For a few days in September 2011, it was the biggest story in the world. The little-known OPERA experiment in Gran Sasso, Italy, had just made an electrifying claim - that subatomic particles called neutrinos had travelled faster than the speed of light. Next year, two experiments - MINOS at Fermilab in Batavia, Illinois, and T2K in Japan (pictured) - will be able to test the claim. If it stands up, how should we meld these misbehaving particles with the rest of physics?
One option is via tachyons, hypothetical particles that are born speeding faster than light. It turns out that the speed limit imposed by Einstein's special theory of relativity isn't so much a cap that nothing can exceed as a barrier that nothing can cross. Tachyons were dreamed up to illustrate this: particles born faster than light pose no problem for special relativity as long as they spend their whole lives in the fast lane.
Born speeding (Image : Kamioka Observatory, Institute for Cosmic Ray Research)
Are neutrinos tachyons? One way they might be is if the universe is filled with a field that interacts with particles as they fly through it. If photons have more drag in that field than neutrinos do, then neutrinos would naturally outpace the speed of light. This idea may feel familiar : light travels slower in glass than in a vacuum, for instance. So the universe might be permeated with a sort of diffuse glass.
If neutrinos do turn out to be tachyons, theorists will still have their work cut out. Though they are born speeding, tachyons interfere with another demand of special relativity: that a particle's behaviour be the same no matter where it is facing or how fast it is going. Meanwhile, there is no shortage of other theories scrabbling to explain this most astonishing of results.