What does a particle still mean?

c. What are quasi-particles?

The term quasi-particle describes a physical concept in which elementary excitations of a solid, such as spin waves, are described as particles. Since these particles are not made of matter, they are called quasi-particles.

At first it must seem strange to refer to elementary stimuli as particles. It might seem less curious if you first make it clear what matter actually is. From Einstein's theory of relativity, we know that matter and energy are, in principle, the same. Matter can be converted into energy. Which, for example, is the case with nuclear fission and fusion reactors or with matter-antimatter annihilation. On the other hand, from energy, matter can be created. This happens artificially in particle accelerators, where the high kinetic energy of the accelerated particles is used to create new particles. Or, of course, in our earth's atmosphere, where high-energy particles and rays arrive and create a whole shower of particles there.

From this point of view, it seems that what we call matter is also an elementary stimulus of a medium called space. Some of these excitations, like muons or pions, have a very short lifespan, similar to the spin waves in the solid. Others, on the other hand, seem to have an infinitely long lifespan, such as protons, neutrons and electrons, from which "normal" matter is made up.

After these considerations, it might not seem so absurd to describe elementary excitations in a solid as particles. In most cases these particles have a finite lifetime, like the magnon or the exciton. However, there are also quasi-particles with an apparently infinite lifespan like the Cooper pairs in superconductors. As you can see there is a great similarity between particles and quasi-particles in this respect. These similarities go even further, so that quasi-particles can also be assigned a mass and a speed. However, it will not be discussed further at this point.

The next section is about the quasi-particle of the spin wave and the question:

What is a magnon?