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 in some cases in which the latter is true, particles can actually act as their own antiparticles. (Credit: kotoffei / Adobe Stock)

Ask Ethan: Do any particles not have antiparticles?

In our Universe, matter is made of particles, while antimatter is made of antiparticles. But sometimes, the physical lines get real blurry.

Here in this Universe, there are certain laws of physics that never appear to be broken. No information-carrying signal, for instance, can ever move faster than the speed of light. Energy, if you account for all of the different types that exist, can never be created or destroyed: only conserved. Electric charge, linear momentum, and angular momentum are all similarly conserved. And, to the best of our knowledge, the only way to create new matter particles is to create an equal number of new antimatter particles, as we’ve never observed a single reaction that has either created or destroyed a net amount of matter over antimatter, or the other way around.

But are all of the entities in our Universe either “matter” or “antimatter” in some sense, or are there particles out there that don’t have antiparticles at all? That’s the question of David Wiser, who wants to know:

“I was wondering if there are any elementary particles that do not have corresponding antiparticles? The only two that seem to fit this category are the photon and graviton. Are there others? Is there any…