The Theory Of Everything

Fellow Idaho blogger Clayton Cramer -- whose 1,000 hits a day (garnered by that insidious combination of "interesting commentary" and "thoughtful insights" he offers) are all that's keeping me from fulfilling my dream of being recognized as "Idaho's Most Visited Blogger" -- asks an interesting question about one of my favorite topics: string theory.

He read an article in the January 2006 Astronomy magazine that questions the validity of string theory and poses these problems:

Problem one: String theory won't work in our reality of three dimensions plus the fourth dimension of time. To make it work, its creators had to invent six or seven additional dimensions, which contradicts our own senses and the rest of science. None of these extra dimensions can be possibly be tested. They have to be taken on faith.
Problem two: String theory is untestable -- no experiment can tell if it's right or wrong.

Clayton goes on to ask: Is string theory untestable--does it have to be "taken on faith"?

Now, I'm not a physicist -- I'm just a nuclear power guy with a strong amateur interest in string theory. My basis for claiming to be able to answer this question is that I've read both of Brian Greene's popular books on string theory: "The Elegant Universe" and "The Fabric of the Cosmos". I also watched the Nova documentary on "The Elegant Universe".

String theory offers the hope of reconciling quantum mechanics with general relativity, a problem that's been insoluble so far -- the math just falls apart. The way string theory does this is by 1) postulating that all particles are one dimensional "strings" in our space, vice no-dimensional points, and 2) postulating the existence of a "graviton" -- a force particle that transmits the force of gravity in the same way that a photon transmits the electromagnetic force and gluons transmit the strong force. If gravitons really exist, then gravitation can be described in terms of more familiar particle interactions, rather than the Einsteinian "warping" of space-time (which, according to string theory, isn't wrong -- it's just an excellent way of describing the effects of gravitons continually issuing out from all particles with mass and interacting with other particles -- the interaction causes the particle interacted with to be pulled slightly towards where the graviton came from).

The "problem" that the extra dimensions can't be tested is valid -- if all particles in our universe are limited to our four dimensions of space-time, by definition we can't go outside of them. (Some versions of string, or "M", theory postulate that gravitons actually don't have this limitation.) Likewise, it seems that it would be problematic to make a machine that would detect gravitons -- since the machine would have to have mass, it would be continually emitting gravitons itself, which would screw with any readings you get.

Probably the best hope for experimentally "proving" that string theory might be valid would come from detecting the postulated "sparticles" that could exist at very high energies. First, some background. Consider a closed vibrating string; it looks like a circle that morphs into an egg and back. If you raise it to the next fundamental resonance level, it becomes kind of a vibrating cloverleaf, looking something like this:

Now, what if you start vibrating it at the next resonance above that, it'll look something like this:

(A good demonstration of how this concept looks in motion can be found here; you can move the "energy" bar on the right to see what the different resonances look like.)

All the particles we know about today, according to string theory, are different vibrational patterns, in 11 dimensions (our normal four, plus seven small "curled up" dimensions of very small size) of the first fundamental resonance of strings. It's thought that the next generation of particle accelerators (after the Large Hadron Collider that's going to open next year) might be able to reach high enough energies to create particles based on the next resonance level. If it does so, and the properties of these particles are as predicted by string theory, it'd be a pretty strong sign that the string theorists are on the right track; the "standard model" of particle physics doesn't predict these at all.

I look forward to seeing what the next 50 years bring...