Why is unification so important in physics?

Answer

Dear Laurent,

Part 1: Why is a unified theory of elementary particles and their interactions so important?

Your question is a little bit philosophical and different physicists will have a different view on this.

It is indeed the case that “unification” (in the form of a theory that can describe all interactions between elementary particles under all circumstances) is an important area of ​​research in physics. But certainly not the only research area; there is also solid state physics, thermodynamics, mechanics, etc.

To understand why so much attention is being paid to a unified theory of elementary particles and their interactions, we need to think for a moment about what physics is. As the name says, physics tries to describe “nature”. Usually we do this in the form of mathematical “laws” that we can apply over and over again (think Newton’s laws, gravity, Lorentz force for electromagnetism, etc.). Very importantly, we can use and apply the same laws over and over again, and we can also use the laws to predict what will happen when we run a particular experiment.

From that point of view, it makes sense that we want a single theory that can describe all interactions. Now we have to choose a different theory under different circumstances. For the gravitational interaction between stars we have to use general relativity, for the other interactions (electromagnetic, strong and weak interactions) we have to use a quantum theory (the standard model). It would be more elegant if we had one theory that we could apply in all circumstances.

In most places in space, either the gravitational interaction is important (stars, galaxies…) or the other interactions. We therefore come a long way with separate descriptions of gravity (general relativity) and the other interactions (quantum theories). However, there are special places, the so-called black holes, where both the gravitational and the electromagnetic interaction are important. We will therefore only be able to really describe black holes once we have a unified theory of the elementary particles. The problem, however, is that general relativity and the standard model are not compatible.

Part 2: The wave-particle duality in quantum mechanics

I also want to pick up on what you write about the wave-particle duality of light (this is actually true of all particles, not just photons), because I feel like you’re misinterpreting that a bit. It is not the case that a wave theory and a particle theory are separate descriptions of light. Photons (light particles) are neither particles nor waves. However, they are “quantum particles” that we can describe extremely well with a theory that we call quantum electrodynamics. The quantum particles sometimes seem to behave like waves (e.g. diffraction) and sometimes like particles (photoelectric effect), but that’s only because we tend to always compare everything to the things we know from classical physics (waves or particles).

So it is not the case that light is sometimes a particle, sometimes a wave. It is neither, but it is a quantum particle that apparently has properties of both. That is an important difference of interpretation for the question you are asking.

Greetings,

Philippe

Physicist Iowa State University & Free University of Brussels