Ultra-High-Energy Photon Defies Physics to Reach Earth

A single photon carrying 300 teraelectronvolts of energy, detected from a gamma-ray burst 2 billion light-years away, has survived a journey that standard physics deems impossible. The detection, made by the Carpet experiment in October 2022, challenges fundamental tenets of Einstein's theory of relativity and has prompted a new theoretical model combining hypothetical particles with a potential flaw in spacetime's fabric.

Researchers Giorgio Galanti of Italy's National Institute for Astrophysics (INAF) and Marco Roncadelli of the National Institute for Nuclear Physics (INFN) authored the study, which has been accepted for publication in Physical Review Letters. They sought to explain how this photon from the record-breaking gamma-ray burst GRB 221009A, nicknamed BOAT for Brightest Of All Time, could have reached Earth.

The Impossible Journey

According to established physics, a photon of such extreme energy should have been annihilated long before completing its multi-billion-year voyage. The universe is filled with the cosmic microwave background radiation, a relic of the Big Bang. A 300 TeV photon traveling through this radiation field would be expected to interact with lower-energy background photons, converting into particle-antiparticle pairs and disappearing. The researchers liken the feat to firing an arrow through an impossibly dense forest stretching for 2 billion light-years without it hitting a single tree.

"We started from a very simple question: How did this photon survive a journey that, according to known physics, should have destroyed it?" explains Giorgio Galanti, the study's first author. Previous explanations involved axion-like particles (ALPs), hypothetical lightweight particles that could allow photons to oscillate into them and back, evading interaction. However, this mechanism alone fails to account for photons at the unprecedented 300 TeV energy.

A New Theoretical Framework

The new model proposes a two-part solution. It retains the ALP oscillation mechanism but couples it with a potential violation of Lorentz invariance, a cornerstone of Einstein's special relativity. Some quantum gravity theories suggest that at the highest energies, this principle-which states that the laws of physics are the same for all observers-could be subtly modified. This modification would alter photon propagation, effectively making the universe more transparent to ultra-high-energy light.

In this scenario, the photon finds a "fast lane" through the cosmos. The effect does not rewrite physics wholesale but suggests that at energies far beyond those produced in terrestrial particle accelerators, new phenomena may emerge. The combined model allows the photon to avoid the destructive interactions that should have occurred.

A Corroborating Delay

The theory's predictive power provides a second clue. The model calculates that the ultra-high-energy photon should arrive at Earth approximately one hour later than the burst of lower-energy photons from the same event. This delay was indeed observed, with the timing of the 300 TeV detection from Carpet lagging behind the main photon shower recorded by China's Large High Altitude Air Shower Observatory (LHAASO).

"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," adds co-author Marco Roncadelli. He notes that if future observations confirm this scenario, the universe itself becomes a natural laboratory for probing quantum gravity at otherwise inaccessible energies.

The study offers a unified explanation for both the photon's survival and its delayed arrival. The researchers emphasize that further observations of similar extreme events will be required to test and potentially validate their proposed theoretical scenario.