(This title sounds like a Ramones parody written by Weird Al Yankovic.)
Yesterday afternoon, a big news story broke about a possible violation of relativity by neutrinos. To summarize, neutrinos produced at CERN (English version: European Organization for Nuclear Research) are detected at a facility in Italy known as OPERA (Oscillation Project with Emulsion-tRacking Apparatus — I think Calvin works for them). The distance between the two labs is about 730 kilometers (about 450 miles) through the crust of the Earth; the research team claims the actual distance is known to 20 centimeters. With this measurement, they found neutrinos to move just slightly faster than the speed of light by a factor of about 0.000025, or 0.0025%.
Neutrinos are difficult particles to study for a large number of reasons. First, they are electrically neutral and non-magnetic, so steering them using magnets and the like doesn’t work. (For more on how neutrinos are detected, see Ethan Siegel’s post on the same subject.) Also, the standard techniques for finding mass fail: to this day, though we know they have mass, we don’t know exactly what that mass is, only that it is very tiny. This combination of factors means that neutrinos move very close to the speed of light, according to what we know about particles (both from basic relativity and from quantum field theory). In Einstein’s theory of relativity, the speed of light is not just a speed limit for particles with mass — it’s a speed barrier. You can’t even reach it, much less pass it.
Now we can see why this story is a big deal: if the neutrinos are actually traveling faster than light, then relativity will need modifications, much as Newtonian physics needed the changes relativity itself provided to account for electromagnetism, especially light. But here’s also why I don’t want to write about the whole situation (despite the fact that I’m doing it anyway, ’cause I’m dumb): it’s simply too soon to make any concrete claims based on the available data. Even the researchers don’t claim relativity violations: they say specifically that they draw no theoretical conclusions. Now that I’ve started, though, I should finish.
- This is not as simple as “Einstein was wrong”. For one thing, other experiments don’t show neutrinos moving faster than light over long distances. Ben Still points out that if neutrinos consistently move as quickly as was measured, the neutrinos from Supernova 1987a (a massive star explosion in the Large Magellanic Cloud) would have arrived 4 years before the light did. However, the neutrinos only were 3 hours ahead of the light, and that discrepancy is because light scatters off the gas in the supernova, while neutrinos don’t interact strongly with anything and so can go right through. Supernova 1987a immediately rules out the idea that neutrinos always move faster than the speed of light.
- Although the effect is larger than the experimenters’ estimated errors, this is a complex chain of measuring apparatus. Because the relative deviation from the speed of light is so tiny, a relatively small error missing from their analysis would push the results into the regime where relativity isn’t violated. (One possible problem is with the time of the neutrino emission.) I’m not sophisticated enough to understand everything in the paper, so I’m not going to try to find flaws. Others will do so, not necessarily to “show OPERA is wrong” — science operates by testing claims, and in fact the OPERA researchers invite people to check their results. As I say over and over again on this blog, it’s about evidence.
- Even with everything taken into account (and they were careful!) the result is still ambiguous. In an email exchange with my Ph.D. advisor, Arthur Kosowsky, he said that he would be more likely to believe a claim of neutrinos moving 100 times faster than light than 1.0025 times faster. Although that obviously falls under the umbrella of my first comment, I have to say I agree: a result so incredibly close to light speed feels too close for comfort. Science doesn’t work on feelings, but I think everyone would be more comfortable declaring the discovery of new physics if the speed was a lot bigger!
So what are we left with? First, time and further checks will tell. Many seemingly ground-breaking results evaporate under scrutiny, and this may be one of them. Second, the press releases were incredibly over-hyped: Einstein’s relativity is not overthrown, though if the OPERA results are borne out, relativity will effectively be shown incomplete. (For a couple of alternative ideas, listen to particle physicist/rock star Brian Cox’s interview on the BBC.) Any alternative explanation will not involve allowing just any particle to go faster than the speed of light, and won’t let neutrinos go faster than light in a consistent manner, which means something is special about this particular set of circumstances. Third, automatically rejecting these results is also a mistake: the hype is there, but the paper itself seems to strike the right tone. Researchers are allowed to believe their results are correct, after all! There are many neutrino detectors in use around the world, and although all neutrino experiments are challenging, I have no doubt we’ll have follow-up tests very soon.
OK, I’ve done my duty and blogged about the neutrinos. Can I do something else now?