Published online 28 October 2009 | Nature | doi:10.1038/news.2009.1044

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A burst of γ-rays lets scientists test quantum theories of gravity.

Space-time behaves like a fluid in the theory of general relativity. Chris Henze / NASA / Science Photo Library

Astronomers have used a high-energy burst of light from a distant galaxy to test the fabric of space and time. The work is the best test yet of attempts to create a 'theory of everything'.

At present, two separate theories dominate the world of physics. General relativity explains gravity and the motion of large objects such as planets, stars and galaxies, whereas quantum-mechanics explains the behaviour of very small things such as atoms.

Both theories do well at explaining their respective worlds, but they don't fit together mathematically. The problem is as fundamental as it gets: the two see space and time very differently, according to Giovanni Amelino-Camelia, a theoretical physicist at the University of Rome La Sapienza in Italy.

“The study of space-time structure in a sense that is meaningful for quantum gravity has started.” Giovanni Amelino-Camelia

University of Rome La Sapienza

The difference is something like the difference between an ocean and a beach. General relativity sees space-time as a vast, continuous fluid, whereas quantum mechanics suggests that it is grainy like sand. Some quantum versions of gravity suggest that the 'grains' of space-time, if they exist, would be vanishingly small, measuring around 10–35 metres. That would make them nearly impossible to detect with instruments on Earth.

But high-energy light particles, known as γ-rays, might be able to tell the difference. γ-rays are powerful photons that are thought to come from extreme astronomical events such as colliding neutron stars.

High energies correspond to short wavelengths, and some γ-rays are so short in wavelength that they might be able to distinguish between sandy and fluid space-time. If space-time is grainy, then shorter wavelength γ-rays might stumble over the grains, perhaps making them travel slightly slower than γ-rays of longer wavelength. "What you need is really a race," Amelino-Camelia says.

Space race

The study, published online today in Nature, reports that an orbiting γ-ray satellite has caught just such a race in action1. On 10 May this year, the Fermi Gamma-ray Space Telescope spotted a short γ-ray burst from a galaxy at a distance of around 2 billion parsecs, or 7 billion light years, from Earth. The burst lasted several seconds, with the γ-rays of shortest wavelength arriving around 0.829 seconds after the first rays were detected.

That's late, but not late enough for the simplest theories of quantum gravity to hold up, according to Jonathan Granot, a member of the Fermi telescope team at the University of Hertfordshire in Hatfield, UK. In other words, for now, space-time would seem to be smooth rather than sandy.

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Granot says that he's a little disappointed the race only confirmed existing theories. "If we could have clearly detected such an effect it would have been worthy of a Nobel Prize," he says. But, he adds, any test of far-out ideas about quantum gravity is "extremely helpful". Fundamental physics is "becoming very detached from observations", he adds. "The mere fact that we can significantly constrain some ... models is very good."

The result doesn't mean that the best efforts to unify gravity with quantum mechanics are wrong. There are still plenty of versions of quantum gravity that wouldn't change the speed of light in the recent burst, according to Lee Smolin, a theoretical physicist at the Perimeter Institute in Waterloo, Ontario, Canada. Nevertheless, he says that the paper "is the best test yet of a general hypothesis about quantum space-time".

Further measurements in coming years could guide theorists as they attempt to unify gravity with quantum mechanics. "The study of space-time structure in a sense that is meaningful for quantum gravity has started," Amelino-Camelia says.