A team of researchers from Penn State and Columbia University has recently observed a quasi-particle that is massless when moving in one direction but acquires mass when moving in a different direction. This quasi-particle, known as a semi-Dirac fermion, was captured by the team inside a ZrSiS crystal and was first theorized 16 years ago. The scientists observed that when the particle travels in one direction at the speed of light, it remains massless. However, when it is forced to change direction, it slows down for the ‘turn’ and gains mass.

This property relates to Einstein’s most famous equation, E=mc², which states that energy and mass are interchangeable, connected by the speed of light squared. According to Einstein’s Theory of Special Relativity, mass traveling at the speed of light would have infinite mass and require infinite energy to maintain its speed, which is impossible. Therefore, only massless particles can travel at the speed of light.

Relativistic effects also come into play when objects approach and attain the speed of light. As an object with mass moves faster, time dilation and length contraction effects become significant. At the speed of light, time would effectively stop for the object, and distances would shrink to zero. These extreme conditions are not physically achievable for objects with mass.

Source: ScienceDaily by Adrienne Berard, 2024. Semi-Dirac Fermions in a Topological Metal. Physical Review X, Shao, et al, 2024.