A 300 TeV Photon Crossed 2 Billion Light-Years It Should Not Have Survived. Two Italians Think Spacetime Bent the Rules.

On Oct. 9, 2022, the brightest gamma-ray burst ever recorded washed over Earth. Astronomers nicknamed it the BOAT, for Brightest Of All Time. It came from an explosion about 2 billion light-years away, and among the flood of photons was one that, according to established physics, should never have made it here at all.

The Carpet detector, an ultra-high-energy cosmic-ray instrument at the Baksan Observatory in the Russian Caucasus, clocked that photon at roughly 300 teraelectronvolts — the highest energy ever measured for a photon from a gamma-ray burst. The problem is that space is not empty. Over a journey that long, a photon at that energy should have collided with photons from the cosmic microwave background, the faint afterglow of the Big Bang that fills the universe, converted into other particles and vanished. It is like firing an arrow through a forest 2 billion light-years deep and having it miss every tree.

Giorgio Galanti of Italy's National Institute for Astrophysics and Marco Roncadelli of the National Institute for Nuclear Physics have proposed an explanation, accepted for publication in Physical Review Letters. "We started from a very simple question: How did this photon survive a journey that, according to known physics, should have destroyed it?" said Galanti, the study's first author. "The new data from the Carpet experiment showed us that the explanations proposed so far were no longer sufficient. We therefore looked for a theoretical scenario capable of consistently describing what we observe, without resorting to arbitrary corrections to the equations."

The leading explanation until now involved axion-like particles, or ALPs — hypothetical particles so light they barely exist. In that picture a photon converts into an ALP partway through its trip, sails through the background radiation untouched because ALPs do not interact with it, and converts back into a photon near the Milky Way. The mechanism works for photons up to a few tens of teraelectronvolts. It does not stretch to 300.

So Galanti and Roncadelli added a second ingredient, and it is not a small one. They combined the ALP mechanism with a possible violation of Lorentz invariance — the property of Einstein's special relativity that says the laws of physics look the same to every observer regardless of how fast they are moving. Several quantum-gravity models predict that at extreme energies this symmetry could be slightly modified, subtly changing how photons propagate. In the combined scenario, the universe becomes far more transparent at these energies, and the photon slips through interactions that would otherwise have destroyed it. It finds, in effect, a fast lane. This does not mean physics behaves differently in general — only that at energies this extreme, effects never before observed may come into play.

The part that makes the paper more than an accommodation of one awkward data point is a second, independent prediction. The same model says the 300 TeV photon should have arrived about an hour later than the lower-energy photons from the same burst, because the modified propagation is energy-dependent. That delay is precisely what China's Large High Altitude Air Shower Observatory recorded.

"The most interesting aspect of our work is that, for the first time, it brings together two ideas that until now had been developed separately," Roncadelli said. "If future observations confirmed this scenario, the universe would become a natural laboratory for studying quantum gravity at energies enormously higher than those achievable by any accelerator built on Earth." One model, two otherwise unconnected observations: a photon that should not exist and a lag nobody had a reason to expect. More bursts will be needed before anyone declares Lorentz invariance broken, but the cheap version of this experiment is simply to keep watching the sky.

The paper is Galanti et al., "Lorentz-violating scenarios for the highest-energy photons from GRB 221009A," Physical Review Letters (2026), DOI 10.1103/zjh2-mc47.