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Re: [Phys-L] light-by-light scattering



On 09/03/2017 10:01 AM, bernard cleyet wrote:

https://www.theguardian.com/science/life-and-physics/2017/sep/03/experiment-reveals-evidence-for-a-previously-unseen-behaviour-of-light

It's an amusing article, but it doesn't really explain what is going on.

The physics is fairly simple, but there's why it's not trivial:
The Maxwell equations are linear, and linearity implies superposition,
so to explain what's going on you need more than just the Maxwell
equations for light in classical empty space.

Meanwhile, we know that the real world is quantum mechanical, and
classical physics is only an approximation. In particular, the
quantum vacuum is not nearly as empty as the classical vacuum.
There are /vacuum fluctuations/ consisting of ephemeral aka virtual
aka evanescent pairs. Mostly we are talking about electron/positron
pairs, since they are the least-massive and hence easiest to
produce. This has been well understood for eons. One manifestation
is the Casimir effect.
http://math.ucr.edu/home/baez/physics/Quantum/casimir.html

It's no big deal to have photon A transfer some momentum to a
virtual pair, which then transfers the momentum to photon B. This
has to be done muy pronto, since the pair doesn't last very long,
so you need a *lot* of light in one place.

That is to say, once you posit that the vacuum isn't quite empty,
you can more-or-less understand what's going on by using your
intuition about the Maxwell equations.

You have to be a bit of a lawyer when you read the Butterworth
article. It says no particles (other than the photons) are
"produced" but the fact is, other particles are /involved/. The
diagram for this is an "electron loop" diagram or "box" diagram,
which is simple and amusing:
https://images.nature.com/full/nature-assets/nphys/journal/vaop/ncurrent/carousel/nphys4208-f1.jpg
which is taken from the recent article
http://www.nature.com/nphys/journal/vaop/ncurrent/full/nphys4208.html
of which Butterworth is a coauthor.

Also I'm not sure what part of this is being claimed as "previously
unseen". There have been reports of light-by-light scattering for
a long time; here's one from 1985:
http://puhep1.princeton.edu/~kirkmcd/accel/malibu.pdf
I semi-recall hearing of light-by-light scattering in intense
laser pulses, although I don't have a good reference for that.
Maybe the recent experiment exhibits the effect more cleanly
or more directly; I don't know.