And though light goes pretty fast – 300m metres in a second – “space”, as the Hitchhiker’s Guide would have it, “is big”. Even at the speed of light, travelling between stars just takes too long without some dodge to evade Einstein’s traffic cops.
Last week, excitement spread when an experiment seemed to show that a certain type of particle could break this rule. The Opera experiment is a super-sensitive particle detector in the Gran Sasso laboratory in Italy. It is deep in a cavern watching out for neutrinos. Neutrinos are a kind of fundamental particle, produced, for example, by the sun.
They are also produced at the Cern particle physics lab in Geneva, home of the Large Hadron Collider. Cern has been firing neutrinos under the Alps and across Italy, towards Gran Sasso, for a few years. Since neutrinos hardly interact with matter, they pass through the rock with ease. A small number of them interact in the Opera detector. And this is where the fun starts.
The experimenters have a fantastically accurate measure, using differential GPS, of the distance between Cern and Gran Sasso. They think they have the 730km distance nailed to within 20cm. Also, they have synchronised atomic clocks at each end. So, since speed is distance divided by time, they can measure the speed of the neutrinos. And they come up with a number bigger than the speed of light.
This is an utterly stunning result. They can hardly believe it themselves, and have put it out there with a kind of shrug: “We know this looks crazy, everyone, but we need the rest of the science community to look at it and find our mistake, because we can’t.” People (including me) have been wading in with suggestions as to possible errors, and cross-checks which could be done to make the measurement more solid. But it is already a careful piece of work and while there are some good ideas, no one has yet shown it wrong for sure.
What might it mean if the neutrinos really are travelling faster than light? Why is it such a big deal?
In Einstein’s theory of relativity time isn’t the same for everyone. It depends upon how fast you are moving. Previously, Newtonian physics assumed time was the same for everyone. Einstein junked that, and instead said that the speed of light was the same for everyone. This led to some stunning consequences. One of them was E=mc2. Another is that nothing can go faster than light, since if you could catch light up and overtake it, then its speed would not remain the same for you. If you caught it up, then its speed relative to you would in fact be zero.
Weird though it sounds, Einstein's theory works. Time really does depend on speed. The very GPS systems which measure the distances need to take it into account or they don’t work.
This weirdness is nothing though, compared to what happens if things can go faster than light.
Imagine a neutrino being created at Cern and later arriving at Gran Sasso. Or you getting on the underground and later getting off again. Someone measuring the time between you getting on and getting off will not get the same answer as you, if they are moving with a different speed. But you and they will always agree on the order of the events. That the neutrino was made before it arrived, and that you got on the train before you got off.
If they were travelling faster than light, this wouldn’t be true anymore. The order could switch. So in the universe from their point of view, you get off the train before you get on. Or the neutrino arrives before it sets out. This is a violation of causality. Causes really ought to happen before effects, no matter how fast you travel.
So if anything travels faster than light, something is clearly broken. We’d need a new theory. Einstein’s theory improved Newton’s, even though Newton still works for lots of things. The new theory would improve relativity. It might involve extra dimensions, which is perhaps a more scientifically respectable way of saying hyperspace, or something else, but it would be very exciting.
So even though I would bet a lot that the experiment is wrong, there’s a part of me still really hopes it’s right.
Professor Jon Butterworth is head of Physics and Astronomy at UCL. He is a member of the High Energy Physics group on the Atlas experiment at Cern's Large Hadron Collider.