We’ve seen particles that are massless only when moving one direction

Strange particles that have mass when moving one direction but no mass when moving in another were first theorised more than a decade ago. Now, these mass-shifting particles have been glimpsed in a semimetal exposed to extreme conditions.

“This [particle] is very bizarre. You can imagine walking on the streets of New York and if you go straight, you are super light, you are massless. But turn 90 degrees east or west, and you become super massive,” says …