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.

Summary

Scientists have discovered semi-Dirac fermions, particles that bizarrely gain or lose mass depending on the direction they travel.

Found in the semi-metal material ZrSiS, these quasiparticles are massless when moving at light speed in one direction but gain mass when slowing down in another, due to resistance within the material’s electronic structure.

This behavior, tied to Einstein’s E=mc², was unexpected and may lead to applications similar to graphene.

Researchers are now studying the unexplained quantum interactions behind this phenomenon, published in Physical Review X.