Geekman explains the universe

or StyleyGeek gets someone else to do her blogging for her. (Do you think this strategy would work for my thesis, too?)

In the comments to a post below, there was a discussion of a public lecture that was recently given on the topic of supersymmetry. Morton T Fogg missed the lecture and made the request that someone explain the lecture in one or two sentences. Miss M then fingered me as "a real live physicist" (as opposed to a dead one, I can only assume) and suggested I have a go. So here I go.

At the lecture the speaker didn't really say a lot about supersymmetry itself, and didn't really explain what it was when he did. This is understandable, since it was a public lecture, and he had to first cover a lot of physics background before he could even start talking about supersymmetry. It's also hard to get lay people excited about supersymmetry since it's really abstract. Still, he gets my admiration for even trying.

So here's my explanation of what supersymmetry is, assuming a moderately scientific background.

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That was a five minute pause where I stared at the screen without writing anything. This is going to be difficult. Geekman adopts a lotus position and attempts to focus his chi.

Right. At the most fundamental level we know about, everything physical can be sharply divided into two classes with nothing left over. This division is normally made at the level of elementary particles (which are particles that appear to be absolutely fundamental and indivisible, i.e. not made up of anything else), but since everything is either one of these elementary particles or made up of collections of them, this division also applies to macroscopic objects.

The two classes are called bosons and fermions, and they have very different properties. Some of these properties are pretty abstract and don't make much sense outside a quantum mechanical analysis, but there is at least one property that has obvious, understandable consquences for everyday life: Bosons are friendly, fermions aren't. You can put as many bosons in the same place at the same time as you like; with fermions you can only have one in one place at any given time.

As you may now have guessed, practically all the stuff you see around you is fermionic. If you try to walk through a wall you will quickly become aware of this. You, the wall, not generally happy being in the same place at the same time. Feel free to try this at home - it's a very cheap and educational physics experiment, and probes fundamental properties of the universe.

Something that is bosonic, on the other hand, and is noticeable in everyday life is the photon (photons are particles of electromagnetic radiation). Light, radio waves, X-rays and so on all pass happily through each other without any problem.

Geekman un-adopts the lotus position as he's getting cramp, but tries to hold on his chi in case it proves useful.

Now, the next thing you have to know is that physicists like symmetry. A lot. A lot lot lot, even. If there's something asymmetric about their equations (making them ugly), they'll often stick in extra terms just to make them more symmetric (and elegant or beautiful), even if there's no justification for it. Amazingly, more often than not, these extra terms, in the fullness of time, turn out to actually be correct and describe extra stuff in the real world. The universe appears to like symmetry too. Good universe. Pretty universe.

I've used the term symmetry a lot, so it's probably time to explain it. There are all sorts of symmetries, and the ones that are most familiar to people are geometric ones. If, for example, I rotate a square through ninety degrees, it looks unchanged. Similarly if I reflect it though a diagonal and so on. But what does symmetry mean when applied to equations? Well, the same sort of thing, really. It means you can apply some transformation to one of the variables in your equation and the equation is still valid. One example might be if you take the solar system and move every object in it a metre to the left. The solar system ends up shifted but behaves exactly the same. This is equivalent to taking your equations that govern the movement of matter in a gravitational field and adding a constant to the variable denoting the position. If your equations don't still work after this change they don't respect this particular symmetry. The beauty about seeking symmetry in equations is that you can use symmetries that are a lot more abstract than the obvious spacial ones.

So, that's symmetry. Unfortunately we have this sharp dichotomy: bosons and fermions. Distinct, different, not symmetric. This makes physicists twitchy. Is is possible, they ask, to change the standard equations of quantum mechanics so that if we turned fermions into bosons the equations would still be correct? The standard equations don't respect this symmetry, but it's possible to construct equations that do, and they are indeed much more elegant. Beautiful, even. And if you look at these equations, you see they predict a whole lot more elementary particles that we haven't seen yet. If past history of physics is any guide, the fact that we have altered the equations to obtain beauty and seen new forms of matter predicted strongly suggests these new forms of matter actually exist.

That's the abstract motivation. Symmetry and beauty in the fundamental equations that describe reality.

There are a number of other motivations, but they're kind of too abstract to describe here. Suffice it to say that making our equations supersymmetric removes a number of inconsistancies in quantum mechanics, and it also appears to be required to make string theory work.

So, there's my quick and dirty description of supersymmetry for non-physicists, although it ran to a lot more than the one or two sentences requested by Morton T Fogg.