One of the strange but crucial features of our world is that every type of atom except hydrogen contains neutrons in its nucleus, even though neutrons, on their own, decay (to a proton, electron and anti-neutrino) within about 15 minutes on average. At first glance this seems puzzling. At second glance too. How can stable matter be made from unstable ingredients?
The reason this is possible has everything to do with Einstein’s special relativity, and the way mass and energy are intertwined there. A crucial role is played by the energy that is most important for binding things together, which I’ve called “interaction energy”.
I’ve now written an article explaining why neutrons inside of nuclei can be stable, giving the example of the deuteron (one proton bound to one neutron) which is the nucleus of “heavy hydrogen”, or “deuterium”. If you understand this example, you’ll basically understand the point for other nuclei as well.
[For those of you in the New York City area: I'll be joined by the wonderfully talented singer-songwriter-pianist Andrea Wittgens in giving a physics/music joint performance/presentation at the storied Cornelia Street Cafe, Sunday May 13th at 6 p.m., as part of their Entertaining Science series. It's entitled Rhapsody for Piano and Universe, and intended for the general public. The place is pretty small, so get reservations in advance.]