Materials science: Semi-Dirac fermions discovered in ZrSiS
The formula E = mc2, energy = mass x (speed of light)2 is well-known. Most people probably have heard of things like a rocket, atom or subatomic particle with mass speeding up to get close to the speed of light, but never reaching light speed due to infinite energy needed. Energy needed to add more speed increases infinitely as anything with mass gets very, very close to the speed of light. For a particle with mass m > 0 and velocity v = c, equations for momentum and energy become inconsistent, indicating that such a state is not physically possible.
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 (Zr), silicon (Si) and sulfur (S), while studying the properties of quasiparticles. These emerge from the collective behavior of many particles within a solid material.
It sounds like an impossible feat – how can something gain and lose mass readily? But it actually comes back to that classic formula that everyone’s heard of but many might not understand – E = mc2. This describes the relationship between a particle’s energy (E) and mass (m), with the speed of light (c) squared.
Squiggly thing
According to Einstein’s theory of special relativity, nothing that has any mass can reach the speed of light, because it would take an infinite amount of energy to accelerate it to that speed. But a funny thing happens when you flip that on its head – if a massless particle slows down from the speed of light, it actually gains mass.
And that’s what’s happening here. When the quasiparticles travel along one dimension inside the ZrSiS crystals, they do so at the speed of light and are therefore massless. But as soon as they try to travel in a different direction, they hit resistance, slow down and gain mass.
“Imagine the particle is a tiny train confined to a network of tracks, which are the material’s underlying electronic structure,” said Shao. “Now, at certain points the tracks intersect, so our particle train is moving along its fast track, at light speed, but then it hits an intersection and needs to switch to a perpendicular track. Suddenly, it experiences resistance, it has mass. The particles are either all energy or have mass depending on the direction of their movement along the material’s ‘tracks.’”
See, it's like a massless particle on a teeny, tiny train track traveling at the speed of light. The track crosses another track going in a different direction. The choo-choo, the massless particle or semi-Dirac fermion, has to slow down to make the curve or it will fly off the track or something, I think. But once it slows down to less than the speed of light it gains mass.
Q: Is going from the fast track to a slower track a one-way trip for an irrationally exuberant speed of light massless particle that slows down, gains mass, becomes less exuberant because it is forever limited to sub-light speeds because it has mass?
By Germaine: Definitely not a physicist
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