So Quantum Fluctuations don't exist?

[Tio Barnabe]

Is it true that when we dig deep into the math, we realize that Quantum Fluctuations and virtual particles are just a heuristic way of explaining certain phenomena to lay public?

 

[Demystifier]

Quantum fluctuations are fundamental and real. Virtual particles are not.

 

[Lord Jestocost]

In 1973, Edward P. Tryon even asked: "Is the Universe a Vacuum Fluctuation?" (Nature 246, 396 - 397 (14 December 1973); doi:10.1038/246396a0)

 

[A. Neumaier]

See the insight article https://www.physicsforums.com/insights/physics-virtual-particles/ for precise explanations of terms, and
the insight article https://www.physicsforums.com/insights/vacuum-fluctuation-myth/ for how it is sold to the lay public.

 

[Vanadium 50]

I don't care much for the title. There is a subset of PF members who like to post false statements, in the hopes they are corrected. This is an inefficient way to learn; it makes many long-standing members cross, and if left unchallenged keeps incorrect information on the board. Better to ask questions.

 

[PhysicsExplorer]

Is it true that when we dig deep into the math, we realize that Quantum Fluctuations and virtual particles are just a heuristic way of explaining certain phenomena to lay public?

They do actually exist.

There seems to be a conflicting set of views from two groups of scientists: The most common view is that fluctuations do exist in spacetime and in fact, they are a consequence of field theory!

A second smaller set of scientists are claiming that virtual particles are nothing but perturbative mathematical artefacts. I have never heard so much nonsense, personally.

The evidence for vacuum fluctuations is very suggestive for their existence. Not to mention the much abused Casimir effect, there are other reasons we believe these fluctuations exist. Let's be clear on something, what do we mean when we talk about fluctuations?

Well, we mean we are talking about the ground state, or the zero point energy of all fields. This is a well-known experimental fact: That quantum fields possesses zero point energies and can never completely ''freeze.'' Such a system was first theoretically introduced by Planck and then later by Einstein as a correction to calculations on hydrogen. How do we really know these fields exist? You can try and freeze a gas down inside a vacuum - theoretically and classically speaking, you should be able to remove all the matter and energy and be left with nothing. This is not the case as it turns out.

A quantum vacuum is different to a Newtonian vacuum. A quantum vacuum, actually never reaches zero Kelvin, no matter how much energy you pump into the system to try and create a perfect vacuum. Physically, the gas still retains a heat due to ''residual kinetic energy,'' in the system. For this very reason, zero point fields are an experimental fact of the system and adds excellent evidence to quantum theory in respect to fluctuations.

There are other reasons to think fluctuations have to exist: For instance, I got into a fair exchange with a physicist recently who said to me fluctuations where nothing but mathematical anomalies from perturbation theory - ''if this was the case how does a black hole lose its mass?'' I asked.

He said, ''through Bogoliubov transformations...'' and continued to say, ''I can link you to Hawking paper, he never once mentions virtual particles.''

I replied, ''that's nice. But if that is the case, why do these transformations have creation and annihilation operators?''

Needless to say, he could not answer me. Yes, physicists came to this picture of virtual particle activity at the horizon for very good reasons. Whether Hawking specifically said they involved virtual particles, seemed irrelevant to me, for why else would the transformations have a definition of the creation and annihilation operators?

My conclusion is, the claim virtual particles/fluctuations are just mathematical abstractions, has no basis.

 

[PhysicsExplorer]

Quantum fluctuations are fundamental and real. Virtual particles are not.

We may need clarity from this, with a definition. A virtual particle, is a fluctuation of the vacuum.

 

[LeandroMdO]

A quantum vacuum is different to a Newtonian vacuum. A quantum vacuum, actually never reaches zero Kelvin, no matter how much energy you pump into the system to try and create a perfect vacuum.

If you construct a field theory at finite temperature via any of the usual formalisms (Matsubara imaginary time formalism, Schwinger-Keldysh, TFD, et al.) temperature is a tunable parameter that can take any positive value. Of course the third law of thermodynamics prevents a physical system from reaching absolute zero in a finite number of steps just as it does in nonrelativistic physics, but nothing stops you from writing down a quantum field theory at zero temperature. In fact, for all intents and purposes, we are pretty much at zero temperature as far as most of the standard model is concerned (just convert a typical temperature of 300 K to eV and compare it to the SM masses).

He said, ''through Bogoliubov transformations...'' and continued to say, ''I can link you to Hawking paper, he never once mentions virtual particles.''

I replied, ''that's nice. But if that is the case, why do these transformations have creation and annihilation operators?''

Because the transformations relate different bases in which to write down a canonically quantized field theory in curved space. We write them down in terms of creation and annihilation operators for the same reason as always: because each mode satisfies a harmonic oscillator equation. What changes is the definition of the modes due to the redshift induced by the BH (the words "positive frequency" and "negative frequency" depend generically on the choice of observer, though all inertial observers agree). We could relate the states directly if we really wanted to, but the expressions turn out rather cumbersome, so it's more convenient to just relate the creation and annihilation operators, which define the particle number basis.

Now, I wouldn't say Hawking radiation works "because" of the Bogoliubov transformations. They too are just a mathematical tool. But personally I have never seen a derivation of Hawking radiation that involved virtual particles. Feynman diagrams are a tool for perturbatively understanding interacting theories, but Hawking radiation occurs even in a free theory---free apart from the coupling to gravity, that is. I'm not saying the virtual particle picture of Hawking radiation doesn't exist, I just personally haven't seen it. If it exists, it's quantum gravity (i.e. hard).

As for the "quantum fluctuations", it's a matter of semantics. Most of the time when textbooks use the words "quantum fluctuations" they seem to mean "quantum effects". In that sense, quantum fluctuations clearly exist. If we're talking about some dynamical particle-popping interpretation of loop diagrams, which seems to be a rather common colloquial meaning, they clearly don't. If we're talking about the fact that a naive quantization gives a non-zero formally divergent ground state energy, then the fluctuations exist again. Point being, the word "quantum fluctuations" has no standard definition, so it seems hopeless to discuss its existence or lack thereof. It's best to stick to well-defined concepts.

 

[atyy]

Quantum fluctuations are fundamental and real. Virtual particles are not.

What do you mean by a quantum fluctuation?

 

[PhysicsExplorer]

If you construct a field theory at finite temperature via any of the usual formalisms (Matsubara imaginary time formalism, Schwinger-Keldysh, TFD, et al.) temperature is a tunable parameter that can take any positive value. Of course the third law of thermodynamics prevents a physical system from reaching absolute zero in a finite number of steps just as it does in nonrelativistic physics, but nothing stops you from writing down a quantum field theory at zero temperature.

But what is the point other than an approximation? The physical interpretation of systems never ceasing to have an intrinsic temperature is very important - sure, most of the vacuum sits on the precipice of zero kelvin, but so what?

Chemists tend to think about systems at zero point, doesn't make it right.

 

[LeandroMdO]

But what is the point other than an approximation? The physical interpretation of systems never ceasing to have an intrinsic temperature is very important - sure, most of the vacuum sits on the precipice of zero kelvin, but so what?

Chemists tend to think about systems at zero point, doesn't make it right.

I don't think that what people typically mean when they talk about "quantum fluctuations" is about the impossibility of reaching the absolute zero in practice. The words that would be used in such a circumstance are "thermal fluctuations" (another term I usually avoid, for the same reasons). The term seems to refer to some property that is present at any temperature, zero or no.

 

[PhysicsExplorer]

I don't think that what people typically mean when they talk about "quantum fluctuations" is about the impossibility of reaching the absolute zero in practice. The words that would be used in such a circumstance are "thermal fluctuations" (another term I usually avoid, for the same reasons). The term seems to refer to some property that is present at any temperature, zero or no.

Not normally no, I grant you that. Though... it is an interpretation of the physics which seems to hold water? I mean, it should have been enough that quantum physics predicted Casimir energy, and in finding it, but clearly, this was not the case, not everyone was convinced. Personally, a whole array of quantum subjects do not make sense to me, without fluctuations.

A good example is the shielding effect of the virtual particles around an electron. There is even mention, by some physicists we can actually measure the effects of the virtual particle, in a physical way.

 

[bhobba]

They do actually exist.

They do not as any of the many professors that teach QFT and post here, or an actual textbook, not pop sci hand-wavy half-truths, but actual textbooks, will tell you.

If you believe otherwise here is an actual textbook:
https://www.amazon.com/dp/019969933X/?tag=pfamazon01-20

Tell me the page where the existence of virtual particles is derived. Not as lines in a Feynman diagram - as that they of course exist. But a Feynman diagram is just a pictorial representation of a Dyson series. That's not what I mean - I mean as actual particles that pop into and out of existence like the pop-sci books say.

Thanks
Bill

 

[PhysicsExplorer]

They do not as any of the many professors that teach QFT and post here, or an actual textbook, not pop sci hand-wavy half-truths, but actual textbooks, will tell you.

If you believe otherwise here is an actual textbook:
https://www.amazon.com/dp/019969933X/?tag=pfamazon01-20

Tell me the page where the existence of virtual particles is derived. Not as lines in a Feynman diagram - as that they of course exist. But a Feynman diagram is just a pictorial representation of a Dyson series. That's not what I mean - I mean as actual particles that pop into and out of existence like the pop-sci books say.

Thanks
Bill

Any of the professors here?

What about top professionals, outside the forum who make a living on the subject, who testify to their existence? I have met more scientists who appreciate their existence, than not.

That says something to me.

 

[PhysicsExplorer]

What do you mean, show you where virtual particles are derived?

I said field theory implies their existence. Field theory is about the creation and annihilation of particles, and zero point fields is just the ground state of those fields.

 

[PhysicsExplorer]

Here's a good example, scientific american, which testifies to the existence of the virtual particles

https://www.scientificamerican.com/article/are-virtual-particles-rea/

So yeah, while the view of the 'professors' here would be valuable, I make it clear, its not a general consensus that virtual particles are not real.

 

[atyy]

I mean, it should have been enough that quantum physics predicted Casimir energy, and in finding it, but clearly, this was not the case, not everyone was convinced.

https://arxiv.org/abs/hep-th/0503158

 

[PhysicsExplorer]

https://arxiv.org/abs/hep-th/0503158

Yes as I said, not everyone was happy... and so... they searched for alternative explanations. It doesn't prove anything though.

 

[bhobba]

There is even mention, by some physicists we can actually measure the effects of the virtual particle, in a physical way.

Its called heuristics. Physicists use it all the time. But as to it being correct - well that's an entirely different matter.

You sound as though you may have read an actual QFT textbook - if so you should know better.

I know that particular heuristic (ie actual virtual particles popping into and out of existence) is often used to explain, for example, the Casmir force. Many calculations use it. But its just a heuristic and is not required at all eg:
https://arxiv.org/pdf/hep-th/0503158.pdf

Thanks
Bill

 

[PhysicsExplorer]

Its called heuristics.

Thanks
Bill

No clearly it isn't. Did you check the link I provided?

 

[PhysicsExplorer]

And its not that the particles popping into and out of existence was used to explain the phenomenon... the Casimir force was predicted by quantum theory before its discovery.

 

[LeandroMdO]

Here's a good example, scientific american, which testifies to the existence of the virtual particles

https://www.scientificamerican.com/article/are-virtual-particles-rea/

So yeah, while the view of the 'professors' here would be valuable, I make it clear, its not a general consensus that virtual particles are not real.

The examples in that article are of quantum effects in field theory, not of virtual particles themselves. The predictions of quantum field theory should be just as good (or just as bad) regardless of the mathematical machinery that we used in order to make the problem tractable. For example, the Casimir effect is commonly calculated without any reference to virtual particles, because it is much simpler to derive it directly from the quantization of the field. It's analogous to Hawking radiation in the sense that the effect occurs even for free fields (free apart from the interaction with the plates), whereas virtual particles are useful for computing the effect of interactions.

More generally, even when there are interactions, the usual wordy descriptions are questionable. The picture of the virtual particle-antiparticle pairs that screen a charge, for example, comes from loop corrections to the photon propagator. To my mind it's much more natural to consider such corrections as properties of the photon, rather than the vacuum. The photon in a magnetic field propagates at different speeds depending on polarization; you can by way of analogy imagine that the vacuum behaves as a dielectric, or you can simply think that the photon acquired an effective interaction with the magnetic field through the electron loop vacuum polarization corrections. It's just words.

Now, I'd like to see this author's interpretation of a Fadeev-Popov ghost... :)

 

[PhysicsExplorer]

The examples in that article are of quantum effects in field theory, not of virtual particles themselves.

Talk about downplaying the main point of the article. The point is, that these fluctuations are real. Why are you brushing over the conclusion, like it was a deliberate attempt to smear the physics? It's really not.

 

[PhysicsExplorer]

Fluctuations, the theory of the creation and annihilation of particles, are part and parcel of field theory. Field theory doesn't make sense without these particles coming into and out of existence. It's basically part of its construction.

 

[LeandroMdO]

Talk about downplaying the main point of the article. The point is, that these fluctuations are real. Why are you brushing over the conclusion, like it was a deliberate attempt to smear the physics? It's really not.

I am undoing a conflation that was done illegitimately. I am not saying it was done maliciously, but it is illegitimate.

For example, it is often useful for computations to introduce a "proper-time" parameter so that the dynamics of a field theory in four dimensions reduce to the dynamics of nonrelativistic quantum mechanics in five dimensions. This is how Schwinger derived the Euler--Heisenberg Lagrangian that gives the effective interaction between photons and an external electromagnetic field. Should I take this formal trick to mean that there are really 5 dimensions? Of course not. There are other ways to derive the Euler--Heisenberg Lagrangian, such as by writing the usual perturbative Feynman diagram expansion and carefully resumming it. I shouldn't confuse Nature with the tricks that I invented to study it.

I emphasize that the Casimir effect is not typically computed via the use of Feynman diagrams. There's no need.

 

[bhobba]

Any of the professors here?

Yes - and I will even name a few:
Vanhees
MFB
Arnold Neumaier

Arnold even wrote an insights paper on it clearly detailing just what going on:
https://www.physicsforums.com/insights/physics-virtual-particles/

What about top professionals, outside the forum who make a living on the subject, who testify to their existence? I have met more scientists who appreciate their existence, than not.

They do? I mean they actually say they exist 100% for sure and make no mistake about it? If so it should be in the textbooks they write. Ok let's go to the big one - Weinberg's textbooks - The Quantum Theory Of Fields. Exactly where does he say that in there?

Physicists are just normal people who will often converse in technical lingo and heuristics. But as for writing textbooks or properly peer reviewed papers they are usually - not always - the peer review process is not 100% effective - but most of the time - a lot more careful

That says something to me.

And me too - but probably not the same as what it tells you.

Thanks
Bill

 

[PhysicsExplorer]

I am undoing a conflation that was done illegitimately. I am not saying it was done maliciously, but it is illegitimate.

For example, it is often useful for computations to introduce a "proper-time" parameter so that the dynamics of a field theory in four dimensions reduce to the dynamics of nonrelativistic quantum mechanics in five dimensions. This is how Schwinger derived the Euler--Heisenberg Lagrangian that gives the effective interaction between photons and an external electromagnetic field. Should I take this formal trick to mean that there are really 5 dimensions? Of course not. There are other ways to derive the Euler--Heisenberg Lagrangian, such as by writing the usual perturbative Feynman diagram expansion and carefully resumming it. I shouldn't confuse Nature with the tricks that I invented to study it.

I emphasize that the Casimir effect is not typically computed via the use of Feynman diagrams. There's no need.

I tried to talk about the Casimir effect as little as possible, due to the disparity of understanding. There seems to be, much more evidence than just the Casimir force. For instance, it should be theoretically possible to provide this evidence. Zero point energy seems to be the best example.

The reason why, is as I have explained before: even if you remove all the visible matter and energy from a region of space by making say, a vacuum, there should still be energy left over. In other words, it doesn't matter if you try and make a Newtonian vacuum, its impossible due to virtual particles in system.

 

[PhysicsExplorer]

They do? I mean they actually say they exist 100% for sure and make no mistake about it? If so it should be in the textbooks they write.

Thanks
Bill

You're keen for textbooks. Yes they are good, but very dry and won't always answer your questions. Yes, there really are top scientists who believe in its existence. We can start with the genius Wheeler who predicted fluctuations, called quantum foam. The idea has not died today. Yes, there are many, if not most of physicists believe in the existence of these particles.

 

[PhysicsExplorer]

Physicists are just normal people who will often converse in technical lingo and heuristics.
Thanks
Bill

But it's not! I have already shown in a link, scientists still seem to believe in their existences. I doubt scientificamerican would be as stupid to get writers into make such drastic errors in lingo.

 

[bhobba]

I am undoing a conflation that was done illegitimately. I am not saying it was done maliciously, but it is illegitimate.

There is no need - people that have actually studied this stuff know what you mean. It is just people that may have chatted with some physicists or read pop-sci accounts or even textbooks that are not actual QFT textbooks get the wrong impression. Here we like to correct those misconceptions so they do not have to 'unlearn' them when they encounter the real deal. Once you have studied the real deal ie an actual QFT textbook you know what's going on.

Thanks
Bill

 

[LeandroMdO]

I tried to talk about the Casimir effect as little as possible, due to the disparity of understanding.

Since we're discussing what quantum theory implies as a matter of principle, the disagreements about the significance of the Casimir effect are not important and can be ignored. We can talk about the Casimir effect in a toroidal geometry, where there are no plates and no van der Waals interactions. There are some stray factors of 2 because the boundary conditions are periodic instead of Dirichlet, but the calculation is pretty much exactly the same.

The reason why, is as I have explained before: even if you remove all the visible matter and energy from a region of space by making say, a vacuum, there should still be energy left over. In other words, it doesn't matter if you try and make a Newtonian vacuum, its impossible due to virtual particles in system.

Let's put it this way. Can you come up with an experiment that can test whether this interpretation is correct? Note that it's not enough to show that quantum theory gives correct predictions; you must show that this interpretation predicts something that others don't. By definition of the word "interpretation" it should be clear that you can't, and that the best you can hope for is to establish that certain equations look simpler or more beautiful on the picture you're advocating for. But that is not the case: the simplest, most elegant derivation of the Casimir effect makes no reference to virtual particles at all, because perturbation theory wasn't necessary to derive it.

Or, as I mentioned earlier in an edit: can someone genuinely argue that Fadeev-Popov ghosts exist?

 

[PhysicsExplorer]

There is no need - people that have actually studied this stuff know what you mean. It is just people that may have chatted with some physicists or read pop-sci accounts or even textbooks that are not actual QFT textbooks get the wrong impression.

Thanks
Bill

It would also give the wrong impression that I am not well-read up on the subject enough, to not have a consistent view. I certainly haven't made the following up:

1) the consensus seems largely the view that the virtual particles exist

2) There is more than Casimir force which indicates their existence

3) the fluctuations are implied in field theory anyway by the use of the creation and annihilation operators. Field theory really is about non-conservation on short time scales.

Let alone, the history I know behind the virtual particle/fluctuation theory. Quantum foam started with Wheeler, but has a much richer history which still survives today.

 

[PhysicsExplorer]

Let's put it this way. Can you come up with an experiment that can test whether this interpretation is correct?

You can try and create a perfect vacuum. Has any scientists attempted to do this, I am sure they must have. Certainly, we have attempted to cool gases down to absolute zero and we fail.

 

[PhysicsExplorer]

Here's a video on zero point temperatures, not seen it, but I recognise the guy, he speaks on science a lot

 

[PhysicsExplorer]

He makes a mention of the uncertainty principle - which is actually related to the idea of fluctuations. But really, he is right, the uncertainty principle alone could explain why a system cannot be completely frozen... however, my best guess is forget that and try and find out if anyone has attempted to make a perfect vacuum and whether it is possible. That would be key, because in absence of ordinary matter and energy, the only thing residual could be virtual activity existing as the ground state.