Late light reveals what space is made of

ON THE night of 30 June 2005, the sky high above La Palma in Spain's Canary Islands crackled with streaks of blue light too faint for humans to see. Atop the Roque de los Muchachos, the highest point of the island, though, a powerful magic eye was waiting and watching.

MAGIC - the Major Atmospheric Gamma-ray Imaging Cherenkov Telescope - scans the sky each night for high-energy photons from the distant cosmos. Most nights, nothing remarkable comes. But every now and again, a brief flash of energetic light bears witness to the violent convulsions of a faraway galaxy.

What MAGIC saw on that balmy June night came like a bolt from the blue. That is because something truly astounding may have been encoded in that fleeting Atlantic glow: evidence that the fabric of space-time is not silky smooth as Einstein and many others have presumed, but rough, turbulent and fundamentally grainy stuff.

It is an audacious claim that, if verified, would put us squarely on the road to aquantum theory of gravity and on towards the long-elusive "theory of everything". If it were based on a single chunk of MAGIC data, it might easily be dismissed as a midsummer night's dream. But it is not. Since that first sighting, other telescopes have started to see similar patterns. Is this a physics revolution through the barrel of a telescope?

Such incendiary thoughts were far away from Robert Wagner's mind when the MAGIC data filtered through to the Max Planck Institute of Physics in Munich, Germany, the morning after. He and his fellow collaborators were enjoying a barbecue. Not for long. "We put our beers aside and started downloading the full data set," says Wagner.

It was easy to pinpoint the source of the data blip - a 20-minute burst of hugely energetic gamma rays from a galaxy some 500 million light years away known as Markarian 501. Its occasional tempestuous outbursts had already made it familiar to gamma-ray telescopes worldwide.

This burst was different. As Wagner and his colleagues analysed the data in the weeks and months that followed, an odd pattern emerged. Lower-energy photons from Markarian 501 had outpaced their higher-energy counterparts, arriving up to 4 minutes earlier (Physics Letters B, vol 668, p 253).

This should not happen. If an object is 500 million light years away, light from it always takes 500 million years to get to us, no more, no less. Whatever their energy, photons always travel at the same speed, the implacable cosmic speed limit: the speed of light.

Perhaps the anomaly has a mundane explanation. We do not really understand the processes within objects such as Markarian 501 that accelerate particles to phenomenal energies and catapult them towards us. They are thought ultimately to have something to do with the convulsions of supermassive black holes at the objects' hearts. It could be that these mechanisms naturally spew out low-energy particles before high-energy ones.

Or they might not. "The more fascinating explanation would be that this delay is not intrinsic to the source, but that it happens along the way from the source to us," says Wagner.

Quantum signature

What piqued the interest of Wagner and his colleagues was that the MAGIC observations were showing just the sort of effect that quite a few models of quantum gravity predict. Physicists have been on the lookout for experimental signposts to the right theory for the best part of a century (see "Quantum gravity: why we care").

Physicists have been on the lookout for signposts to the right theory of quantum gravity for the best part of a century

"All approaches to quantum gravity, in their own very different ways, agree that empty space is not so empty after all," says theorist Giovanni Amelino-Cameliaof Sapienza University of Rome in Italy. Many models based on string theory suggest that space-time is a foamy froth of particles, and even microscopic black holes, that spark up out of nothing and disappear again with equal abandon. The alternative approach favoured by Amelino-Camelia, loop quantum gravity, posits that space-time comes in indivisible chunks of about 10^-35metres, a size known as the Planck length.

Last year, it was suggested that the signature of just such a quantum space-time had popped up in unexplained noise plaguing a gravitational-wave detector in northern Germany (New Scientist, 17 January 2009, p 24). But that interpretation is far from a done deal, and most experts agree that a more substantive sighting could only come from observing the possible interactions of space-time with particles passing through it.

According to many string theory models, particles of different energies should speed up or slow down by different amounts as they interact with a foamy space-time. A minimum size for space-time grains, as predicted by loop quantum gravity, could violate the cherished principle of special relativity known as Lorentz invariance, which states that the maximum speed of all particles, regardless of their energy, is the speed of light in a vacuum.