Are Quasiparticles Real?

I'm posting this because someone here (@dougnatelson) can respond to this. This is based on this article that asks if quasiparticle are real.

In much the same way, "quasiparticles can only exist within some medium or material, because they are built up from the response of that material's constituents or building blocks," Natelson said; in contrast, particles such as electrons and protons "can exist in free space."

In other words, quasiparticles can't exist in a vacuum and they can't exist on their own. They're reliant on particles acting together and being in a material where they can emerge, just as a wave carried out in a stadium can only exist when there is a group of people there who can perform it.

As a condensed matter physicist, I'm quite familiar with the concept of "quasiparticles", since we deal with electrons, "holes", etc. that undergo many-body interactions. It is why, for example, charge carriers in solids have "effective mass", which is typically not the "bare mass" of the charge carrier, but the "normalized" mass due to such many-body interactions. In essence, we have reduced one many-body problem into many one-body problem by lumping the complicated many-body interactions into this new particle's mass. This particle is often called the "quasiparticle".

But the above is a highly simplified picture. Some people would say that this is the Fermi Liquid model of quasiparticles where the strength of the coupling between the particle and the many-body interactions are "weak", allowing for such simplification. This is no longer true for particles with strong interactions where the Fermi Liquid model breaks down (marginal Fermi Liquid and so on). Therefore, it can get very complicated, especially when you add particles in reduced dimensions (2D and 1D).

I like this article because it highlights a very basic concept in condensed matter physics that does not get the appropriate publicity. However, the one issue that is missing is the explanation on what we mean by "real". Is an electron "real" based on what we measured the "bare electron" properties? After all, if we buy into the idea of quantum field theory, even a bare electron has its own vacuum interactions. This leads to the crazy idea that particles actually are all the result of some kind of many-body interactions themselves, i.e. they are quasiparticles!

Instead of worrying whether something is "real", whatever that means, we should be focusing on fact that we can measure such-and-such properties of such-and-such a thing. The measurements, or rather, the outcome of the measurements are real for they can be reproduced. The measurements represent the characteristics and properties of the entity that we are measuring. Maybe this is carrying empiricism to the extreme, but it beats trying to convince people that it is "real".

😊

Zz.