Different wavelengths of light from a distant gamma-ray burst travel at the same speed, down to quantum scales (Illustration: NASA/SkyWorks Digital)

A hint that quantum fluctuations in the fabric of the universe slow the speed of light has not been borne out in observations by NASA’s Fermi telescope. The measurements contradict a 2005 result that supported the idea that space and time are not smooth.

Einstein’s theory of special relativity says that all electromagnetic radiation travels through a vacuum at the speed of light. This speed is predicted to be constant, regardless of the energy of the radiation.

Yet in 2005, the MAGIC gamma-ray telescope on La Palma in the Canary Islands suggested the speed of light might not be constant after all. The telescope, which measured the light released by a galaxy around 500 million light years away, found that higher energy photons arrived four minutes behind their lower energy counterparts.

Grainy universe

The discovery hinted that the speed of light may change depending on its energy. This effect could be a consequence of some theories of quantum gravity, which attempt to unify Einstein’s theory of gravity with the laws of quantum mechanics. These models postulate that space and time are not smooth. Instead space-time is inherently grainy, fluctuating rapidly over distances of about 10-35 metres, a length called the Planck scale.


If space-time is grainy, higher-energy photons would move more slowly than their lower-energy counterparts. That’s because higher-energy photons have smaller wavelengths, which makes them more sensitive to tiny fluctuations in space-time.

However, the MAGIC lag was apparently too large to be easily explained by graininess on the quantum scale. If the delay were caused by fluctuations in space-time, they would have to occur on scales more than 10 times larger than the Planck scale.

“This intriguing evidence has been wandering around in the quantum gravity community for more than a year now, with hope on the progressive side, and stomach aches on the conservative side,” says physicist Giovanni Amelino-Camelia of Sapienza University of Rome in Italy.

Now new observations suggest quantum gravity cannot be responsible for the time delay observed by MAGIC. The light from a powerful, 7-billion year old gamma-ray burst detected by NASA’s Fermi Gamma-ray Space Telescope shows no evidence of a lag between photons of a range of energies.

“We have fewer stomach aches now,” says Amelino-Camelia. “The Fermi data has pushed the limit where it’s now proven the MAGIC data cannot be interpreted in that way.”

Light artefact

Fermi’s measurement is the most stringent direct limit on how much the speed of light might vary with energy, says Jonathan Granot of the University of Hertfordshire in the UK, who led the analysis of the burst. “For the first time, we can put the limit [down to] the energy scale in which quantum effects would alter the geometry of space time.”

The MAGIC time delay may be down to an astrophysical process where particles are accelerated to enormous energies within the hearts of galaxies. Follow-up calculations after MAGIC’s 2005 result showed that is possible to produce flares that release lower-energy radiation before higher-energy radiation, according to MAGIC collaborator Robert Wagner of the Max Planck Institute of Physics in Munich, Germany. “I think what we can say for the time being is quantum gravity effects cannot be the dominant effect,” he says.

Knockout blow?

The result does not necessarily strike a blow to quantum gravity. Only a subset of models predict the effect, and “while it seems reasonable to expect that the variation of the speed of light with energy is a sign of quantum space-time, there is no well developed theory of quantum space-time that cleanly makes this prediction,” says Lee Smolin of the Perimeter Institute for Theoretical Physics in Waterloo, Canada.

What’s more, it will require even more precise measurements to completely exclude the possibility that light may change its speed depending on its energy. “If there is an effect, the experiment is now at the threshold of scales where the effect is expected, and there is the exciting prospect that it could be discovered over the…next few years,” Smolin says.

Journal reference: Nature (doi:10.1038/nature08574)