Those tachy neutrinos

Like pretty much every other physicist in the world, I’ve been inundated with people sending me links to articles about neutrinos timed to be moving faster than the speed of light (thank you BBC, The Guardian, The Age, Slashdot, Bad Astronomy, Starts With a Bang, etc.).

And just like every physicist, I’m obliged to say that we’ll have to wait to see whether this is confirmed as a truly universe-shattering discovery, or some sort of statistical fluke or experimental error.

The implications of it being true are so great that we have to be careful about jumping to conclusions; plus, there are good results from previous experiments that showed neutrinos don’t move faster than the speed of light.

First though, the gist of today’s result: this was part of the OPERA experiment to measure neutrino oscillations (that’s where they change from one flavour to another).

Neutrinos are subatomic particles related to electrons. They have nearly zero mass, no electric charge, and they only interact via the weak nuclear force. This makes them very hard to detect, but it also means there are a lot of unanswered questions about them, and hence potential for revealing more about the fundamental nature of the universe.

The neutrinos in this experiment were emitted from the CERN facility in Switzerland and detected in Gran Sasso, 732 km away in Italy. Using atomic clocks to time their emission and detection, and accurate GPS measurements of the distance, the particles were found to arrive about 60.7 nanoseconds earlier than they should have if they were travelling at the speed of light.

Diagram of the neutrino beam CNGS, or CERN neutrinos to Gran Sasso, passing through the Earth
Diagram of the neutrinos' subterranean path, known as CNGS, short for "CERN neutrinos to Gran Sasso" (Image from OPERA)

This means that they must have been moving at a speed of 1.0000248 ± 0.0000028 (stat.) ± 0.0000030 (sys.) times the speed of light (c).

Those numbers with all the zeroes after the plus-minus sign ± are the statistical and systematic uncertainties respectively. The statistical uncertainty is calculated from the standard deviation of the number of measurements made, whereas the systematic uncertainty is an estimate of inaccuracies inherent in the experimental equipment or methodology.

It’s pretty clear that the speed measured is well out of the range of the uncertainties, which is why the result is so worth commenting on. But that still doesn’t mean it’s right…

According to Albert Einstein’s Special Theory of Relativity, normal matter cannot travel faster than the speed of light. Anything that does would hypothetically be what we call a tachyon, unable to travel slower than the speed of light and capable of travelling backwards in time. Which raises serious questions about causality.

Alternatively, Einstein could be wrong. But that would also be very troubling, because in every other case relativity works very well. It’s really not clear what you could replace it with that would agree with relativity in every known instance except this one.

But apart from these theoretical reasons to doubt the result, the speed of neutrinos was previously measured to great accuracy in the explosion of Supernova 1987A. When a star explodes as a supernova, it first emits a burst of neutrinos from its collapsing core; the core then rebounds off itself, triggering an incredible explosion. And about 3 hours later, it emits a burst of light that can be seen across the universe.

And that’s exactly what was observed in 1987. A burst of neutrinos was detected and then, about 3 hours later, the light of the explosion became visible.

The timing was so close to theoretical predictions that it puts a limit on any faster-than-light – or superluminal – neutrinos of less than 1.000000002 times the speed of light. Or to put it another way, if the OPERA experiment is correct, the neutrinos should have arrived more than 4 years before the light did, not 3 hours.

So what happens now? Do we just dismiss the OPERA result?

No, we do the same thing we always do: see if other scientists can reproduce the result (which of course plenty of them are clamouring to do). And we find out what explains this discrepancy, whether it’s an error in one of the experiments, or if there are unique conditions, like different energy neutrinos behaving in different ways, or something weird happening in the rocks between Switzerland and Italy.

After all, even though there are some really wacky theoretical ideas being put forward – like neutrinos taking a shortcut through higher dimensions – it’s unexpected results like this that have the chance of giving us new physics. Especially now that we’re staring down the barrel of a possible deadlock with the Higgs not being found at LHC, we have to hope for mysteries like these to show us where to look next.

To find out more, read the full OPERA paper at the physics e-print repository, (PDF 4.7 MB).


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