The Opera experiment last September made headlines around the world with their announcement that neutrinos sent from the CERN laboratories to the Gran Sasso cavern appeared to be moving at superluminal speed.
The claim was fortified two months later, when results of a custom-made narrow-bunched beam of neutrinos sent for a week to Opera in October proved that the result of the timing measurement was not an artifact of the ill-understood bunch structure of the beam which had provided Opera with about 15,000 neutrino interactions in the past two years -the statistics on which the original claim had been based.
The Opera analysis apparently showed that neutrinos were arriving to the Gran Sasso cavern after a 720km trip through the earth's crust a full 60 nanosecond earlier than if they were traveling at light speed in vacuum. This fostered all kinds of speculations from theorists, but also a lot of scepticism. Many questioned several details of the measurement, pointing their finger to unaccounted-for systematic uncertainties. The measurement is of course a very hard one to perform with the required accuracy, despite being a "simple" measurement of a time delay, resulting from a distance determination and two timing measurements.
Other experiments had measured the speed of neutrinos in the past, always finding results compatible with the predicted motion at light speed. In particular, the detection of a few neutrinos after the burst of the Supernova 1987A had shown that electron neutrinos had moved at exactly light speed to reach the earth after a very long trip; however the Opera neutrinos are of the muon kind, and they have a much larger energy, so they could still be thought to behave differently.
If neutrinos were superluminal particles, some theorists argued they should radiate energy in the form of electroweak interactions. A measurement of the energy spectrum of the neutrinos detected by the ICARUS detector - another facility invested by CERN neutrinos at the Gran Sasso cavern - had shown last fall that there was no such energy loss. If neutrinos were indeed superluminal, they therefore needed to be supposed to produce further exceptions to the rules of quantum physics, besides breaking the rules of Einstein's theory of relativity.
Then, a month ago Opera released information about two sources of systematic uncertainties that had previously been unaccounted for. These could or could not invalidate the measurement, and more studies were therefore needed to ascertain their effect. However, by that time the scientific community had already largely understood that the Opera claim of superluminal neutrinos was hardly believable. Indeed, the simultaneous measurement of cosmic ray muon signals in Opera and a nearby detector had shown that the two experiments saw a difference of about 60 nanoseconds in their timing measurements: the analysis of the additional systematic uncertainties was in a way redundant at that point.
Lastly, the ICARUS collaboration performed a independent, full-fledged measurement of the neutrino speed using the same methodology as the one of the original Opera measurement. They found a result perfectly compatible with neutrinos moving at light speed, and utterly incompatible with Opera's 60 ns effect.
Two days ago a workshop was held at the Gran Sasso laboratories, where the various experiments reported their findings and discussed them. I have no report from the workshop, but it is clear that the superluminal signal of Opera is as dead as it can be. Following the workshop, the Opera collaboration is reported to have voted on removing Ereditato from the leadership position. The motion did not pass, but the voting showed that the collaboration was split, and this must eventually have led Ereditato to step down today.
Physics is a hard science, and it progresses by trial and error. I see no particular exception in the case of Opera neutrinos. However, the affair does force us to question whether it was reasonable for the Opera management to come up with such a steep claim, which was bound to make headlines around the world. They did have a significant effect, but the claim was of such extraordinary size that additional caution might have been a good alternative. However, I dissent from that reading of the matter.
I believe we are focusing on the wrong issue if, horrified with the distrust that the general public must be feeling after having seen a steep claim made and retracted in the matter of a few months, we conclude that scientific results should be produced, digested, and sat on for enough time to make us fully confident they are "exact" before being released to the public. The issue we should address is not how to avoid distrust from the public, in my opinion. The issue is how we educate the public!
The release of the preliminary Opera results last September was made after the news had leaked from the experiment to a few scientists not belonging to the collaboration. This is natural and unavoidable -I could cite a gazillion examples and explain to you why it is not possible, in the era of cellphones, email, and twitter, to keep scientific results private for long. Should we try to avoid the unavoidable ? No reason. Let us instead try to educate the public on the fact that what happened to Opera's superluminal neutrino claim is good science: we study an effect, find something unexpected, and then try to kill the effect with all our means by studying it in more detail and with all the other tools we have available. What survives this kind of treatment is usually only real, trustable effects.
- Opera's Final Word On Opera's Superluminal Neutrinos
- ICARUS Refutes Opera's Superluminal Neutrinos
- A Six-Sigma Signal Of Superluminal Neutrinos From Opera!
- ICARUS: Neutrinos Travel At Light Speed. Period.
- OPERA Confirms Faster Than Light Neutrinos And Indicates Ultra Superluminal Small Initial Jumps