read misconceptions, experts point out it's a misconception, but you don't believe them, then don't want to learn the detail to get to the bottom of it. Instead you ask how it came about. It is explained its to give a feel to lay people - but you don't accept it an keep asking why. It really is silly and to be blunt downright annoying to those that have taken the time to study the detail.

Thanks
Bill

 

[ddd123]

I've actually understood the detail regarding Feynman diagrams. Hawking radiation is a little more complicated for me because it is quantum gravity. But by doing so I sure understand why virtual particles don't exist, but I don't understand why they use them as popularization as it's not needed.

Why not just say it's due to the gravitational field? Or other fields? It's not more complicated than making a story up about the vacuum. What's the difference between that and saying it's due to a cosmic dinosaur? Not sure if I'm getting the point across.

 

[bhobba]

Wave particle duality is a half truth because of Hilbert space (as atyy has explained a few times),

It's at best a half truth because it's WRONG ie with a wave multiplying it by a phase factor makes a difference - in QM it doesn't, nor are real waves complex valued. It was known to be wrong when Dirac came up with his transformation theory in 1926, likely 1925 when Heisenberg came up with matrix mechanics and Dirac q numbers a little after.

Thanks
Bill

 

[ddd123]

I mean, taking these bookkeeping mechanisms so literally, isn't it as if I owed you two apples, and these enter a mysterious state of negative reality thereby becoming negapples, from which they pop out again when I return the apples? At this point I don't think making this story up is justified, it's sensationalism isn't it?

Are you afraid of saying, e.g., Stephen Hawking has to round up his budget by selling books? :P

 

[bhobba]

but I don't understand why they use them as popularization as it's not needed.

What precisely don't you get about loose heuristic thinking for pictorial vividness? Seriously it's not hard.

Thanks
Bill

 

[ddd123]

What precisely don't you get about loose heuristic thinking for pictorial vividness? Seriously it's not hard.

Because for me a heuristic still involves a calculation. Like when you use Newtonian formulas to derive the Hawking temperature of a black hole. Or old quantum theory when you quantize electron "orbits".

 

[bhobba]

At this point I don't think making this story up is justified, it's sensationalism isn't it?

In case you haven't figured it out yet in physics there are many incorrect concepts you learn when starting out that later you have to unlearn. Feynman commented on it. He didn't like doing it but realized you can't tell the truth from the start because you don't have the background to understand it. You don't have the background to understand a Dyson series, asymptotic divergences, perturbation theory, complex integrals etc etc. So they are loose with these concepts and speak of the lines in a Feynman diagram as being real. Later you learn they are not.

Thanks
Bill

 

[ddd123]

But there's no such picture in Hawking radiation. In fact, from the site that was linked earlier: http://math.ucr.edu/home/baez/physics/Relativity/BlackHoles/hawking.html

we read:

you'll find Hawking radiation explained this way in a lot of "pop-science" treatments: Virtual particle pairs are constantly being created near the horizon of the black hole, as they are everywhere. Normally, they are created as a particle-antiparticle pair and they quickly annihilate each other. But near the horizon of a black hole, it's possible for one to fall in before the annihilation can happen, in which case the other one escapes as Hawking radiation.

In fact this argument also does not correspond in any clear way to the actual computation. Or at least I've never seen how the standard computation can be transmuted into one involving virtual particles sneaking over the horizon, and in the last talk I was at on this it was emphasized that nobody has ever worked out a "local" description of Hawking radiation in terms of stuff like this happening at the horizon. I'd gladly be corrected by any experts out there... Note: I wouldn't be surprised if this heuristic picture turned out to be accurate, but I don't see how you get that picture from the usual computation.

 

[bhobba]

Because for me a heuristic still involves a calculation.

Heuristic: involving or serving as an aid to learning, discovery, or problem-solving.

And that is exactly what treating them as real is and why even experts are sometimes loose about it.

Thanks
Bill

 

[A. Neumaier]

Where does exactly this vacuum fluctuations causing the Hawking radiation heuristic come from?

It comes from taking pieces of intuition and connecting them with a plausible narrative.

A slightly improved version of the following is now an Insight Article.

The starting point is the sound knowledge that there are technical notions of vacuum fluctuations (= nonzero vacuum expectation values), virtual particles (=internal lines in a Feynman diagram), and that in bare quantum field theory with a cutoff, the vacuum is a complicated multiparticle state depending on the cutoff - though in a way that it diverges when the cutoff is removed, so that nothing physical remains. Then the question arises: is there anything about it to convey a bit of this to ordinary people? It is highly unsatisfactory not to be able to talk about what one is doing in one's research...

So one goes for analogies and images. Already calling internal lines ''virtual particles'' is a step in this direction. Allow yourself a little more liberty and combine it with Feynman's classical absorber theory of radiation; after all Feynman also invented the diagrams bearing his name, possibly even inspired by this analogy. The lines defining the virtual particles look like world lines in a classical process, so why not interpret them (in one's imagination) as the quantum remnants of the classical world lines of Feynman's earlier (later abandoned) theory? This happy accident makes the story possible. It is not completely accurate but plausible (in the absence of correction of the intuition by mathematical formulas) because both classical particles and virtual particles are represented pictorially by lines, and it is something that ordinary people can imagine. This is the beginning of the myth. An extra reassurance that you are on a good path is that the arrows that physicists draw on their diagrams (to indicate the sign of conserved quantum numbers) happen to match Feynman's classical idea that antiparticles are just particles moving backward in time.

To bring in more physics one has to be able to interpret complete Feynman diagrams. Tree diagrams are easy but bring in a new aspect. They talk about real and virtual particles. On an electron line containing two vertices, the electron changes its status from being real (external) to being virtual (internal) and back (external) again. We learn from it a new fact - a virtual particle can become real, and conversely. The interpretation as world lines teaches us other things: A single Feynman diagram should in fact be considered just as a tiny snapshot of an extended web containing all particles in the universe; after all, world lines do not begin and end nowhere. Thus ''in reality'' (meaning in the simplified reality pained for the general public) all particles should be viewed as virtual until they are observed (where they obviously are real). This matches a version of the Copenhagen interpretation: Unobserved particles have a sort of ghost existence, since properties emerge only when they are subjected to a quantum measurement. You are pleased by this coincidence - it seems to say that there is a coherent story to be told. Also, since most of the lines in Feynman diagram end, you have a layman's picture for decaying particles: What you see in a bubble chamber is just a Feynman diagram made visible! This is the first serious manifestation of the myth. In spite of lacking any grounding in real physics (being grounded instead in visual analogy), you feel entitled to make this identification since it serves your final goal, to make some of the intricacies of microphysics accessible to the general public.

The next thing is to interpret the bare multiparticle state. It is obviously a complex superposition of bare particles. Make the next move to identify bare particles with virtual particles; after all both are unobservable but appear in some version of the formalism. Then you have the picture of the vacuum as teeming with particles. From the form of the Feynman diagrams (now looking at loop diagrams) you can read off that in order to make sense of the narrative these particles pop in and out of existence. This is the birth of the next item in the myth. That in a superposition nothing dynamical happens is a small nuisance that you happily sacrifice in order to be understandable to your intended audience. After all you can now give an illusion of having conveyed something of the complexities of the naive perturbative approach without having to talk about perturbation theory. In addition, without asking for it, you have found an unexpected visual interpretation of the notion of a vacuum fluctuation - clearly a teeming vacuum where particles constantly pop in and out of existence fluctuates, and each single act of popping may rightfully be regarded as a fluctuation of the vacuum. Another piece of the myth has found its place. Never mind that there is not the slightest way of justifying this analogy on the level of mathematical formulas. What counts is how the picture appeals to the general public, and it is obvious that drastic simplifications are needed to achieve this goal.

Now one needs to worry about the basic principles of physics in all this. After all, one doesn't want to talk about particles alone but convey some general physics as well. Let us bring in conservation laws. Everyone knows that energy is conserved in Nature. But wait, doesn't the creation of particles require some energy? Don't mind, quantum mechanics comes to the rescue. People will have heard of the Heisenberg uncertainty relation, and if they haven't this is an opportunity to make your audience acquainted with it. It states the intrinsic uncertainty of position and momentum in nonrelativistic mechanics. What does it tell about energy conservation? Nothing at all, but analogy comes to the rescue. In relativistic physics time is the 4th coordinate of position and energy the 4th coordinate of momentum. Thus we don't make a big blunder if we consider a time-energy uncertainty relation. (Though time is nowhere in mainstream physics an operator observable.) Uncertain energy can be liberally interpreted as a slightly inaccurate conservation law. After all, one can derive from quantum mechanics only that the expectation of the energy operator is conserved. Expectation brings to mind that whatever you measure inaccurately must be measured many time for getting an improved accuracy. Thus only the average energy needs to be conserved. Reinterpret the average (in the service of simplifying the physics to give your audience a coherent story) as an average in time.

Thus you found the solution: Energy can be borrowed for a short period of time if it is returned on the average. The next item of the myth arrived. Now you are quite confident that you'll be able to get a full and rich story (for laymen only, so all the small blunders made can be excused) and continue to turn it into something you'll tell in public (or write in a book). You hope that the attentive audience will not ask where the energy is borrowed from, but unfortunately you told the story first a colleague with an unbiased mind and he insisted on that this should be clarified first. You need to look at some more pieces of information to get the next input. Fortunately you soon find it: The zero-point energy of a harmonic oscillator had in the past always been ignored by saying that only energy differences are observable. Maybe it is the bank from which the virtual particles lining up for popping into existence can borrow their energy. And yes - it turns out that the bare quantum field has a huge amount of zero point energy - an infinite amount if you take the physical limit. Clearly this must be the source - and no ordinary person will be interested to question it. Thus the final piece of the myth arrived. You are happy - it will be a really good story conveying a lot of physics while still understandable to ordinary people.

That there is no physical mechanism for how the borrowing works is a small nuisance that (for the layman) can be ignored - after all, they want a simple story that they can believe, not a technical discussion of all the problems involved - they know that quantum mechanics is full of unresolved problems. At this point your story is already so convincing that you don't mind that all observable quantities also become infinite in the limit considered, and that when you instead do a proper renormalization (needed to get the high accuracy predictions quantum field theory is famous for) the whole capital of the vacuum energy bank shrinks to zero.

Now the particle philosophy for the laymen is essentially complete. Only a few - to laymen imperceptible - jumps of the imagination were needed in the service of understandability. Like in a cinema, where the pictures jump in discrete steps but provide a sufficient illusion for the audience to see a continuous story. To make sure that the audience, captured by the imaginative illusion, will not take it for physical reality, and to ensure that your status as a respected scientist is preserved, you begin with a caveat (like Steve Carlip did - see post #5 - before he entered the mythical narrative): ''Be warned - the explanations here are, for the most part, drastic oversimplifications, and shouldn't be taken too literally.'' But in spite of this you can instead be sure that most of your audience will ignore this sentence said in the first few seconds in favor of the nice pictures that you took a whole hour to explain and make intelligible.

When Hawking discovered what was later called Hawking radiation this picture for the general public was already well entrenched. So he only had to figure out how his discovery would fit in - and it fitted well. Instead of talking about gravitational energy (not visible, hence a sort of vacuum) creating a particle-antiparticle pair one partner of which escapes there is only a small step to saying what the educated general public expects. Since the particles are not (yet) observable by the far away observer seeing only the radiation, they must be sold according to the philosophy developed above as virtual particles created (hence vacuum fluctuations in action). Years later, when one of the particles is finally observed by the far away observer, it becomes real as a piece of the observable Hawking radiation.

Thus if you want to summarize to lay people the Hawking effect in a single phrase, what is more natural than to say that ''vacuum fluctuations cause the Hawking radiation'' without repeating the warning ''This shouldn't be taken too literally''?

 

[ddd123]

Thus if you want to summarize to lay people the Hawking effect in a single phrase, what is more natural than to say that ''vacuum fluctuations cause the Hawking radiation'' without repeating the warning ''This shouldn't be taken too literally''?

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?

 

[stevendaryl]

That is indeed the modern view.

Thanks
Bill

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.