Delbrück scattering results in ether?

[nomadreid]

According to http://en.wikipedia.org/wiki/Delbrück_scattering

" ... based on the relativistic quantum mechanics of Dirac according to which the QED vacuum is filled with ... electron-positron pairs ...[which] should be capable of producing coherent-elastic photon scattering..."

If this is the case, then don't we arrive at a sort of ether, in which the velocity of photons could occasionally be affected by the vacuum?

 

[Bill_K]

If this is the case, then don't we arrive at a sort of ether, in which the velocity of photons could occasionally be affected by the vacuum?

No, not the vacuum. As the article states, Delbrück scattering is the scattering of a photon by a Coulomb field.

 

[nomadreid]

Right, the Coulomb field which is continuously being produced by the electron-positron pairs which is continuously being produced in the vacuum. That is, as a photon goes through the vacuum, this Coulomb field would scatter the photon, so that the net effect is such that one could talk of a scattering in the vacuum if not by the vacuum. The end result would be the same: a scattering of photons as they pass through the vacuum. But this is what the infamous aether was supposed to do, and didn't. No?

 

[Bill_K]

Right, the Coulomb field which is continuously being produced by the electron-positron pairs which is continuously being produced in the vacuum. That is, as a photon goes through the vacuum, this Coulomb field would scatter the photon, so that the net effect is such that one could talk of a scattering in the vacuum if not by the vacuum. The end result would be the same: a scattering of photons as they pass through the vacuum. But this is what the infamous aether was supposed to do, and didn't. No?

If you're confused about the difference between a Coulomb field and vacuum, I probably can't help you much.

I'd just point out that every process ultimately takes place "in the vacuum". That doesn't mean it was "caused by" the vacuum. When a neutron or a muon decays, it takes place in a vacuum, but you can't attribute it to the vacuum acting like the ether.

Also, Coulomb fields scatter other things. More fundamental than Delbrück scattering is Compton scattering and/or Thomson scattering, the scattering of a photon by an electron. Delbrück scattering is just the first-order radiative correction to Compton scattering.

 

[nomadreid]

I understand the difference between the Coulomb field and the vacuum, so I am apparently not expressing myself very well. Let me try again. I am not saying that the vacuum is the cause of any scattering. In fact, the scattering issue is secondary to my main question: the excerpt cited in my original post seemed (to me) to be saying that QED says that the photon will be able to interact with the virtual electron-positron pairs -- as an example, scattering (Compton, Thompson, Delbrück...). This interpretation of the excerpt (which is where I suppose I am going wrong) makes me queasy, because it seems to run counter to my intuition as to what a virtual entity is: precisely something that cannot interact

 

[Jilang]

This paper suggests there may "beautiful connection" between the speed of light and the concept of virtual particles.
http://arxiv.org/pdf/1005.0131.pdf

 

[strangerep]

[...] the excerpt cited in my original post seemed (to me) to be saying that QED says that the photon will be able to interact with the virtual electron-positron pairs -- as an example, scattering (Compton, Thompson, Delbrück...). This interpretation of the excerpt (which is where I suppose I am going wrong) makes me queasy, because it seems to run counter to my intuition as to what a virtual entity is: precisely something that cannot interact

Yeah, and the vacuum is also filled with unicorn-antiunicorn pairs.

More seriously, you can safely assume that any explanation of a physical effect that relies on virtual particles being somehow real is just another opiate for the masses. Explaining that the vacua of the free and interacting theories are inequivalent is not something that can be done quickly -- that's a topic in advanced field theory, hence often evaded with the help of arm-waving fictions

These effects are simply higher-order effects in QED. The Feynman diagram in that Wiki page illustrates the particular higher-order term in the perturbation series, but the explanation of the internal lines suggests that they somehow have physical reality. That's nonsense. Feynman diagrams are just a pictorial representation of term(s) in a perturbation series.

 

[strangerep]

This paper suggests there may "beautiful connection" between the speed of light and the concept of virtual particles.
http://arxiv.org/pdf/1005.0131.pdf

Study it more carefully. They use properties of relativistic charged fermions, and then "derive" speed of light results. (Sigh)

 

[nomadreid]

Thanks, strangerep and Jilang.

strangerep's responses put me more at ease on two related issues. First, my original question. Strangerep confirmed my intuition about virtual pairs (although I did wonder where all those unicorns came from in my living room, where I keep things like the miniature black hole I found in the flea market). I shall see how far I can get with the topic of the relevant perturbation series, but as strangerep used the expression "advanced field theory", that may take a while.

Then, a related issue: I had earlier puzzled about the article http://arxiv.org/abs/1302.6165 , a 2013 article which (I found out upon receiving Jilang's post) was doing something similar to the 2010 article which Jilang mentioned (http://arxiv.org/pdf/1005.0131.pdf), and indeed in the references for the 2013 article appears the 2010 paper. (But a different set of authors.) Apparently, a juicy circular argument never dies. But just as in the case of the faulty arguments at the basis of the paradoxes in set theory, maybe there is something more positive that can eventually be derived from those ideas which are non-circular in these papers.

So again thanks to both.

 

[strangerep]

I shall see how far I can get with the topic of the relevant perturbation series, but as strangerep used the expression "advanced field theory", that may take a while.

Only introductory QFT is needed to understand the relationship between Feynman diagrams and the (Dyson) perturbation series.

The "advanced field theory" thing was in reference to unitarily inequivalent vacuua.

 

[nomadreid]

thanks, strangerep, that gives me hope...