So the name of the game with KATRIN is to measure the energy of electrons coming out of the tritium decays using its giant spectrometer. (Remember, according to Einstein's famous equation E=mc^2, energy is equal to mass multiplied by a constant number, the speed of light squared.) And that rest-mass energy will never, ever be available to the electron. But if the neutrino does have mass, then it will always have its rest-mass energy, meaning the energy stored inside a neutrino at rest due to its mass. If the neutrino is massless, then there's no lower limit to the energy it can have, just like there is no lower limit to the energy a photon can have. Whatever is leftover must go to the electron. Sometimes the reaction will give more energy to the neutrino, and sometimes less. Because tritium is such a light atom, this is one of the lowest energies possible for neutrinos to have, which makes the measurements of the tiny neutrino mass as easy as possible. The amount of energy released by the reaction is set by the nuclear energy of the tritium atom, and so the electron and neutrino must share a combined total of 18.6 keV of energy between them.

The result? The transformation results in the emission of an electron and an electron antineutrino, the antiparticle partner of the electron-neutrino. It naturally breaks down through a process called beta decay, in which one of the neutrons inside the nucleus spontaneously transforms into a proton (through an interaction involving the weak nuclear force).

Tritium is a rare, radioactive isotope of hydrogen containing one proton and two neutrons. The device features an absurdly large amount of tritium and a gigantic, 200-ton (180 metric tons) spectrometer, which measures the energy of electrons. In Germany, the Karlsruhe Institute of Technology's KATRIN (Karlsruhe Tritium Neutrino Experiment) device is designed to do exactly that. There are lots of theoretical models out there, but we don't know which is correct. Nailing down the mass of any of the neutrino species would be a big help in particle physics, because we don't know how they have mass. They only have limits provided by various experiments on the total combined neutrino mass and some of the differences in masses between different ones. To date, physicists do not know the masses of the three neutrinos. What we see as an electron-neutrino, for example, is a complex combination of three different neutrinos with three different masses. Each mass does not, however, line up with an individual flavor and instead each flavor is composed of a mix of these masses.

If the masses were all zero, neutrinos would travel at the speed of light and wouldn't have a chance to oscillate. That way, the three masses travel at different speeds, and the flavors oscillate depending on the quantum state of the three masses. For the oscillation to work the three masses must be greater than zero, and all different. And it turns out that, like flavors, there are three different neutrino masses. In order to oscillate between flavors, neutrinos need mass. But then in the 1960s, physicists began to notice that these three neutrino species can 'oscillate,' or change from one flavor to another as they travel. This idea of flavors fit fine with a massless neutrino. And that idea of a massless neutrino worked in the field of physics for some time, even after more information was learned about neutrinos, such as the fact that they come in three kinds, or 'flavors,'one for each kind of interaction they can participate in: Electron-neutrinos appear along with electrons muon-neutrinos pair with muons and tau-neutrinos go along with tau particles.

The neutrino is