Beauty, beliefs and GUTs

When I discuss a previous blog on the controversial topic of religion and science, the issue of belief without evidence sometimes comes up. Some people take the viewpoint that you should not believe in anything without evidence (I might caricature this as militant agnosticism :P), but this strikes me as a bit over constraining and does not really match how scientists actually behave.

Although we must try and objectively assess the evidence and make clear scientific statements about it, this does not mean we have to be opinion less about everything else. Indeed my experience of scientists is they are more likely to be opinionated (I am for sure ;) ) than opinion less. But even with regard to the science, when we are doing research at the frontiers of knowledge we do not yet have scientific evidence to know what will be the right approach, how nature will have solved the problem we are interested in. What motivates us in our work can be opinion based on our expectations about nature, and this is why different people look at different possibilities to solve the same problem.

In particle physics, for example, I have a (fairly dilute) belief in both Grand Unified Theories (GUTs) and Supersymmetry (SUSY). Others have a stronger belief, while some seem sure they are not realised in nature. Nonetheless everyone in the field knows that ultimately these are scientific theories which need to be tested. But where does our prior belief or theoretical prejudice spring from?

I have already talked briefly about motivations for supersymmetry so I will focus on GUTs here. As I previously discussed the Standard Model of particle physics describes three fundamental forces (strong, electromagnetic and weak) through gauge symmetries. The different forces have different 'strengths' by which the fundamental particles interact, and the fundamental particles are sensitive to different forces. Quarks, the particles which form hadrons like the proton and neutron, are sensitive the all three forces, while leptons, like the electron, to only the electromagnetic and weak forces. The fact that the strong force is much, well... stronger than other two explains why quarks are confined into hadrons and cannot be observed as free states.

So we have three (not counting gravity which is so weak we don't notice it) different forces with different strengths and fundamental particles which interact with different forces. This description matches the data incredibly well giving some of the most precisely tested predictions in all of science. But why do we have these different forces? Perhaps nature is simply like this, some scientists may well believe that is all there is to it. But for me this seems to leave nature looking rather arbitrary and messy.

Grand Unified Theories are a possible explanation for where these different forces come from. In these GUTs the different symmetries are embedded into a larger symmetry group which unifies all forces into a single unified one. All fields describing matter particles then transform together under this larger symmetry and interact via this single force. The different forces can then arise as a consequence of the breaking of this GUT symmetry. Additionally the larger symmetry of this gauge group means it has fixed charges, allowing for the possibility that the different values of electric charge we observe at low energies (unexplained in the SM) are fixed by the breaking of this GUT.

The simple elegance and beauty of Grand Unified Theories fits well into my expectations about nature. There is no proof that nature can be described in increasingly simple ways (the idea that it can is often called reductionism) but the aesthetic beauty of it appeals to me. As does my hope that I can explain or understand the forces and the electric charges we observe in nature.

This hope may seem in stark contradiction with the observation of three separate forces with different interaction strengths. However the vacuum is not empty but filled with 'virtual' particle and anti-particle pairs which are being created and annihilated and when particles interact across the vacuum such virtual particles are affected. For the case Quantum Electro-Dynamics (QED) the positively charged particles in the vacuum are drawn closer towards a negative charge and vice versa, and this effectively screens the interaction strength from the charges between two particles separated across the vacuum. The further apart the particles are the weaker the interaction strength. Since how close together particles can get in collisions depends on how energetic they are, interaction strengths depend on the energy at which the interactions take place.

For more complicated symmetries describing the weak and strong force there is a similar effect, but one also gets anti-screening from the gauge bosons so that interactions interactions can either increase or decrease with distance depending on the particle content of the model. This is the basis for having quarks which are confined into colorless hadrons at low energies while behaving like free particles at high energies, known as asymptotic freedom, for which the 2004 nobel prize for physics was awarded.

This also allows for the possibility that the separate three forces we observe at low energies have a common interaction strength at higher energies. This possibility is precisely realised in models with Supersymmetry at the TeV energy scale (the scale we are probing at the LHC), as is shown by the red lines in the figure to the left.

To motivate Supersymmetry I previously described the hierarchy problem and pointed out that this motivates new physics at the TeV scale based on our lack of understanding as to how two unrelated masses could be tuned to be incredibly close, but not equal. While this is not a fundamental proof of new low energy physics it is at least a physical question which we want to understand and the Supersymmetric solution is very elegant and therefore in my mind compelling.

I am personally very attracted to this picture of a simple Supersymmetric Grand Unified Theory at high energies which as we move down to lower energies has some of it's elegant symmetry spontaneously broken, leaving us with the complicated set of massive particles with different interactions which we have already observed. I find this idea beautiful, fascinating and compelling.

Others do not find it so convincing, but that is fine. This does not make either them nor me poor scientists who misunderstand the evidence, we simply have different theoretical prejudices based on our expectations of nature. Ultimately questions like this will be resolved by data and this is what we excitedly await from the LHC.