Einstein’s theory of relativity is built upon two essential postulates that have profoundly shaped our understanding of the universe. The first postulate, derived from the work of Dutch physicist Hendrik Lorentz, states that the laws of physics are identical for all observers travelling in a straight line at a constant speed with no acceleration. This principle, known as “Lorentz invariance,” suggests that physical phenomena appear the same to anyone in an inertial frame of reference, whether a butcher, baker, or candlestick maker moving in a vacuum.
The second postulate is equally intriguing: the speed of light in a vacuum is constant and independent of the observer’s motion or direction. No matter how fast one travels, whether at a standstill or approaching the speed of light itself, the speed of light will always be measured as “c,” approximately 300,000 kilometres per second. This concept defies common sense but has withstood extensive experimental scrutiny.
The Quest to Test Lorentz Invariance
Physicists have long been fascinated by Lorentz invariance and have rigorously tested it across numerous experiments. So far, it has held up without exception. However, new questions arise as the frontiers of physics expand, especially in the realm of quantum gravity. Some quantum gravity theories suggest that at extremely high energies, the vacuum of space might not be as empty as it seems but could act as a medium that affects the speed of photons.
This potential deviation from Lorentz invariance is particularly intriguing because it challenges one of the most fundamental aspects of Einstein’s theory. To test whether this postulate holds at the highest energies, researchers have turned to one of the universe’s most extreme phenomena: gamma-ray bursts (GRBs).
Gamma-Ray Bursts: Nature’s Cosmic Beacons
GRBs are the most powerful explosions in the universe, releasing as much energy as our Sun will emit in mere seconds over its entire lifetime. These bursts, often resulting from supernovae or neutron star mergers, produce highly energetic gamma rays that travel vast distances across the cosmos, making them ideal candidates for testing relativity limits.
In October 2022, astronomers detected the brightest GRB ever observed, GRB 221009A. This event, located in a distant galaxy 2.4 billion light-years away, lasted just over 10 seconds but remained observable for 10 hours after its initial detection. The gamma rays emitted by GRB 221009A had travelled through the vacuum of space for 2.4 billion years before reaching Earth, providing a unique opportunity to test whether Lorentz invariance holds at such extreme energy levels.
LHAASO’s Investigation into GRB 221009A
A research team from the Large High Altitude Air Shower Observatory (LHAASO) in Sichuan, China, took on the challenge of analyzing this extraordinary GRB. Positioned 4,410 meters above sea level, LHAASO’s Water Cherenkov Detector Array was ideally situated to capture the afterglow of GRB 221009A, specifically its high-energy photons.
Within 100 minutes of the burst’s initial detection, the LHAASO detectors recorded over 64,000 photons with energies reaching up to 7 trillion electron-volts (TeV). In perspective, the GRB released more energy in its brief existence than the Milky Way galaxy in 500 million years.
The research team employed two methods to search for violations of Lorentz invariance. First, they measured the time delays between 10 gamma-ray energy bands containing TeV-scale photons. Second, they extracted energy-dependent arrival-time delays from the data to see if photons of different energies arrived at different times. A violation of Lorentz invariance would manifest as a delay in the arrival of photons with different frequencies, indicating they had travelled at different speeds through the vacuum.
Einstein’s Relativity Passes Another Test
The results were conclusive: no statistically significant violations of Lorentz invariance were detected. All photons, regardless of their energy, arrived simultaneously after their 2.4 billion-year journey. This lack of photon dispersion suggests that the speed of light remains constant even at these extreme energy levels, reinforcing Einstein’s second postulate.
From this observation, the researchers placed two new lower limits on the energy scale at which quantum gravity effects might appear. The first limit matched previous findings from other GRB observations, while the second limit raised the previous threshold by a factor of five, further constraining the potential for new physics beyond Einstein’s theory.
Looking Forward: The Future of High-Energy Physics
While these findings reaffirm the robustness of Einstein’s theory of relativity, the quest to explore the unknown continues. The researchers concluded that future observations of high-energy prompt emissions from GRBs, rather than just their afterglows, could offer even more sensitive tests of Lorentz invariance. As technology advances and we observe more of these cosmic events, the possibility remains that we may find evidence that challenges or extends our current understanding of the universe.
For now, however, Einstein’s theory of relativity remains an unshakable foundation in the ever-expanding frontier of physics.
