“If you could send a light beam through the Earth, it should arrive at the same instant as the neutrino – if the neutrino travels at light speed – or slightly before it (if the neutrino travels slower than light) but not later, as that would require the neutrino to travel faster than light. If we could do that experiment, it would be clear cut. The problem is, we cannot. The Earth is transparent to neutrinos, but opaque to light.
If we know the distance from Cern to Rome precisely enough, and the time that the neutrino took to get there, then the ratio of distance to time – kilometres per second – gives the speed. In effect this is what the experiment does, but even this is not straightforward.
Measuring the time to accuracies of nanoseconds involves accounting for the time that electronic signals take to pass through circuits, into readouts and onwards to further parts of the complex of counters, computer chips and the myriad pathways of the nanoworld. If you have all of these measured, and if they are indeed everything you need to know, then you can determine the time elapsed – with some uncertainty. This they have done. However, if there is some unexpected bottleneck, unrecognised and hence unaccounted for, the timing might be a few nanoseconds amiss.
Then there is the measurement of the distance. Determining this to an accuracy of about 10 centimetres in 730km is required – and, apparently, is possible by geodesy. But precisely how this is done is, to me at least, still one of the many mysteries in this experiment. You certainly don’t do it with a tape measure, even if you had one that was accurate to atomic sizes. Sending a radio signal up to a satellite, at the instant the neutrino leaves Cern, which then passes it on down to the receiver in Rome, and comparing which arrives first, and by how much, has its own difficulties. The speed of radio waves through the atmosphere is affected by magnetic fields, and by other phenomena; it is far from simply a radio beam passing through a vacuum at “the speed of light”.
I would bet that a subtle error in the measured distance or time is more likely than that their ratio – the inferred speed – exceeds Einstein’s speed limit.
Ultimately nature knows the answers and we have to find them by experiment. If it is possible to travel faster than light – in a vacuum – then it doesn’t matter how many physicists say nay: the truth will out. And if it is true? I shall rewrite Neutrino and replace email with numail (neutrino-mail) – it’s faster.”