Untangling the origin of string theory
In the summer of 1968, while a visitor in CERN’s theory division, theoretical physicist Gabriele Veneziano wrote a paper titled “Construction of a crossing-symmetric, Regge behaved amplitude for linearly-rising trajectories”. He was trying to explain the strong interaction, but his paper wound up marking the beginning of string theory.
“The reaction of the physics community came to me as a shock,” explains Veneziano in an interview at CERN earlier this year. “As soon as I had submitted the paper I went on vacation and did not think much about it. At the end of August 1968, I attended a conference in Vienna and found out, to my surprise, that the paper was already widely known and got mentioned in several summary talks.”
The paper was an instant hit, states Veneziano, because the model answered several questions at once. But it was not apparent then that it had anything to do with strings, let alone quantum gravity. It took until 1973 for other theorists to prove this crucial link.
“At that point it became clear that the original model had a clear physical interpretation of hadrons being quantised strings. Some details were obviously wrong: one of the most striking features of strong interactions is their short-range nature, while a massless state produces long-range interactions. The model being inconsistent for three spatial dimensions (our world!) was also embarrassing, but people kept hoping.”
During the following decade, describes Veneziano, most people stayed away from string theory; the Standard Model of particle physics had just come to life and there was so much to do in order to extract its predictions and test it. “But fifty years on, the enthusiasm of young theorists is still clear and the field is atypically young,” he says. “Perhaps what motivates these scientists is the mathematical beauty of string theory, or the possibility of carrying out many different calculations, publishing them and getting lots of citations.”
But is string theory any closer to describing reality? “People say that string theory doesn’t make predictions, but that’s simply not true. It predicts the dimensionality of space, which is the only theory so far to do so, and it also predicts, at tree level (the lowest level of approximation for a quantum-relativistic theory), a whole lot of massless scalars that threaten the equivalence principle (the universality of free-fall), which is by now very well tested. If we could trust this tree-level prediction, string theory would be already falsified. But the same would be true of quantum chromodynamics (QCD), since at tree level it implies the existence of free quarks.”
“The usual argument is that you need unconceivably high energies to test string theory,” says Veneziano. “But the new incarnation of string theory can be falsified by large-distance experiments, provided we can trust the level of approximation at which it is solved. On the other hand, in order to test string theory at short distance, the best way is through cosmology. Around (i.e. at, before, or soon after) the Big Bang, string theory may have left its imprint on the early universe and its subsequent expansion can bring those to macroscopic scales today.”
This is an extract of the full interview with Gabriele Veneziano in the CERN Courier.