Whether elementary or composite, all known particles can annihilate with their antiparticle counterparts. In some cases, particles are matter and antiparticles are antimatter; in other cases, particles and antiparticles are neither matter nor antimatter, and sometimes particles are their own antiparticle. We do not yet know if the Standard Model particles are truly fundamental and indivisible, or if they are composite at some high-energy scale. (Credit: kotoffei / Adobe Stock)

Ask Ethan: Why do matter and antimatter annihilate?

From forming bound states to normal scattering, many possibilities abound for matter-antimatter interactions. So why do they annihilate?

The matter that we’re made out of here on planet Earth is composed of atoms: protons and neutrons in the nucleus, orbited by electrons, that bind together in countless possible ways to produce what we experience as the world around us. For every fundamental and composite particle of matter that exists, however, there’s also its antimatter counterpart: antiprotons for protons, antineutrons for neutrons, positrons for electrons, etc. When matter and antimatter collide and interact, they annihilate, producing pure energy and whatever particles are allowed by the quantum laws that govern nature as well as Einstein’s most famous equation, E = mc².

But do matter and antimatter always annihilate away when they interact? Aren’t other types of interactions possible, and even likely? That’s what Brian Vant-Hull wants to know, writing in to ask:

“What physics principle mandates that matter an antimatter MUST annihilate when brought together? It makes sense that when matter is created antimatter could (but does not need to) be created to satisfy the conservation laws. Time reversal suggests…