Physicists reconfirm that light speed stays constant, even at extreme energies and colors

Physicists have checked again whether light of different colors travels at different speeds, and once more it seems not to budge.

They timed energetic photons from distant cosmic blasts to see whether high energy ones ever reached their detectors ahead of the rest.

The project draws on data from space based and ground based gamma-ray observatories watching sources billions of miles away.

Working across institutions in Spain and Portugal, the team used bursts with photon energies far beyond anything produced in accelerators on Earth.

Testing light speed

In modern physics, this constancy is folded into Lorentz invariance, meaning physics looks identical for all constant speed observers.

If light traveled slightly faster or slower depending on its energy or direction, that symmetry would crack and many theories would need repair.

The work was led by Merce Guerrero, a theoretical physicist at the University of Aveiro (UA) in Portugal. Her research focuses on using high energy astrophysics to test tiny departures from exact Lorentz symmetry.

Over a century ago, the Michelson Morley experiment used an interferometer, a device comparing light paths, to search for Earth’s motion through an invisible ether.

The original Michelson Morley interferometer experiment, described in a classic entry, found no difference at all, forcing physicists to rethink space and time.

Einstein took that stubborn null result as a clue that light’s speed is the same for every inertial observer. Built on top of this, the Standard Model, the theory describing known particles and forces except gravity, has survived every precision test so far.

Why light speed varies

The trouble starts when gravity enters the picture at very small scales, where quantum rules usually dominate.

Physicists call the missing unified description quantum gravity, a theory that would merge gravity with quantum mechanics in a single framework.

General relativity treats gravity as curves in spacetime, while quantum field theory prefers flat backgrounds where probabilities flow cleanly.

Modeling particles that both ride on this curved geometry and also help shape it has turned out to be mathematically messy and conceptually confusing.

Many candidate theories predict that at extremely high energies, Lorentz symmetry might fail by a minuscule amount.

Those energies cluster around the Planck energy, a huge scale where quantum gravity effects are expected to become important.

One of the cleanest possible signals would be a tiny link between photon energy and the speed of light in empty space.

Over everyday distances the effect would be invisible, but over cosmic travel times it could add up to detectable delays.

Timing light speed

Astronomers get their best tests from short, bright outbursts like gamma-ray bursts (GRBs), brief explosions that flood detectors with extremely energetic photons.

They also use flares from active galaxies and the steady ticking of pulsars, treating each source as a fast astronomical clock.

In these observations, researchers track the time of flight, tiny shifts in arrival times between low energy and high energy photons from one outburst.

One landmark analysis of Fermi data set strong limits on both linear and quadratic dispersion in light speed.

The new study tackles a different angle by pooling many earlier measurements into one global analysis. It expresses every possible deviation in the language of the Standard Model Extension, a framework that catalogues tiny Lorentz violating terms as testable coefficients.

To keep things consistent, the team compared their coefficients with long running tables summarizing hundreds of Lorentz symmetry tests across many particle species.

That lets them say not only that light still respects Lorentz invariance, but also how much tighter each individual coefficient is now bounded.

What results show

What these new bounds really provide is a sharper ruler for testing Lorentz symmetry in the photon sector. Compared with previous global fits, the allowed range for each key coefficient shrinks by about one order of magnitude in this work.

They corrected missing factors in earlier formulas, folded in instrument uncertainties, and converted one sided limits into symmetric confidence intervals before combining them.

That work is less flashy than finding new particles, but it decides exactly how strongly existing data can rule out each possible deviation.

A separate paper showed how delays created inside gamma-ray burst engines can mimic or hide Lorentz violation signals.

By treating time delays in a global statistical way, the new study helps keep those source effects from being mistaken for new physics.

The upshot is that, within the sensitivity of current instruments, light speed does not depend on photon energy to any measurable degree.

Any remaining violation would need to be so tiny that even photons traveling billions of miles arrive nearly perfectly together.

What comes next

Design studies for the Cherenkov Telescope Array (CTAO), a new observatory made of ground based telescopes, suggest it could sharpen time delay tests dramatically.

Even a null result matters because it tells future experiments where to focus, and which speculative ideas are already running out of room.

Next generation detectors will catch more flares from pulsars, active galaxies, and bursts, filling in parts of the sky that are currently poorly sampled.

When combined with methods like those in Guerrero’s work, that broader coverage can isolate whether any deviation prefers particular directions or source types.

There’s a common question of whether the speed of light is truly universal or only an approximation. So far, every precise test from colliders to cosmic light curves points toward a picture where Lorentz invariance is a deep symmetry of nature.

Yet theorists have to reconcile that symmetry with a quantum description of gravity, so cleaner bounds like these narrow which ideas are worth pursuing.

The universe is not giving up its secrets easily, yet each null result tightens the box around models that try to bend space and time.

The study is published in Physical Review D.

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