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Researchers at TU Wien were the first to successfully detect Weyl particles in strongly correlated electron systems – that is, materials where the electrons have a strong interaction with each other. In materials like this, the Weyl particles move extremely slowly, despite having no mass.

“The strong interactions in such materials usually lead, via the so-called Kondo effect, to particles behaving as if they had an extremely large mass”, explains Sami Dzsaber. “So it was astonishing for us to detect Weyl fermions with a mass of zero in this particular type of material”. According to the laws of relativity, free massless particles must always spread at light speed. This is, however, not the case in solid states: “Even though our Weyl fermions have no mass, their speed is extremely low,” says Bühler-Paschen. The solid state lends them its own fixed ‘light speed’ to a certain extent. This is lower than 1000 m/s, i.e. only around three millionth of the speed of light in a vacuum. “As such, they are even slower than phonons, the analogue to the water wave in the solid state, and this makes them detectable in our experiment”. (1)

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Low speeds. High speeds.

What is the difference?

The light is fast. But not for light.

Weyl particles are slow. But not for Weyl particles.

The limits you imagine are not there.

Imagine a Weyl particle.

Fast as 10 m/s…

Massless particles. Heavy particles.

High speed particles. Low speed particles.

Depending on the environmental interactions.

Remove them and see.

Everything is fast. Everything is slow…

Imagine a Weyl particle. Fast as light…

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In the beginning everything was still and fast as light at the same time. Until we came. And started observing… The cosmos was once still and, thus, fast like lightning. Then the cosmos started moving. And everything came to a halt.

Note: Weyl particles are not particles which can move on their own (like electrons or protons), they only exist as ‘quasiparticles’ within a solid material. “Quasiparticles are not particles in the conventional sense, but rather excitations of a system consisting of many interacting particles”, explains Prof. Silke Bühler-Paschen from the Institute of Solid State Physics at TU Wien. In some sense, they are similar to a wave in water. The wave is not a water molecule, rather it is based on the movement of many molecules. When the wave moves forward, this does not mean that the particles in the water are moving at that speed. It is not the water molecules themselves, but their excitation in wave form that spreads. After physician Paul Dirac had arrived at his Dirac equation in 1928, which can be used to describe the behavior of relativistic electrons, Hermann Weyl found a particular solution for this equation – namely for particles with zero mass, or ‘Weyl fermions’. The neutrino was originally thought to be such a massless Weyl particle, until it was discovered that it does indeed have mass. The mysterious Weyl fermions were, in fact, detected for the first time in 2015.

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