Scientists have made a satisfying and intriguing physics discovery some 16 years after it was first predicted to be a possibility: a quasiparticle (a group of particles behaving as one) that only has an effective mass when moving in one direction.

In physics, mass generally refers to a property of particles that relates to things like their energy and resistance to movement. Yet not all mass is built the same – some describes the energy of a particle at rest, for example, while mass may also take into account the energy of a particle’s motion.

In this case, the effective mass describes the quasiparticle’s response to forces, which varies depending on whether the movement through the material is up and down, or back and forth.

Scientists have accidentally discovered a particle that has mass when it’s traveling in one direction, but no mass while traveling in a different direction. Known as semi-Dirac fermions, particles with this bizarre behavior were first predicted 16 years ago.

The discovery was made in a semi-metal material called ZrSiS, made up of zirconium, silicon and sulfur, while studying the properties of quasiparticles. These emerge from the collective behavior of many particles within a solid material.

“This was totally unexpected,” said Yinming Shao, lead author on the study. “We weren’t even looking for a semi-Dirac fermion when we started working with this material, but we were seeing signatures we didn’t understand – and it turns out we had made the first observation of these wild quasiparticles that sometimes move like they have mass and sometimes move like they have none.”