High School So Quantum Fluctuations don't exist?

[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.

 

[bhobba]

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.

And I showed you a link explaining exactly what's going on.

Scientific American is written for a semi-lay audience. It fits into the textbook category, a bit below an actual QFT textbook, and slips into informal imprecise language.

This is no big deal, but for some reason this particular misconception people get really worked up about. There are a number of misconceptions in QM (see section 9.3 for this one which I will quote):
https://arxiv.org/pdf/quant-ph/0609163.pdf
'The calculational tool represented by Feynman diagrams suggests an often abused picture according to which “real particles interact by exchanging virtual particles”. Many physicists, especially nonexperts, take this picture literally, as something that really and objectively happens in nature. In fact, I have never seen a popular text on particle physics in which this picture was not presented as something that really happens. Therefore, this picture of quantum interactions as processes in which virtual particles exchange is one of the most abused myths, not only in quantum physics, but in physics in general. Indeed, there is a consensus among experts for foundations of QFT that such a picture should not be taken literally. The fundamental principles of quantum theory do not even contain a notion of a “virtual” state. The notion of a “virtual particle” originates only from a specific mathematical method of calculation, called perturbative expansion. In fact, perturbative expansion represented by Feynman diagrams can be introduced even in classical physics [52, 53], but nobody attempts to verbalize these classical Feynman diagrams in terms of classical “virtual” processes. So why such a verbalization is tolerated in quantum physics? The main reason is the fact that the standard interpretation of quantum theory does not offer a clear “canonical” ontological picture of the actual processes in nature, but only provides the probabilities for the final results of measurement outcomes. In the absence of such a “canonical” picture, physicists take the liberty to introduce various auxiliary intuitive pictures that sometimes help them think about otherwise abstract quantum formalism. Such auxiliary pictures, by themselves, are not a sin. However, a potential problem occurs when one forgets why such a picture has been introduced in the first place and starts to think on it too literally.'

People who have studied QM know them, they have had to unlearn them as they progressed in their education and while regrettable they had to unlearn stuff most just accept its simply a by-product of the usual way its taught and don't get too worried about it. I went through most of those stages and just laughed when the truth was finally 'revealed'. Without doubt I have misconceptions now that will be corrected later - its just part of how things are.

But this virtual particle thing as actual particles that pop in and out of existence for some reason many people will not let it go. They do after studying an actual textbook of course - but for some reason will not believe those that have - its very perplexing.

Thant's
Bill

 

[bhobba]

You're keen for textbooks. Yes they are good, but very dry and won't always answer your questions.

Yes they can be dry - there is a reason for that and it is a bit regrettable. Some texts try to do something about it with varying levels of successes. And they don't always answer you questions - that's one reason we have this forum. But this one they do answer - the paper I referenced by Dr Neumaier gives the detail.

Thanks
Bill

 

[PhysicsExplorer]

And I showed you a link explaining exactly what's going on.

Thant's
Bill

Again, you're speaking to me like I am not aware of the arguments.

I have heard far too much conflicting ideas from both sides. I am not convinced virtual particles are not real. You put this down to lingo, but its not. The idea of fluctuations is ingrained in the physicists mind, from the earliest attempts of quantum gravity.

 

[LeandroMdO]

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

That video is something of a conceptual salad, and I would encourage you to unlearn its contents. Absolute zero is not the temperature in which every position is fixed and all momenta are zero. Absolute zero is the temperature in which the system is sitting in its many-particle ground state. The impossibility of reaching absolute zero in a finite number of steps is not directly related to the Heisenberg uncertainty principle in the way he asserts, but rather is a consequence of a purely thermodynamic argument. The entropy approaches a constant as T -> 0, one which is independent of any other parameters that might be varied in the system (pressure, magnetic field, etc), so an infinite number of steps is required (see this picture, where X represents the external parameter).

Now, the third law actually tends to fail in classical physics because excitations of arbitrarily small amplitude are allowed and thus the specific heat (C = T.dS/dT) never vanishes. So quantum mechanics is actually important, but it has nothing to do with a pedestrian argument such as "you'd known position and momenta to infinite precision". You would not.

 

[PhysicsExplorer]

That video is something of a conceptual salad, and I would encourage you to unlearn its contents. Absolute zero is not the temperature in which every position is fixed and all momenta are zero.

Actually, that is the classical definition. He is right to mention it in this context.

 

[PhysicsExplorer]

Hence why, deviation from the classical theory into phase space gives us two solutions

1) fluctuations
2) that positions cannot be well defined due to the phase space

These are quantum properties.

 

[bhobba]

the consensus seems largely the view that the virtual particles exist

Of course they do. But not as actual particles - they are just lines on a Feynman diagram which is just a pictorial representation of something called a Dyson series:
https://en.wikipedia.org/wiki/Dyson_series.

You could have called them Jaberwocky's - but we call them virtual particles and therein lies the rock bottom essence of this whole misconception. Heuristically in solving some problems and understanding quantum processes some like to think of them as actual particles. For example its a nice pictorial way of thinking of how the charge of an electron gets bigger as you get closer to it - its screened by this sea of virtual particles. That's fine to tell a lay audience and even in a beginning level non QFT textbook. But that's not what is really going on at all - its the re-normalization group which John Baez explains, even though since its written for a non-technical reader he also uses the virtual particle analogy:
http://math.ucr.edu/home/baez/renormalization.html

BTW - note what John quotes at the end:
The author feels that this technique of deliberately lying will actually make it easier for you to learn the ideas. - Donald Knuth



Thanks
Bill

 

[LeandroMdO]

Actually, that is the classical definition. He is right to mention it in this context.

He's asserting that you can't get to absolute zero because you'd know the position and momenta of the particles to infinite precision, which is disallowed by the uncertainty principle. This is not correct in any context.

 

[PhysicsExplorer]

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

Of course they do. But not as actual particles
Thanks
Bill

Again, that is not generally what physicists think. You can repeat saying what you are saying, but I could collect loads of papers that could contradict the claim they are not real. The idea they are not real, even conflicts with the earliest attempts of quantum gravity - the general view back then was that this was a very serious line of investigation (quantum foam). So much so, the question of the foam still exists todayhttps://en.wikipedia.org/wiki/Quantum_foam

 

[PhysicsExplorer]

He's asserting that you can't get to absolute zero because you'd know the position and momenta of the particles to infinite precision, which is disallowed by the uncertainty principle. This is not correct in any context.

In the absolute zero picture, the classical picture does indeed say that positions of particles could be well defined. Infinite precision? Fancy words... but... yeah... the classical picture does freeze the particles of the system - this is in conflict with actual observed phenomenon - systems will not completely freeze and the positions of systems cannot be defined. It certainly is disallowed by the uncertainty principle...

 

[PhysicsExplorer]

Also, some take that gamma ray bursts provides some evidence for a smooth vacuum, but this may not be the case. Depends on how a zero point field couples to light. Without a complete theory, how can we say for sure radiation is effected by zero point noise?

 

[PhysicsExplorer]

Look at it this way, physicists considered fluctuations so seriously, they even went as far to model a universe like a fluctuation! And its not fancy lingo, it really was modeled like a fluctuation. This is known as the Vilenkin model and has been considered, 'a beautiful model.' Was even taken seriously by Hawking.

 

[bhobba]

Again, you're speaking to me like I am not aware of the arguments.

Well then you know they are just lines on a Feynman diagram.

Exactly why do you think such are 'real'?

Not an appeal to authority - we have plenty of those around here that disagree with you - but your exact reasoning. In particular point to the flaw in logic in the quote I gave from the paper about QM misconceptions. I have pointed to the flaw in logic in the usual account - but exactly where does my flaw lie?

Thanks

 

[PhysicsExplorer]

Well then you know they are just lines on a Feynman diagram.

Exactly why do you think such are 'real'?
Thanks

Because as I have said many times over, physicists have clearly treated them as more than just lines on a Feynman diagram. I have seen many posters over the years who have tried to wash away this obvious fact. That particle physicists considered fluctuations long before you didn't.

 

[Nugatory]

Again, that is not generally what physicists think. You can repeat saying what you are saying, but I could collect loads of papers that could contradict the claim they are not real.

Perhaps you can, but so far you have suggested only two references:
https://en.wikipedia.org/wiki/Quantum_foam
https://www.scientificamerican.com/article/are-virtual-particles-rea/
Neither is a serious peer-reviewed paper, neither is an acceptable source under the Physics Forums rules, and both are deliberately oversimplified to cater to people who lack the inclination or mathematical background to learn the real thing.

 

[bhobba]

He's asserting that you can't get to absolute zero because you'd know the position and momenta of the particles to infinite precision, which is disallowed by the uncertainty principle. This is not correct in any context.

It comes from a misunderstanding of the uncertainty principle that has been discussed here many times.

Ballentine gets it right in his textbook - page 223 - but many don't. It was one of those things I had to unlearn.

Thanks
Bill

 

[PhysicsExplorer]

But as for why I believe in them, I have already explained, that zero point fields understand fluctuations as a ground state of the field. The earliest theories, considered these fluctuations - in fact, so did cosmology, they renamed the primordial fluctuations as the cosmic seeds, something I have investigated as well.

Lots of physics, just simply doesn't make sense without them, such as the virtual particle shielding of electrons. Or that physical effect of virtual particles we can measure which I cannot remember any link to. Just everything, every part of my understanding of field theory, is based on the existence of these particles.

 

[PhysicsExplorer]

Perhaps you can, but so far you have suggested only two references:
https://en.wikipedia.org/wiki/Quantum_foam
https://www.scientificamerican.com/article/are-virtual-particles-rea/
Neither is a serious peer-reviewed paper, neither is an acceptable source under the Physics Forums rules, and both are deliberately oversimplified to cater to people who lack the inclination or mathematical background to learn the real thing.

Well I will if you want when I have time... but since fluctuations have been written on since 1955, I can honestly admit finding them is a no-brainer.

 

[PhysicsExplorer]

It comes from a misunderstanding of the uncertainty principle that has been discussed here many times.

Thanks
Bill

If by uncertainty principle, you mean its application to the metric which would give rise to fluctuations, then no, its not a misunderstanding as such. We add such fluctuations to the metric in general relativity like

g = g + h

Lawrence Crowell has written a lot on fluctuations, you might want to read about his investigations.

 

[Nugatory]

This thread has reached the limits of useful discourse and has been closed.