New paper claims the Nobel committee got its neutrino physics wrong
A theoretical physicist has published a paper claiming that the Nobel committee got the physics wrong when it comes to the 2015 Nobel Prize in Physics.
In case you need a refresher, the covetable physics prize was awarded in 2015 “for the discovery of neutrino oscillations which show that neutrinos have mass”.
To be clear, the new paper isn’t arguing that the 2015 prize winners didn’t deserve the prize, or that their research was wrong – but that how their breakthrough has been interpreted by the Nobel committee is incorrect.
The prize-winning research was on neutrinos – weird, ghost-like particles that are born out of nuclear interactions, like the ones happening at the core of our Sun, or as cosmic rays smash into Earth’s atmosphere.
They’re described as “ghost-like”, because they hardly interact with matter. In fact, trillions of neutrinos are flowing through you right now and you have no way of detecting them.
The only way scientists can detect the presence of neutrinos is through their interactions with the weak subatomic force and gravity, using underground detectors such as the Super-Kamiokande (SuperK) particle detector in Japan, or the Sudbury Neutrino Observatory (SNO) in Canada.
To make neutrinos even more fascinating, they come in three different ‘flavours’ – electron, muon, and tau. And they can also switch between these three flavours, so an electron neutrino can change to a tau neutrino and back again. This is what’s known as oscillating.
The Super-K detector, which only detects high-energy muon neutrinos generated by cosmic rays striking our atmosphere, reported in 1998 that there were more of those muon neutrinos coming up through Earth than raining down from the atmosphere on their side of the planet.
This suggested that the neutrinos were oscillating along the way to become electron or tau neutrinos that the Super-K couldn’t detect.
The SNO team followed up these observations in 2001 and 2002 by looking at the lower-energy electron neutrinos flowing from the Sun.
One of their techniques could only detect electron neutrinos, but another could detect all three flavours. Their results showed that by the time the electron neutrinos reached Earth, only 34 percent of them were still electron neutrinos, suggesting that they were also shifting along the way.
Together, the Nobel committee took these two findings as proof that neutrinos can oscillate along their travels, and therefore that they have mass.
If neutrinos didn’t have mass – something that had long been suspected – then they would move at the speed of light in a vacuum, and if that was the case, time would stand still for them, so they wouldn’t be changing at all.
So far so good. And no one is questioning that the discovery proved that neutrinos have mass.
What a new paper by physicist Alexei Smirnov from the Max Planck Institute for Nuclear Physics in Germany is calling out is the Nobel committee’s use of the word “oscillation” in their description of the work.
He thinks that the Japanese team’s Super-K experiment successfully showed oscillations in action, but that the SNO results show something else happening with the neutrinos from the Sun – a more subtle type of shape-shifting.
“While Super-Kamiokande, indeed, has discovered oscillations, SNO observed effect of the adiabatic (almost non-oscillatory) flavour conversion of neutrinos in the matter of the Sun,” Smirnov writes in his paper over on arXiv.org.
“Oscillations are irrelevant for solar neutrinos apart from small [neutrino] regeneration inside Earth.”
If you don’t speak physicist, that basically means Smirnov thinks the neutrinos from the Sun do change their type, but not through oscillation. And so the Nobel committee got it wrong.
“No question the experiment deserves to be awarded a Nobel Prize,” Smirnov told Adrian Cho at Science magazine. “It’s just a question of what they actually saw.”
To be clear, Smirnov’s paper hasn’t been peer-reviewed as yet. His 18-page critique, “Solar neutrinos: Oscillations or No-oscillations?” has been published on arXiv.org for other physicists in the field to scrutinise and discuss.
So for now, this is just one person’s opinion. But it’s already triggering controversy.
“Certainly, he is right that the citation is essentially wrong,” Giorgio Gratta, a neutrino physicist at Stanford University, who wasn’t involved in the study, told Cho.
But Olga Botner, another independent neutrino physicist from Uppsala University in Sweden, and a member of the Nobel committee, disagreed. “[T]he citation for the Nobel Prize is by necessity short and cannot reflect all details of the discoveries being recognised,” she told Cho.
Smirnov’s argument centres around the different ways in which the Super-K and SNO neutrinos appear to change flavour. The Super-K high-energy neutrinos oscillate, or flick, flavour over time.
But the SNO low-energy solar neutrinos seem to change flavour gradually due to their interaction with mass, Smirnov argues.
His hypothesis is that, inside the Sun, the environment is rich with electrons, and that environment makes neutrinos become electron neutrinos.
But as they travel out of the Sun, the flavour of these neutrinos change as the amount of electrons they comes into contact with decrease. Smirnov says this isn’t a true back-and-forth oscillation, but rather a shift in response to electron density, which he calls “adiabatic”.
This is also how the SNO team actually described their results, as an “adiabatic flavour conversion”, rather than an oscillation. But the Nobel team referred to them as “observing neutrino oscillations”, and Smirnov says that’s wrong, by his definition at least.
Again, no one is arguing that the team didn’t deserve the Nobel Prize, or that there was any problem with the research. The issue in question is how we describe what neutrino oscillations are.
For the general public, that doesn’t affect things very much. We now know that neutrinos have mass, and we know they change flavour.
But for those in the physics world who are still struggling to define exactly how neutrinos work and how they behave, figuring out exactly when the subatomic particles are oscillating and when they aren’t is a big deal.
There’s no clear answer to this one, and again, this is just one physicist’s opinion. But part of the scientific method is having other researchers probe and disagree with your work (or other people’s interpretations of it), so that in the future our understanding of the world can be more accurate.
So bring on the lengthy debates and discussions about this Nobel citation, because at the end of it, we’re bound to get better science as a result, and that’s what we’re here for.
You can read Smirnov’s full paper over at arXiv.org