{"id":5874,"date":"2011-09-24T17:43:42","date_gmt":"2011-09-24T17:43:42","guid":{"rendered":"http:\/\/scienceblogs.com\/principles\/2011\/09\/24\/faster-than-a-speeding-photon\/"},"modified":"2011-09-24T17:43:42","modified_gmt":"2011-09-24T17:43:42","slug":"faster-than-a-speeding-photon","status":"publish","type":"post","link":"http:\/\/chadorzel.com\/principles\/2011\/09\/24\/faster-than-a-speeding-photon\/","title":{"rendered":"Faster Than a Speeding Photon: “Measurement of the neutrino velocity with the OPERA detector in the CNGS beam”"},"content":{"rendered":"

\"ResearchBlogging.org\"<\/a>There have been a lot of pixels spilled over this faster-than-light neutrino business, so it might not seem like something I should take time away from pressing work to write up. It is the<\/em> story of the moment, though, and too much of the commentary I’ve seen has been of the form “I am a {theorist, journalist} so hearing about experimental details gives me the vapors” (a snarky paraphrase, obviously). This suggests that there’s still room for a canine-level write-up going into a bit more depth about what they did and where it might be wrong.<\/p>\n

So, what did those jokers at CERN pull this time? Isn’t it bad enough that they want to feed us all into a black hole, now they’re messing with the speed of light?<\/strong> First of all, this wasn’t a CERN experiment in the same way that the LHC is. The experiment reporting on this uses a particle accelerator at CERN, but it’s actually an Italian collaboration who did the experiment and data analysis, with the principal detector based at the Gan Sasso underground laboratory in the Alps. The name of the collaboration is OPERA, which is one of those ghastly pseudo-acronyms where they use the second<\/em> letter of one of the words in order to force it to spell something.<\/p>\n

OK, fine, what have the Italians done?<\/strong> Well, the goal of their experiment is to look for an “oscillation” of neutrinos. The neutrinos are created at CERN in one of their three varieties, and on the way to Gran Sasso, they can change character and end up being detected as a different type. (They start as muon neutrinos and end up as tau neutrinos, or at least that’s the plan. It’s not terribly important for this experiment.)<\/p>\n

As part of the preliminary analysis for their main experiment, they looked at about three years worth of data, and noticed something odd: the neutrinos in their experiment seem to be moving slightly faster than the speed of light. The difference is pretty big in absolute terms– about 7500 m\/s, or nearly 17,000mph– but it’s only about 1\/40,000th of the speed of light. Still, the difference they see is many times larger than their uncertainty, and they can’t figure out why, so they’re making their results public.<\/p>\n

Wow. How do they measure that, anyway?<\/strong> Conceptually, what they did is the most basic kind of velocity measurement, the sort of thing we talk about in the first few weeks of introductory physics. They measured the distance between CERN and Gran Sasso, and divide that by the time between when the neutrinos are created and when they’re detected to get the speed at which the neutrinos covered that distance.<\/p>\n

The implementation, of course, is a little more complicated than that…<\/p>\n

<\/p>\n

But if they know when a neutrino was created and when it was detected, how could they screw this up?<\/strong> That’s the first complication. Neutrinos are ridiculously difficult to detect, so they have to throw a whole lot of them at their detector to see anything. In the entire 3-year dataset they analyzed, they only have around 16,000 neutrinos total.<\/p>\n

This means that they don’t actually have the ability to say when a specific neutrino was created, because there are huge numbers of neutrinos created for every one that they detect.<\/p>\n

How can they possibly claim anything about the speed, then?<\/strong> What they do is to compare the distribution of times when they detect neutrinos to the distribution of times when neutrinos were created in the source. The way this works is that they blast a moderately high-energy proton beam into a stack of graphite, which creates some exotic particles that eventually decay into other things, spitting out neutrinos along the way. The other decay products get blocked by the several thousand kilometers of rock between CERN and Gran Sasso, but the neutrinos interact so weakly with ordinary matter that they go right through, and a handful of them get detected.<\/p>\n

They can’t tell much of anything about exactly when the neutrinos are created, but they know that the neutrinos had to come from one of the protons in their original beam. And they have very good diagnostics on the proton beam. So, they argue that the distribution of arrival times of the the neutrinos ought to have the same shape as the distribution of arrival times of the protons, so they compare those distributions as a whole, which looks sort of like this:<\/p>\n

\"i-1e8528da88cf64d648ad1128974a8d19-neutrino_distributions.png\"<\/p>\n

(That’s Fig. 11 from the freely available arxiv preprint<\/a>.)<\/p>\n

The black points (with error bars) are the total number of neutrinos detected in a given 150 ns window around a particular arrival time (on the horizontal axis). The red lines show the shape of the proton beam distribution in time. At the top, there’s no correction for the time of flight, and you can see that the two distributions are shifted relative to one another (the two graphs are for different subsets of their data). At the bottom, they’ve put in a correction for the time of flight (basically, moving all the times for the red curve left by a microsecond), and you see that the red line fits the observed distribution of neutrinos pretty nicely.<\/p>\n

So, that’s the thing they call the time of flight?<\/strong> Exactly. If you look at that time, and compare it to what you expect for particles moving at the speed of light, you find that it’s about 60ns too short, suggesting the neutrinos were moving faster than they speed of light.<\/p>\n

OK, so how did they get the time wrong?<\/strong> Well, that’s the problem. They think they can account for all the uncertainties in the timing process, and it’s not enough to cover the observed shortfall. In fact, the fact is about six times bigger than they think their measurement uncertainty is.<\/p>\n

How do you get that kind of timing accuracy, anyway? I mean, I can’t buy a watch that’s good to better than a second. How do you keep track of things to the nanosecond?<\/strong> Nanosecond timing isn’t all that difficult by itself– you can get an atomic clock easily enough. What’s tricky is synchronizing<\/em> the timing at two different places. That’s a fiendishly difficult problem, and it’s more or less that sort of thing that led to the whole theory of relativity.<\/p>\n

And you have a book about that coming soon, we know. How do they do the synchronization?<\/strong> They take advantage of GPS for that. The Global Positioning System is a network of satellites containing atomic clocks, each beaming out a signal saying what time it is according to that clock. Measuring the difference in times between several satellites lets you figure out how far you are from each of them, and since the orbits of the satellites are well known, that tells you where you are on the surface of the Earth.<\/p>\n

They use a “common view” GPS system to coordinate their timing. They have an atomic clock at each end of the experiment, and as the name suggests, they use the time broadcast by GPS satellites that are visible to both ends at the same time as a reference to make sure their clocks are synched up. The difference between the two sites is around two nanoseconds, much smaller than the arrival time shift they see.<\/p>\n

So, is that the only problem with the timing?<\/strong> Not quite. There’s also some ambiguity about where the neutrinos are created, which might cause a problem. The exotic particles created by the proton beam pass down a 1-km tunnel, and the decay that produces the neutrinos happens somewhere in there, but they can’t tell exactly where.<\/p>\n

Wait a minute– their source is a mysterious 1km tunnel, but that doesn’t throw off their timing accuracy?<\/strong> The claim is that the particles that decay to make the neutrinos are also moving at very nearly the speed of light, and so pass down the tunnel at almost the same speed as the neutrinos. This makes the measurement less sensitive than you might think to exactly where the neutrinos are created– what matters is not the total uncertainty in the position, but the difference in travel time for a neutrino created partway up the pipe versus one created all the way at the end, and the speed of the parent particles is so close to the speed of the neutrinos that there’s very little difference in travel time. They’ve checked this with simulations, and say it’s a tiny effect– a fraction of a nanosecond.<\/p>\n

OK, that’s two places the timing might be wrong but they say it isn’t. How about the distance? Could they have screwed that up?<\/strong> That’s the other obvious source of error, but it’s hard to see how. Again, they have GPS to use for this, and while the accuracy of the position obtained by GPS for a moving receiver, like in your phone, is only several meters, if you’re trying to measure the distance between two fixed points, and monitor it over a long time, you can get really<\/em> good accuracy. They claim to have the distance down to 20cm, which is a bit less than a nanosecond at the speed of light. <\/p>\n

Twenty centimeters? Really?<\/strong> Really. They even provide a graph showing their measurements over the three-year run, which pick up a slow change due to continental drift, and a dramatic jump due to an earthquake in 2009:<\/p>\n

\"i-83eb9c74bbc44b77b2340035978c4780-neutrino_position.png\"<\/p>\n

(That’s Fig. 7 from the paper, and the greyish background is part of the figure. do not adjust your monitor.)<\/p>\n

You know what? Living in the future is pretty awesome?<\/strong> Yes, yes it is.<\/p>\n

OK, so, what, did the distance shrink in the winter or something?<\/strong> It doesn’t look that way. They looked for changes in the arrival time for different times of day, different times of year, and so on, and didn’t find any significant difference. They’ve also checked the distance by other methods– sending light down fiber optics, etc.– and their answers agree. As far as they can tell, they know the distance to twenty centimeters out of 730 km, full stop.<\/p>\n

So, what else could be wrong?<\/strong> Well, that’s the problem. They’ve checked all the obvious things, and they all seem to hang together. Which is why they’re putting this result out there, knowing full well that it disagrees with just about everything else. They’re hoping that some clever person will spot a mistake, or, failing that, that another experiment will do the same test (there’s one in Japan and one in the US), and see if they get the same result.<\/p>\n

Doesn’t this conflict with the supernova stuff Macho Ethan<\/a> was on about?<\/strong> Probably, but maybe not. The neutrinos involved there had very low energies, compared to the ones used here. It could be that really high-energy neutrinos travel at a different speed than really low-energy ones. They don’t see any such dependence on energy in their results, but they can’t check that much of an energy range, so it might be that there’s something going on there.<\/p>\n

Could it really be that neutrinos are moving faster than light?<\/strong> Maybe. Theoretical physicists, particularly theoretical particle physicists, are nearly infinitely flexible. If another experiment finds the same result, I have no doubt that theorists will find a way to accommodate it.<\/p>\n

It’d be deeply, deeply weird, though, not least because the existence of superluminal particles that interact with ordinary matter (as neutrinos do, albeit weakly) opens the door to violations of causality– effects happening before the things that caused them, and that sort of thing. This wouldn’t be a big loophole– the speed difference is tiny, and neutrinos interact extremely weakly– but it’s the kind of philosophical problem that would really bother a lot of people.<\/p>\n

So, if you had money to bet on it<\/a>, bet that this result is wrong. But these guys aren’t complete chumps, and if something is wrong with their experiment, it’s something pretty subtle, because they’ve checked all the obvious problem areas carefully.<\/p>\n

The OPERA Collaboraton: T. Adam, N. Agafonova, A. Aleksandrov, O. Altinok, P. Alvarez Sanchez, S. Aoki, A. Ariga, T. Ariga, D. Autiero, A. Badertscher, A. Ben Dhahbi, A. Bertolin, C. Bozza, T. Brugi\u00c3\u00a9re, F. Brunet, G. Brunetti, S. Buontempo, F. Cavanna, A. Cazes, L. Chaussard, M. Chernyavskiy, V. Chiarella, A. Chukanov, G. Colosimo, M. Crespi, N. D’Ambrosios, Y. D\u00c3\u00a9clais, P. del Amo Sanchez, G. De Lellis, M. De Serio, F. Di Capua, F. Cavanna, A. Di Crescenzo, D. Di Ferdinando, N. Di Marco, S. Dmitrievsky, M. Dracos, D. Duchesneau, S. Dusini, J. Ebert, I. Eftimiopolous, O. Egorov, A. Ereditato, L. S. Esposito, J. Favier, T. Ferber, R. A. Fini, T. Fukuda, A. Garfagnini, G. Giacomelli, C. Girerd, M. Giorgini, M. Giovannozzi, J. Goldberga, C. G\u00c3\u00b6llnitz, L. Goncharova, Y. Gornushkin, G. Grella, F. Griantia, E. Gschewentner, C. Guerin, A. M. Guler, C. Gustavino, K. Hamada, T. Hara, M. Hierholzer, A. Hollnagel, M. Ieva, H. Ishida, K. Ishiguro, K. Jakovcic, C. Jollet, M. Jones, F. Juget, M. Kamiscioglu, J. Kawada, S. H. Kim, M. Kimura, N. Kitagawa, B. Klicek, J. Knuesel, K. Kodama, M. Komatsu, U. Kose, I. Kreslo, C. Lazzaro, J. Lenkeit, A. Ljubicic, A. Longhin, A. Malgin, G. Mandrioli, J. Marteau, T. Matsuo, N. Mauri, A. Mazzoni, E. Medinaceli, F. Meisel, A. Meregaglia, P. Migliozzi, S. Mikado, D. Missiaen, K. Morishima, U. Moser, M. T. Muciaccia, N. Naganawa, T. Naka, M. Nakamura, T. Nakano, Y. Nakatsuka, D. Naumov, V. Nikitina, S. Ogawa, N. Okateva, A. Olchevsky, O. Palamara, A. Paoloni, B. D. Park, I. G. Park, A. Pastore, L. Patrizii, E. Pennacchio, H. Pessard, C. Pistillo, N. Polukhina, M. Pozzato, K. Pretzl, F. Pupilli, R. Rescigno, T. Roganova, H. Rokujo, G. Rosa, I. Rostovtseva, A. Rubbia, A. Russo, O. Sato, Y. Sato, A. Schembri, J. Schuler, L. Scotto Lavina, J. Serrano, A. Sheshukov, H. Shibuya, G. Shoziyoev, S. Simone, M. Sioli, C. Sirignano, G. Sirri, J. S. Song, M. Spinetti, N. Starkov, M. Stellacci, M. Stipcevic, T. Strauss, P. Strolin, S. Takahashi, M. Tenti, F. Terranova, I. Tezuka, V. Tioukov, P. Tolun, T. Tran, S. Tufanli, P. Vilain, M. Vladimirov, L. Votano, J. -L. Vuilleumier, G. Wilquet, B. Wonsak, J. Wurtz, C. S. Yoon, J. Yoshida, Y. Zaitsev, S. Zemskova, & A. Zghiche (2011). Measurement of the neutrino velocity with the OPERA detector in the CNGS
\n beam CERN<\/span> arXiv: 1109.4897v1<\/a><\/span><\/p>\n","protected":false},"excerpt":{"rendered":"

There have been a lot of pixels spilled over this faster-than-light neutrino business, so it might not seem like something I should take time away from pressing work to write up. It is the story of the moment, though, and too much of the commentary I’ve seen has been of the form “I am a… Continue reading Faster Than a Speeding Photon: “Measurement of the neutrino velocity with the OPERA detector in the CNGS beam”<\/span><\/a><\/p>\n","protected":false},"author":2,"featured_media":0,"comment_status":"open","ping_status":"1","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[19,33,7,141,225,11],"tags":[],"class_list":["post-5874","post","type-post","status-publish","format-standard","hentry","category-experiment","category-in_the_news","category-physics","category-relativity","category-researchblogging","category-science","entry"],"_links":{"self":[{"href":"http:\/\/chadorzel.com\/principles\/wp-json\/wp\/v2\/posts\/5874","targetHints":{"allow":["GET"]}}],"collection":[{"href":"http:\/\/chadorzel.com\/principles\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"http:\/\/chadorzel.com\/principles\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"http:\/\/chadorzel.com\/principles\/wp-json\/wp\/v2\/users\/2"}],"replies":[{"embeddable":true,"href":"http:\/\/chadorzel.com\/principles\/wp-json\/wp\/v2\/comments?post=5874"}],"version-history":[{"count":0,"href":"http:\/\/chadorzel.com\/principles\/wp-json\/wp\/v2\/posts\/5874\/revisions"}],"wp:attachment":[{"href":"http:\/\/chadorzel.com\/principles\/wp-json\/wp\/v2\/media?parent=5874"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"http:\/\/chadorzel.com\/principles\/wp-json\/wp\/v2\/categories?post=5874"},{"taxonomy":"post_tag","embeddable":true,"href":"http:\/\/chadorzel.com\/principles\/wp-json\/wp\/v2\/tags?post=5874"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}