Scientists tried to break Einstein’s speed of light rule
Einstein’s claim that the speed of light is constant has survived more than a century of scrutiny—but scientists are still daring to test it. Some theories of quantum gravity suggest light might behave slightly differently at extreme energies. By tracking ultra-powerful gamma rays from distant cosmic sources, researchers searched for tiny timing differences that could reveal new physics. They found none, but their results tighten the limits by a huge margin.
In 1887, a landmark experiment reshaped our understanding of the universe. American physicists Albert Michelson and Edward Morley attempted to detect Earth's motion through space by comparing how fast light traveled along different directions. Their experiment found no difference at all. This unexpected null result became one of the most influential outcomes in scientific history. It led Albert Einstein to propose that the speed of light is constant, a cornerstone idea behind his theory of special relativity.
Special relativity rests on the principle that the laws of physics remain the same for all observers, regardless of how they are moving relative to one another. This idea is known as Lorentz invariance. Over time, Lorentz invariance became a foundational assumption in modern physics, especially within quantum theory.
Why Question a Principle That Works So Well
Quantum theory evolved with Lorentz invariance at its core. This is especially true for quantum field theory and the Standard Model of Particle Physics, which is the most thoroughly tested scientific theory ever created and has passed experimental checks with extraordinary precision. Given this track record, it may seem strange to question Lorentz invariance after more than a century of success.
The motivation comes from another of Einstein's breakthroughs. His theory of general relativity explains gravity as a bending of spacetime itself. Like special relativity, it has been confirmed with remarkable accuracy across many environments, from weak gravitational fields to extreme cosmic conditions.
The Clash Between Quantum Theory and Gravity
Despite their individual successes, quantum theory and general relativity do not fit together smoothly. Quantum physics describes reality using probability wave functions, while general relativity describes how matter and energy shape the geometry of spacetime. These two approaches struggle to coexist when particles move through curved spacetime while also influencing that curvature.
Efforts to combine the two theories into a single framework known as quantum gravity often run into the same obstacle. Many proposed solutions require small violations of Lorentz invariance. These violations would be extremely subtle but could offer clues about new physics beyond current theories.
Testing Einstein With Light From the Cosmos
One prediction shared by several Lorentz-invariance-violating quantum gravity models is that the speed of light may depend slightly on a photon's energy. Any such effect would have to be tiny to match existing experimental limits. However, it could become detectable at the highest photon energies, specifically in very-high-energy gamma rays.
A research team led by former UAB student Mercè Guerrero and current IEEC PhD student at the UAB Anna Campoy-Ordaz set out to test this idea using astrophysical observations. The team also included Robertus Potting from the University of Algarve and Markus Gaug, a lecturer in the Department of Physics at the UAB who is also affiliated with the IEEC.