THE light from a spectacular gamma-ray burst has been used to set the most stringent constraints yet on the size of the "grains" of space-time. The conclusion has been praised for pushing boundaries in the search for a theory of everything but has also drawn criticism for exaggeration.
A big challenge in physics is how to combine Einstein's general relativity, which describes gravity, with quantum mechanics to create a theory of quantum gravity. The best efforts on this front include string theory, which argues that everything in nature arises out of the vibration of tiny strings in 10-dimensional space-time, and loop quantum gravity (LQG), which shows mathematically that the fabric of space-time is woven out of gravitational field lines. Both approaches lack experimental support, however.
So cosmologists have been looking for signs of quantum gravity in the distant universe. One prediction of both string theory and LQG is that space-time is not smooth but "grainy" at extremely small scales. In theory, this graininess can be observed by studying particles from the same source but with different energies, to see if they are affected differently by the structure of space-time.
Over the past few years, terrestrial telescopes such as the Major Atmospheric Gamma-ray Imaging Cherenkov Telescope (MAGIC) in the Canary Islands and the High Energy Stereoscopic System (HESS) in Namibia have seen low-energy photons from a gamma-ray burst (GRB) arriving before their high-energy counterparts. While this could be due to delays in emission at the source, it could also be caused by the interaction of photons with the structure of space-time (New Scientist, 15 August 2009, p 26).
Now a team has used data from the Integral satellite, run by the European Space Agency (ESA), to study an entirely different effect: the polarisation of light of different energies from a GRB. In December 2004, Integral observed one of the brightest GRBs ever recorded. "We were lucky because the GRB was in the field of view of the instrument," says Philippe Laurent of CEA, the French government's atomic energy research institute in Saclay.
Earlier this year, astronomers used the Canada-France-Hawaii Telescope on Mauna Kea, Hawaii, to estimate the distance to the galaxy that produced this GRB, putting it some 275 million light years away.
Now Laurent and his colleagues have used the combined data to look for effects of quantum gravity. One idea is that if space-time is composed of indivisible grains, then this would polarise photons in a way that depends on their energy. This effect would accumulate over cosmological distances and so be observable from Earth. Yet the team found no such effect (Physical Review D, DOI: 10.1103/physrevd.83.121301).
Considering the extremely high quality of Integral's data, this puts an important limit on the size of the grains of space-time. According to an ESA press release, it means they must be smaller than 10-48 metres, many orders of magnitude smaller than the Planck length of 10-35 metres, the universe's smallest length scale.
However, this conclusion is drawing criticism. Carlo Rovelli, who studies LQG at the Centre for Theoretical Physics in Marseille, France, points out that grains of space-time cannot be smaller than the Planck length and says that the limit is "wrong".
The strange conclusion is a consequence of the way in which the data was analysed with respect to a very specific model of quantum gravity, which has fallen out of favour with some theorists. The main versions of both string theory and LQG don't support such a model. "This result does not contradict the main expectations [of these theories]," says Rovelli. Laurent agrees that the work has limitations. He admits that it doesn't affect theories that make more subtle predictions. These are still beyond the reach of today's experiments.
Nonetheless, the work demonstrates astonishing progress. Giovanni Amelino-Camelia, who studies quantum gravity at Sapienza University of Rome, Italy, is thrilled with the quality of observations and the analyses carried out by Laurent's group. He says this kind of work was unthinkable just a decade ago and raises hopes that more subtle effects could soon be tested.
This would rely on astronomers finding ever more distant GRBs and studying photons that are widely separated in energy. "The pace of improvement is well beyond the most optimistic expectations a decade ago," Amelino-Camelia says. Even Rovelli agrees that the future looks promising. "I share this excitement," he says.
A big challenge in physics is how to combine Einstein's general relativity, which describes gravity, with quantum mechanics to create a theory of quantum gravity. The best efforts on this front include string theory, which argues that everything in nature arises out of the vibration of tiny strings in 10-dimensional space-time, and loop quantum gravity (LQG), which shows mathematically that the fabric of space-time is woven out of gravitational field lines. Both approaches lack experimental support, however.
So cosmologists have been looking for signs of quantum gravity in the distant universe. One prediction of both string theory and LQG is that space-time is not smooth but "grainy" at extremely small scales. In theory, this graininess can be observed by studying particles from the same source but with different energies, to see if they are affected differently by the structure of space-time.
Over the past few years, terrestrial telescopes such as the Major Atmospheric Gamma-ray Imaging Cherenkov Telescope (MAGIC) in the Canary Islands and the High Energy Stereoscopic System (HESS) in Namibia have seen low-energy photons from a gamma-ray burst (GRB) arriving before their high-energy counterparts. While this could be due to delays in emission at the source, it could also be caused by the interaction of photons with the structure of space-time (New Scientist, 15 August 2009, p 26).
Now a team has used data from the Integral satellite, run by the European Space Agency (ESA), to study an entirely different effect: the polarisation of light of different energies from a GRB. In December 2004, Integral observed one of the brightest GRBs ever recorded. "We were lucky because the GRB was in the field of view of the instrument," says Philippe Laurent of CEA, the French government's atomic energy research institute in Saclay.
Earlier this year, astronomers used the Canada-France-Hawaii Telescope on Mauna Kea, Hawaii, to estimate the distance to the galaxy that produced this GRB, putting it some 275 million light years away.
Now Laurent and his colleagues have used the combined data to look for effects of quantum gravity. One idea is that if space-time is composed of indivisible grains, then this would polarise photons in a way that depends on their energy. This effect would accumulate over cosmological distances and so be observable from Earth. Yet the team found no such effect (Physical Review D, DOI: 10.1103/physrevd.83.121301).
Considering the extremely high quality of Integral's data, this puts an important limit on the size of the grains of space-time. According to an ESA press release, it means they must be smaller than 10-48 metres, many orders of magnitude smaller than the Planck length of 10-35 metres, the universe's smallest length scale.
However, this conclusion is drawing criticism. Carlo Rovelli, who studies LQG at the Centre for Theoretical Physics in Marseille, France, points out that grains of space-time cannot be smaller than the Planck length and says that the limit is "wrong".
The strange conclusion is a consequence of the way in which the data was analysed with respect to a very specific model of quantum gravity, which has fallen out of favour with some theorists. The main versions of both string theory and LQG don't support such a model. "This result does not contradict the main expectations [of these theories]," says Rovelli. Laurent agrees that the work has limitations. He admits that it doesn't affect theories that make more subtle predictions. These are still beyond the reach of today's experiments.
Nonetheless, the work demonstrates astonishing progress. Giovanni Amelino-Camelia, who studies quantum gravity at Sapienza University of Rome, Italy, is thrilled with the quality of observations and the analyses carried out by Laurent's group. He says this kind of work was unthinkable just a decade ago and raises hopes that more subtle effects could soon be tested.
This would rely on astronomers finding ever more distant GRBs and studying photons that are widely separated in energy. "The pace of improvement is well beyond the most optimistic expectations a decade ago," Amelino-Camelia says. Even Rovelli agrees that the future looks promising. "I share this excitement," he says.
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