Ultra-energetic gamma rays from distant cosmic explosions have reached Earth with no measurable energy-based delay, showing that light’s speed does not change even at extreme energies.
By pushing this test to its sharpest limits yet, the study narrows the space for theories that predict spacetime should alter how fast the highest-energy light can travel.
Einstein and gamma ray bursts
Timing marks from flaring galaxies and brief cosmic bursts gave scientists a natural clock for the test.
Working with physicists at Universitat Autonoma de Barcelona (UAB), physicist Merce Guerrero combined those records and hunted for energy-linked delays.
Her team matched photons that left together and arrived years later, then saw the same arrival pattern at every energy.
With no delay signal to chase, the group turned that null result into stricter limits on how spacetime can distort light.
A famous null test
In 1887, the experiment that Michelson and Morley ran found no change in light’s speed with direction.
That clean zero pushed physicists to drop the ether idea and accept that motion cannot change light’s speed.
Two decades later, Einstein set the constant speed of light at the heart of his 1905 paper on special relativity.
Modern tests still chase the same core claim, but they now use the universe itself as the apparatus.
Einstein and the Lorentz invariance
At the center sits Lorentz invariance, a rule that states physics looks identical in every steady motion. Break that symmetry and light could act like it has a preferred direction or a speed that depends on energy.
Some versions of quantum gravity, efforts to unite quantum physics with gravity, predict that tiny cracks in Lorentz invariance exist.
Even a tiny violation would force researchers to rewrite parts of modern physics, so tighter limits matter in practice.
Quantum physics treats particles as probability waves, while general relativity treats gravity as the shape of spacetime.
Mix those ideas in extreme places, such as near black holes, and the math stops agreeing on what happens next.
To bridge that gap, theorists add new terms that slightly relax Lorentz invariance, especially for very energetic photons.
Astrophysical light offers a rare place to look, because Earth-based labs cannot reach the same energies or travel times.
Gamma rays as probes
High-energy gamma rays, the most energetic form of light, carry enough energy for the timing tests.
If spacetime altered their travel, higher-energy photons would reach Earth slightly earlier or later than lower-energy ones.
Over billions of years, a delay of less than a second at the source could grow into seconds at arrival.
Only fast, bright events like bursts and flares can mark a start time sharply enough for that comparison.
Cleaning up the clocks
Different observatories tag photon energy differently, so small calibration errors can look like a physics effect.
Guerrero’s team at UAB rechecked older results, added missing factors, and folded in detector uncertainties before comparing sources.
Those details mattered because earlier reports did not always measure uncertainty in the same way, and some overlooked small instrument errors.
Once those issues were corrected, the limits became clearer and less likely to be mistaken for signs of new physics.
Translating into one language
Physicists often summarize these results with a single number, but that shortcut hides exactly which part of Einstein’s symmetry is being tested.
To make the findings more precise, the team translated them into the Standard-Model Extension, a framework that lists specific ways Lorentz symmetry could fail.
Those coefficients sit beside thousands of others in the annual tables kept by theorists Alan Kostelecky and Neil Russell.
With a common bookkeeping system, results from many telescopes can stack together, narrowing the space for any real Lorentz violation.
Limits tighten sharply
In a new paper, the UAB team pushed the limits about an order of magnitude tighter than earlier Lorentz-violation conversions.
One gamma-ray burst known as GRB 221009A provided the strongest limit in the analysis, but it remains a single exceptional event rather than part of a broader pattern.
“We urge the community to publish their full likelihoods, plus expectation values and variances.” Guerrero wrote.
Because some cosmic sources may delay photons before they even begin their journey, future studies will need many more events to separate true travel effects from delays caused at the source itself.
Next telescopes watch
Bigger instruments will soon track gamma rays with sharper timing, turning more cosmic outbursts into usable tests.
The Observatory called the Cherenkov Telescope Array Observatory (CTAO) will spot very-high-energy flashes by catching their brief atmospheric glow.
With dozens of telescopes spread across two hemispheres, it should capture more rare bursts and flares with clean time tags.
As that sensitivity improves, physicists can push the same test into higher energies where some Lorentz-breaking ideas would show up first.
Einstein survives again
Cosmic timing tests continue to confirm Einstein’s claim that the speed of light is constant, even when photons carry the highest energies ever observed.
As new observatories collect cleaner data, the next advances will come from testing that principle across many events rather than relying on a single extraordinary burst.
The study is published in Physical Review D.
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