The speed of light has long been a cornerstone of modern physics, serving as a constant in the equations that describe the universe. For over a century, the notion that light travels through space at a fixed speed has underpinned many of physics’ most crucial theories. Yet, the scientific community continues to test this principle, seeking to uncover new truths about the universe. A recent study has synthesized decades of observations, offering a new perspective on this fundamental law of nature and paving the way for future research.
The study revisits the concept of Lorentz invariance, a principle that emerged from the famous 1887 experiment by Albert Michelson and Edward Morley. Their attempt to detect changes in light speed, which would have indicated the Earth’s motion through space, resulted in a “null” outcome. This unexpected result laid the groundwork for Einstein’s theory of special relativity, which posits that all observers must agree on the laws of physics and that light moves at a constant speed in a vacuum.
The Legacy of Lorentz Invariance
Lorentz invariance is not only central to special relativity but also to quantum field theory and the Standard Model of particle physics, both of which describe the behavior of matter and forces with remarkable precision. However, a longstanding conflict exists between quantum theory and Einstein’s later theory of general relativity. Quantum theory relies on probability waves to describe particles, while general relativity treats gravity as a distortion of spacetime. These differing views clash at extremely small scales, prompting efforts to reconcile them through a theory of quantum gravity.
Many such attempts suggest that Lorentz invariance might break down at very high energies. If true, this could mean that photons released simultaneously from a distant cosmic event might not reach Earth at the same time. To explore this possibility, scientists have turned to high-energy light from distant pulsars, active galaxies, and gamma-ray bursts, which release light across vast energy ranges and distances. A slight variation in photon speed could result in a measurable delay over such distances.
How High-Energy Light Becomes a Testbed
Researchers have previously examined these delays by correlating a photon’s energy with its travel time, setting limits on an energy scale linked to quantum gravity. While lower-order effects have been pushed to extremely high energy limits, higher-order effects remain elusive. However, recent bursts and improved detectors have bolstered these estimates, with various quantum gravity models predicting energy-dependent shifts.
Concurrently, theorists have developed the Standard Model Extension, a framework that describes potential Lorentz violations using numerous coefficients. Each coefficient represents a specific deviation in photon behavior. Historically, converting astrophysical measurements into this framework has been challenging.
A New Framework for a Complex Problem
The recent study addresses this challenge by reviewing the strongest measurements from past observations and translating them into limits on the Standard Model Extension. The focus is on nonbirefringent coefficients, which avoid polarization-related complications. The researchers demonstrate how commonly used parameters for photon dispersion can be expressed in terms of spherical harmonics, allowing each burst or flare to be mapped to a weighted sum of the Standard Model Extension coefficients.
To ensure consistency, the team corrected past measurements, accounting for previously omitted terms and systematic uncertainties. They updated uncertainties for instruments like the Fermi Large Area Telescope and LHAASO, and 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 such as GRB 190114C and GRB 221009A, were included. Notably, GRB 221009A provided the strongest limit yet, surpassing earlier constraints by a factor of ten.
Turning Many Observations Into One Clear Picture
Given that the Standard Model Extension allows for direction-dependent effects, each astrophysical source samples a different part of the sky. The study compiled 65 measurements to solve for 25 different coefficients, treating each measurement as a probability distribution and combining them into a multidimensional Gaussian that could be rotated into an orthogonal basis. This approach enabled the extraction of individual limits for each coefficient, resulting in improvements of roughly an order of magnitude across the board.
The authors emphasize the importance of consistent reporting and access to full likelihood curves, as these are crucial for precise conversions. They caution that a single burst cannot rule out delays caused within the source itself, underscoring the need for a broad dataset to achieve definitive answers. With additional strong measurements, sensitivity to certain coefficients could improve by five more orders of magnitude.
This study bridges two major paths in the search for Lorentz invariance violations. By updating past measurements, correcting inconsistencies, and providing a unified translation method, the researchers have created a roadmap for future comparisons. Their findings highlight the power of light that has traversed the universe for billions of years, carrying clues about the fundamental laws of nature.
Research teams from institutions such as the Universitat Autònoma de Barcelona, the Institute of Space Studies of Catalonia, and the University of Algarve are at the forefront of this effort. As researcher Mercè Guerrero and colleagues report, their goal was to test Einstein’s theories with the most precise astrophysical data available. While the study did not overturn relativity, it refined the constraints more than ever before. Upcoming instruments, including the Cherenkov Telescope Array Observatory, promise even more sensitive measurements.
Practical Implications of the Research
This work strengthens the foundation for testing quantum gravity theories by providing a consistent framework for comparing results from various observatories. Improved limits on Lorentz invariance violations guide theorists toward models that more accurately reflect the universe. As detectors advance, scientists may finally determine whether light always travels at a constant speed or if a deeper layer of physics remains hidden within the cosmos.
The research findings are published in the journal Physical Review D.
