Gamma Ray Experiment on Top of Extinct Volcano Suggests Speed of Light Is Constant Throughout Universe

An experiment on the side of an extinct volcano in Mexico involving gamma rays is helping scientists answer a fundamental question in physics: Is the speed of light the same throughout the universe?

The High Altitude Water Cherenkov (HAWC) observatory on the slopes of the Sierra Negra volcano in Puebla, Mexico, detects gamma rays and cosmic rays from distant galactic sources that enter Earth's atmosphere.

Gamma rays are produced by the hottest and most energetic objects in the universe, including supernova explosions, neutron stars and black holes. Gamma ray bursts are the most energetic events to take place, releasing more energy in 10 seconds than the Sun will over its entire lifetime.

Studying gamma rays provides an opportunity to understand these high energy objects. But detecting them is difficult as they have very small wavelengths. As NASA notes: "Gamma-ray wavelengths are so short that they can pass through the space within the atoms of a detector."

The HAWC observes gamma rays using hundreds of huge tanks of purified water. The gamma rays are detected by recording the level of Cherenkov light produced as the particles pass through the water. As the HAWC explains: "Cherenkov light is the electromagnetic equivalent of a sonic boom. It occurs when a charged particle travels through a medium faster than light can travel through that medium."

In the latest experiment, led by Andrea Albert, from the Los Alamos National Laboratory, New Mexico, researchers with the HAWC were looking to test the Lorentz Invariance.

This is part of Albert Einstein's theory of special relativity that says the speed of light is constant across the universe, regardless of where you are or how fast you are moving. This constant forms part of the Standard Model of particle physics—our best, but as of yet incomplete, explanation for how the universe works.

It is thought, however, that this theory may break down at the highest energies. If the Lorentz Invariance broke down, it would mean several different phenomena become possible.

"How relativity behaves at very high energies has real consequences for the world around us," Pat Harding, an astrophysicist from the Los Alamos National Laboratory, said in a statement. "Most quantum gravity models say the behavior of relativity will break down at very high energies."

Results of the experiment are published in the journal Physical Review Letters. The team were looking at recently detected gamma rays that came from four galactic sources producing photons above 100 TeV. That is equivalent to a trillion times the energy of visible light. The maximum collision energy produced at the Large Hadron Collider is around 14 TeV.

Findings showed that even at these extreme energies, none of the sources violated the Lorentz Invariance. "Our observation of such high-energy photons at all raises the energy scale where relativity holds by more than a factor of a hundred," Harding said in the statement. "Detections of even higher-energy gamma rays from astronomical distances will allow more stringent checks on relativity."

Discussing the findings with Gizmodo, Ralf Lehnert, from the Indiana University Center for Spacetime Symmetries, said the findings were good but this is just one way to measure violations of the Lorentz invariance. However, he also said it adds to many other tests that have looked for violations and found none.

Harding said the team will continue to investigate gamma rays and the Lorentz invariance "As HAWC continues to take more data in the coming years and incorporate Los Alamos-led improvements to the detector and analysis techniques at the highest energies, we will be able to study this physics even further," he said.

gamma ray water tanks
Illustration showing the four gamma ray sources with the water tanks at the HAWC. Jordan Goodman