Threats to eat one's boxer shorts on live television if one is proved wrong should not be made lightly by scientists, however confident they might be in their pronouncements. Not because of the health risks, the potential for public humiliation, or because it somehow trivialises the scientific process, but because it leads the public to think that science is about vested interest or a closed-minded reluctance to embrace new discoveries.
Nevertheless, my recent light-hearted remark concerning a sartorial diet did hit the headlines, and I am more than happy to use the opportunity to discuss both the thrill and the process of scientific research.
Let me bring you up to speed (as it were). Two European labs – Cern in Geneva, and Gran Sasso in northern 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 as though it weren't there because they hardly ever interact with anything. 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, sophisticated 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 produced at Cern, 730km away, 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 quite an incredible result.
According to our understanding of the laws of physics, nothing can exceed the speed of light, an impressive billion kilometres an hour. And in my experience, there is nothing that annoys people more about Einstein's theory of relativity (for that is where this notion originates) than its claim to this cosmic limit. Since Einstein's work in 1905, thousands of experiments have only confirmed it – and indeed much of the beautiful edifice of modern physics rests on it being correct. The crucial point is not 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 is there to be shot down – to be shown to be false by new experimental evidence, or to be replaced with a better, more accurate theory that explains more. But extraordinary claims require extraordinary evidence, and the scientists working on Opera – who cannot be faulted for the thoroughness of their experimental work – 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 potential source of 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. It had been pointed out by theorists very soon after Opera's initial announcement that 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. Both results are experimental measurements, not discoveries. A proper test would involve a new experiment carried out independently by another lab, and plans are under way in Japan and the US, but they will take some months at least.
I would 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.
And my boxers? Hold on to that ketchup for now; 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?
By Jim Al-Khalili, professor of physics and professor of the public engagement in science, at the University of Surrey.