New analysis challenges Einstein’s theory on the speed of light

The speed of light has been one of science’s most trusted markers of stability. For more than a century, the idea that light moves through space at a constant speed has supported some of physics’ most important theories. Yet many scientists still test this idea with the hope that the universe might reveal something new.

A recent study reviews those efforts and offers a clearer path for future work, bringing together decades of observations to examine one of the most basic laws of nature with sharper tools than ever before.

The search traces its roots back to 1887, when Albert Michelson and Edward Morley tried to measure changes in light speed that would reveal the Earth’s motion through space. Their attempt produced a “null” result that helped inspire Einstein’s theory of special relativity. The theory states that all observers must agree on the laws of physics and that light moves at a constant speed in a vacuum. This idea, known as Lorentz invariance, also lies at the heart of quantum field theory and the Standard Model of particle physics, which together describe the behavior of matter and forces with remarkable accuracy.

Despite this success, there is a long standing conflict between quantum theory and Einstein’s later theory of general relativity. Quantum theory uses probability waves to describe particles. General relativity treats gravity as a distortion of spacetime. When combined, these views clash in ways that become severe at extremely small scales.

Many attempts to build a theory of quantum gravity have found that Lorentz invariance might break down at very high energies. If this is true, even slightly, photons released together from a distant explosion might not arrive on Earth at the same moment.

To look for signs of this breakdown, scientists study flashes of radiation from distant pulsars, active galaxies, and gamma ray bursts. These events release light across a wide energy range and sometimes from billions of light years away. A tiny change in photon speed can grow into a measurable delay across such distances. Earlier work examined this delay by connecting a photon’s energy to its travel time. This allowed researchers to place limits on an energy scale linked to quantum gravity.

Lower order effects have been pushed to extremely high energy limits, near or beyond the Planck scale. Higher order effects are harder to pin down, but new bursts, along with improved detectors, have strengthened those estimates. Different quantum gravity ideas make different predictions, but many expect some form of energy dependent shift.

At the same time, theorists have developed a larger framework called the Standard Model Extension. Instead of predicting one single energy scale, it describes Lorentz violation with many coefficients. Each one represents a specific kind of possible deviation in photon behavior. Until now, converting astrophysical measurements into this framework has been a messy task.

A new study tackles that challenge by reviewing the strongest measurements from past observations and translating them into limits on the Standard Model Extension. The focus is on a family of nonbirefringent coefficients that avoid complications related to polarization. The team shows how the commonly used parameters for photon dispersion can be rewritten in terms of spherical harmonics. This makes it possible to map each burst or flare to a weighted sum of the Standard Model Extension coefficients.

To build consistent limits, the researchers corrected past measurements. Some older studies left out key terms or did not include systematic uncertainties. The team added updated uncertainties for instruments such as the Fermi Large Area Telescope, LHAASO, and several ground based observatories. They also converted older one sided limits into two sided limits at the 95 percent confidence level.

Several new sources were included, such as the Crab Pulsar, the active galaxy Mrk 421, and two recent gamma ray bursts known as GRB 190114C and GRB 221009A. The latter produced the strongest limit yet, improving earlier constraints by more than a factor of ten.

Because the Standard Model Extension allows direction dependent effects, each astrophysical source samples a different part of the sky. The study gathered 65 measurements to solve for 25 different coefficients. The authors treated each measurement as a probability distribution and combined them into a multidimensional Gaussian that could be rotated into an orthogonal basis. This made it possible to extract individual limits for each coefficient.

The results show improvements of roughly an order of magnitude across the board. Much of this progress comes from stronger recent bursts and a more complete selection of sources. The paper also highlights the need for more consistent reporting and better access to full likelihood curves. Without them, conversions remain approximate.

The authors stress that a single burst cannot rule out delays caused within the source itself. Only by combining a wide set of events can the field move toward firmer answers. With about a dozen more strong measurements, sensitivity to certain coefficients could improve by five additional orders of magnitude.

This work links two major paths in the search for Lorentz invariance violations. By updating past measurements, correcting inconsistencies, and providing a unified translation recipe, the researchers have built a roadmap for comparing future results. Their findings show the power of light that has crossed the universe for billions of years, carrying clues about the deepest rules of nature.

Teams at institutions such as the Universitat Autònoma de Barcelona, the Institute of Space Studies of Catalonia, and the University of Algarve continue to lead this effort. As researcher Mercè Guerrero and colleagues report, their goal was to test Einstein with the most precise astrophysical data available.

The result did not overturn relativity, but it sharpened the limits more than ever before. Upcoming instruments, including the Cherenkov Telescope Array Observatory, promise even more sensitive measurements.

This work strengthens the foundation for testing quantum gravity theories by offering a consistent way to compare results from many observatories.

Better limits on Lorentz invariance violations guide theorists toward models that match the real universe. These methods may help future instruments probe physics far beyond the reach of laboratories.

As detectors improve, scientists may finally learn whether light always moves at one constant speed or whether the universe still hides a deeper layer of physics.

Research findings are available online in the journal Physical Review D.

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