A Bet Concerning Neutrinos (Part 3)

As you’ve probably heard, an experiment called OPERA measured how fast neutrinos go from a particle accelerator in Switzerland to a detector in Italy. They got a speed slightly faster than light. This got a lot of people excited.

As a conservative old fart, I made a bet with Frederik De Roo saying that no, neutrinos do not go faster than light.

Since then, various reports have been zipping across the internet at near light-speed, claiming that neutrinos don’t go faster than light. But I think they’re a bit premature. Much as I’d like to win, I don’t think I’ve won just yet.

For example, last week someone who works on artificial intelligence at a university in the Netherlands said that the OPERA team made a mistake in their use of special relativity—a mistake that explains away their result:

• Ronald A.J. van Elburg Times of flight between a source and a detector observed from a GPS satellite, 12 October 2011.

Two days later, a pseudonymous blogger who works for MIT’s Technology Review said the argument was “convincing”:

• KentuckyFC, Faster-than-light neutrino puzzle claimed solved by special relativity, The Physics arXiv Blog, 14 October 2011.

The popular news media got all excited! But they may have been getting ahead of themselves. After all, the OPERA team includes a bunch of particle physicists. Special relativity is child’s play for them. Would they really screw up that bad, after years of checking and rechecking their work? Chad Orzel suggests not:

• Chad Orzel, Experimentalists aren’t idiots: The neutrino saga continues, Uncertain Principles, 16 October 2011.

And none of the physicists I know find Elburg’s argument very convincing.

But that’s not all! A couple weeks earlier, Cohen and Glashow did a calculation:

• Andrew G. Cohen, Sheldon L. Glashow, New constraints on neutrino velocities, 29 September 2011.

According to this, faster-than-light neutrinos would lose energy by emitting lots of electron-positron pairs, a bit like how a supersonic jet makes a sonic boom. Two days ago, another team of physicists doing experiments on neutrinos at Gran Sasso claimed that together with their experiment, this result refutes the existence of faster-than-light neutrinos:

• ICARUS team, A search for the analogue to Cherenkov radiation by high energy neutrinos at superluminal speeds in ICARUS, 17 October 2011.

At least one good physics blogger has taken this work as “definitive”:

• Tomasso Dorigo, ICARUS refutes Opera’s superluminal neutrinos, A Quantum Diaries Survivor, 18 October 2011.

He says:

The saga of the superluminal neutrinos took a dramatic turn today, with the publication of a very simple yet definitive study by ICARUS…

And so, the news media are getting excited again, saying that now the OPERA result is really dead, like a vampire with two stakes through its heart.

But how “definitive” is this result, really? I’m a bit disappointed that the Cohen–Glashow paper doesn’t clearly state the assumptions that go into their argument. They zip through the calculation in an offhand way that suggests they’re using standard principles of physics to their heart’s content—in particular, special relativity. Normally that’s fine. But not here. After all, if faster-than-light neutrinos were signalling a breakdown of any of these principles, their calculation might be invalid.

Of course I don’t believe neutrinos are going faster than light: that’s why I made that bet! If you don’t want to believe it either, that’s fine. But if you want to entertain this possibility, in order to disprove it, you’d better be clear on the logic you’re using.

Without actually measuring the speed of neutrinos, the best you can hope for is something like this: “If theoretical principles X and Y and Z are true, then our experiment shows neutrinos don’t go faster than light.” So neutrinos could still go faster than light… but only if X or Y or Z is false.

Maybe X, Y and Z are principles we hold sacred—maybe even more sacred than the principle that nothing goes faster than light! But shocking discoveries can have shocking consequences. Sacred truths can fall like dominoes.

Given this, I think the only truly definitive way to hammer the nail in the coffin of the OPERA experiment is to either

1) find a mistake in the experiment that convincingly explains its result

or

2) do more measurements of the speed of neutrinos.

And maybe Dorigo acknowledges this, in a way. He says:

So, forget superluminal neutrinos. Or maybe not: what remains to be seen is whether other experiments will find results consistent with v=c or not. That’s right: regardless of the tight ICARUS bound, every nerd with a neutrino detector in his or her garage is already set up to produce an independent confirmation of the startling OPERA result… We’ll soon see measurements by MINOS and Borexino, for instance. Interesting times to be a neutrino expert are these!

(Emphasis mine.)

So, I’m going to wait and see what happens. I want to win my bet fair and square.