Related Quantum Physics News on Phys.org

#### 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!

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#### klotza

Gold Member
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

Gold Member
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!

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#### Nugatory

Mentor
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:
wikipedia said:
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.
wikipedia said:
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]
wikipedia said:
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.

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#### 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/ [Broken]

“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] [Broken] 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.

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.

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#### 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.

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#### bhobba

Mentor
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

Mentor
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

Staff Emeritus
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

Mentor
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

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#### mfb

Mentor
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

Staff Emeritus
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

Mentor
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

Mentor
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.

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