In Part One of this article I told the story of the remarkable results from an experiment which seemed to show that neutrinos were traveling faster than light. I closed by asking the question “Can neutrinos travel faster than light?”
The answer is, in short, no.
At the time of reporting the results the head of the OPERA experiment, Professor Antonio Ereditato, was reported to have said:
Speaking at the time, Professor Ereditato added “words of caution” because of the “potentially great impact on physics” of the result.
“We tried to find all possible explanations for this,” he said.
“We wanted to find a mistake – trivial mistakes, more complicated mistakes, or nasty effects – and we didn’t.
“When you don’t find anything, then you say ‘well, now I’m forced to go out and ask the community to scrutinize this’.”
The scientific community responded to the announcement by suggesting contrary evidence as well as many potential sources of experimental error.
The strongest contrary evidence was rather significant – that of observations of SuperNova 1987A – as explained here by Dr Phil Plait, an astronomer, on his blog called “Bad Astronomy”:
There’s another point that actually is quite important here. If neutrinos travel faster than light, then we should’ve detected the neutrinos from Supernova 1987A before we saw the explosion itself. That exploding star was formed when the core of a massive star collapsed, detonating the outer layers. The collapsing core blasted out a furious wave of neutrinos strong enough to be seen here on Earth, over 160,000 light years away.
The distance from the detector in Italy to the source in Geneva is about 730 km. The travel time at the speed of light is about 2.43 milliseconds, and the neutrinos appear to have outraced that speed by 60 nanoseconds. If true, that means they were traveling just a scosh faster than light, by about 1 part in 40,000. The neutrinos from SN1987A traveled so far that had they been moving that much faster than light, they would’ve arrived here almost four years before the light did. However, we saw the light from the supernova at roughly the same time as the neutrinos (actually the light did get here later, but it takes a little while for the explosion to eat its way out of the star’s core to its surface, and that delay completely accounts for the lag seen).
Further checking by the OPERA researchers actually found (amongst other things) a significant source of error relating to communications in the GPS timing equipment which could account for the 60 nanosecond difference.
However the conclusive evidence that the original results were wrong came in March, as described in Nature magazine:
Now another experiment located just a few meters from OPERA has clocked neutrinos traveling at roughly the speed of light, and no faster. Known as ICARUS, the rival monitored a beam of neutrinos sent from CERN in late October and early November of last year. The neutrinos were packed into pulses just 3 nanoseconds long. That meant that the timing could be measured far more accurately than the original OPERA measurement, which used 10-microsecond pulses.
So that’s it folks. Here you’ve see the scientific process laid bare to the public as it rarely is. This is a rare thing only because its rare for the public to take an interest in these processes. Such rigor and review are the daily stuff of scientific practice. As a rule, the scientific community values objective truth above all else and if they can find fault with research (their own or, even better, someone else) then they are happy to do so.