Neutrinos are hard things to study. These elementary particles aren’t only elusive, but they don’t interact very much with their surroundings, and they are picky about partners. Their existence was first postulated in the 1930s and was only confirmed in 1942, when neutrinos were directly observed for the first time.
Neutrinos are very small and light, with Enrico Fermi naming them the “little neutral one” — neutrino being an Italian diminutive. But while they may be small and light, neutrinos are immensely important for our understanding of the universe.
Apart from the photon, which has no mass, the neutrino is the lightest particle we know of. They typically emerge from a type of radioactive decay called beta decay, which makes them unreactive to most of the forces in the universe. This means neutrinos travel thousands of light-years with nearly zero interaction with the medium whatsoever.
The sun bombards the planet with neutrinos all the time since there is a continuous nuclear reaction in our star, they pass through our bodies without interacting with anything. So then, since it doesn’t really react with almost anything, how do you catch a neutrino?
Catching a neutrino
First of all, you need to be patient and wait for neutrinos to come from the universe to us. Then, you need to have the right equipment — a neutrino observatory. The most famous one is in Japan, the Super-Kamiokande (previously called the Kamioka Observatory). The observatory lies inside a mine in Kamioka, Hida, and probably looks nothing like you imagine.
The observatory consists of a cylindrical stainless steel tank about 40 m (131 ft) in height and diameter holding 50,000 metric tons (55,000 US tons) of ultrapure water. In this tank, a neutrino interacts with the electrons or nuclei of the pure water and produces an electron (or positron) that moves faster than the speed of light in water (which is slower than the speed of light in a vacuum). This in turn creates something called Cherenkov radiation — think of it like a subsonic boom in the particle world. By detecting this radiation, researchers can indirectly detect a neutrino.
In time, the Kamiokande (Kamioka Nucleon Decay Experiment) evolved to Kamiokande-II in 1982 and now the current detector is the Super-Kamiokande — more than 10 times bigger than the original version. Other observatories are also under construction. Most notably, the Deep Underground Neutrino Experiment (DUNE), which is being constructed by Fermilab.
But these observatories, and our current understanding of neutrinos, may not have been the same had it not been for the fortuitous meeting of two physicists.
A gambling process
In the late 1930s, George Gamov was visiting Brazil to give a few astrophysics lectures. The Russian physicist visited Rio de Janeiro where he met the Brazilian physicist Mário Schenberg. The two scientists spent friendly times in a casino in a place called Urca beach, where they conversed (and presumably, gambled a bit).
Because World War II was ongoing, Schenberg couldn’t travel to Europe for collaborations. Lucky for him, Gamov was right there, and they would make the best of their time together. Gamov was interested in the evolution of stars, he was one of the first physicists to speculate that thermonuclear reactions happen in stars.
After some time, an opportunity to work at George Washington University appeared, and Schenberg traveled to the US to work officially with Gamov. Gamov was focusing on supernovae phenomena and asked Schenberg for assistance. The two fixed the gaps in the idea by adding a very fast neutrino emission to the process.
White dwarfs are very dense and hot stars, an environment that’s perfect to release electrons and enable them to react with a stable nucleus (protons and neutrons) — a reaction that forms an unstable nucleus and emits neutrinos. The instability allows the stable nucleus to decay quickly emitting electrons which permits more reactions like these, the process makes the star lose such an amount of energy that it cools the inner parts of the star. This is called the URCA process, in honor of their time in the casino losing as much money as the star loses energy. URCA also means gangster in Odessa (which used to be a Russian region, but it is a Ukrainian city today) slang, where Gamov grew up. So the name was probably chosen by Gamov, and it fits the anecdote very well.
Very massive stars, in which the URCA process takes place, have their interior cooled down compared to the outer shells. Therefore, the inner part cannot endure the weight of the outer parts. The star contracts, and as the material contracts it heats up causing a tremendous explosion with enough luminosity to outshine a galaxy.
When the interior of the white dwarf cools down, its outer parts compress, putting more pressure on the interior, which can eventually collapse the entire star into a supernova. The blast is colossal, energetic enough to outshine the supernova’s own host galaxy.
How do we know that? We have tons of evidence from thousands of supernovae data. During most of these observations, astronomers have recorded everything from the beginning of the event, until it reaches a peak of luminosity, before ultimately decaying. This is done through powerful telescope lenses such as the Magellan Telescopes in Las Campanas Observatory in Chile.
The trouble is to find a supernova event and detect its neutrino emission at the same time. For that we need luck, and we were lucky in the South American summer of 1987. The astronomer working at Las Campanas detected a new bright dot in the Large Magellanic Cloud region. First, he thought something was and went outside to check himself, it was there, a very bright new shiny thing in the sky. No one had seen one since Johannes Kepler, this was a moment to change the history of astrophysics.
The supernova later named SN 1987A was also detected by Kamiokande II. The neutrinos traveled nearly 170,000 light-years and reached the Kamioka Observatory. The funny thing is that they reached Earth around three hours before telescopes detected the light. Out of the estimated billion billion billion billion billion billion (10^58) neutrinos that came out of the supernova, only 12 were registered in Kamiokande-II.
We’ve only started to scratch the surface of neutrinos. We know they exist, we know they matter, but there’s still much we don’t know about them.
For instance, neutrinos come in three flavors: electron neutrino, muon neutrino, and tau neutrino. They also have corresponding antiparticles, collectively called antineutrinos. But we cannot measure the mass of these neutrino flavors as they constantly change from one to the other. The current mass of a neutrino is a combination of the three types of neutrinos. It’s hard to understand why the neutrino’s mass works like this.
Yet even so, with our limited understanding, we may be able to use neutrinos to great effect. In July 2018, the IceCube Neutrino Observatory in Antarctica announced the detection of a high-energy neutrino that hit their research station in September 2017. They managed to trace it back to its point of origin to an active galactic nucleus located 3.7 billion light-years away in the direction of the constellation Orion. This is the first time that a neutrino detector has been used to locate an object in space and that a source of cosmic rays has been identified, and suggests that neutrinos could be used to probe some of the deeper parts of the universe. In fact, some researchers have proposed that neutrinos can be used to probe environments that other radiation (such as light or radio waves) cannot penetrate.
Neutrinos are some of the most abundant particles in the universe, but we’ve only recently started learning about them in the past century. Who knows what we’ll discover next?