Neutrinos: faster than the speed of light?
By Frank Close
To readers of Neutrino, rest assured: there is no need yet for a rewrite based on news that neutrinos might travel faster than light. I have already advertised my caution in The Observer, and a month later nothing has changed. If anything, concerns about the result have increased.
The response to my article created some waves. There were a couple of cogent remarks on The Observer’s comments section. Firstly, it was questioned that if neutrinos could indeed travel faster than light, the neutrinos from the supernova in 1987 (which had travelled 187,000 light years across space) should have arrived 4 days before the supernova was seen by eye, contrary to what was observed. One person also pointed out that if neutrinos could really go faster than light, then they could have gone at any speed, up to infinite. So the fact that the experiment “only” found them travelling at 1.00005 times light speed suggested that there was some underestimated error somewhere, and that the true answer would turn out to be 1. I agree; finding out what is the challenge.
I already mentioned some of the problems with the experiment – how it measures the time and the distance involved at huge accuracy, and then takes the ratio to get a speed. I read the paper on how they determine the distance, and the methods involved details of geodesy, which are outside my expertise. I admitted in the article that this was a “mystery to me”. One commenter, under the cloak of anonymity, took this to mean that I had not read the paper, and insisted on repeating this. Comments are subject to the laws of libel no less than printed media; the offending item was removed by the moderator.
This aspect of my personal mystery typifies the problems that the actual experimenters have. The team is primarily a group of high-energy physicists, and computer experts; however, the experiment requires expertise in many other areas. This is what inspired the team to go public in the first place: to advertise their concerns in the hope that other experts might come up with ideas. And there have been several.
A neutrino is detected in Italy, 500 miles from CERN, and the time is recorded. Surprisingly the precise time that that particular neutrino left CERN is harder to know than some first thought. CERN does not provide a beam of neutrinos. Instead it makes pions, which decay, producing neutrinos. The pions decay on the average in a hundred-millionth of a second, but some live longer and others die sooner. Precisely when this happens on a case-by-case basis is a statistical result, which is taken into account in the experiment. And the pions themselves are not primary particles, but are themselves produced by collisions between beams of high energy protons and a target. The protons that start all this emerge from the CERN accelerator in bunches – pulses spread over a short period of time. Precisely when in this time interval a proton does its work, leading to the pions and subsequent neutrinos is a big worry in some people’s opinion. The resulting beam of neutrinos then spreads out, over a mile wide by the time they arrive at the target 500 miles away in Italy, and one has to try and match an arriving neutrino with an initial proton back at CERN. This is all done statistically. If you saw the BBC2 television programme Faster than the speed of light on 19 October in the UK, check out Jon Butterworth’s comments about how tricky some of this experimental analysis is, and his questions about whether everything has yet been fully looked into.
More theoretical perhaps, but from a Nobel Laureate, Sheldon Glashow, comes evidence of an inconsistency in the evidence for super-luminal neutrinos. If neutrinos travelled faster than light, they would radiate electrons and positrons, and lose energy – and speed. Nothing like this has been seen. Of course, one can say that if neutrinos could travel beyond the speed at which current theory allows, all bets are off: the implications that they radiate electrons and positrons could also fail. But one has a sense of Occam’s Razor: inventing one excuse on top of another as one is squeezed into ever smaller corners.
Ultimately though, as I said in The Observer article, it is experiment that decides and it doesn’t matter how many theorists say nay. The truth will out. Science is built upon reproducibility. If and when this result is confirmed by independent experiments, and systematic errors ruled out, then I will be excited; but that time is not yet.
Frank Close is a particle physicist, author and speaker. He is Professor of Physics at the University of Oxford and a Fellow of Exeter College, Oxford.
Close was formerly vice president of the British Association for Advancement of Science, Head of the Theoretical Physics Division at the Rutherford Appleton Laboratory and Head of Communications and Public Education at CERN. He is the author of several books, including Neutrino, Nothing: A Very Short Introduction, Particle Physics: A Very Short Introduction, and Antimatter. His latest book, The Infinity Puzzle, publishes this week.