Mass direction paradox
Particles can sometimes behave as if they have no mass when their energy comes entirely from motion, making them pure energy moving at the speed of light. For example, photons, or light particles, are massless because they travel at light speed.
According to Einstein’s theory of special relativity, anything moving at the speed of light cannot have mass. However, in solid materials, the collective behavior of many particles, called quasiparticles, can behave differently. This unique behavior can result in particles having mass in one direction only.