In class, we have spent most of our time looking at the theoretical side of string theory.  This is all well and good, except that it doesn't really help explain what string theory will be used for.  My interest with this project is to learn more about the practical side of string theory, of how it will be used in its potential role as a grand unified theory, and about what it means to our physical universe, as we know it now.

  The first uses of our knowledge of string theory will, of course, be attempts to verify string theory as a whole, comparing its predicted results to a known-good set of data.  Until we have fairly conclusive evidence indicating that string theory is correct, we won't really be able to safely use string theory to predict unknown results.  Many of the predictions of string theory occur at scales that are far too small, or energies that are far too large, to be measured with the use of any foreseeable new technology.  But there is one portion of string theory whose measurement is not so impossible: Supersymmetry.  If supersymmetry can be measured, we can be fairly sure that string theory is, at least in part, correct.

  So what, exactly, is supersymmetry?  Essentially, it states that every force-transmitting particle (a boson, such as the photon or the graviton) has a 'superpartner', an as-yet-undetected particle with properties that are predicted by string theory.  These pairs of particles would always exist together; each should always be detectable in the presence of its superpartner.  So verifiably detecting the superpartner of a particle would be a good verification of string theory.

Here is a chart containing particles and their predicted superpartners:

Force-transmitting particles and their superpartners: Name Spin Superpartner Spin Graviton 2 Gravitino 3/2 Photon 1 Photino 1/2 Gluon 1 Gluino 1/2 W+,- 1 Wino+,- 1/2 Z0 1 Zino 1/2 Higgs 0 Higgsino 1/2

Matter-composing particles and their superpartners: Name Spin Superpartner Spin Electron 1/2 Selectron 0 Muon 1/2 Smuon 0 Tau 1/2 Stau 0 Neutrino 1/2 Sneutrino 0 Quark 1/2 Squark 0

  If string theory is, at some point, verified, there are many experiments that can be done to test the many of its very odd aspects.  One of the strange things anticipated by string theory is that there are more than the four dimensions that we know to exist today.  And yes, there are four dimensions; time, as Einstein demonstrated, is just as much a dimension as the three spatial dimensions.  But string theory requires that there be ten dimensions!  There are, at present, several different theories about how such a thing could be the case.

  One such theory, the Kaluza-Klein theory, essentially states that the additional dimensions are simply too small to see.  It presents the idea that there are several very small dimensions, each of which wrap around themselves or each other.  These dimensions would be wrapped up in a vaguely doughnut-like shape; at least, that is what the present theory indicates.  One analogy that I have found that explains how this compression works is this: Say you have a garden hose on a reel.  If you look at it really close up, it is clear that its surface is two-dimensional (let's discount time and the inside of the hose for the moment, just for simplicity).  But if you look at it from really far away, it looks like a series of concentric circles, or like a spiral.  The component line of circles and spirals is one-dimensional, so by zooming out enough, you have effectively removed one dimension from the picture.  Now, zoom out even more.  As you get farther and farther away, the reel of hose will get smaller and smaller, until it is a single point.  A point has no size, and therefore no dimension; by looking from a great enough distance, you can reduce an entire dimension, or even two, down to an apparent nothing.  One theory states that every point in our universe is just like this hose; if we could zoom in far enough (past the Planck scale, ~10^-35 meters), we would see these odd twists in the universe.  But we are many, many orders of magnitude off from being able to see beneath the Planck scale, so direct observation of this phenomenon is highly unlikely.

  These extra dimensions would be so small that they would be immeasurable by modern equipment; this is the explanation for why we cannot detect them.  It is not clear what is meant by a 'small' dimension; all dimensions that we can observe are, as best as we can tell, infinite in size.  At least, no perceptible signal, travelling at the speed of light, has yet returned from any boundary representing the end of the universe, nor has such a signal looped around the whole universe and returned to the other side, as it would if the universe is, as some theorize, a 'hyperdoughnut' (think the surface of a doughnut, then add a dimension or two), or something similar.  Also, the methods of limiting a process to a subset of the overall ten dimensions turns out to be a very unwieldy process under the Kaluza-Klein theory.  As a result, scientists have continued to look for better ways of looking at multiple dimensions.  One such way is the Brane theory.

  In the Brane theory, we are, to a degree, constrained to exist within a subset of the overall ten dimensions.  A common analogy for explaining this involves the computer monitor that you are, most likely, looking at right now while reading this webpage.  The monitor's display panel does have a length, width, and height, so it exists in three dimensions.  But it is not possible to move the images presented in more than two dimensions: up/down and left/right.  The monitor's image represents a subset of three-dimensional space; it represents only two dimensions of space.  Light, in this theory, is restricted to the 'brane', or region of four-dimensional space-time, in which it is created (guess where 'the Brane theory' got its name); gravity can leave its brane, but it is restricted to a very local region outside of it, so its effects would likely be miniscule, undetectable by modern equipment.

  The difference these two models, in practical terms, is that the Kaluza-Klein model limits the size of alternate dimensions, while the Brane model limits the range in which we can interact with other dimensions.  But both are consistent with the idea of having the full 10 dimensions required by string theory.  In terms of which is actually true, the proof of one or the other is caught up somewhere in the extremely complex formulations of string theory; it is believed that a study of supersymmetry, discussed earlier, would allow us to determine which is a more accurate description, but such proof will, alas, have to wait until we have more accurate detectors.

  String theory, clearly, is as yet an unproven science.  But, as we have discussed in class, it shows a great deal of promise, in that it matches up to a great degree of accuracy with many facets of proven science.  But because it has not yet ben proven true, and because our technology is not quite advanced enough to test the validity of its assertions, string theory is not really used at all experimentally.  There are, of course, experiments planned to test string theory; to determine if it is, or can be true.  But we don't quite yet have the technology to carry out even the simplest of these experiments; all that we can really do for the moment is continue to refine the theoretical details of string theory, so that when the technology becomes advanced enough to take advantage of string theory, we already know exactly how to use it.

Major Sources:

  The Official String Theory Website. http://superstringtheory.com/.

  Curling up Extra Dimensions in String Theory. http://focus.aps.org/story/v1/st7.