Vacuum Fluctuations in Experimental Practice - Comments

[A. Neumaier]

A. Neumaier submitted a new PF Insights post

Vacuum Fluctuations in Experimental Practice

Continue reading the Original PF Insights Post.

 

[sanman]

Sir, you write:
"what fluctuates in the experiment is the electro-optical signal detected, not the vacuum."

Sir, by what experimental/observational means can we discern that the fluctuations are not part of the Vacuum?

It's one thing to say that "vacuum fluctuations" are an unsupported assertion, but it's another thing to be able tor rule them out, particularly by experimental means. How can this "unsupported assertion" be ruled out, experimentally?

 

[A. Neumaier]

It's one thing to say that "vacuum fluctuations" are an unsupported assertion, but it's another thing to be able tor rule them out, particularly by experimental means. How can this "unsupported assertion" be ruled out, experimentally?

Claims require proof, otherwise they can be ignored. That's the rule in science. Nothing is ruled out in science; the demarcation line between science and speculation is the proof, not the disproof.

The electro-optical signal is the only thing measured, and it exhibits fluctuations. Thus they are fluctuations of the signal, not of the vacuum.
To understand what this means consider fluctuations of a visual signal seen on an oscilloscope in an ordinary experiment, and someone claims that these are a visual proof of fluctuations of the vacuum. Nobody would take such a claim serious without a proof.

To argue that fluctuations of the vacuum are measured one would have to give theoretical evidence that the signal fluctuates in the same way as the vacuum. Lack of this evidence is enough to reveal the claim as pure speculation.

 

[Greg Bernhardt]

Thanks Arnold!

 

[mfb]

Nice article!

Compared to QED, QCD has the advantage that it is asymptotically free at large energies, with the consequence that – unlike QED – it can be studied in a lattice approximation, with enormous numerical effort ultimately rewarded by reasonable (few digit) accuracy.

Sometimes we would be happy to have a 10% accuracy...

 

[A. Neumaier]

Nice article!Sometimes we would be happy to have a 10% accuracy...

I know. I was thinking of optimistic figures (low lying baryon spectrum to $1-9$%), and deliberately used the vague expression ''few''.

As you probably know (I write this for @atyy who thinks no continuum limit is needed), a lot needs to be done to get values that can be compared with experiment, not just calculations on a fixed lattice. From the paper just cited:

p.29: What one would ideally like to do then is to fix the N_f + 1 dimensionless bare parameters of the lattice theory, the bare quark masses and the gauge coupling, such that the N_f dimensionless observables on the lattice assume their physical values exactly and the lattice spacing a is of the desired size. One could then measure any observable on the lattice for a range of lattice spacings a and, with the appropriate functional form that is given by the discretization effects of the specific action used, extrapolate them into the continuum a = 0. [...]
p.31: The removal of the cutoff, also known as continuum extrapolation, is an unavoidable part of any lattice calculation that wants to make a statement about the underlying fundamental continuum theory. The severity of the continuum extrapolation however depends very strongly on both the action used and the combination of scale setting observable and measured observable. [...]
p.46: While ground state non-singlet hadron masses can be computed to a few percent accuracy today, reaching the same level of precision for excited states or singlet hadrons is still a challenging task.

The final results for the baryon masses in the infinite volume limit, together with their error margins, are given in Table 1 on p.38.

 

[Mordred]

I've been reading up on QFT itself and I am beginning to see where Neumaiur is coming from. Still not sure I have everything straight though. Actually positive that I don't lol, the majority of the math itself isn't the issue though. I'm currently on the Ultraviolet cutoffs and the imaginary i. Though I still need to work on the LSZ reduction formula.

Point being is from what I've gotten so far I can see the validity behind this article. I've gotten far enough to realize what I thought of as vacuum isn't what I thought

 

[PeterDonis]

Moderator's note: some off topic posts and responses have been deleted.

 

[Daniel Kellis]

I think virtual particles are OK in diagrams if they help solve the problem. But it is not OK to let virtual particles break the fourth wall, as is done with Hawking radiation. One virtual particle has positive energy and the other has negative energy. Hawking radiation relies on Maxwell's demon to ensure that the negative energy virtual particle always goes into the black hole and the positive energy virtual particle always escapes.

 

[mfb]

The derivation of Hawking radiation does not use virtual particles at all. That is just a pop-science description that leads to misconceptions.

 

[Nugatory]

Hawking radiation relies on Maxwell's demon to ensure that the negative energy virtual particle always goes into the black hole and the positive energy virtual particle always escapes.

We have another live thread on this misconception right now: https://www.physicsforums.com/threads/how-does-hawking-radiation-work.904630/
The links in the first two replies are worth reading.

 

[Jeronimus]

Sir, you write:
"what fluctuates in the experiment is the electro-optical signal detected, not the vacuum."

Sir, by what experimental/observational means can we discern that the fluctuations are not part of the Vacuum?

It's one thing to say that "vacuum fluctuations" are an unsupported assertion, but it's another thing to be able tor rule them out, particularly by experimental means. How can this "unsupported assertion" be ruled out, experimentally?

I don't think that this question has been answered fully.

Can you give us an example of an experiment(thought experiment) as YOU would imagine it, such that it would provide sufficient proof for vacuum fluctuations being "real" and being "directly" detected according to your standards? (You are allowed to use futuristic devices which do not exist yet, but may in the future)

edit: In particular. What kind of experiment would you propose/imagine for -> "...one would have to give theoretical evidence that the signal fluctuates in the same way as the vacuum. "As far as i am concerned, real and direct is quite a mouth full. Special relativity for example is a nice and elegant theory which is capable of predicting some of our experiences of the world quite accurately as long as there is no gravity involved.
In the end however, it is just a method to predict experiences. There are other theories/methodologies which have the equivalent predictive power to SR. We simply cannot know if spacetime out there is as we imagine it to be. Nor can we know _directly_ if particles or fields are "real" as we imagine them to be.
All we CAN know is that by following the methodologies of the according/more successful theories, we can predict our experiences more accurately than following less successful theories.

So to me, the question on if virtual particles are real or can be directly observed is nonsensical. What matters to me is, if the theory which is based on virtual particles and their fluctuations is capable of predicting what i will experience in the future more accurately or equally accurate as other competing theories.

 

[vanhees71]

Since vacuum fluctuations are not existing (at least not accoring to standard relativistic QFT), you cannot even define an experiment to measure them. For the conception of measurements you need at least some model!

 

[Karolus]

A. Neumaier submitted a new PF Insights post

Vacuum Fluctuations in Experimental Practice

Continue reading the Original PF Insights Post.

I did not understand exactly, I'm sorry, but the topic is very interesting. For all I knew, the Casimir experiment would prove the existence of "a quantum vacuum." In addition, other authors, such as Feynman and Hawking argue explicitly a state of vacuum in which particles and anti-particles "virtual" is created and destroyed. In the second quantization are introduced proper operators of "creation" and "destruction. "
Now the article casts doubt this description? How are and things?

Thank you very much

 

[weirdoguy]

In addition, other authors, such as Feynman and Hawking argue explicitly a state of vacuum in which particles and anti-particles "virtual" is created and destroyed.

Can you show any textbook/peer reviewed article on QFT where they say that?

 

[Karolus]

This article for example:
Particle Creation by Black Holes

S. W. Hawking ​

Department of Applied Mathematics and Theoretical Physics, University of Cambridge, Cambridge, England Received April 12, 1975

https://www.brainmaster.com/software/pubs/physics/Hawking Particle Creation.pdf

At page 202 :
Just outside the event horizon there will be virtual pairs of particles, one with negative
energy and one with positive energy

but there are many other, very technical, and not simply informative, for example on the vacuum polarization, or the effect Lamb-shift etc.

 

[mfb]

From Hawking about that picture:

It should be emphasized that these pictures of the mechanism responsible for the thermal emission and area decrease are heuristic only and should not be taken too literally.

You can make up those things to describe something in English, but the actual calculations don't have any virtual particles or vacuum fluctuations.

Same for vacuum polarization, Casimir effect, Lamb shift and everything else. Sometimes virtual particles are a nice (but not necessary) tool in calculations, but that is different from vacuum fluctuations.

 

[weirdoguy]

but there are many other, very technical, and not simply informative, for example on the vacuum polarization, or the effect Lamb-shift etc.

And all of them treat virtual particles as a mathematical tool, not physical 'reality' (whatever that means). And that is the whole point A. Neumaier is bringing up.

 

[Karolus]

And all of them treat virtual particles as a mathematical tool, not physical 'reality' (whatever that means). And that is the whole point A. Neumaier is bringing up.

does not convince me. So whatever it is nothing more than a mathematical tool. Even quarks have no physical reality, are only mathematical tricks, but then whatever. Even the light, we have only the equations of Maxwell there is no other reality than the Maxwell equations. Has anyone ever seen a "photon"? or a "quark"? What is the reality of a photon? Only mathematical tool...

 

[PeterDonis]

Has anyone ever seen a "photon"? or a "quark"?

Yes. We have detectors that can detect single photons (they're called photomultipliers). We ran experiments called deep inelastic scattering experiments in the 1960s where we shot high energy electrons into nuclei and watched them bounce off quarks; that's how the quark model was developed.

But now ask the question: are photons and quarks "particles"? I would answer no: "particle" is just a mathematical tool we use to construct our models. Photons and quarks are photons and quarks. They are not little billiard balls or pointlike masses. But there are real things there that we see in experiments like the ones I mentioned above, and "photon" and "quark" are the names we give to those real things.

So if you want to say that "virtual particles" are the name you will give to "whatever real thing it is that explains the Casimir effect, Hawking radiation, Lamb shift, etc.", sure, you can do that. But then you are using the term "virtual particles" to describe something that isn't a particle any more than photons or quarks are particles. The term "vacuum fluctuations" is really no better in that regard, because it invites the inference that whatever it is is fluctuating, like a wave--but it isn't a wave any more than it is a particle. It is something for which we have no intuitive analogy at all; it's not like anything we have an ordinary language word for.

This is a major reason why many (like myself) keep emphasizing that to really understand physics, you have to use math, not ordinary language. There is no dispute about the math: we know how to construct mathematical models that make accurate predictions about all of the experiments I referred to above. The only disputes are about which ordinary language words are the "right" ones to use to refer to the physics; but that dispute is ultimately pointless because no ordinary language words are "right".

 

[weirdoguy]

does not convince me

So just take QFT textbook and learn it properly by yourself. Then you'll see what everyone here is talking about, because:

to really understand physics, you have to use math, not ordinary language.

 

[Karolus]

:to really understand physics, you have to use math, not ordinary language.

ok, then teach me the first law of Newton, or any other physical argument, without using a single word of "ordinary language", ie using only the language of mathematics. There is no discussion in physics, (and even math!) no matter how advanced, you do not use ordinary language, because, after all, mathematics, I think, is just a language

 

[Karolus]

Yes. We have detectors that can detect single photons (they're called photomultipliers).

not entirely true. The photomultiplier detects only the intensity of the current ... where is the "photon"?

 

[PeterDonis]

There is no discussion in physics, (and even math!) no matter how advanced, you do not use ordinary language

You might be using words like "energy", "work", "force", etc., but those words do not refer to ordinary language concepts. They refer to particular parts of the math.

mathematics, I think, is just a language

But not the same kind of language as ordinary language, because mathematical terms have precise referents.

The photomultiplier detects only the intensity of the current

The intensity of the current is the output of the photomultiplier, not the input. "Photon" is the name we give to whatever-it-is that goes into the photomultiplier and causes the current to be output.

 

[sanaullah]

A. Neumaier submitted a new PF Insights post

Vacuum Fluctuations in Experimental Practice

Continue reading the Original PF Insights Post.

sir when we say light is photon and it has certain frequency then how can we visualize the vibration, cycles of photon

 

[A. Neumaier]

sir when we say light is photon and it has certain frequency then how can we visualize the vibration, cycles of photon

The photon is the vibration! Visualization is by a sine wave.

 

[David Neves]

Here is a recent article.

https://physics.aps.org/synopsis-for/10.1103/PhysRevLett.118.204801

Scattering from the Quantum Vacuum

According to quantum theory, the vacuum swarms with particles that pop in and out of existence. While they are virtual, these particles are at the root of observable quantum phenomena like the Casimir effect. James Koga and Takehito Hayakawa at the National Institutes for Quantum and Radiological Science and Technology, Japan, have now detailed a way to measure with unprecedented accuracy a difficult-to-isolate quantum-vacuum effect known as Delbrück scattering. The approach may allow sensitive tests of the theory of quantum electrodynamics (QED).

Delbrück scattering has analogies with the better-known form of scattering responsible for the color of the sky—Rayleigh scattering. Rayleigh scattering arises from the interaction of photons with bound electric charges in the scattering particles. Delbrück scattering instead derives from the interaction of photons with virtual electron-positron pairs in the presence of the Coulomb field of an atomic nucleus. First observed in the 1970s, the effect remains hard to characterize because it occurs in combination with four other types of scattering, including Rayleigh.

Koga and Hayakawa propose a method to isolate and measure Delbrück scattering. The key to their solution is the use of polarized gamma rays. According to their calculations, an appropriate choice of scattering angle, photon polarization, and photon energy would make Delbrück scattering 2 orders of magnitude stronger than that of the other three forms of scattering. Assuming the use of tin as the scattering material and a high-flux gamma-ray source like ELI-NP (Extreme Light Infrastructure - Nuclear Physics)—a facility under construction in Romania—the team predicts that, using data collected over 76 days, the method could double the accuracy achieved in previous experiments.

Now, at one point it says, "Delbrück scattering instead derives from the interaction of photons with virtual electron-positron pairs in the presence of the Coulomb field of an atomic nucleus." Now, if virtual particles are just a mathematical tool, then how would you explain Delbrück scattering without invoking virtual particles?

 

[LeandroMdO]

Now, if virtual particles are just a mathematical tool, then how would you explain Delbrück scattering without invoking virtual particles?

Virtual particles are a mathematical tool with which we can predict the outcomes of real experiments. There's no conflict whatsoever: nobody's asserting that it's wrong to use calculations with virtual particles, just that wordy descriptions like "the vacuum swarms with particles that pop in and out of existence" do not correspond to what is mathematically described by the theory (as has been convincingly argued in the original post), and are rather illegitimate heuristics that seem to spread for sociological reasons.

Delbrück scattering in particular is nothing special, and is related to things like vacuum birefringence, photon splitting, or the Schwinger effect. The basics of the calculation were already known in the 1930s due to Heisenberg and Euler, though some pieces of the puzzle required Schwinger's contribution circa 1950. In either case the words "virtual particles" were nowhere to be seen.

The electron propagator is modified in the presence of an external field and one-loop corrections lead to effects that don't happen in ordinary conditions. Nothing justifies interpreting these one-loop diagrams as a photon scattering off "particles that pop in and out of existence". It's better to interpret it as the photon having a weak response to external electromagnetic fields because of its interaction with charged fields.

 

[dextercioby]

Here is a recent article.

https://physics.aps.org/synopsis-for/10.1103/PhysRevLett.118.204801

Scattering from the Quantum Vacuum

According to quantum theory, the vacuum swarms with particles that pop in and out of existence. [...]

Just because the reviewers from the (otherwise reputable) journal accepted for publication this article with an introduction/abstract full of inaccurate clichés, this does not mean the authors are right to use them.

 

[PeterDonis]

how would you explain Delbrück scattering without invoking virtual particles?

"Delbruck scattering derives from the interaction of photons with the field produced by the nucleus, which is not the exact Coulomb field that appears in classical physics, because of QED corrections, and therefore can produce effects that a classical Coulomb field would not produce."

The observable fact is that the field of something like an atomic nucleus is not an exact classical Coulomb field (or more generally an exact classical electromagnetic field). "Virtual particles" are just a name we give to one particular aspect of one particular theoretical model that helps us to predict that observable fact, as well as many other observable facts.

 

[Cthugha]

As already pointed out in the other thread, I disagree with many of the conclusions drawn in this insights article. Maybe the most important point I disagree with is:

What is called (not only in this paper, but everywhere where quantum field theory is used) the vacuum is just a mathematical state used in the computations of quantum electrodynamics (QED) with which predictions about experimentally realizable situations are computed in perturbation theory.

As most people outside of high energy physics, here they do not go beyond the simple Jaynes-Cummings model of cavity QED, which of course shares part of the name of the all-out relativistic quantum field theory QED is, but is usually not even treated relativistically.

In cavity QED, accordingly people are interested in vacuum states of cavity modes. For any realistic scenario, these will have a finite quality factor. This may be insanely high as in Haroche's experiments on resonators with millisecond lifetimes or it may be as bad as a piece of glass where reflections occur at the facets. In any case, the fundamental photon mode of the system is now one of finite spectral width and of finite lifetime. And most importantly, it is necessarily coupled to the environment and accordingly an open quantum system. One can easily see this in the finite coherence time one gets for the cavity field. And for open quantum systems I do not see any problems with converting the ensemble average into a time average as long as the averaging time window is much longer than the coherence time of the system.

Outside of high energy physics, any reasonable article on vacuum fluctuations (yes, of course there are also plenty of bad and unreasonable ones) either considers open quantum systems or situations similar to those considered by Glauber in his seminal paper on quantum optics in dielectric media (https://journals.aps.org/pra/abstract/10.1103/PhysRevA.43.467), where he finds that the ground state of the light field inside a dielectric becomes a squeezed state, when analyzed using empty-space photon operators. People usually talk about stuff like that, when they talk about virtual particles in dielectric media and not about internal lines in Feynman diagrams.

Similar things can be said about reference 32 which you discredit for unclear reasons. While active, Zimmermann was one of the most distinguished many-body theorists out there and questioning this is a very daring claim. And of course virtual states have a different meaning when considering ultrafast optics as compared to high energy QED. Edit: this should not have read virtual, but vacuum states.

One does not have to find this terminology useful or elegant, but one should at least acknowledge that in contrast to all-out relativistic QED which is important for precise predictions of energies, cavity QED is interested first and foremost in dynamics and open systems. Of course this results in different meanings in different fields for the same terminology. If you really think that the Science paper is actually wrong, it would be good scientific practice to write a rebuttal.

 

[A. Neumaier]

In any case, the fundamental photon mode of the system is now one of finite spectral width and of finite lifetime.

I was not aware of this. Can you give me a good reference where this and its implications are discussed in some detail?

By the way, your quote environment in the previous post misses a ".

 

[Cthugha]

I was not aware of this. Can you give me a good reference where this and its implications are discussed in some detail?

I am not sure, I have a reference at the level you might be interested in. If I get it right, you are working on the really well defined mathematical side of physics, while I am an experimentalist that does too much semiconductor physics to be a quantum optics guy and too much quantum optics to be a semiconductor guy. What I guess might be something along the lines you could be interested in, are the lecture notes "An Open Systems Approach to Quantum Optics: Lectures Presented at the Université Libre de Bruxelles" by Howard Carmichael. Unfortunately, I am not sure whether there is an easily accessible version on the web. The RMP article by Plenio and Knight (https://journals.aps.org/rmp/abstract/10.1103/RevModPhys.70.101) is also interesting, but might not be what you are looking for (However, it also focuses on quantum jump approaches, which you seem to be interested in). I remember some book chapter or article about the question of how to quantize light fields subject to loss, but I do not remember where exactly I found it. I will let you know when I find it.

edit: This was not the article, I was looking for, but at least it goes into the right direction: https://arxiv.org/abs/quant-ph/9702030
It might have been some other article by Gardiner.

By the way, your quote environment in the previous post misses a ".

Oh, thanks a lot. I corrected that.

 

[A. Neumaier]

I am not sure, I have a reference at the level you might be interested in.

Thanks for the references. It may take a while to find time to do the reading (and then also to respond to your previous post) as I have an upcoming deadline mid of June that takes a lot of my time.

If I get it right, you are working on the really well defined mathematical side of physics, while I am an experimentalist that does too much semiconductor physics to be a quantum optics guy and too much quantum optics to be a semiconductor guy.

I am interested both in mathematical physics (the really well defined mathematical side of physics) and in the modeling of applications in physics by the concepts from theoretical physics.

I'd need something that explains in quantum optics (or if necessary semiconductor) terms how the finite lifetime of the fundamental photon mode (a) arises, and (b) affects the modeling of typical experimental situations. I am not so much interested in experimental details, rather in the way the experiments are modeled. I understand the theory of quantum optics quite well.

 

[A. Neumaier]

Glauber in his seminal paper on quantum optics in dielectric media (https://journals.aps.org/pra/abstract/10.1103/PhysRevA.43.467), where he finds that the ground state of the light field inside a dielectric becomes a squeezed state, when analyzed using empty-space photon operators. People usually talk about stuff like that, when they talk about virtual particles in dielectric media and not about internal lines in Feynman diagrams.

Similar things can be said about reference 32 which you discredit for unclear reasons. While active, Zimmermann was one of the most distinguished many-body theorists out there and questioning this is a very daring claim. And of course virtual states have a different meaning when considering ultrafast optics as compared to high energy QED.

There is an important difference between virtual particles and virtual states. Do you agree with my definitions in https://www.physicsforums.com/insights/physics-virtual-particles/ ? If not, what is different in the usage of these terms in quantum optics?