In my post titled “On the photon frequency” I emphasized the fact that photons do not interact with other photons, they pass through each other. Why? Because photons do not have electric charge.
Some of you may have seen statements like “photon-photon interaction” or read about proposals for photon-photon colliders. What are they talking about? How is this possible? How can you collide photons if they pass through each other?
At CERN LHC (Large Hadron Collider) certain periods of machine time are dedicated to heavy ion collisions. The main purpose of this is to study the properties of the unique form of matter known as the quark-gluon plasma.
At LHC, Pb (Lead) ion beams are collided with Pb (Lead) ion beams at very high energies. Such collisions – even the off-axis ones – create extremely strong electric and magnetic fields as well as high frequency (gamma ray) photons. These energetic photons may interact indirectly through quantum processes. In other words, in photon–photon scattering, one photon scatters from the transient vacuum charge fluctuations of the other.
If the ion beams collide head-on then the strong nuclear force between the nucleons (protons and neutrons) inside the Pb ions dominate the scene and it becomes difficult to isolate the rare processes corresponding to photon-photon interactions. This is why off-axis collisions also known as ultra-peripheral collisions are studied. In ultra-peripheral collisions electromagnetic force dominates the scene.
In the presence of strong electric and magnetic fields energetic photons may result in electron/positron pairs (real or virtual). There are many other possibilities: virtual quarks or leptons (and the antiparticles associated with them) may be created as well. These entities interact (via electromagnetic force or via strong-nuclear force) and then transform into real photons again. The entire process would have the appearance of elastic photon-photon scattering. This is an extremely rare process. Only few photons will interact in such a way.
Interestingly, the ATLAS experiment at CERN LHC recently announced that they found evidence for such a rare process
“Protons that are accelerated to nearly the speed of light produce a very strong electromagnetic field. The generated field is even stronger when Lead ions are used rather than protons . When two such ions pass each other in a so-called ultra-peripheral collision, two photons can scatter off one another while the ions themselves stay intact. The scientists then observe two low-energy photons with specific kinematic properties and no additional activity in the detector. Based on the data taken in 2015 at the LHC, physicists at the ATLAS experiment conducted a search for light-by-light scattering and found 4.4σ evidence for the phenomenon. The σ-value describes the statistical significance of a scientific result. Physicists usually speak of a “discovery” if they find a 5σ result and call a 3σ result “evidence” for something new. Light-by-light scattering has a very small cross-section, which means that it happens very rarely. Thus in four billion analysed events, only 13 candidates for such diphoton events were observed.” – PhysOrg
In the explanations above the “pair creation” from photons was mentioned. Even though the matter creation from light was known from the early days of quantum mechanics the direct observation of this basic phenomenon of nature was only made in the 1990’s. Here’s Richard Ruiz’s reporting regarding history:
“…Despite recent claims, the first evidence for direct production of matter from photon beams came in 1994 from the DELPHI experiment at the Large Electron Positron (LEP) Collider, the LHC’s predecessor at CERN. There are earlier reports of photon-photon scattering at colliders but I have been unable to track down the appropriate citations. Since 1994, evidence for photon-photon scattering has been observed by the Fermilab’s CDF experiment at the Tevatron, as reported by the CERN Courier, and there is even evidence for the pair production of muons and W bosons at the CMS Experiment. ” – Richard Ruiz
Here’s the reference for the pair production of muons from photons: