How heavy is a neutrino?

The question is more complicated than it seems. Neutrinos are elementary particles first discovered six decades ago. Over the years, scientists have learned several surprising things about them. But they have yet to answer what might sound like a basic question: How much do neutrinos weigh? The answer could be key to understanding the nature of the strange particles and of our universe. To understand why figuring out the mass of neutrinos is such a challenge, first you must understand that there...