Rumors say that string theorists are vacuum cleaner door-to-door salesmen. Perhaps. So I thought it could have been interesting to shed some light on this topic.
Once upon a time (1968), Gabriele Veneziano, an italian physicist, was struggling to find a consistent model for nucelar dynamics. He found a model, but it wasn’t as consistent as he hoped. Anyway, his model treated nuclear constituents not as pointlike particles, but like “strings” (that was so because the force between quarks acts quite like a spring). Unfortunately, this model turned out to predict the existence of some weird particles which has been never observed. They are spin-2 massless bosons, and they look like not playing any role in nuclear physics…But two years later Nambu, Susskind (not the one who wrote “The perfume”) and Nielsen (not the one who sells soap) independently discovered that the same formulas describe relativistic quantum vibrating strings, and that orphan boson was the quantum of gravity. That’s only the beginning.
In the past, magnetism and electricity seemed to be completely different kind of phenomena. Now we call them electromagnetism. In the past, electromagnetism and nuclear decays seemed very different processes. Now we call them electroweak interactions. Today, electroweak and strong interactions on a side, and gravity on the other, seem to be separate forces. Are they, really? We don’t know. A lot of physicists now climb on the shoulders of Newton, Maxwell and Weinberg trying to see a new landscape, in which there’s only one bigbigbig force, still nameless. Some of them are called “string theorists”.
String theory is not really a theory of unification, but it can provide some clues of it. As a matter of fact, if you make the assumption that everything is made up of tiny vibrating strings, then you must have gravity and you can have fermions and bosons as side dishes through the famous supersymmetry (without supersymmetry you can have only bosons). Strings themselves can be open (like in a guitar) or closed. Depending on the way they vibrate, they can describe different particles. For examples, at the lowest energy closed strings have the same properties (mass and spin) of the quanta of the gravity: the gravitons. So, if you have a volume filled with gravitons, then you have a curved space. But there can be a lot of other esoteric vacuum states called for examples dilatons, which can explain what happened during the “inflation era”, just after the Big Bang (has inflation era really happened, after all?). Another really striking and startling feature of the strings is that they require more than the usual 3+1 dimensions to rest in peace. Gravity seems to be self-consistent only in 10+1 dimensions. Now, the question is: where are those extra-dimensions? Nobody really knows. Maybe they are packed, rolled and very tiny so we can’t see them, or better we do live in 11 dimensions, but our senses are “constrained” in a 3+1 world. In fact, open strings can be “attached” to vibrating multi-dimensional membranes (called Dirichelet-branes or D-branes for short) and closed strings can travel between two D-branes and then be “absorbed” by one of them, causing gravity. So our world is a D-3 brane.
String theorists introduced the metaphysical concept of “principle of elegance”. That is: they believe that Mother Nature is simple and elegant in her very building blocks. So a theory describing The Nature must be mathematically elegant, indeed. Everything should be derived as a “natural” consequence, with no phenomenological-driven assumptions. After all, it is an answer to the Einstein’s cosmic question ”Had God any choice in creating the Universe?” It was his fondest hope that the answer was no. So, if He had no choice, string theory can be a one-way mathematic pathway.
Today, string theory is still “redundant”: we have 6 theory, linked in pairs by a symmetry. None of them describes correctly our Universe. Some have no fermions, some have tachyons (particles that travel faster than light in vacuum). Theorists do believe that they are very close to the “central” theory that links all of them. They call it M-Theory (magical? mesmerizing? metaphysical? mysterious?). I’m waiting.