The Trouble with Neutrinos
The science and even the popular press are filled with excitement at the moment after the OPERA experiment at Europe’s giant particle physics laboratory, CERN (to which I applied for a summer job when I was 16, but that’s another story). Apparently, neutrinos sent from CERN and captured at Italy’s INFN Gran Sasso Laboratory about 730 km away are arriving faster than scientists thought physically possible – faster than the speed of light travelling in a vacuum.
I had to write about this because the news reporting has really annoyed me. Every announcement has said that Einstein might be wrong because he (special relativity) says nothing can travel faster than light in a vacuum. Poppycock! (As I’m being polite.) What the theory says is that nothing that has what scientists call “rest mass” can travel at the speed of light – there isn’t any block on things travelling faster. It’s always slightly surprised me that in a discipline where mathematical physicists are used to things called discontinuous functions, I rarely hear of people willing to accept that something could go from “slower” to “faster” without having to “equal”, but it might be possible.
One argument against travelling faster than light is that, although there are solutions to Einstein’s equations, they contain the square root of minus one which we sometimes call an “imaginary” number (as opposed to other numbers that are called “real”). This is a brilliant example of mathematical spin and how it has actually damaged our understanding of mathematics and the universe. There is nothing less real about these imaginary numbers than what are called the real ones. It’s actually by combining both set that we achieve a far deeper understanding of the mathematical and physical universe. But way back when they were first introduced, French mathematician and philosopher Rene Decartes was very distrustful of them so coined the term imaginary as a pejorative description, hoping it would mean they didn’t catch on. He’s got a lot to answer for.
What is a neutrino? Like the similarly named neutron, a neutrino carries no net electric charge (compared with other familiar subatomic particles such as electrons (-1) or protons (+1). Unlike the neutron, the neutrino has almost (but not quite) no mass. Having no charge and almost no mass makes a neutrino extremely difficult to detect.
Back to relativity! Anything travelling faster than light in relativity yields solutions including the square root of minus one which people have interpreted as meaning travelling backwards in time. That’s the reason for the joke that’s currently doing the rounds on the twittersphere:
Barman: “I’m sorry, sir. We don’t serve neutrinos in here.”
A neutrino walks into a bar.
The idea of time travel in physics isn’t as unusual as nonscientists might think. In fact travelling into the future is completely straightforward and not disputed. Even if the time comes when relativity is superseded by a better theory, it will have to allow for the possibility of time travel into the distant future as we know full well how to do this (you just move very quickly) and have demonstrated it experimentally. However, as I mentioned in a recent article on the Johnny Mackintosh website, we can consider an antimatter particle to be the same as a particle of normal matter but travelling backwards in time – that’s how a notation called Feynman Diagrams actually work.
One of the reasons relativity came about was due to the unexpected results of the Michelson-Morley experiment back in 1887 which showed the speed of light didn’t vary, even if you changed the way you yourself were moving. Could it be that people will look back on these OPERA results in a similar fashion? Although we can’t yet rule it out, I doubt it. Nowadays we realize that there is an amount of uncertainty in every observation and experiment – statisticians possibly occupy the most pivotal role in the interpretation of results. And in an experiment such as this which is so incredibly complex, there are a lot of uncertainties that have to be quantified. It only takes the most fractional error somewhere for these results to be off by enough to bring the results back into line.
For instance, CERN can’t make the neutrinos in short enough bursts that there’s a gap between them leaving Switzerland and arriving in Italy (so they use statistical methods to infer which are which). Then the neutrinos don’t actually travel round the Earth – they go through it – so we can’t directly measure the distance they travel. We use light to measure distances accurately around Earth by bouncing it off satellites, but the speed of light is only constant in a vacuum – it is slowed down in different media and by different phenomena so it becomes hard to measure this distance as precisely as we want. And how can we know exactly how far away the satellite is. In fact I’ve just published a book on a technique called Nonlinear Filtering that tries to answer these sorts of questions and will be a technique the CERN scientists have used. And then, probably far more important, there are delays in electrical circuitry and clock speeds.
But it’s brilliant that CERN has released the conundrum – it shows a problem in action and is especially brave because everyone thinks the results are almost certainly wrong. I applaud them for asking the world to scrutinize this when they couldn’t find the error themselves and, whatever the final outcome, new things will be learnt as a result. And it’s always a great day when theoretical physics makes it into the news bulletins.