Theories of the Universe: A Brief History of String Theory

A Brief History of String Theory

String theory began with the observation that elementary particle resonances (the different energies at which new elementary particles are produced in the colliding beams from particle accelerators) form regular patterns, not unlike the overtones from a plucked string. This led the Italian physicist Gabriele Veneziano to propose in 1968 that the hadrons, the strongly interacting elementary particles, are in fact energy vibrations of incredibly small strings. In geometry, the most elementary unit is a point, like the period at the end of this sentence. But Veneziano thought that the most elementary unit in geometry was not a point in space, but a tiny extended string.

His theory was pretty ingenious, but soon ran into difficulties. It was discovered that the only way for the mathematics of the theory to satisfy both quantum mechanics and general relativity was if the strings existed in a space of 26 dimensions for bosons or 10 dimensions for fermions. At the time, physicists persisted in trying to connect string theory to the theory of quarks, suggesting that quarks are actually the ends of strings. The reason that isolated quarks have never been seen became immediately obvious—break a string in half in the hope of capturing a free end and all you end up doing is creating a new end.

Let me explain this in another way, just in case the last paragraph left you feeling a little unclear. The quarks inside hadrons are held together by the exchange of gluons, and the effect is as if two quarks are joined by a piece of elastic. The force between quarks (the color force, which also indirectly gives rise to the strong interaction) is so strong that the energy in the “elastic” is comparable to the mass energy in the quarks themselves. Under these conditions, a pair of quarks is joined by the color force and behaves in many ways like a stretched piece of string. A good image for this would be the chain shot used in sea battles in the days of sail. A pair of cannonballs joined by a chain would whirl around one another as they stuck the rigging of a man-of-war ship, and would end up doing far more damage than two single balls passing through the sails.

But to get back to our story, this first version of string theory was soon superceded by the development of supersymmetry. The basic concepts developed by string theory about the unification of fermions and bosons through extra dimensions replaced the notion of strings, and supersymmetry replaced the theory that had given it birth. Once the idea of supersymmetry had been placed in the minds of physicists, it was easy to incorporate it into the then standard model of the particle world, as we discussed in the last section. As a matter of fact, that is the way generations of students after 1976 were introduced to supersymmetry, without any mention of strings at all. In the early 1980s, the English physicist Michael Green and the American John Schwarz married the ideas of string theory to those of supersymmetry to create, yes you guessed it, superstrings.

The Cosmic Onion

Between 1984 and 1986, thousands of research papers were written by physicists from around the world. This three-year period is regarded as the first superstring revolution. What was this first theory about? Why was it replaced with the second revolution in superstring theory? And what are these strings made of? To answer these questions let's look at some of the basic concepts that define this first revolution.

To clarify the most essential point of string theory, we need to change how we view the whole set of elementary particles. In SST, which is the abbreviation I'll use when discussing superstring theory, the idea of a particle as just a point in space, which is the most general concept associated with a particle, is replaced with the idea of a tiny vibrating string, a string that's connected together to form a loop. That's it! You now have the secret of the theory of everything. But as with all theories, there is more to the whole picture.

What are these strings made of? In truth, no one knows, and I'll tell you why in a moment. I can tell you that they are the size of the Planck length. Remember that unit of measurement from the last section? That was 10^-35 meter, or about 100 billion billion (10²⁰) times smaller than a proton. Now, regarding their composition, well that's the trick. If they are truly the most fundamental units out of which everything else is made, then to say that they are composed of something would mean that there was something even smaller than these strings, and then, of course, they wouldn't be what they are. We would have the dilemma of a cosmic onion, in which each layer that's peeled away just reveals another layer.

Musical Strings

A deeper look at the analogy of a vibrating musical string can take us to a better understanding of how the first SST revolution saw string theory as a possible answer to unification. If we use a violin as our musical instrument to explain how strings work, I think you'll get a clear picture of how physicists understand SST. The four strings on a violin can each vibrate at almost an infinite number of vibrational patterns called resonances. These are the wave patterns that fit between the two fixed ends of the violin string. When we hear these different vibrational resonances, we experience them as the different musical notes. The way the strings vibrate in string theory is very similar. Each closed loop can support almost an infinite number of resonant vibrational patterns within its structure. Instead of having a string fixed on both ends, the closed loop provides the same kind of structure so that it can vibrate in the same way as the violin string.

So how does this analogy help explain the central concept of SST? What this means is that just as the different vibrational patterns of a violin string create the different sounds that we hear, so, too, the different vibrational patterns of the strings in string theory create the different masses and force charges.