“Where there is no patrol car, there is no speed limit.” –Peter Beckmann
The way they did it was pretty straightforward. Shoot a pulse of protons moving at ultra-high energies — at speeds indistinguishable from the speed of light — into a fixed target.
They’ll produce all sorts of high energy particles: baryons, mesons, electrons, positrons, muons, and more. But don’t just aim them at a detector; shoot them into the Earth itself.
All the baryons and mesons, all the matter and antimatter, will be blocked by the intervening ground. Everything, that is, except the neutrinos.
Because they interact so very rarely, the neutrinos will pass — virtually untouched — through the hundreds of miles of Earth, towards your underground neutrino detector.
And once arriving there, you can detect some tiny fraction of them: maybe just 1-in-1016 or so.
Good thing for you that you made so many of them.
Over the past two years, OPERA made on the order of 1020 neutrinos, detecting about 16,000 of them. The question, of course, is were they speeding or not?
They initially emitted pulses of protons that were about 10,500 nanoseconds in length, and tried to match that up with the neutrinos they detected. Based on the distance from the source to the target, it should have taken a certain amount of time.
And based on their understanding of their electronics, there should have been an offset of 988 nanoseconds.
But what they found was that, on the whole, the neutrinos had an offset of 1048 nanoseconds, which means they took 60 nanoseconds less time than they were supposed to! And this was in an experiment with a claimed uncertainty of just ten nanoseconds, so this result is well outside of their reported experimental error.
So what happened? As I went through in great detail, there were four main possibilities.
- There was a systematic error in their measurements, and their measurements are simply systematically off by 60 nanoseconds (or thereabouts).
- The errors are much larger than they claim, and they’re not actually measuring the arrival time of these neutrinos to their claimed accuracy.
- There’s a bias in the detection of their neutrinos, and the pulse shape of the arriving neutrinos doesn’t match the pulse shape of the things that created them. Or…
- They really did break the speed of light, and the laws of physics don’t work the way we think they do, and in your face, Einstein!
Of course, if you really want to believe in that fourth possibility, you had better go and make sure that you didn’t make a mistake, and rule out those first three possibilities.
The first one’s going to be hard to rule out, and will likely take either the OPERA team finding their own error (or releasing the full details of their methods to the world), or another experiment replicating theirs and confirming it.
How so? Let’s recap one of the things that can go wrong when you design an experiment the way OPERA did, and then we’ll tell you how they’re going to fix it!
When you start with an initial set of particles that are spread out over some large amount of time (or distance), you should still know where they’ll all be at some later time.
After all, they’re moving at speeds indistinguishable from the speed of light! If they move at the speed of light, then they should arrive at their destination — a known distance away — at a very specific time!
The problem, however, arises from the fact that you’re not detecting all of the particles at the end.
In fact, you’re only detecting a tiny fraction of these particles. And if the ones you detect are — in any sort of way — biased, whether towards higher energies, earlier times, or any other fathomable way, you won’t observe an average speed-of-flight equal to the actual speed of flight.
And all of this happened because of the big spread of your initial pulse.
So what OPERA is going to do over the next few weeks — and it should only take a few weeks to observe the dozen-or-so neutrinos necessary to test this — is to change the initial pulse shape. Rather than the 10,500 nanosecond-wide pulse, they’re going to make the pulse only 1-to-2 nanoseconds wide!
You need, of course, a great number of these pulses in order to get the 1016 initial protons necessary to make one neutrino show up in your detector.
But that’s okay; they’re sending these 1-2 ns wide pulses every 500 ns!
This “space-between-pulses” is sufficiently large that, when you do detect a neutrino in your detector, you know which pulse it came from!
And because you know the initial time of the pulse so precisely well, you know — equally as precisely — when that pulse ought to arrive in your detector!
And now, there are really only three possibilities for what they’ll see. The first one is that OPERA may find that they’ve underestimated their error in their experiment, and will see that the neutrino-arrival-time is not as precise as they had claimed.
They will certainly have a spread in the arrival times of their neutrinos. If the spread is more than about 8-to-10 nanoseconds, then their uncertainty is much greater than the initial claim was. If the spread is on the order of 30 nanoseconds or more, the initial claim is totally invalidated, and the actual errors are much greater than was previously reported.
This is a very good check, though, because if the spread of these neutrinos comes in to be a small value, they can rule out option #2 — large arrival-time uncertainty — as the culprit.
Once they’ve nailed down a narrow spread in time-of-flight for these neutrinos, they may also find that they arrived when they were supposed to. (Or, if not, that they were early by much less than 60 nanoseconds.)
This is another great check, because if the neutrinos do arrive when you expect them tom or if they do anything other than arrive 60 ns early, you know that there was a bias in your detection of the earlier (10,500 nanosecond) pulse.
But there is one more possibility.
OPERA could see — in the detector — that a narrow, low-uncertainty pulse arrives 60 nanoseconds early. This is what they’d like to see, and that would not only rule out both options #2 and #3, above, it would demonstrate that their earlier analysis was very good!
If this happens, the only possibilities left are that you have a systematic uncertainty in your timing measurement, or these neutrinos are breaking the speed of light.
The team has stated that they’re going to perform this test before all of the members will sign their names to the paper, and submit it for publication. It’s a great idea, and we should await the results — in just the next few weeks — with bated breath.
But you’ve got to wonder: why didn’t they do this test before releasing this result to the world?