Whither Supersymmetry?

Friday, October 29, 2010 at 9:05AM

Vatche Sahakian in frosh, quantum

Vatche Sahakian in frosh, quantum

All particles that physicists have so far discovered - the building blocks of matter and energy in the universe - carry a quantum mechanical property called "spin". The nomenclature is purposefully chosen to suggest that this property is akin to imagining tiny particles spinning around. However, this is not really a genuine picture since particles in quantum mechanics are fuzzes of probability - instead of tiny spinning billiard balls. Yet, this spin property of particles does behave very much like the good old notion of spin that we know and love - except for one important peculiarity: quantum mechanical particle spin is quantized… This means that, when measured, its value comes out as a discrete multiple of a universal number: spin 1, spin 2, spin 3, etcetera times the universal number. But it's more interesting than that. We also find particles with spins that are half-integer multiples of the same universal value: spin 1/2, spin 3/2, spin 5/2, etcetera. And nothing else. So, spin values arrange themselves on discrete levels - like the steps of a ladder - and come in two categories: integer spins and half-integer spins.

These two classes have very different properties. Integer spin particles - called **bosons** - can be compressed together without resistance when there are no forces acting between them. Eventually, at high densities, they form an intriguing new form of matter called a Bose condensate. Superconductivity and superfluidity involve cooling things down to a point where such Bose condensates emerge - along with very peculiar macroscopic properties (see posts on superconductivity and superfluidity). Half-integer spin particles - called **fermions** - resist such compression: no two fermions can occupy the exact same state (this is known as the Pauli exclusion principle). The accompanying video shows a simulation of two fermions in a box. Quantum mechanically, the particles are simply probability lumps: red for high probability, blue for low. As the fermions are pushed together, they repel quantum mechanically; that is, there is no actual force law acting between these two particles! The repulsion is statistical in Nature. Notice in particular the interesting fringing pattern upon collision: remember these are supposed to be particles… that's quantum mechanics craziness for you (see post on visualizing quantum mechanics).

All visible matter in the universe happens to be fermonic: for example, electrons, proton, and neutrons are all spin 1/2 particles. All forces of Nature are transmitted through the mediation of force particles: and all such force particles are for some reason bosonic. For example, the electromagnetic force is transmitted through a spin 1 particle known as the photon. These two classes of lego pieces of the subatomic world then have very different roles and character. A few decades ago, some theoretical physicists started contemplating the possibility that fermions and bosons may in fact be related; that there is a deep symmetry in the natural laws that connects these what otherwise are very different types of particles. For a lack of a better term, we call it supersymmetry. There is no experimental evidence for this symmetry yet. However, interesting things happen when such a symmetry is hypothetically considered. For one, one realizes that it is possible to talk about all the known forces of Nature as a single force law (see post on Grand Unification)… very elegant and perhaps very true. Another consequence has to do with understanding gravity within the crazy quantum world: string theory, the only theoretical framework that purports to combine gravitational and quantum physics - requires Supersymmetry to be logically self-consistent. While supersymmetry does not require string theory, string theory does need supersymmetry.

An immediate consequence of the existence of supersymmetry in Nature is the necessity of a plethora of additional building block particles of fundamental physics yet to be discovered. These particles are needed so as to be paired with the ones we already know in a manner that makes the catalogue of fundamental particles more symmetric - supersymmetric. The new particle physics accelerator in operation in Switzerland, the Large Hadron Collider, may be able to see some of these additional particles - and hence confirm the existence of Supersymmetry in Nature (see post 1 and post 2 on the LHC). The discovery of Supersymmetry in the next few years would undoubtedly be the greatest scientific discovery of the new century. But then, the century is rather young.

Article originally appeared on Physics feed for your imagination (http://schrodingersdog.net/).

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