The speed of light has long been regarded as one of science’s most steadfast constants. For over a century, the notion that light travels through space at a constant speed has underpinned some of the most critical theories in physics. Yet, scientists continue to test this principle, hoping to uncover new insights about the universe. A recent study has reviewed these efforts, offering a clearer path for future research by synthesizing decades of observations to scrutinize one of nature’s fundamental laws with unprecedented precision.
The Legacy of Lorentz Invariance
The quest to understand the speed of light traces its origins back to 1887, when Albert Michelson and Edward Morley attempted to measure changes in light speed that would reveal Earth’s motion through space. Their “null” result inspired Einstein’s theory of special relativity, which asserts that all observers must agree on the laws of physics and that light travels at a constant speed in a vacuum. This concept, known as Lorentz invariance, is central to quantum field theory and the Standard Model of particle physics, which together describe matter and forces with remarkable accuracy.
Despite its success, a longstanding conflict exists between quantum theory and Einstein’s later theory of general relativity. Quantum theory uses probability waves to describe particles, while general relativity treats gravity as a distortion of spacetime. These perspectives clash at extremely small scales, where many attempts to develop a theory of quantum gravity suggest that Lorentz invariance might break down at very high energies. If true, photons released from a distant explosion might not arrive on Earth simultaneously.
High Energy Light as a Testbed
To investigate potential breakdowns in Lorentz invariance, scientists study radiation bursts from distant pulsars, active galaxies, and gamma-ray bursts. These events emit light across a broad energy spectrum, sometimes from billions of light-years away. A minuscule change in photon speed can result in a measurable delay over such vast distances. Previous research examined this delay by linking a photon’s energy to its travel time, setting limits on an energy scale associated with quantum gravity.
Lower order effects have been pushed to extremely high energy limits, near or beyond the Planck scale. Higher order effects remain elusive, but new bursts and improved detectors have refined these estimates. Different quantum gravity theories predict various outcomes, but many anticipate some form of energy-dependent shift. Meanwhile, theorists have developed the Standard Model Extension, a framework that describes Lorentz violation with numerous coefficients, each representing a potential deviation in photon behavior.
A New Framework for a Complex Problem
A new study addresses the challenge of translating astrophysical measurements into the Standard Model Extension framework. The focus is on nonbirefringent coefficients, which avoid complications related to polarization. The researchers demonstrated how parameters for photon dispersion could be rewritten using spherical harmonics, enabling each burst or flare to be mapped to a weighted sum of the Standard Model Extension coefficients.
To establish consistent limits, the researchers corrected past measurements, addressing omissions and updating uncertainties for instruments like 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. New sources, including the Crab Pulsar, the active galaxy Mrk 421, and recent gamma-ray bursts GRB 190114C and GRB 221009A, were included. The latter produced the strongest limit yet, surpassing previous constraints by over a factor of ten.
Turning Observations Into a Clear Picture
The Standard Model Extension allows for direction-dependent effects, meaning each astrophysical source samples a different part of the sky. The study compiled 65 measurements to solve for 25 different coefficients. The authors treated each measurement as a probability distribution, combining them into a multidimensional Gaussian that could be rotated into an orthogonal basis, enabling the extraction of individual limits for each coefficient.
The results show improvements of roughly an order of magnitude across the board, largely due to stronger recent bursts and a more comprehensive selection of sources. The paper emphasizes the need for consistent reporting and better access to full likelihood curves, as conversions remain approximate without them. The authors stress that a single burst cannot rule out delays caused within the source itself. Only by combining a wide array of events can the field progress toward definitive answers. With about a dozen more strong measurements, sensitivity to certain coefficients could improve by five additional orders of magnitude.
This work connects two major paths in the search for Lorentz invariance violations, updating past measurements, correcting inconsistencies, and providing a unified translation recipe. The researchers have created a roadmap for comparing future results, demonstrating the power of light that has traversed 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 findings did not overturn relativity but refined the limits more than ever before. Upcoming instruments, including the Cherenkov Telescope Array Observatory, promise even more sensitive measurements.
Practical Implications of the Research
This research strengthens the foundation for testing quantum gravity theories by providing a consistent method for comparing results from various observatories. Improved limits on Lorentz invariance violations guide theorists toward models that align with the real universe. These methods may enable future instruments to probe physics far beyond the reach of laboratories. As detectors improve, scientists may finally determine whether light always travels at a constant speed or if the universe conceals a deeper layer of physics.
The research findings are available online in the journal Physical Review D.
Related Stories
- Scientists prepare to launch tiny lifeforms into space at near the speed of light
- Cutting-edge rocket engine could accelerate to 99% the speed of light
- Can you time travel by going faster than the speed of light?
Like these kinds of feel-good stories? Get The Brighter Side of News’ newsletter.
