Italian physicists propose how a photon defied the universe to reach Earth

A 300-trillion-electronvolt photon from the brightest gamma-ray burst ever detected should have been absorbed by background radiation en route. Researchers from Italy's INAF and INFN now suggest a combination of axion-like particles and a violation of Lorentz invariance could explain its survival – and its one-hour delay.

Bottom line — If confirmed, the finding would let the universe test quantum gravity at energies no Earth accelerator can reach.

Go deeper (6)

• The photon was detected on 9 October 2022 by Russia's Carpet detector at the Baksan Observatory, coming from GRB 221009A – nicknamed 'BOAT' (Brightest Of All Time) – about 2 billion light-years away, per Scientific American.
• According to standard physics, a photon of that energy should have been absorbed by interactions with the cosmic microwave background, making its arrival 'virtually impossible', write Galanti and Roncadelli in Physical Review Letters, per Phys.org.
• The researchers propose that, at extreme energies, axion-like particles allow the photon to temporarily convert and evade destruction, combined with a violation of Lorentz invariance – a fundamental pillar of Einstein's relativity, per Phys.org.
• The model also predicts a one-hour delay compared with lower-energy photons from the same burst, which matches data from China's LHAASO observatory, per Scientific American.
• An alternative explanation, reported by ScienceFeed, suggests the photon travelled through a region of unexpectedly low background radiation density, creating a 'transparency window' – though this is not mentioned in the peer-reviewed study.
• The authors stress that further observations are needed to test the scenario, per the paper.

Read the reporting