Faster than light neutrinos? A quick calculation
by dave on September 22nd, 2011
A number of people on twitter and elsewhere (including my grad student, Austen) alerted me to an interesting story making its way across the interwebs. A number of sources have reported on a press release put out by the OPERA collaboration.
Here’s the basic idea: OPERA has a detector in Gran Sasso Italy, about 730 km from the LHC. The LHC produces neutrinos in abundance, and neutrinos have a few important properties:
1. They are very nearly massless and thus, even at moderate energies, we’d expect them to travel essentially at the speed of light.
2. They can “oscillate” or change identities, which means that the mu neutrinos produced at the LHC can turn into tau neutrinos which the OPERA detector is designed to measure.
3. They are very weakly interacting, which means that they can pass through solid earth unimpeded.
Light should make the journey from the LHC to Gran Sasso in about 3 ms, but according to the OPERA collaboration press release, they are detecting the neutrinos as making the journey in about 60 ns less than expected.
In other words, they are claiming that, unless there is some hitherto undetected systematic, neutrinos are traveling faster than light.
A couple of caveats:
1. The actual paper doesn’t seem to be up on the arXiv yet, so I don’t know exactly what their measurement is.
2. Even once it is, I’m a theorist, not an experimentalist, so I’m unlikely to be able to identify the systematics.
That said, I am pretty darn certain that this result is flawed. Neutrinos have mass, which is why they oscillate in the first place, so if it turned out that a massive particle could travel faster than light (and it wasn’t some sort of issue with not correcting for general relativistic effects or something like that), that would pretty much overturn special relativity.
More to the point, I have a simple calculation that makes me extremely skeptical.
Remember that the neutrinos are supposed to beat light by about 60 ns over a travel time of 3 ms. That’s
Now consider a supernova explosion. In particular, consider Supernova 1987A.
This was an explosion about 160,000 light years from earth. The thing is, the neutrinos and the photons from the explosion reached us at almost exactly the same time. In the cause of intellectual honestly, I need to point out that the neutrinos were detected first, by about 3 hours, but this is because the envelope of the explosion was optically thick and the photons had to bounce around a while, while the neutrinos just streamed right out.
But how much of a delay between neutrinos and photons would we expect if the OPERA result applied?
In other words, if the effect really were this large, we would have seen the neutrinos from SN 1987A way back in 1984. Yeah, we would have noticed that.
I don’t want to be too glib, however. There are a couple of key differences:
1. The neutrinos detected from 1987A were (anti) electron neutrinos, not tau neutrinos. However, since neutrinos oscillate from one flavor to another, I’d be surprised if this was the key difference.
2. The energies are quite different. In 1987A, neutrino energies were typically a few 10′s of MeV. The neutrinos measured by OPERA are a factor of 100 higher. It could very well be that this is a sensitive function of energy.
Of course, since the expectation is that neutrinos should NEVER travel faster than light, there’s no way to compute what we’d expect.
I, for one, am not going to hold my breath.
I posted this before getting my hands on the actual paper. In the cause of fairness, I need to mention that the authors of the paper talk about the constraints from 1987A themselves. They rightly refer to this result as a low-energy limit. As you can see from above, I find it unlikely that the differences between the high and low energy limit will have such a huge effect.