Thread by @GastonGiribet

Motivado por los posts de @MininniPablo sobre electrodinámica, que están muy buenos, me atrevo a resucitar un viejo post que escribí en 2019, aquí extendido, sobre la interacción de la luz con la misma luz. Algo así como cruzar Jedi lightsabers. Aquí va el hilo:

Light-by-light scattering is a phenomenon in which two photons, i.e. two particles of light, interact one with each other. Classically, light doesn't hit light, but because of quantum mechanics effects there exists a small probability that two light beams can actually interact by

interchanging electrons and positrons just before crossing paths. This is described by quantum electrodynamics (QED); actually, this is one of the first predictions made by the theory.
The direct observation of light-by-light scattering at high energy had proven elusive for

decades, until the Large Hadron Collider (LHC) began its second data-taking period of activity, known as Run 2. Collisions of lead ions at LHC has shown to provide a uniquely clean environment to study the light-by-light interaction [1].
Last year, on March 17th 2019, ATLAS

collaboration at LHC reported the observation of light-by-light scattering with a significance of 8.2 standard deviations [2]. The result uses data from the most recent heavy-ion operation of the LHC, which took place in November 2018.

ATLAS collaboration was the first in seeing, back in 2016, the light-by-light scattering using the strong electromagnetic field of the hadrons colliding at LHC.
Light-by-light scattering had been observed before, but indirectly. In 1998, for instance, the phenomenon was observed

in experiments involving the interaction of electron beams with terawatts pulses of laser at 527 nm wavelength. In that case, an excess of positrons appearing in the interaction between the two beams was interpreted as the signal of positron-electron pairs produced by the

scattering of photons [3].
The photon-photon (i.e. light-by-light) interaction is also what is behind the so-called Delbrück scattering, namely the deflection of high-energy photons when passing by sufficiently close to an atomic nuclei to feel its strong Coulombian field.

The latter phenomenon has been observed in the second half of 20th century. Photon-photon interaction is also important in high-energy astrophysics as it places bounds on the gamma ray spectrum we can observe, due to the interaction between gamma photons and photons from the CMB.

[1] home.cern/.../atlas-obse…
[2] atlas.cern/.../atlas-obse…
[3] journals.aps.org/.../10.1103/Ph…

Any update, welcome.