Wissenschaftler haben zufällig ein Teilchen entdeckt, das Masse hat, wenn es sich in eine Richtung bewegt, aber keine Masse, wenn es sich in eine andere Richtung bewegt | Teilchen mit diesem bizarren Verhalten, die als Semi-Dirac-Fermionen bekannt sind, wurden erstmals vor 16 Jahren vorhergesagt.

From the article: The discovery was made in a semi-metal material called ZrSiS, made up of zirconium, silicon and sulfur, while studying the properties of quasiparticles. These emerge from the collective behavior of many particles within a solid material.

“This was totally unexpected,” said Yinming Shao, lead author on the [study](https://journals.aps.org/prx/abstract/10.1103/PhysRevX.14.041057). “We weren’t even looking for a semi-Dirac fermion when we started working with this material, but we were seeing signatures we didn’t understand – and it turns out we had made the first observation of these wild quasiparticles that sometimes move like they have mass and sometimes move like they have none.”

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.

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.