3.2 “Who Ordered This?”

summary

We have a specific group of particles, we know their properties, and have the math to explain their behavior. Yet many questions remain.

Why this particular group? Why these properties? Why does matter seem to come in a family, and why do we have three families, roughly “small, medium, large?”
We know some of the laws, but we don’t yet know the specific solution to these laws. For instance, Newton’s laws of gravity are universal, whereas our solar system is merely an example of those laws.
Is there some kind of special intrinsic way that nature picked our specific symmetry groups? And how unique is this model?

With so many questions left unanswered, speculations arise and theorists take over.

Physicists theorize potential new particles and alternate, all-encompassing symmetries.
If we look at particles & antiparticles, and we use the fact that a neutrino has a small mass, it predicts that there also exists a very heavy neutrino, one which has not yet been discovered.
But if we include all of the particles, both real and theoretical, perhaps we can rearrange them in a sort of way to show that our symmetry group is actually a small part of a larger, Grand Unified symmetry.
We do need new theories that can explain things outside the Standard Model—dark matter and dark energy, for instance. We are on the verge of finding out what exactly 95% of the universe is made of, and at the same time the theory is crying out to be extended.

Properties of the four fundamental forces of nature provide hints at high-energy unification.

The four fundamental forces are the strong nuclear force, weak nuclear force, electromagnetism, and gravity.
It turns out that the “character” of these forces changes as you move to higher and higher energies.
Consider the electric force between two electrons. Its strength has a $\sim\frac{1}{r^2}$ dependence, and is characterized by a coefficient called the fine-structure constant, $\alpha$: $$\alpha=\frac{e^2}{\hbar c}\approx \frac{1}{137}$$What is so special about this number, 137?
The fact is that the value of the fine-structure constant (and thereby, the strength of the electric force) is a function of the energy at which you measure it. The properties of matter change depending on what energy they have—or, in a sense, the “magnification” you are looking at the particles with.
As you follow the strength of the three forces, they seem to converge at a very high energy. With supersymmetry, the convergence seems even more defined. Perhaps these are three elements of one unique phenomenon.

Supersymmetry might provide answers to some remaining mysteries in particle physics.

The potential convergence of the strengths of the three fundamental forces that act on elementary particles is a strong hint that there is something convincingly like unification at very high energies.
Supersymmetry is a new type of symmetry, one which we haven’t observed in nature, but we do know that it is consistent with everything else we know about nature. It relates the two “halves” of which nature is composed.
On one half we have forces and fields. Technically, these are particles with integer spins—bosons. The Higgs particle has spin-0, the gauge forces consist of spin-1 particles, and gravity is theoretically comprised of a spin-2 particle.
On the other half, we have particles with half-integer spins—fermions. These are constituents of ordinary matter: protons, neutrons, electrons, neutrinos, and quarks.
There is a saying in quantum mechanics, “Anything allowable is obligatory.” Supersymmetry, in the context of gravity, predicts a spin-³/₂ particle, the gravitino. In fact, every particle would have a mirror particle, but none of them have been found.