String Theory: An Historical Narrative
I offer my understanding of a historical narrative for the development of string theory. I am not a specialist in string theory, nor in quantum field theory, nor in general relativity. But I have cobbled this together from my various sources and will try not to make too many mistakes (feel free to correct any errors you encounter).
Billiard balls scatter off of one another by simply bouncing, what physicists would normally call an elastic collision. Seen from a distance, one can pretend that quantum mechanical particles scatter off of each other in similar fashion. However, the resulting distribution of scattering angles suggests this is not the case, so to understand scattering quantum mechanically requires a closer look at the details of how one particle interacts with another. That leads to the idea of a resonance state. When two particles collide and bounce (i.e., scatter) off of each other, they actually merge together in a very short lived resonance state, and then go their merry way.
With that context in mind, the scattering of pions was a particular problem some years ago (and may still be for all I know) where theory & experiment simply did not agree. In 1968,
Gabriele Veneziano tried to solve the problem with a radical notion - he replaced the single resonance state with a dual resonance state (
Veneziano, 1968). I call this the
moment of conception for string theory.
A scant two years later, in 1970,
Yoichiro Nambu,
Leonard Susskind (
Susskind, 1970) &
Holger Bech Neilsen, all acting independently, hit on the same idea. They thought of the dual resonance of Veneziano as the ends of a string, suggesting that fundamental particles might be properly described as tiny strings. This was the first time anyone had suggested deviating from the usual idea of representing fundamental particles as point like.
This marks the birth of what is now called bosonic string theory; it was able to describe the physics of bosons, at least qualitatively, but could not deal with fermions at all, and so was considered to be a not very useful idea. At this point string theory, such as it was, remained virtually unknown outside a small community of high energy particle physicists.
But it did not take long to overcome the boson problem. In 1971
Pierre Ramond (
Ramond, 1971), and the team of
John Schwarz (who is much more pleasant in person than his webpage picture implies) &
André Neveu (
Neveu & Schwarz, 1971), independently discovered the idea of supersymmetry. This new particle symmetry allowed string theory to expand into the realm of fermions, and marks the birth of what is now properly called superstring theory (today the term string theory as we commonly use it actually refers to superstring theory). But string theory required the presence of a particle known to be impossible as a hadron by particle physicists (a massless spin-2 particle)
So far, string theory has remained a theory for quantum mechanical particles only, and still of no interest outside the community of particle physicists. That all changed in 1974, when the team of John Schwarz &
Joel Scherk (
Scherk & Schwarz, 1974), and
Tamiaki Yoneya (
Yoneya, 1974) acting independently, decided on a radical interpretation of the impossible particle. It turns out that massless and spin-2 describe the graviton particle required by any quantum mechanical theory of gravity, but which those searching for quantum mechanical theories of gravity had been unable to properly derive. Schwarz, Scherk and Yoneya were able to interpret the massless spin-2 particle as a graviton and use that interpretation to prove that the low energy limit of superstring theory is general relativity. This marks the transition of string theory from a narrow theory of particle physics to a fundamental theory of everything, a quantum mechanical version of Einstein's elusive unified field theory. To me, this is a point worth emphasizing:
The low energy limit of superstring theory is general relativity. This means that if Einstein had never been, but string theory had been invented by someone, then general relativity could have been discovered by way of string theory.
In 1976 a parallel development to string theory, one of some significance, took place. The team of
Daniel Freedman,
Peter van Nieuwenhuizen and
Sergio Ferrera (
Freedman, van Nieuwenhuizen & Ferrera, 1976) and the team of
Stanley Deser &
Bruno Zumino (
Deser & Zumino, 1976) combined general relativity with supersymmetry to invent
supergravity. The idea is to work towards a quantum theory of gravity with general relativity as the starting point by adding supersymmetry to the mix. So supergravity is a direct competitor with string theory as
the quantum theory of gravity.
At this point both string theory & supergravity are known to reduce to general relativity, an obvious requirement for any quantum theory of gravity. But it was not yet known how to prove that string theory was in fact entirely self consistent. In 1984 the team of
Michael Green and John Schwarz successfully proved that string theory was entirely self consistent, but only if it were allowed 9 spatial dimensions, which makes it a 10-dimensional theory when 1 dimension of time is included (
Green & Schwarz, 1984a;
Green & Schwarz, 1984b). John Schwarz calls this the
First Superstring Revolution. Allowing for the fact that he might be somewhat biased by his own role in this revolution, it is nevertheless true that string theory rather suddenly gained in popularity overnight.
Now string theory takes a few years on break. One string theory became 5 string theories, and people began to despair that string theory would continue to advance.
But after 11 years, in 1995, two big things happened. First,
Joseph Polchinski showed that structures lurking around in the higher spatial dimensions of 10-dimensional string theory, called
d-branes (short for
Dirichlet membranes, and discovered by Polchinski and Horava a few years prior), could be identified with similar structures in 10-dimensional supergravity (
Polchinski, 1995). Second,
Edward Witten discovered the duality relationships which implied that all 5 string theories, and supergravity were in fact different aspects of one single underlying theory, which we now call
M-theory, and which required the addition of one more spatial dimension, making string theory an 11-dimensional theory (
Witten, 1995). Schwarz calls this the
Second Superstring Revolution.
As far as I can tell, that basically brings us up to today. M-theory is the primary focus of string theory research. Theorists do not know the true equations of M-theory (or any other string theory), only the approximate forms of the equations. This is the major stumbling block. How does one predict, for instance, the true mass of a supersymmetric partner particle, if one does not know the true equations for the theory, as opposed to the approximate equations?
To the extent that my narrative does not suffer from major errors on my part, I think it reveals an important point overlooked by those who think we have done this long enough, without finding the true equations, and we should quit now. String theory advances in fits & starts, but advance it does. The historical narrative shows a steady advance, from the simple particle models of Veneziano and others, to the surprising and unexpected emergence of general relativity from string theory, to the surprising connection between supergravity and superstring theory. These are not inconsequential steps of progress in understanding string theory. I don't understand how it can make any sense to see this narrative, and respond by throwing hands up in despair and abandon string theory for greener pastures. Couple this history of consistent advance in knowledge with the fact that it took 228 years for Einstein to solve Newton's problem of general relativity, and I think string theorists are really on a roll here.
Obviously, nobody is in a position to declare string theory to be the true unifying theory of physics. But I think neither is anyone in a position to declare that it definitely is not.
