Neutrinos are the one elementary particle that give scientists a big headache. Two physicists get a Nobel Prize for solving one more mystery about them: neutrinos change their identity.
They come out of space. Thousands of billions of them pass through your body every second. But don't worry: they aren't dangerous. Neutrinos are tiny elementary particles that hardly ever interact with anything.
That's also how the nickname for the neutrino came to be: they are also known as ghostly particles or even poltergeist.
These monikers reflect the problem that physicists face when dealing with neutrinos: how do you investigate something that is practically invisible, not only to the naked eye but also to most everything else?
Takaaki Kajita of the University of Tokyo in Japan and Arthur B. McDonald of Queen's University in Kingston, Canada, took on the challenge. Now they have been rewarded with the Nobel Prize in Physics.
The laureates made "a fundamental step in unveiling the nature of the neutrino," said Olga Botner, member of the Nobel Prize committee at the announcement in Stockholm.
Previously, three other Nobel prizes in Physics were awarded for discoveries concerning the 'ghostly particle:' in 1988, 1995 and 2002.
Enthusiasm in the research community
Neutrino researchers in Europe are full of joy. "It is great that our field was awarded," Björn Wonsak of the neutrino research physics group at University Hamburg told DW. "It can definitely use some more publicity."
At Karlsruhe Institute of Technology (KIT) "we are all over the moon," said Johannes Blümer of KIT's center for elementary particle and astroparticle physics (KCETA). "Both laureates are well known here at the institute - they are very nice people."
In 2013, KCETA awarded a physics prize, the Julius Wess award, to Takaaki Kajita for his discovery - "it's almost like we anticipated [that he would get the Nobel Prize later]," Blümer told DW with a laugh.
Antonio Ereditato from the University of Bern, Switzerland, worked together with both laureates, and especially close with Takaaki Kajita. "He is a good friend and a sweet person," he said. "I am very happy he got the Nobel Prize, it is a well-deserved recognition."
Takaaki Kajita himself told the Nobel committee that winning the award was "kind of unbelievable" for him, while Arthur McDonald said the first thing he did when he heard the good news was to give his wife a hug.
Neutrinos are so light that scientists always thought they didn't have a mass at all - like photons, which make up light.
After years of research, huge machines in the ground could at least show that neutrinos really existed.
The particles can only be detected underground, because here the radiation that continuously hits Earth from space does not interfere with the measurement. Underground mines are thus perfect for neutrino research.
"I believe that there are some crazy people, including me, of course, who go inside mines to study the stars," Arthur McDonald said at a lecture at the University of California at Berkeley in 2010.
The big mystery
The ghostly particles also form in our sun.
There was still one issue that baffled scientists: when they calculated how many neutrinos should form in the sun and thus reach Earth and compared that with the number of neutrinos that were actually detected - the two didn't match up. Only 30 to 50 percent of the calculated amount was actually detected on Earth.
"There were huge debates between astrophysicists and particle physicists about who got it wrong," Blümer said. "Was the established theory of the processes inside the sun incorrect? Was the energy of the sun different from what we thought?"
Finally, in 1998, Takaaki Kajita found the answer to this puzzle. When he presented the solution at a physics conference for the first time "there was standing ovation" by everyone present, according to Blümer.
Improved detectors to solve the puzzle
Three types of neutrinos exist. But normal neutrino detectors couldn't detect all three kinds at the same time.
Kajita had an idea: maybe neutrinos were able change their identity on their long way from the sun to the detector on Earth. The sun only produces one kind of neutrino. But if they transformed into another kind invisible to the detector during their journey, it would explain the differences that had puzzled physicists for years.
So Kajita developed the Super-Kamiokande detector (pictured atop the article) that could detect two different kinds of neutrinos - and found he was right: neutrinos do indeed change. This process is called oscillation.
Arthur McDonald later experimented with a detector that could detect all three kinds of neutrinos. Now 100 percent of neutrinos that left the sun were found on Earth - mystery solved.
What is this good for?
Arthur McDonald explained the practical use of this discovery to journalists at the Nobel Prize announcement.
"We were able to verify the understanding of the processes in the core of the sun," he said, adding that these processes are similar to the ones experts want to apply when creating a nuclear fusion reactor. "It might help us to understand the process of generating fusion power."
Björn Wonsak of Hamburg University has another example: "Neutrino detection might some day help to monitor nuclear power plants." A country might say, for example, that it's generating nuclear material for civil use only, but it's really building a nuclear bomb. "The energy of the neutrinos that form during both processes are different - and that could be detected."
The fact that neutrinos can change their state means that the three types all have different masses, which in turn means that that their mass cannot be zero - even though it is indeed very small.
That is one of the biggest conclusions from the Nobel laureates' discovery.
Whoever measures the exact mass of the neutrino will probably get another Nobel Prize in Physics - that would be the fifth in the investigation of the 'poltergeist' particle.
So there is still much more to find out - or as Blümer puts it: "Neutrino physics is simply fascinating."