Quantum vacuum in proximity of matter

[markuslig]

This quantum vacuum thing... I'm thinking about these virtual particles popping in and out of existence. Does that happen everywhere including inside atoms(within electron orbitals), protons, quarks and whatever is smaller? Or just in the space between Atoms... what i would call vacuum?

im a complete amature, so please don't slap a formula with symbols i can't even read in my face thanks :P

 

[Avodyne]

Yes, it happens everywhere, at least to the extent that it happens at all.

The key point is that various people use various words to describe what the theory of quantum electrodynamics "means". This notion of "virtual particles popping in and out of existence" is one way some people choose to describe what is "happening". My personal opinion is that this is highly misleading, but many others disagree.

But everyone agrees on how to do calculations in quantum electrodynamics, and the results are in spectacular agreement with experiment.

 

[WannabeNewton]

My personal opinion is that this is highly misleading, but many others disagree.

Could you expand upon this, if you don't mind? I have seen such a view espoused by others as well. I ask because while textbooks tend to describe with full conviction the usual picture of virtual particles fluctuating in the vacuum in the context of Feynman propagators in perturbation theory, I don't really see any talking about to what extent this picture is tied down to perturbation theory and absent from the exact formulation of QED.

 

[The_Duck]

Could you expand upon this, if you don't mind? I have seen such a view espoused by others as well. I ask because while textbooks tend to describe with full conviction the usual picture of virtual particles fluctuating in the vacuum in the context of Feynman propagators in perturbation theory, I don't really see any talking about to what extent this picture is tied down to perturbation theory and absent from the exact formulation of QED.

Here are some problems with virtual particle picture:

* It naively suggests that the vacuum is a time-dependent state when in fact it is time-invariant.

* It naively seems to violate things like energy conservation, which confuses laymen.

* Similarly, describing forces as the result of virtual particle exchange leads to all sorts of misconceptions that are discussed weekly on this forum. (How does this explain attractive forces? How do gravitons escape from black holes? And so on.)

* It is rooted in perturbation theory, which is just an approximation technique. The notion of "virtual particles popping in and out of existence" is very appealing when you look at, say, the one-loop Feynman diagram for the vacuum polarization in perturbation theory. But if you do a nonperturbative lattice field theory calculation on a computer, virtual particles do not appear at any step.

Of course, virtual particles are a very useful concept if you know enough not to fall into any of these traps. But in explanations for laymen, they sometimes confuse as much as they help.

 

[WannabeNewton]

Brilliant, thank you The_Duck!

 

[bhobba]

It is rooted in perturbation theory, which is just an approximation technique. The notion of "virtual particles popping in and out of existence" is very appealing when you look at, say, the one-loop Feynman diagram for the vacuum polarization in perturbation theory. But if you do a nonperturbative lattice field theory calculation on a computer, virtual particles do not appear at any step.

Yes - that is the issue with virtual particles - some think its simply an artefact of the perturbative methods normally used in QFT.

In a non perturbative approach they do not appear so their 'reality' is open to question.

I am no expert in this but my understanding is perturbative QFT is derivable from lattice QFT by Taylor expanding and taking the continuum limit. Since that continuum limit is responsible for the infinities that plague QFT (pertubatively that is - I have also read that an axiomatic treatment avoids them in the first place) such as the infinite energy of the vacuum, and why Effective Field Theory (EFT) has a cut-off.

But, as I said, I am no expert - the above I have gleaned from various stuff I have read over the years.

Thanks
Bill

 

[Avodyne]

I agree with The_Duck. But note that many experts, including Nobel Laureate Frank Wilczek, are happy with the "popping in and out" picture: http://books.google.com/books?id=oa2Ddk3K0TQC&pg=PA404&lpg=PA404&dq=Frank+Wilczek+virtual+particles

Also, I think it would best if the word "real" and its derivatives ("really", "reality", etc) were just not used in physics discussions. There is simply no definitive answer to the question of what is "really" going on in realms far removed from direct human experience.

 

[bhobba]

Also, I think it would best if the word "real" and its derivatives ("really", "reality", etc) were just not used in physics discussions. There is simply no definitive answer to the question of what is "really" going on in realms far removed from direct human experience.

Agree entirely.

Trouble is we get quite a few people without a lot of background in physics. They haven't cottoned onto physics in general is really about mathematical models - not about 'reality', what's really going on etc etc.

I try to explain it by saying that as far as we can tell today the mathematical description is the 'reality'.

Thanks
Bill