Einstein’s general theory of relativity describes the properties of gravity and assumes that space is a smooth, continuous fabric. Yet quantum theory suggests that space should be grainy at the smallest scales, like sand on a beach.
One of the great concerns of modern physics is to marry these two concepts into a single theory of quantum gravity.
Now, Integral has placed stringent new limits on the size of these quantum “grains” in space, showing them to be smaller than some quantum gravity ideas would suggest.
According to calculations, the tiny grains would affect the way gamma rays travel through space. The grains should “twist” the light rays, changing the direction in which they oscillate, a property called polarization.
High-energy gamma rays should be twisted more than the lower-energy ones, and the difference in the polarization can be used to estimate the size of the grains.
Philippe Laurent from the Commission of Atomic Energy (CEA) in Saclay, France, and his collaborators used data from Integral’s IBIS instrument to search for the difference in polarization between high- and low-energy gamma rays emitted during one of the most powerful gamma-ray bursts (GRBs) ever seen.
GRBs come from some of the most energetic explosions known in the universe. Most are thought to occur when massive stars collapse into neutron stars or black holes during a supernova, leading to a huge pulse of gamma rays lasting just seconds or minutes, but briefly outshining entire galaxies.
Laurent and colleagues searched for differences in the polarization at different energies, but found none to the accuracy limits of the data.
Some theories suggest that the quantum nature of space should manifest itself at the “Planck scale”: the minuscule 10^-35 of a meter, where a millimeter is 10^-3 (0.001) m.
However, Integral’s observations are about 10,000 times more accurate than any previous and show that any quantum graininess must be at a level of 10^-48 m or smaller.
“This is a very important result in fundamental physics and will rule out some string theories and quantum loop gravity theories,” said Laurent.
Integral made a similar observation in 2006 when it detected polarized emission from the Crab Nebula, the remnant of a supernova explosion just 6,500 light-years from Earth in our own galaxy.
This new observation is more stringent, however, because GRB 041219A was at a distance estimated to be at least 300 million light-years.
In principle, the tiny twisting effect due to the quantum grains should have accumulated over the large distance into a detectable signal. Because nothing was seen, the grains must be even smaller than previously suspected.
“Fundamental physics is a less obvious application for the gamma-ray observatory, Integral,” said Christoph Winkler from ESA. “Nevertheless, it has allowed us to take a big step forward in investigating the nature of space itself.”
Now, it’s over to the theoreticians, who must re-examine their theories in the light of this new result.