More than a century ago, two American scientists, Albert Michelson and Edward Morley, carried out a famous experiment that changed the course of physics.
They tried to measure whether Earth’s motion through space affected the speed of light.
To their surprise, it didn’t. Light refused to change its speed, no matter the direction it traveled.
That puzzling result helped inspire Albert Einstein to propose one of the most important ideas in modern science: the speed of light is constant for all observers. This idea became a cornerstone of his theory of special relativity.
Since then, countless experiments have confirmed that light always travels at the same speed in a vacuum — about 300,000 kilometers per second.
This principle, known as Lorentz invariance, is also at the heart of quantum physics and the Standard Model of particle physics, which describes the fundamental particles and forces of nature with incredible precision.
So why are scientists still testing it 115 years later?
The reason lies in another of Einstein’s groundbreaking ideas — general relativity, which describes gravity as the bending of space and time.
While both quantum physics and general relativity work extremely well in their own realms, they do not fit neatly together.
Many physicists believe there must be a deeper theory, often called “quantum gravity,” that unites them. Some versions of this future theory suggest that Lorentz invariance might be slightly broken under extreme conditions, meaning the speed of light might depend ever so slightly on the energy of the light itself.
Detecting such a tiny effect is impossible in ordinary laboratory settings. However, nature provides an ideal testing ground: the distant universe.
Very-high-energy gamma rays produced by powerful cosmic events travel billions of years before reaching Earth. If photons of different energies travel at slightly different speeds, the difference would build up over this vast distance and cause a tiny delay in their arrival time.
A research team led by Mercè Guerrero and Anna Campoy-Ordaz, with collaborators from universities in Spain and Portugal, used this approach to push the limits of our understanding.
They analyzed data from very-high-energy gamma rays captured by observatories and applied a new statistical method to test specific predictions from theories that allow for small violations of Lorentz invariance.
Their results, published in the journal Physical Review D, showed no evidence that the speed of light changes with energy.
Once again, Einstein’s theory held firm. However, the study achieved something important: it improved previous limits on possible deviations by a factor of ten. In other words, if any violation exists, it is even smaller than scientists had previously thought.
Although the researchers did not overturn Einstein’s ideas, their work represents a major step forward. With new, more powerful instruments being developed — such as the Cherenkov Telescope Array Observatory — scientists will soon be able to observe even higher-energy gamma rays from even farther away. These future observations will continue the century-old quest to see whether the speed of light is truly constant under all conditions.
So far, the universe keeps giving the same answer: no matter how hard we try to prove it otherwise, light still refuses to play by any different rules.
Source: KSR.
