Two European labs — Cern in Geneva, and Gran Sasso in Italy — have collaborated in a study of the behaviour of subatomic particles called neutrinos. These tiny entities barely weigh anything at all, don't have electric charge and can travel through solid matter. In fact, billions of neutrinos, mainly produced in the sun, are at this moment streaming through your body without you noticing.
Opera (Oscillation Project with Emulsion-tRacking Apparatus, if you must know) is the name of a large instrument at Grand Sasso that can catch a tiny fraction of these elusive particles. In September, its scientists announced they had measured a beam of neutrinos arriving so fast that they must have been travelling faster than the speed of light. Although this was only 60 billionths of a second quicker than light over the same distance, it was still an incredible result.
According to the laws of physics, nothing can exceed the speed of light, an impressive billion kilometres an hour. The crucial point isn't that light is so special but rather that this speed limit is written into the fabric of space and time.
But what if Einstein was wrong? Is there a way of understanding the findings of Opera? The whole point of a scientific theory is that it's there to be shot down, to be shown to be false by new experimental evidence, or to be replaced with a more accurate theory. But extrao-rdinary claims require extraordinary evidence, and the scientists working on Opera are the first to admit they have no idea how their result is possible. They also know where potential faults in the experiment still lie and, so far, they have ruled out one systematic error. But they admit there may well be others.
After the media hype claiming Einstein was wrong came the next twist in this drama. A rival experiment at Gran Sasso, called Icarus, also captured some of the Cern neutrinos, but it measured their energy rather than journey time. If the neutrinos were indeed superluminal, they would have to be emitting radiation throughout their journey and hence losing energy. Not doing so would be a bit like an aircraft that manages to break the sound barrier without a sonic boom. It just shouldn't be possible. The Icarus collaboration announced this week that they found no evidence of this radiation, since the neutrinos arrived at their destination with the same energy as when they had left. They could not have been travelling faster than light.
The point is that Icarus no more proves Einstein right than Opera proves him wrong. A proper test would involve a new experiment carried out independently by another lab, and plans are underway in Japan and the US.
I'd love it if neutrinos could indeed travel faster than light. Such a discovery, if confirmed, would be heaven for physicists around the world: blackboards will be scrawled on, heads scratched, and Nobel prizes in the offing for a new Einstein. But my money is still on Einstein. He was, after all, pretty smart. And I am not prepared to rewrite my lecture course on relativity just yet. But what fun, eh?