The post below made reference to the theory of supersymmetry and this weblog is long overdue a post on the subject. However, as supersymmetry is proposed as an extension of the Standard Model (SM) of particle physics, we’d better have a few words about the SM first…
As we said before, one of the big discoveries of 20th century physics is that there exist only four independent forces or interactions. These are gravity, electromagnetism (the unification of electricity and magnetism achieved by Maxwell in the 19th century), the strong nuclear force (that holds the protons and neutrons together in the nucleus), and the weak nuclear force (responsible for nuclear decay and radioactivity).
Physicists have long suspected that the four fundamental forces are not truly independent, but deeply connected. The idea is that at the tremendous energies of the Big Bang, a single superforce existed, which gradually split off into the four seperate entities we see today as the universe cooled. This idea received a great boost in the 1970s, when Salaam, Weinberg and Glashow established a strong theoretical connection between the electromagnetic and the weak nuclear interactions, using the methods of gauge symmetry. The theory predicted the existence of new particles (W and Z bosons), which were subsequently discovered in high-energy experiments at CERN in the 1980s…ever since we talk about the electro-weak interaction as a single entity.
Shortly before this, the first comprehensive theory of the strong nuclear force had also emerged – the key idea being Gellman’s prediction that the nuclear particles (protons and neutrons) are in fact made up of quarks, and the strong nuclear force is really an interquark force. This was verified by scattering experiments at Stanford in 1979, and the theory of the strong interaction is now known as quantum chromodynamics
Putting the two theories together gave rise to the Standard Model - a model that has been fantastically accurate at predicting the masses and properties of all particles discovered so far. However, the model contains several shortcomings
- there is no real unification between the electro-weak and strong interactions, they are treated in parallel
- gravity doesn’t appear at all
These shortcomings led to new theories that attempted to unify the strong nuclear force with the electro-weak interaction (known as Grand Unified Theories), and even more ambitious attempts to unify all three with gravity (Theories of Everything). To accomplish either of these, some new mathematical approaches would be needed….see next instalment…
I forgot to mention another shortcoming of the Standard Model – namely that one particle, necessary to the model, has never been observed (thanks, tankers!). The Higgs boson plays a central role in the SM as the Higgs field gives the mechanism for other particles to acquire the masses we observe. Unfortunately, no evidence of the Higgs particle has been seen in accelerator experiments so far. Most theoreticians are convinced this is simply because we need higher energies than currently available to create it (i.e. it has a large mass), and expect to see evidence of Higgs bosons in the next round of accelerator experiments due to begin at the new accelerator in CERN next year – the Large Hadron Collider.
The alternative is that we’ll see something quite different, which would be even more interesting!