How did an ‘impossible’ photon survive 2 billion light-years? Scientists point to a transformation involving axion-like particles

A photon from one of the most powerful explosions ever observed has left physicists with a problem: according to standard physics, it should not have survived the journey to Earth.

The event, known as GRB 221009A or the “BOAT," short for Brightest Of All Time, erupted more than 2 billion light-years away. Yet detectors on Earth picked up an extraordinarily energetic photon associated with the burst.

The latest research suggests that something unusual may have happened to allow such high-energy light to make the journey.


Scientists are investigating whether photons could have temporarily transformed into hypothetical particles known as axion-like particles (ALPs) and then changed back into photons before reaching Earth.

But the newest study adds another twist: ALPs alone do not fully explain the observation. The researchers say a combination involving ALP-photon conversion and a possible breakdown of Lorentz invariance, a principle at the heart of Einstein’s relativity, offers a more consistent explanation.

The cosmic explosion that stunned astronomers

GRB 221009A was detected on October 9, 2022, and quickly became known as the BOAT because of its extraordinary brightness.

Gamma-ray bursts are among the most energetic explosions in the universe. They can release enormous amounts of radiation in a matter of seconds, producing powerful jets that travel across vast distances.

GRB 221009A was particularly remarkable because scientists detected gamma rays at energies far beyond what they normally expect to survive such a long trip through space.

The Carpet experiment at Russia’s Baksan Observatory detected a photon-like event initially reported at around 251 teraelectronvolts (TeV). Further analysis by the Carpet collaboration later put the energy at roughly 300 TeV, with uncertainties.

That is an extraordinary amount of energy for a single photon arriving from a cosmological distance.

Why should the photon have disappeared?

The difficulty begins with the fact that intergalactic space is not truly empty.

As extremely energetic photons travel through the universe, they can interact with background radiation and produce pairs of particles. The universe is filled with low-energy photons, including radiation left over from the early universe and other forms of extragalactic background light.

At sufficiently high energies, these interactions can effectively block gamma rays from travelling enormous cosmic distances.

For GRB 221009A, standard propagation models predict that photons at the energies detected by Carpet should be heavily absorbed before reaching Earth. The new Physical Review Letters study describes the observation as being in strong tension with conventional expectations.

So the question becomes surprisingly simple:

How did the photon get here?

The proposed cosmic escape route

One possibility involves axion-like particles.

ALPs are hypothetical particles that are related to axions, which were originally proposed in particle physics to address a fundamental symmetry problem. Unlike ordinary photons, ALPs would not interact with background light in the same way.

Under certain conditions, a photon can potentially convert into an ALP and later convert back into a photon.

That creates an intriguing possibility for distant gamma rays.

Instead of remaining a high-energy photon throughout its journey — and being absorbed along the way — part of the radiation could switch into a form that is much less affected by the cosmic radiation field.

Earlier research had already proposed ALP-photon mixing as a possible way to explain the unusually energetic photons associated with GRB 221009A. A 2023 Physical Review Letters study found that suitable ALP parameters could significantly reduce the effective attenuation of TeV photons.

But there is a problem with the ALP explanation

The newest research makes the picture more complicated.

Giorgio Galanti and Marco Roncadelli examined the latest Carpet result and tested several possibilities for explaining the unusually energetic photon.

Their conclusion was that the specific ALP-only scenarios they considered were strongly disfavoured by the new observation.

Instead, the researchers found that certain models involving Lorentz-invariance violation (LIV) could accommodate the photon. They also examined scenarios in which standard photon-ALP oscillations are combined with LIV-related changes to photon propagation.

That distinction is important.

The research does not establish that axion-like particles exist, nor does it prove that Einstein’s theory is wrong. It shows that the unusual observation can be explored using models involving physics beyond the standard picture.

What does Einstein have to do with it?

Lorentz invariance is a fundamental principle underlying special relativity.

In simple terms, it says that the basic laws of physics should not depend on the constant velocity of an observer.

Some theories attempting to describe physics at extremely high energies, particularly approaches connected to quantum gravity, have explored whether this symmetry could be modified under certain conditions.

If Lorentz invariance were violated at extremely high energies, photons might propagate differently from what conventional relativity predicts.

That could potentially help explain why an ultra-high-energy photon appears to have travelled farther than standard models allow.

The new Physical Review Letters paper finds that specific LIV frameworks are compatible with the Carpet observation, while combined scenarios involving both ALP-photon conversion and LIV can provide a consistent interpretation of the highest-energy observations from GRB 221009A.

The photon arrived unusually late

There is another intriguing feature of the observation.

The ultra-high-energy photon was detected roughly an hour after lower-energy radiation from the same burst.

That timing matters because some models involving modified photon propagation can produce energy-dependent travel times.

In other words, the energy of a photon could potentially affect how it travels across enormous distances.

The researchers note that the observed timing is compatible with the type of behaviour predicted in the models they investigated.

Still, compatibility is not the same as proof.

Scientists need more observations to determine whether the effect is genuinely pointing toward new physics or whether another explanation will eventually account for the data.

This does not mean Einstein has been disproved

The phrase “defied Einstein” makes for an eye-catching headline, but the scientific reality is more cautious.

The observation is not proof that relativity has failed.

Instead, researchers are asking whether extremely energetic photons could reveal subtle effects that ordinary experiments cannot easily detect.

The study places constraints on possible Lorentz-violating energy scales and explores how such effects could work alongside photon-ALP conversion.

There is also an important caveat: the entire interpretation depends on the high-energy event genuinely being associated with GRB 221009A and being correctly identified as a photon.

That is why independent observations and future gamma-ray bursts will be crucial.

FAQ about the BOAT and its impossible photon

What is the BOAT?

The BOAT is the nickname given to gamma-ray burst GRB 221009A, one of the brightest and most energetic gamma-ray bursts ever observed. It was detected on October 9, 2022, from more than 2 billion light-years away.

Why was the photon from GRB 221009A considered impossible?

A photon with an energy of around 300 TeV should be strongly absorbed while travelling such a vast distance because of interactions with background radiation. Its detection therefore creates a significant problem for standard models of photon propagation.

Could axion-like particles explain how the photon reached Earth?

They may contribute to an explanation. Photons could theoretically convert into axion-like particles, which would be less affected by background radiation, before converting back into photons. However, the latest study found that ALP-only scenarios do not adequately explain the new observation.

Does this prove Einstein’s theory is wrong?

No. The research does not prove that special relativity is incorrect. It explores whether extremely high-energy photons could reveal possible Lorentz-invariance-violating effects that would represent physics beyond the standard model. More observations are needed before scientists can draw such a conclusion.