Quantum fluctuations & 'virtual particles'

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A question about what quantum fluctuations in the vacuum actually are
I've been reading about how language around virtual particle fluctuations is metaphorical. This is helpful:

https://www.physicsforums.com/insights/vacuum-fluctuation-myth/

I'm just trying to understand a bit more from a layman's point of view. I found Matt Strassler's article 'Virtual Particles: What are they?' helpful too.

He explains virtual particles in terms of interacting 'ripples' between two 'real' particles.

I initially pictured 'quantum fluctuations' as the random movement or appearance of energy within the vacuum but I think that's mistaken.

Am I understanding correctly that what are referred to as 'quantum fluctuation' or 'virtual particles' are actually interactions between the fields of two 'real' particles? And that the uncertainty comes in because of the uncertainty about the position, momentum, etc of 'real' quantum particles? It's not a new level of uncertainty where random energy pops up somewhere in those fields?

So when people talk about 'random quantum fluctuations in the vacuum' there's isn't some random energy popping up somewhere, the interactions are unpredictable because of the uncertainty around the location / momentum of the real particles?

Any answers much appreciated, I know I am probably many layers away from understanding this as a non-physicist but I just wanted to know if I'm going in the right direction!
 

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  • #2
PeroK
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TL;DR Summary: A question about what quantum fluctuations in the vacuum actually are

I've been reading about how language around virtual particle fluctuations is metaphorical. This is helpful:

https://www.physicsforums.com/insights/vacuum-fluctuation-myth/

I'm just trying to understand a bit more from a layman's point of view.
In simple terms, virtual particles are a mathematical tool/trick used in certain calculations. They don't exist in any physical sense.
I found Matt Strassler's article 'Virtual Particles: What are they?' helpful too.

He explains virtual particles in terms of interacting 'ripples' between two 'real' particles.
Virtual particles are not real, so they are not ripples either.
I initially pictured 'quantum fluctuations' as the random movement or appearance of energy within the vacuum but I think that's mistaken.
Yes, because energy only really exists when we measure something. Fundamentally, you can't talk about appearance and disappearance of energy from the vacuum. Instead, fluctations are again a mathemetical trick used within certain calculations.
Am I understanding correctly that what are referred to as 'quantum fluctuation' or 'virtual particles' are actually interactions between the fields of two 'real' particles? And that the uncertainty comes in because of the uncertainty about the position, momentum, etc of 'real' quantum particles? It's not a new level of uncertainty where random energy pops up somewhere in those fields?
It's not a question of uncertainty. The calculations in QM, whether you invoke virtual particles or not, produce probabilities (technically probability amplitudes) for each possible outcome. This applies to what particles emerge from a high energy interaction and the probability of different scattering angles. These calculations may use Feynman diagrams and virtual particles to calculate the probability of an outcome.
So when people talk about 'random quantum fluctuations in the vacuum' there's isn't some random energy popping up somewhere, the interactions are unpredictable because of the uncertainty around the location / momentum of the real particles?
It's more fundamental than that. E.g. two high energy particles may simply scatter off each other or produce a new set of output particles. You can't think classically about these processes. Yes, there is uncertainty involved, but it's the energy of the interaction that determines the range of probabilities.
Any answers much appreciated, I know I am probably many layers away from understanding this as a non-physicist but I just wanted to know if I'm going in the right direction!
Yes, it's hard without some sort of mathematical understanding. Ultimately, if you were to learn particle physics as a graduate-level physics course, the mathematics is all you would have. You wouldn't have a classical picture of what's happening. You would just have an understanding of the mathematics involved.

From that point of view, any descriptions you hear like "ripples" or "energy fluctuations" are just something someone made up because they liked the sound of it and possibly it felt (in a remote way) something like a classical approximation of quantum mechanics.
 
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  • #3
Demystifier
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In short, quantum fluctuations and virtual particles are totally unrelated concepts. Quantum fluctuations are there even without interactions, virtual particles are not. Quantum fluctuations are measurable, virtual particles are not. Quantum fluctuations are a quantum effect, "virtual particles" appear mathematically even in classical field theory (except that in this context they are not called so).
 
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PeroK
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Quantum fluctuations are measurable, virtual particles are not.
How would you measure a quantum fluctuation?
 
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How would you measure a quantum fluctuation?
Quantum fluctuation is nothing but a different name for quantum uncertainty. You measure it by repeated measurements of the same observable on an ensemble of identically prepared systems. If you get a distribution different from a delta-function, you have measured a fluctuation.
 
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PeroK
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Quantum fluctuation is nothing but a different name for quantum uncertainty. You measure it by repeated measurements of the same observable on an ensemble of identically prepared systems.
Are you sure that's what's commonly meant by fluctuations? Most popular sources seem to use the term as an explanation for vacuum energy, and involving virtual particles. E.g.

https://en.m.wikipedia.org/wiki/Quantum_fluctuation
 
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Are you sure that's what's commonly meant by fluctuations? Most popular sources seem to use the term as an explanation for vacuum energy, and involving virtual particles. E.g.

https://en.m.wikipedia.org/wiki/Quantum_fluctuation
The goal here is to correct misconceptions appearing in popular sources. In professional physics literature, fluctuation and uncertainty are the same thing.
 
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PeroK
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The goal here is to correct misconceptions appearing in popular sources. In professional physics literature, fluctuation and uncertainty are the same thing.
Agreed. I've never seen fluctuation used in that sense. Only uncertainty, variance or standard deviation.
 
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I've never seen fluctuation used in that sense. Only uncertainty, variance or standard deviation.
Compare the vacuum in free QFT with the ground state of the quantum harmonic oscillator. The former is said to have vacuum fluctuations of the field, while the latter is said to have position uncertainty. But those are essentially the "same" thing, free QFT is just an infinite set of quantum harmonic oscillators.
 
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vanhees71
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Fluctuations are of course nothing else than uncertainties in random experiments, i.e., the statistical fluctuations around the expectation value of some observable.

There is no vacuum energy or, rather, its value within special relativistic or Newtonian physics is entirely arbitrary. Nothing in the physics changes when you add arbitrary values to the total energy. The only state in relativistic QFT which does not fluctuate is the ground state aka the vacuum.

What's often discussed as "virtual particles" are in fact fluctuations of fields. E.g., having an atom in an excited state is quantum-field theoretically a state with the atom in this state and no photons. Since this is not an energy eigenstate and since photon number is not conserved there is some probability for the atom to emit photons and go to a lower state. This is a random process and in this sense a "fluctuation". This spontaneous emission is one of the simplest processes which are only describable in quantum field theory, i.e., it's showing the necessity to quantize the electromagnetic field in addition to the (charged) particles and that the semiclassical approximation, where the electromagnetic field is treated as a classical field and only the particles are quantized.

Another argument for "vacuum fluctuations" is usually the Casimir force between uncharged conductors. The usual calculation in introductory chapters of QFT books (e.g., Itzykson+Zuber) is, however, an approximation in the limit of infinite charge of the conduction electrons. A more detailed treatment of the Casimir force shows that it's in fact due to fluctations of the charges and the em. field and has nothing to do with "vacuum fluctuations". Ironically, the full vacuum of the interacting theory doesn't fluctuate at all.
 
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  • #11
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All the replies much appreciated! I shall go and ponder... :smile:
 

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