“This is to show the world that I can paint like Titian: [See drawing, above.] Only technical details are missing.” –Wolfgang Pauli
First theorized by Pauli all the way back in 1930, neutrinos are some of the most mysterious and puzzling particles ever discovered in nature. For starters, they weren’t even first detected until 1956 (by Reines and Cowan), 26 years after they were predicted to exist! Coming in three flavors — electron, muon and tau — and in both particle and anti-particle type, these neutrinos have the smallest but non-zero masses of any particle ever discovered.
The history of these elusive particles is literally a treasure trove of riches into the fundamental nature of our physical world. The picture above — the very first of a neutrino in a hydrogen bubble chamber — is a fantastic example. Over to the right of the image, you can see what looks like a bunch of tracks all originating at some vertex.
This image — generated in 1970 — is a surefire sign of a neutrino striking a proton, something we can definitively tell by the tracks of the particles coming off. (None of which, by the way, is a neutrino!)
Based on the curvature and radii of the tracks left by the particles that come out of this interaction, since they’re all in a known magnetic field, we can determine their masses, charges, and velocities, and hence we can reconstruct that this was a muon neutrino that struck a proton, producing a negatively-charged muon (to conserve lepton and lepton family number), an oppositely-charged pion (to conserve electric charge) and kicking the proton off (to the upper right)! You can tell the charge of any particular particle by the direction it curves in the magnetic field: positive ones curve counterclockwise, negative ones curve clockwise!
Neutrinos are incredibly difficult to detect: we need to literally make quadrillions of them at the highest energies achievable just to have a reasonable chance at detecting one neutrino, and the lower in energy your neutrinos are, the more steeply their cross-section drops, as the graph below shows.
And, as you all know, recently a beam of very high-energy neutrinos was sent from CERN to Gran Sasso. Something remarkable like 1020 neutrinos were generated and launched towards the OPERA detector, hundreds of miles away beneath an Italian mountain, and a few thousand of them were detected. Oddly enough, on average, they were detected about 60 nanoseconds too early, resulting in rampant speculation that these neutrinos were traveling faster than the speed of light!
I wrote a large number of articles about this, and even went on TV to talk about it. Most recently, I was invited to be a guest href=”http://scienceblogs.com/startswithabang/2011/11/weekend_diversion_neutrinos_on.php”>on the radio to talk about these fascinating particles (episode available here), and as is often the case, didn’t get a chance to answer everyone’s questions.
Well, the skeptics over at Skeptically Speaking were kind enough to send me the unanswered questions, and I’m pleased to take them all on here for you! Let’s get to it…
1.) Early on there was an initial response that this result had been explained because of a measuring error involving special relativity and GPS. What was this criticism and do we know yet if the OPERA team had accounted for this during their initial experiments?
It’s better to start by reminding you what a GPS satellite is: it’s basically a satellite in space a known distance away from the surface of the Earth, with an atomic clock on board. You take an atomic clock at your location on Earth, and you can tell what the light-travel-time (and hence the distance) from your location to the satellite is. By taking a number of GPS satellites (usually four), you can precisely know your position and the amount of time that’s elapsed from any particular event on Earth. But you need to be smart about it: both the effects of special relativity (how quickly the satellites and you, on Earth, are moving as the Earth rotates on its axis and revolves around the Sun) and general relativity (the gravitational effects of the Solar System) must be taken into account! If such effects weren’t taken into account, a GPS satellite could botch your position by as much as 30 meters.
How long does it take light to travel 30 meters? 100 nanoseconds. So it’s conceivable that this systematic, 60 nanosecond shift is entirely due to a GPS error. However, this is well-known physics, so it’s pretty reasonable to assume that the OPERA scientists know how to account for this. (Quite honestly, it would be shocking if this were where the error lied.) But, as they didn’t release this information publicly, the only way to check is to have a different team do the experiment over.
2.) Are the extra dimensions theorized as part of string theory a possible part of the explanation for these neutrinos?
Although this is one of people’s favorite areas of speculation, the answer is “probably not.” Sure, a journey through an extra-dimension could provide a short-cut through our Universe’s spacetime, shaving off those 60 nanoseconds easily. But if those extra dimensions were accessible at these relatively low energies that the OPERA neutrinos have, we would have seen see lots of other telltale phenomena at both the LHC and at Fermilab. The fact that what we’ve seen at both places agrees so strongly with the simple Standard Model and nothing more — no SUSY, no Extra Dimensions, etc. — tell us that string theory is probably not playing a role in this experiment.
3.) When the paper was first announced, information about this neutrino experiment was everywhere, and now I have to search to find up to date information about the status of these experiments and the science community’s reaction to it. Why isn’t the media continuing to cover this story?
The science community is — rightfully — skeptical of these results. If they hold up to repeat, follow-up experiments at other locations, such as MINOS in Minnesota and K2K in Japan, this will be very big news. But we’ve learned all we can learn from OPERA. They see neutrinos arriving 60 nanoseconds early in their detectors, they’ve been unable to find an error, and other experiments find that OPERA’s results, when combined with other predictions of the Standard Model, do not correctly predict what they see. So either OPERA’s results are wrong or our current theories about how particles and fields work are wrong when applied to OPERA’s neutrinos. If we want to learn more, we need to do something new to find it out, and that’s why the media coverage has dropped off.
4.) When the news first broke on the possibility of faster than light neutrinos there was also a lot of talk about the associated possibility of time travel. What do faster than light neutrinos have to do with the theories around time travel?
It’s a simple matter of economics: it’s cheaper to build the OPERA experiment than it is to perform the annual maintenance on a DeLorean DMC-12.
No, no, I’m kidding. Special relativity tells you that the laws of physics should be the same in all inertial (non-accelerating) reference frames. So if I send a signal from point A to point B, then all observers, no matter where they are, in what direction or how quickly they’re moving, will see that the signal was sent from point A before they see the signal arrive at point B. And this is true, so long as the signal moves at or below the speed of light.
But if that signal moves faster than light — i.e., if the neutrinos arrived at the OPERA detector before a photon moving at the speed-of-light-in-vacuum would have — then some observers would see that signal arrive at point B before they see you send it from point A. Hence the joke,
We don’t allow faster-than-light neutrinos in here, said the bartender. A neutrino walks into a bar.
5.) What was the result of the experiment’s initial purpose, to test if one kind of neutrino could turn into another kind, and what are the implications of that result? Could these two different results be related in any way?
There are three types of neutrinos — electron, muon, and tau — but they all have, and this is very important, the same quantum numbers. Same charge, same lepton number, same baryon number, same spin, same isospin, and almost the same mass. (A little more background on neutrinos here and here.) The way it works in quantum mechanics is that if you have the same quantum numbers, you mix together. So there might be three distinct masses for the things that make neutrinos, and we’ll give them some clever names, like m1, m2 and m3. (Technically, we call these mass eigenstates.)
What we see as an electron neutrino might be 70% m1, 20% m2 and 10% m3, while the muon and tau neutrinos would have different percentages. Mixing is something that’s well known to happen for the weak interactions in both quarks and neutrinos, but it’s been much more difficult to measure for neutrinos than it is for quarks. So that was the original goal of this experiment: the measurement of neutrino oscillations was designed to measure exactly how neutrinos (and anti-neutrinos) do their mixing with one another.
6.) How can neutrinos oscillate when they have different masses?
So this is actually really interesting: the way that they oscillate between electron, muon, and tau types allows us to determine what the mass differences are between the different types! (Technically, the mass-squared differences.) By measuring the way the different oscillations take place, we can also tell whether there are only three fundamental types of neutrino (there must be at least three) or more than three: a fourth type would mean there must be sterile neutrinos, or a neutrino beyond what the Standard Model predicts! In other words, we were looking for new physics in this arena when we (may have) found it in another.
7.) Where do the neutrinos that miss the detector end up?
First off, this is almost all of the neutrinos. A little math: we generated about 1020 neutrinos in the OPERA experiment, and about 16,000 of them were caught by the detector, which is 20 meters long on its longest side. So if we wanted to catch half of these neutrinos, we would need about 5 x 1019 of them to interact. Assuming we just stacked identical copies of OPERA, one-after-another, until half of the neutrinos interacted, know how many we’d need?
3,000,000,000,000,000 of them! Or, for those of you who’d rather have one giant detector, you’d need a version of OPERA that was right around seven light years long. Sooooo…. most of them don’t miss the detector so much as they simply pass right through it, and they continue to pass through the mountain, through the atmosphere, and go off into space, where they pretty much continue to pass through every star, planet, gas cloud and galaxy they encounter.
For what it’s worth, a single neutrino would have to pass through about forty million Suns before it had a 50% chance of having one interaction with any of the particles inside.
Don’t wait up.