Insights Misconceptions about Virtual Particles - Comments

[A. Neumaier]

A. Neumaier submitted a new PF Insights post

Misconceptions about Virtual Particles

Continue reading the Original PF Insights Post.

 

[A. Neumaier]

Just in case you read the version from 40 minutes ago - it was by mistake an old one. I just uploaded the correct version - it is much more informative. I am now working on a third post called ''The virtual reality of particles" - which will be the most entertaining one of the trilogy - science fiction pure!

 

[klotza]

Interesting; I will have to read this in greater depth when I get home. One of the ideas floating around in my head is about phenomena where a helpful or heuristic description becomes mis-interpreted as the actual mechanism of a phenomenon. These including vacuum fluctuations for the Casimir effect, virtual particles for Hawking radiation, etc.

 

[Buzz Bloom]

Hi Arnold:

Would you please explain 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 which due to great tidal forces of the black hole would separate, one particle falling towards the black hole, and the other escaping and somehow becoming transformed by this into a real particle.

Regards,
Buzz

 

[A. Neumaier]

misconceptions related to Hawking radiation and virtual particles?

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. A valid description is given on p.645 of the book
B.W. Carroll and D.A. Ostlie, An Introduction to Modern Astrophysics, 2nd. ed., Addison Wesley 2007.

A corresponding animated (hence much more impressive) virtual ghost story for the general public - with all the common misconceptions characterizing these - can be found on Steve Carlip's site. Note that he warns his readers: ''Be warned - the explanations here are, for the most part, drastic oversimplifications, and shouldn't be taken too literally.'' Those who copy from him (or similar sources with similar caveats) usually take the fiction painted for scientific fact. But just because the fiction stems from a well-known scientist, it doesn't have to be science!

 

[Nugatory]

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 which due to great tidal forces of the black hole would separate, one particle falling towards the black hole, and the other escaping and somehow becoming transformed by this into a real particle.

Take a look at page 4 of http://www.itp.uni-hannover.de/~giulini/papers/BlackHoleSeminar/Hawking_CMP_1975.pdf; what you heard was the "heuristic" explanation for non-specialists although the rest of the paper will give you the whole story. Also try this link: http://math.ucr.edu/home/baez/physics/Relativity/BlackHoles/hawking.html

 

[Jeronimus]

"They cannot be said to exist in space and time, have no position, no meaningful probabilities to be created or destroyed anywhere, no life-time, cannot cause anything, interact with anything or affect anything."

If this is the case, then can you explain what is pushing two highly polished surfaces/mirrors together in a vacuum(absent of gravity i assume as well) with a force which increases greatly when reducing the distance?

From wikipedia

The Casimir force per unit area

for idealized, perfectly conducting plates with vacuum between them is

where

(hbar, ħ) is the reduced Planck constant,

is the speed of light,

is the distance between the two plates

 

[A. Neumaier]

If this is the case, then can you explain what is pushing two highly polished surfaces/mirrors together in a vacuum(absent of gravity i assume as well) with a force which increases greatly when reducing the distance?

The pushing is done by the Casimir force caused by the two surfaces - not by the space in between. Note that this space between the surfaces - what is informally called a vacuum - is not truly empty, it is still filled with the quantum fields emanating from the surfaces. Just like the space between the sun and the planets is not empty but filled with the gravitational field.

The Casimir force is explained correctly as a van der Waals force - the same force that holds an argon cluster together. Van der Waals forces are residual forces due to partial cancellation of the electromagnetic quantum field of the nuclei and elecrons making up the surfaces.
The wikipedia article on the Casimir effect acknowledges this:

Casimir's original goal was to compute the van der Waals force between polarizable molecules" of the conductive plates. Thus it can be interpreted without any reference to the zero-point energy (vacuum energy) of quantum fields.[5]

[5] is a famous paper by Jaffe 2005 where the physically sound explanation is discussed in detail without any virtual magic.

From wikipedia

Unfortunately the policy of wikipedia that in case of controversy all points of view must be discussed in a neutral way implies that wikipedia necessarily spreads an amount of nonsense proportional to that held in the general public. What counts in the eyes of wikipedia is not the correctness of a view but whether the view exists and how frequent it is.

People of all ages, cultures and backgrounds can add or edit article prose, references, images and other media here. What is contributed is more important than the expertise or qualifications of the contributor. What will remain depends upon whether the content is free of copyright restrictions and contentious material about living people, and whether it fits within Wikipedia's policies, including being verifiable against a published reliable source, thereby excluding editors' opinions and beliefs and unreviewed research. [wikipedia source]

All encyclopedic content on Wikipedia must be written from a neutral point of view (NPOV), which means representing fairly, proportionately, and, as far as possible, without editorial bias, all of the significant views that have been published by reliable sources on a topic. [wikipedia source]

There is no clear definition of what a ''reliable source'' is, but sources from the popular science literature (which are full of misinformation about virtual particles) are definitely not excluded.

 

[Jeronimus]

The pushing is done by the Casimir force caused by the two surfaces - not by the space in between. Note that this space between the surfaces - what is informally called a vacuum - is not truly empty, it is still filled with the quantum fields emanating from the surfaces. Just like the space between the sun and the planets is not empty but filled with the gravitational field.

As far as i understand it, if virtual particles have an effect on the two surfaces, pushing them together, it is because of the space outside the surfaces, not inside. More virtual particles hitting the outer side of the surfaces than the inside, the closer the surfaces are moved together.

As for a vacuum not being truly empty, i guess here is where everyone agrees, except according to you, if i understand you properly, the vacuum IS truly empty when there are no objects around. According to you, no virtual particles pop in and out of existence for a short period of time supposedly allowed by the
Heisenberg uncertainty principle as some claim.

But if all above was the case, which experiment would you propose or know of, that would show or shows that there aren't any virtual particles popping in and out of existence for a short period of time with real effects on other matter because if there were, A, B, C etc should be the case if they existed, but is not?

In other words. What should be the case IF they existed and had real effects on other matter, but is not the case, because those virtual particles do not exist or if they do, have no effect whatsoever.

 

[A. Neumaier]

if virtual particles have an effect

They cannot have any causal effect since they don't exist in a spatial-temporal sense, as explained in the Insight article. They affect something only in the same platonic sense as each contribution $x^n/n!$ in the power series expansion of $e^x$ has an effect on the value of the exponential function at $x$, although the value of the latter is independent of the way it is computed. (No sensible computer program computes $e^{-10}$ from the power series.)

What should be the case but is not, because those virtual particles do not exist or if they do, have no effect whatsoever.

As the paper by Jaffe shows, the Casimir effect is independent of the notion of a virtual particle. Thus nothing changes whether you add or don't add empty talk about the latter.

the vacuum IS truly empty when there are no objects around. According to you, no virtual particles pop in and out of existence for a short period of time supposedly allowed by the Heisenberg uncertainty principle as some claim.

Indeed. The latter is only what popular science says.

The former is what quantum field theory says (and hence what I say): The vacuum is the state containing exactly zero particles anywhere in space and at all times. Since it is an eigenstate of the number operator, there is no uncertainty at all about this.

Read the Insight article and the earlier one on the same subject, and you'll understand the reasons for the difference in the points of view. If you then still take sides with the popular view, you'll have understood why popular science is much more popular than real science - no amount of explanation can help.

 

[Jilang]

On the subject of the Casimir force and its relationship to the Van der Waals force I recently encountered this and wondered how far it is valid.
http://www.nature.com/nphoton/journal/v5/n4/fig_tab/nphoton.2011.39_F1.html

 

[Jeronimus]

Here is one more article which seems to undermine the theory of vacuum fluctuations/virtual particles having a real effect.

http://resonance.is/quantum-vacuum-fluctuations-harnessed-in-a-propellant-less-engine-tested-by-nasa/

“This paper describes the test campaigns designed to investigate and demonstrate viability of using classical magnetoplasmadynamics to obtain a propulsive momentum transfer via the quantum vacuum virtual plasma. This paper will not address the physics of the quantum vacuum plasma thruster (QVPT)…”

-Anomalous[/PLAIN] Thrust Production from an RF Test Device Measured on a
Low-Thrust Torsion Pendulum. 2014.More recently, on April 5, 2015, NASA Eagleworks reported a new computational simulation that models the EmDrive’s thrust as a three-dimensional magnetohydrodynamic flow of electron-positron pairs of the quantum vacuum – the polarizable structure of the vacuum.

http://resonance.is/wp-content/uploads/2015/06/thruster.jpg

In effect, the NASA team believes that the engine produces thrust via a momentum transfer with polarizable structure of the quantum vacuum. The central idea is that space is not empty, it is filled with energetic oscillations and as well, means that there is no truly isolated system, and hence no violation of the conservation of momentum if an equal force is being transferred to the quantum vacuum opposite to the forward thrust of the engine.

It is a good thing to have skeptics like OP not jumping too quickly onto conclusions but i believe that it is also a good thing to not dismiss any theory unless you can falsify it by an experiment.

If QFT doesn't allow any vacuum fluctuations/virtual particles(or virtual particle fields to go with QFT) popping in and out of existence in space out of seemingly nowhere, hence as OP stated, the vacuum is REALLY completely empty when there are no objects around, then this is fundamentally different from the theory which assumes virtual particles(with real effects) popping in and out of existence at all times even when nothing is around.
Then an experiment has to be proposed to settle this. Falsify one or the other theory.

 

[A. Neumaier]

Here is one more article which seems to undermine the theory of vacuum fluctuations/virtual particles having a real effect.

This is a typical popular science article (count the ratio of formulas to text to get a first idea about this), and fits perfectly what I am discussing in the Insight article.

it is also a good thing to not dismiss any theory unless you can falsify it by an experiment.

One cannot falsify unscientific stuff - precisely this makes it unscientific, and is sufficient ground to dismiss it.

Since virtual particles are objects in diagrams drawn on paper (or other drawing media) without any state that would give them properties in space and time, one cannot do any experiments to test their properties.

 

[bhobba]

The Casimir force is explained correctly as a van der Waals force - the same force that holds an argon cluster together. Van der Waals forces are residual forces due to partial cancellation of the electromagnetic quantum field of the nuclei and elecrons making up the surfaces.

Thanks
Bill

 

[Jeronimus]

Since virtual particles are objects in diagrams drawn on paper without any state that would give them properties in space and time, one cannot do any experiments to test their properties.

Well, sure, if you define virtual particles to have no state or properties, of course you can not test for them. But that is certainly not the theory you are supposed to falsify and test for.

Obviously, the theory in which virtual particles are responsible for pushing two highly polished conducting plates together in a vacuum, is based on virtual particles which do have properties and affect "stuff". Hawkins radiation is supposed to be 1 virtual particle falling into the black hole while the other is accelerated away of it, becoming a "real" particle.
Is Hawkins just a pop scientist?

Now maybe he is, but your reply to my post seemed rather unscientific in my opinion. You simply defined virtual particles to have no properties and therefore we cannot test for them.
That would be similar to saying "Your theory about quarks is wrong, because in my theory, protons and neutrons are indivisible, therefore quarks cannot exist and one cannot test for them"

 

[Nugatory]

Hawkins radiation is supposed to be 1 virtual particle falling into the black hole while the other is accelerated away of it, becoming a "real" particle.
Is Hawkins just a pop scientist?

"Hawking" not "Hawkins". No, he is not a pop scientist, but you are mistaken about the relationship between virtual particles and Hawking radiation. It's a very common misunderstanding (and one that Hawking himself is partly responsible for), but it's a misunderstanding. Take a look at post #6 of this thread for more.

Your misunderstanding about the relationship between virtual particles and Casimir forces is similar; the description of the force as arising from virtual particle interactions is just a heuristic.

 

[stevendaryl]

Well, sure, if you define virtual particles to have no state or properties, of course you can not test for them. But that is certainly not the theory you are supposed to falsify and test for.

Obviously, the theory in which virtual particles are responsible for pushing two highly polished conducting plates together in a vacuum, is based on virtual particles which do have properties and affect "stuff". Hawkins radiation is supposed to be 1 virtual particle falling into the black hole while the other is accelerated away of it, becoming a "real" particle.
Is Hawkins just a pop scientist?

Now maybe he is, but your reply to my post seemed rather unscientific in my opinion. You simply defined virtual particles to have no properties and therefore we cannot test for them.
That would be similar to saying "Your theory about quarks is wrong, because in my theory, protons and neutrons are indivisible, therefore quarks cannot exist and one cannot test for them"

I don't think that analogy quite works. Your reasoning seems to be:

1. Somebody came up with a theory of virtual particles.
   
2. According to that theory, it's not possible to observe virtual particles.
3. But the theory might be wrong, or incomplete, so maybe it actually is possible to observe virtual particles.

But there is no theory of virtual particles that can be right or wrong. A virtual particle is a calculational tool used to solve problems in quantum field theory. It isn't a distinct theory. It's an artifact of how people solve problems. It's hard for me to come up with a really good analogy, but here's my feeble attempt: You know how some people use "tic marks" to keep track of counting items. (I assume people still do that.) You're counting dandelions in your yard, and every time you find a new one, you make a vertical slash on your piece of paper, and every fifth slash you make is diagonal to mark a completed group of five. I don't think it would make much sense for you to say: "Okay, your theory says that there is one diagonal slash every five marks. But maybe your theory is wrong---maybe every 6th slash is diagonal, or every 4th slash." No, you're not going to discover that your slash convention is wrong. It's just a convention, it's not an empirical theory.

 

[bhobba]

Well, sure, if you define virtual particles to have no state or properties,

By definition virtual particles are the pictorial representation of terms in something called a Dyson series:
https://en.wikipedia.org/wiki/Dyson_series

This is what they are. They are not real, they do not cause anything. Statements otherwise are either populist half truths or professionals being loose.

Obviously, the theory in which virtual particles are responsible for pushing two highly polished conducting plates together in a vacuum,

There is no such theory.

I will repeat it again. Statements otherwise are NOT correct. There have been many threads on this forum explaining it as well as insight articles (not just professor Neumaier's) eg:
https://www.physicsforums.com/insights/struggles-continuum-part-5/
'Each of these diagrams is actually a notation for an integral! There are systematic rules for writing down the integral starting from the Feynman diagram. To do this, we first label each edge of the Feynman diagram with an energy-momentum, a variable. The integrand, which we shall not describe here, is a function of all these energy-momenta. In carrying out the integral, the energy-momenta of the external edges are held fixed, since these correspond to the experimentally observed particles coming in and going out. We integrate over the energy-momenta of the internal edges, which correspond to virtual particles, while requiring that energy-momentum is conserved at each vertex.'

You have two choices - you can accept half truths from sources that are being loose to convey difficult concepts to the lay reader, or you can believe what the numerous professors of physics and mathematics on this site will tell you (I am not one but have studied QFT and can assure you what they say is true) - virtual particles are simply the name for mathematical objects - they are not particles - they would have been better called Jaberwocky's but since they are called virtual particles we are stuck with a great deal of populist confusion. They do not cause anything. Of course you are free to choose whatever you like, but why you would choose popularisations over experts not watering it down for a lay audience beats me.

Even better you can actually study it
https://www.amazon.com/dp/019969933X/?tag=pfamazon01-20

Thanks
Bill

 

[mfb]

Is Hawkins just a pop scientist?

No, Hawking is doing both science and trying to explain science to laymen. If you check his publications, you won't find any "virtual particle is falling into a black hole", because there is no such thing. 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.

 

[friend]

There are such things as real particles, right? And there was a time before there were these real particles, during inflation, for example, right? It does seem that real particles do pop into existence from the vacuum when acceleration is involved such as in Hawking radiation, or the Unruh effect, or during reheating after inflation. If the vacuum does not consist of virtual particles, then how did these real particles come into being from the vacuum during these effects that I mentioned?

 

[stevendaryl]

There are such things as real particles, right? And there was a time before there were these real particles, during inflation, for example, right? It does seem that real particles do pop into existence from the vacuum when acceleration is involved such as in Hawking radiation, or the Unruh effect, or during reheating after inflation. If the vacuum does not consist of virtual particles, then how did these real particles come into being from the vacuum during these effects that I mentioned?

Well, the view of quantum field theory is that the fundamental property is the field (of various types), which exists through all of space. For example, the electromagnetic field. These fields can have fluctuations or waves through them, which propagate according to some wave equation. But these are quantum fields, not classical fields, so these fluctuations are quantized, in the same way that the energy for a harmonic oscillator is quantized. Perturbations in the fields due to inflation or whatever causes fluctuations, and these fluctuations manifest themselves as particles. It's possible that a description in terms of virtual particles acquiring enough energy to become real particles might be a useful heuristic, but it's not fundamentally what's going on. The field-theoretic view says that even in vacuum, these fields are present, it's just that vacuum is the lowest energy state of these fields.

 

[Nugatory]

There are such things as real particles, right? And there was a time before there were these real particles, during inflation, for example, right? It does seem that real particles do pop into existence from the vacuum when acceleration is involved such as in Hawking radiation, or the Unruh effect, or during reheating after inflation. If the vacuum does not consist of virtual particles, then how did these real particles come into being from the vacuum during these effects that I mentioned?

Particles are created from energy through the process of pair production (Google for it) - this has nothing to do with virtual particles.

 

[A. Neumaier]

If the vacuum does not consist of virtual particles, then how did these real particles come into being from the vacuum during these effects that I mentioned?

In every treatment of quantum field theory, the vacuum is defined as the eigenstate of all number operators with corresponding eigenvalue zero. This implies that everywhere and at any time the vacuum contains exactly zero particles (in any interpretation of quantum mechanics), without the slightest uncertainty.

The early universe never has been a vacuum but initially a quantum field state with extremely high energy density and hence extremely high temperature. As the system cools down, real particles and ultimately stars appear roughly in the same way as rain drops appear when a cloud cools down. This has nothing to do with virtual particles (or with virtual raindrops popping in and out of existence in a cloud).

 

[friend]

Isn't it true that virtual particles are just another name for quantum fluctuations from which these real particle come when there's acceleration? It seems I'm always hearing one term being used synonymously for the other. And it seems we aren't able to actually measure the quantum fields but only the particles they produce, right?

 

[mfb]

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

This is not true, and I don't see where you would get such a misconception from. This discussion has nothing to do with accelerations.

And it seems we aren't able to actually measure the quantum fields but only the particles they produce, right?

You can measure an electric field, for example, without problems.

 

[stevendaryl]

This is not true, and I don't see where you would get such a misconception from. This discussion has nothing to do with accelerations.

I don't think his comments were completely out of the blue. According to the Wikipedia article on "Quantum fluctuation":

In quantum physics, a quantum fluctuation (or quantum vacuum fluctuation or vacuum fluctuation) is the temporary change in the amount of energy in a point in space,[1] as explained in Werner Heisenberg's uncertainty principle.

According to one formulation of the principle, energy and time can be related by the relation[2]

This allows the creation of particle-antiparticle pairs of virtual particles. The effects of these particles are measurable, for example, in the effective charge of the electron, different from its "naked" charge.​

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

So it's not that mysterious why someone might think that there is a connection between virtual particles and quantum fluctuations: there are articles saying that there is such a connection.

As for the connection with acceleration, there is Unruh radiation, which is similar to Hawking radiation, which popularizers (including Hawking) connect with virtual particles.

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.

 

[A. Neumaier]

I don't think his comments were completely out of the blue.

Yes, popular science is full of this, respected physicists promote these fantasies in their popular science books, and wikipedia's neutrality policy forces the article writers to represent the popular science fantasies as facts. Therefore they are widely believed and hard to eradicate.

Neverteless, as my two insight articles explain in much detial, these popular science fantasies have no basis in physics, only in informal physics talk for the mathophobic.

 

[Jilang]

Well, the view of quantum field theory is that the fundamental property is the field (of various types), which exists through all of space. For example, the electromagnetic field. These fields can have fluctuations or waves through them, which propagate according to some wave equation. But these are quantum fields, not classical fields, so these fluctuations are quantized, in the same way that the energy for a harmonic oscillator is quantized. Perturbations in the fields due to inflation or whatever causes fluctuations, and these fluctuations manifest themselves as particles. It's possible that a description in terms of virtual particles acquiring enough energy to become real particles might be a useful heuristic, but it's not fundamentally what's going on. The field-theoretic view says that even in vacuum, these fields are present, it's just that vacuum is the lowest energy state of these fields.

Even the buckyball field? I doubt it.

 

[A. Neumaier]

Even the buckyball field? I doubt it.

Its density is zero everywhere when no buckyballs are around. In this sense the field is always present.

it is like the number of people in a room, which is always defined even if no one is there. The number is then simply zero.

Fields $\phi(x)$ are like the notion ''number of people in room $x$'', and the states assign (among others) to each $x$ a particular value like ''zero''. Thus fields are present everywhere but in the vacuum state their value is zero.

 

[Jilang]

Thank you. It makes it a lot clearer. They are constructs.

 

[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.