We have seen in our previous posts how string theory and the observations made by MAGIC are helping scientists to really understand the space-time continuum. We continue further investigating the research of AmeIino-Camelia.
In their paper, Amelino-Camelia and his team report four other GRBs (Gamma-Ray Bursts) whose behaviour was consistent with the equation, although not conclusively in support. Others find no such evidence. Just days after Amelino-Camelia’s paper came out, Jacholkowska and her colleagues published their analysis of four other, less energetic GRBs observed by the Fermi telescope. They found no hint of time lags.
In Jacholkowska’s view, we cannot draw any firm conclusions, because Amelino-Camelia’s interpretation assumes, like the Markarian 501 analysis before it, that the gamma rays were emitted simultaneously regardless of their energy. This is always going to be a problem as long as interpretations are based on single observations of one type of source, Ellis says. “If you found an effect that was similar in two, you’d really begin to think you had found something,” he says.
One test that might clear things up involves neutrinos. These ghostly particles travel at virtually the speed of light, interacting with hardly anything. Because they carry energy, however, they should interact with space- time, and, if Amelino-Camelia is correct, suffer an energy-dependent time lag – although one that is only measurable if we can find neutrinos that have travelled far enough. That was always a problem.
Nuclear fusion reactions make the sun such a prodigious neutrino factory that it washes out almost all signals from further away. Besides solar neutrinos, the only cosmic neutrinos ever seen have been from the supernova SN1987A, a star that just happened to explode in our cosmic backyard, in the Large Magellanic Cloud some 170,000 light years away. This is still too close for its neutrinos to manifest any measurable time lag. Decisive help could now be at hand.
IceCube is a neutrino detector buried in a cubic kilometre of Antarctic ice that came fully on stream in 2011. In April 2012, it found two neutrinos that set tongues wagging. Called, in a fit of whimsy, Bert and Ernie, after two characters from the TV show Sesame Street, they were far more energetic than those generated by the sun. For that reason alone, Dan Hooper of Fermilab in Batavia, Illinois, thinks it’s likely that they come from a gamma-ray burst. “There aren’t that many things that can make that amount of energy in a single particle. GRBs top the list,” he says. In May 2013, IceCube announced the discovery of a further 26 neutrinos whose energies possibly betrayed an extragalactic source.
Amelino-Camelia thinks he has found three more in earlier IceCube data – ones that perfectly fit the idea of quantum space-time effects taking place. They all arrived from the general direction of three independently verified GRBs – but, if they are indeed associated with the bursts, got to Earth thousands of seconds earlier than the gamma rays.
Neutrinos are expected to escape from a collapsing star sooner than the light of a GRB because they don’t interact, whereas the visible blast has to fight its way through the collapsing gas before speeding through space. But even taking this into account, Amelino-Camelia maintains that the huge size of the gap between the neutrinos and gamma-ray light is consistent with the different effects of a space-time interaction on them.
Ellis remains sceptical. “Every once in a while, somebody gets a little bit excited but I don’t think there’s any statistically solid evidence yet,” he says. “One of the problems is that extraordinary claims require extraordinary proof, so you have to do something that is really convincing.” That will inevitably require larger telescopes capable of spotting more gamma rays and neutrinos more quickly.
Wagner is involved in an international collaboration of more than 1000 researchers from 23 countries that is aiming to build a giant successor to MAGIC and HESS. The Cherenkov Telescope Array would be 10 times as sensitive, and capable of seeing between 10 and 20 active galaxy flare-ups every year.
After spending years developing technology and looking at possible locations, with funding so far mainly from the governments of Germany, Spain and the UK, the collaboration will now be looking for the €200 million needed to turn the telescope into a reality.
Will it finally open our eyes to the landscape around us? Those involved hope so. “There is no reason to be pessimistic,” says Wagner. To find any kind of structure in space-time would be a revolution to rival Einstein’s, and could show the way forward when physics is struggling to see its next step. “It would be hard to overstate how important that would be,” says Hooper.
Space-time is the fabric of the universe, perhaps of reality itself. But no one knows what it is.