Using a gamma-ray burst to search for violations of Einstein’s postulates of relativity

Einstein’s theory of relativity is based on two assumptions, or postulates. The first is that the laws of physics appear to be the same for anyone moving in a straight line without acceleration.

Einstein borrowed this idea from the Dutch physicist Hendrik Lorentz, who in the late 1800s presented a theory of electrodynamics with this property, giving rise to the notion of an “inertial frame”—the coordinates used by a butcher, baker, or candlestick maker as they move in a straight line in a vacuum, relative to each other, possibly at different speeds. This supposed equivalence is called “Lorentz invariance.”

The second assumption is that the speed of light will be measured the same by anyone in an inertial frame. No matter how fast you are moving or what direction you are heading (in a vacuum), you will see light approaching at a speed of “c,” or just under 300,000 kilometers per second, and the same speed will be observed if it passes nearby. Even though the baker is moving at 0.99999 percent of the speed of light relative to the butcher, both measure light at a speed of “c.” (Yes, this is not intuitive at all.)

Since then, physicists have sought to test Lorentz invariance. It has been strictly verified in all experiments carried out so far. Today, a group of Chinese have observed the most powerful gamma-ray burst ever observed and discovered that the photons it emitted at the same time arrived at their telescope at the same time, even if they had different frequencies.

With this result, the lower energy limit where quantum gravity appears has been multiplied by five. Their work was published in the journal Physical Exam Letters.

Interest in Lorentz invariance has increased in recent years because some theories of quantum gravity predict that for high-energy photons, the vacuum does not appear empty but as a non-empty medium. This prediction, when it occurs in a theory of quantum gravity, occurs near the Planck scale of about 10¹⁹ billion electron volts, where spacetime itself is expected to have to be treated according to the rules of quantum mechanics.

Does Lorentz invariance hold even at such huge energies, or do the laws of physics start to look different for different inertial frames?

To test this theory, a research group from the Large High Altitude Air Shower Observatory (LHAASO) in China observed the afterglow of the brightest gamma-ray burst ever observed, 221009A. This gamma-ray burst, discovered in 2022 and lasting just over 10 seconds but observable for 10 hours after its detection, was in a distant galaxy 2.4 billion light-years away, meaning its highly energetic gamma rays took 2.4 billion years to reach Earth.

A violation of Lorentz invariance would occur if light of different frequencies arrived at Earth at different times, that is, if it had different speeds as it traveled through the long void that separates it from there. This phenomenon is called “photon dispersion” and is observed when light passes through materials such as water or glass, but until now it has never been detected in a vacuum.

The research team used data collected during the October 9, 2022, gamma-ray burst recorded at the Sichuan Observatory in China, at an altitude of 4,410 meters. Orbiting gamma-ray observatories were first triggered by the initial low-energy photons, and LHAASO turned out to be pointed in the right direction to measure the “afterglow” of the high-energy photons from the burst.

Within 100 minutes of 221009A’s triggering, their array of water Cherenkov detectors measured more than 64,000 photons with energies up to 7 trillion electron volts. In its short lifetime, the GRB is estimated to have released as much energy as the entire Milky Way galaxy would have released in 500 million years.

The GRB’s peak intensity occurred about four minutes after its onset. To examine any signs of Lorentz invariance violation, the group used two methods: measuring the delays between 10 gamma-ray energy bands, each band containing photons in the TeV range, and extracting energy-dependent arrival times from the data.

Their analysis revealed no statistically significant violation of Lorentz invariance, that is, no significant delay of GRB photons at different frequencies. (According to Planck’s relation, frequency is proportional to energy.)

From this lack of photon scattering, they obtained two lower limits on the energy where quantum gravity effects could appear, one identical to that observed in previous GRB observations, and the second increasing the previous lower limit by a factor of five for inertial frames traveling below the speed of light.

They conclude: “Future observations of very high energy prompt emissions instead of afterglow emissions from GRBs could further improve the sensitivity to Lorentz invariance using time-of-flight tests.” These limits could be even higher if the initial stage of a future GRB were examined in the same way.

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