Inside a giant spectrometer in Karlsruhe, shaped like a huge metal barrel, scientists are catching particles that are nearly impossible to catch. Neutrinos are the ghosts of the microcosm: every second, trillions of these particles fly through your body, and you don't even notice. But they do have mass, and scientists want to measure it.
To do that, KATRIN uses radioactive tritium gas. When a tritium atom decays, it emits an electron and a neutrino. The electron's energy depends on how much energy the neutrino carried away, which in turn depends on the neutrino's mass. By measuring the electrons with the highest energy, you can work out the neutrino's mass. It's like weighing an invisible person by seeing how much lighter their visible half becomes.
The experiment is now close to its goal. Next, they plan to search for sterile neutrinos — hypothetical particles that interact with matter even more weakly. Then, they'll boost sensitivity using quantum technologies and atomic tritium. Maybe then we'll finally pin down the exact mass of these elusive particles and better understand how the Universe is built.
Every second, about 60 billion neutrinos from the Sun pass through your thumbnail.