Q: “Why don’t physicists shield themselves from neutrinos?”
A: “Because they never see them coming.” #neutrinojokes
Over the past two months, we’ve talked more about neutrinos than ever before thanks to an extraordinary claim that neutrinos have been observed to move faster-than-light!
And as you well know, no particle is allowed to travel through spacetime faster than the speed of light in vacuum, no matter how much energy you put into it!
(Unless, that is, you count what happens in video games.)
Here’s a brief refresher for you as to what’s been going on. CERN, the home of the world’s most powerful particle accelerator, has (as sort of a side-project) been producing a beamline of high-energy protons, and firing them in a very specific direction.
What happens to these protons? They smash them into a “fixed target,” and the protons are moving at such high energies that they can produce all sorts of extremely high energy, unstable particles!
More specifically, the vast majority of these particles wind up moving at somewhere around 99.999% the speed of light, with a good number of them decaying into muons and — to be specific — muon neutrinos.
These neutrinos then travel through the Earth for about 732 km, before winding up at the OPERA detector beneath the mountain of Gran Sasso in Italy. The very interesting thing, of course, is that the OPERA team found that the neutrinos arrived about 60 nanoseconds too early!
What basically happened is they created a series of large pulses of muons and neutrinos over a time period of a couple of years. They calculated, based on their measurement of the distance between the source and the detector and their understanding of their electronics, just how long it should have taken the neutrinos to arrive at the detector.
What they determine is that, if you take the light-travel-time between the source and the detector, then factor in a 986 nanosecond delay for all of the known factors that can affect the arrival time (e.g., the electronics, GPS timing, effects of general relativity), you should get a perfect match of the detected neutrino signal with the source signal.
But they didn’t! They found that you need a 1043 nanosecond delay, for a surprising result that the neutrinos arrive around 60 nanoseconds early. If this is true, it means that the neutrinos traveled just a tiny bit faster than what we think of as the speed limit of the Universe: the speed-of-light in a vacuum. How much faster? Just 0.002% faster than light.
Like many others, I immediately speculated that this team was fooling themselves with some sort of error, spoke publicly about it, and kept reporting on the latest tests and developments. (Even my car got in on the action.)
After all, their delay was based on the data shown in the above graphs. Is it really compelling that you couldn’t shift that graph by 50 or 60 nanoseconds — just one of those bins — and still get a very good fit? Hardly.
So, they did a very clever re-test, which would basically test whether my ideas numbered 2 or 3 were the culprits. The results are in a new release of their paper (pdf), which is hard to find (the arxiv show only the older version), so the link to download it is a copy that I directly uploaded to this site. Other very interesting takes and writeups are available from Chad Orzel, Matt Strassler, Tommaso Dorigo (who has some objections, all of which are very weak) and Sascha Vongehr, as well as even making mainstream media news! Here’s what they did.
(All subsequent images taken from this new version.)
Instead of creating one long pulse, and then reconstructing the shape of that pulse at the end, they instead sent a series of very short, three nanosecond pulses of high-energy protons instead. Moreover, they spaced these pulses out by some large amount of time — over 500 nanoseconds — that’s much longer than the uncertainty in the arrival time. This way, when you observe a neutrino in the detector, you can know definitively which pulse of protons it came from!
Again, these neutrinos were expected to arrive in the detector at the light travel time, offset by 986 nanoseconds. That corresponds to the “zero” mark on the graph below. Where did they show up?
Even with this new procedure, with these extremely short pulses, the neutrinos still all come in earlier than expected. So there’s no detection bias, which means that their earlier results are positively confirmed.
We can also take a look at their statistical error, which is a bit unusual.
Normally, you’d see a bell-shaped curve in the distribution of the arriving neutrinos. What we instead see is flat. OPERA, however, explains that there’s an observable jitter of ±25 nanoseconds due to “the tagging of the external GPS signal by the OPERA master clock.”
This was glossed over in the original version of their paper, but knowing this now, it totally explains why they put their data into 50 nanosecond bins! Now, it’s worth pointing out that a 50 nanosecond shift in the right direction would all but nullify this result.
And that’s the last major skeptical argument for what’s going on here. It is conceivable that they’ve got a systematic error in their expected delay calculations, which may be due to something like the atomic clocks, the measurement of the baseline distance, an electronics triggering mishap, or some other mundane reason akin to these.
But if there isn’t anything like that, then neutrinos really are traveling faster than we expect them to!
Now, let me ask you this: if you’re a good scientist, what’s the next thing you’d demand?
You’d independently check the results by performing an even more precise test of this exact effect, and try to reproduce it! That’s exactly what MINOS is going to do over the next two years, and that’s going to be a very interesting result!
Because if they disagree, we’ll be able to say that one team likely made a mistake. But it’s highly unlikely that both teams will make exactly the same unexpected, unaccounted-for mistake, and so MINOS may very well confirm that these neutrinos are, in fact, moving faster than they ought to!
In which case, we’re going to have a brilliant, legitimate theoretical conundrum on our hands, trying to reconcile why neutrinos from different sources (like supernovae, particle accelerators, nuclear reactors, etc.) appear to travel at significantly different speeds from one another! Some people are speculating already; I’m going to exercise a little patience for now.
But this is a very interesting time to follow what’s going on in the world of neutrinos, and the next round of experiments will either confirm OPERA’s bold findings, or they will wind up with a fair amount of egg on their faces…