“Nothing travels faster than light, with the possible exception of bad news, which follows its own rules.” –Douglas Adams
What’s going on here? A group (a large group, mind you) of physicists known as the OPERA collaboration have made a neutrino beam, and have been studying it for the past few years.
Making a neutrino beam is the easiest type of beam to make, by the way. All you do is shoot a bunch of high-energy particles into the Earth, like so.
You shoot a high-energy beam of protons into a fixed target, and you make all sorts of unstable particles — things like pions, kaons and other mesons — which have a lifetime of at most a paltry few nanoseconds.
You focus this beam very tightly, so that the decay products you get out travel in a narrowly collimated beam as well. What are these decay products?
Among other things, you get a bunch of high-energy muon neutrinos. And if you fire it through the Earth, everything that isn’t a neutrino gets wiped out in short order by the intervening atomic material.
But the muon neutrinos, for the most part, pass straight through the Earth uninhibited. Why? Because neutrinos hardly interact with anything at all! We’ve built neutrino beams like this before: from Fermilab (in Batavia, Illinois) to Minnesota, from KEK (in Japan) to Super-Kamiokande, and others.
And what we’d expect, based on measurements of neutrino mass, is that these particles should be traveling at almost, but just a hair under the speed of light!
And then you go and detect your neutrino.
But I just said they don’t interact with anything! So how do you do this?
You build a giant tank of something liquid for neutrinos to interact with. And although nearly all of your neutrinos pass right through it, every once in a while, one neutrino undergoes an interaction (through the weak force) with one of the atoms in your detector!
And when it does, because of how hugely energetic these neutrinos are, you produce either a muon (for a mu-neutrino) or an electron (for an electron-neutrino) that’s moving close to the speed of light in vacuum, and faster than the speed of light in your liquid!
When you move faster than the speed of light in a medium, you give off a special type of light known as Čerenkov radiation. If you line the outer rim of your neutrino detector tank with photomultiplier tubes, you can not only detect this radiation, you can use the information from it to reconstruct exactly where and when, in your tank, this neutrino interacted with one of your atoms!
Now, in the past, we’ve found that these neutrinos move, more or less, at the speed of light in vacuum (c), as expected. One experiment based out of Chicago, a few years ago, found marginal evidence that neutrinos might move just a tiny bit faster than the speed of light, at 1.000051 (+/- 0.000029) c.
Of course, this result is consistent with neutrinos moving at or slower than the speed of light; the errors are not significantly smaller than the measured difference from c. So OPERA, whose detector is shown below, performed this measurement with great care, and announced their results today.
The 730 kilometer trip should have taken these neutrinos 2.43 milliseconds, were they traveling at the speed of light. But according to the OPERA collaboration, the neutrinos arrived 60 nanoseconds earlier than expected, with a claimed uncertainty of only ten nanoseconds!
Translating that into a measurement for the speed of neutrinos, that means they are traveling at 1.0000247 (+/- 0.0000041) c.
Now, measurement at this level of precision is not easy, and I am certainly not going to be the first person to come out and say I don’t believe, based on this, that neutrinos move faster than the speed of light. (But, as one of many, I don’t.)
Because there’s a much better constraint out there on the speed of high-energy neutrinos from some time ago. Above is a Hubble Space Telescope time-sequenced image of the closest supernova in my lifetime: Supernova 1987A, which took place in the Large Magellanic Cloud 168,000 light-years away.
This supernova was discovered, optically, on February 24, 1987. About three hours earlier, 23 neutrinos were detected over a timespan of less than 13 seconds. The reason for the 3 hour delay? When the core of a star collapses (in a type II supernova; see here), most of the energy is radiated away in the form of neutrinos, which pass freely through the outer material of the star, while the emission of visible light occurs only after the shock wave reaches the stellar surface.
Even if you assume that the light and neutrinos were created at the same time, but the visible light moved at c and the neutrinos moved faster than light, which is why they got here first, know what value you’d get for the speed of these neutrinos?
1.0000000020 c, which is inconsistent with the results from the OPERA collaboration.
Now, something fishy and possibly very interesting is going on, and there will certainly be scientists weighing in with new analysis in the coming weeks. But in all the excitement of this group declaring that they observe neutrinos moving faster than the speed of light, don’t forget what we’ve already observed to much greater precision! And be skeptical of this result, and of the interpretation that neutrinos are moving faster than light, until we know more.