Light is a stream of photons, and ordinarily a photon has no magnetic moment: it doesn't deflect in a magnetic field like iron filings. But in a very strong magnetic field, comparable to the Schwinger critical field (about 4.4×$10^{13}$ Gauss — billions of times stronger than a fridge magnet), a photon develops an anomalous magnetic moment. It's as if a neutral particle suddenly starts feeling the magnet, even though it used to pass right through it.
The authors calculated how this moment depends on the field strength. They found that at a field 30 times the critical value, the photon's anomalous magnetic moment is about 8/3 times larger than at 0.5 times the critical field. This is similar to how an ordinary magnet is attracted more strongly to an iron sheet when brought closer, but here the effect only appears under extreme conditions.
These calculations can be tested experimentally. Recently, the ATLAS experiment detected light-by-light scattering with a significance of 8.2 sigma, and the X-ray polarimeter IXPE measured polarization of magnetar emission up to 80%. The new data on the photon's magnetic moment will help predict how light polarizes in such fields and compare with future measurements.
The Schwinger critical field is so strong that if it were created in a laboratory, it would tear the vacuum apart into particle-antiparticle pairs.
In the series 'Star Trek', strong magnetic fields are used to contain antimatter, but no one there considered that light in such fields itself becomes slightly magnetic.