“I have difficulty to believe it, because nothing in Italy arrives ahead of time.”
-Sergio Bertolucci, research director at CERN, on faster-than-light neutrinos
You know the story. Last year, the OPERA experiment at CERN announced, to the shock and surprise of practically everyone, that they had observed what appeared to be neutrinos moving faster than the speed of light.
How did the experiment conclude this? Let’s refresh your memory.
A beam of high-energy protons, moving very close to the speed of light (but not quite there thanks to Einstein’s relativity) is smashed into a target, creating a whole bunch of debris. Some of that debris consists of neutrinos (and unstable particles that will decay into neutrinos), which are incredibly light — millions of times lighter than a single electron — but not quite massless.
Because neutrinos hardly interact at all, we can fire them through the Earth pretty much at will. While all the other particles will be blocked by the particles in the ground, the neutrinos continue to travel as if in free-fall, affected only by the gravity of the objects around them.
From CERN, these neutrinos travel through around 732 kilometers of Earth, until they arrive at the OPERA detector, buried beneath the Italian mountain of Gran Sasso. The OPERA detector is huge, and that makes it good enough to detect about one out of every 1016 (that’s ten quadrillion) neutrinos that pass through it.
Over the course of more than a year, OPERA detected somewhere around 16,000 of these neutrinos, measuring a number of their properties, including their arrival times. What they found, when they took a look at their data, was absolutely shocking. Based on how energetic these neutrinos were and how far they had traveled, they were able to calculate exactly how long it should have taken for these neutrinos to travel the distance between where they were created at CERN to when they arrived in the OPERA detector.
And the result should have been a time corresponding to a speed indistinguishable from the speed of light in a vacuum.
At least, that’s what it should have been in theory, if physics behaves as we expect it to. What they found, instead, is that the neutrinos arrived about 60 nanoseconds early, which is incredibly fishy. This doesn’t appear to happen for either significantly lower-energy or higher-energy neutrinos, as other experiments (and Matt Strassler) showed.
But when an experiment claims to have been done well and gives you a surprising result, you have to investigate exactly what’s going on here. Now, we thought we understood neutrinos, and if we did, they shouldn’t be arriving early. Certainly nowhere near this early. We immediately wondered if we were fooling ourselves with the results, and while some errors have been ruled out, there are still many questions remaining. The big one, of course is why did these neutrinos arrive earlier than we expected?
The OPERA collaboration could have made an experimental error that they haven’t accounted for. (Some plausible explanations are as mundane as faulty wiring in the experiment.) This is sort of the default position that many people — including myself — take: that there’s some error in the experiment somewhere. But this could also be an indicator of some potentially revolutionary new physics. If we want to know what’s going on, you know how science works: we test it again, in different ways!
And that’s exactly what’s going on at two different locations, unconnected to the OPERA experiment. In Japan, they’re creating neutrinos at similarly high energies to the OPERA experiment, and sending them from Tokai to Kamioka, over a distance of 295 kilometers.
We’re also, in the United States, sending a beam of neutrinos underground from Fermilab to Soudan Mine in Minnesota. The distance from Fermilab to Soudan Mine? An uncanny — wait for it — 732 kilometers.
At the terminal point in both experiments, giant neutrino detectors await. Now, what they see will teach us a tremendous amount — assuming that everyone is a competent experimentalist — about what’s going on with these neutrinos. Let’s run through some of the most likely possibilities of what these experiments may see, and what that will point to!
Option 1: MINOS neutrinos arrive on time, T2K neutrinos arrive on time.
This is perhaps the most boring option, and also perhaps the most expected. If neutrinos really don’t move faster-than-light, and the OPERA collaboration achieved their results because of a unique fault in their experiment, the other collaborations won’t see it. (This will also be what we see if the recent faulty cable theory turns out to be true.)
If this happens, the OPERA collaboration will be left with a lot of egg on their faces, and many jokes like Bertolucci’s, at top, will ensue.
Option 2: MINOS neutrinos arrive ~60 ns early, T2K neutrinos arrive ~60 ns early.
This possibility — that everyone sees their neutrinos arrive about 60 nanoseconds early — is also really interesting, although it could be happening for two very different reasons.
On one boring hand, everyone doing these experiments could be making the same systematic error. We’ve never done timing at this precision over these distances before, and there could be some sort of universal error related to the type of equipment or the setup used that affects everyone equally.
There are also some exciting, theoretical possibilities that the neutrinos travel incredibly fast initially, resulting in a very fast traversal of maybe the first 18 meters or so, followed by normal, roughly light-speed travel over the remaining distance. This idea, currently considered to be extremely fringe, would suddenly be thrust into the spotlight.
Option 3: MINOS neutrinos arrive 60 ns early, T2K neutrinos arrive 24 ns early.
If this happens, the idea that neutrinos move at some speed faster-than-light through the Earth, or that the Earth acts as a medium with an index of refraction slightly less than 1 for neutrinos, would gain a lot of traction. In other words, neutrinos in a vacuum would move at the speed of light, but neutrinos moving through the Earth somehow move faster than light in a vacuum by some small amount; maybe 0.0025% faster.
This would be incredibly interesting from a theoretical perspective, and is quite possibly the result that many of the OPERA experimentalists are hoping for; if this came to pass, theoretical physics would certainly be very busy trying to explain why this was happening.
Option 4: MINOS neutrinos arrive some time other than on time or 60 ns early, T2K neutrinos arrive at some other time than on time, 24 or 60 ns early.
If this happens, we’ll know that there are either experimental errors somewhere or some really bizarre theoretical things going on, but nothing that was reasonable to expect.
You can, of course, make your bets as to what you think is most likely, but there’s a reason we do the experiments; don’t state any definitive conclusions until the results come in! For my own part, the faulty cable theory is a pretty lame explanation that would be shocking at this point. We have to assume some base level of competence when experiments are performed, and it would be a tremendous blow to everyone involved if this turned out to be the culprit. But with the experiment at MINOS already running, we should have our first check, and hence the first information leading us towards a definitive answer in just a few months!
So if you want to know whether neutrinos are moving faster than light, whether there is there some new physics lurking in high-energy neutrinos, whether the Earth is a faster-than-light material to neutrinos, or whether this whole fiasco is just an experimental blunder, you have to stay tuned! Any guesses?