When first I heard about string theory, my initial response was like that of many others. The question needed to be asked: why strings? What about strings makes them capable of being the fundamental constituents of not just all matter, but all forces as well? Anything mathematically abstract that has the capability to describe nature at its most basic level instantly draws me closer.
During the late 1950s and into the 1960s, physicsts examining hadronic behavior discovered that the plot of angular momenta of particles as a function of the square of the particle’s mass (or energy) resulted in very nearly linear trajectories, later known as Regge trajectories (named for Tulio Regge, their discoverer). What’s more, the trajectories had the same slope for pi-mesons as for rho-, or any other type of meson. This indicated to physicists at the time that there existed something almost elastic between quarks that could never break. As it turned out, the slope of the Regge trajectories are directly related to the tension of a string.
Secondary reasoning for the study of strings came from different interpretations of Feyn- man diagrams. The number of pi-pi scattering diagrams looking like this:
was approximately equal in number to the diagrams that looked like this:
So several physicist began drawing diagrams more like this
to describe quark-antiquark interactions, where slicing this diagram top to bottom or side to side gives either of the two Feynman diagrams. The question of what was in the middle sparked further interest in the study of strings.
It is interesting to note that the initial studies were at a much larger scale than the Planck length, which is closer, although not identical, to the scale considered in more recent theories.
As these ideas developed, Bosonic String Theory was born; however, some issues arose from the first theories: the appearance of a spin-2 massless particle that could not be removed; the existence of particles with imaginary mass called tachyons; the necessity of 26 dimensions instead of 4 to preserve quantum Lorentz invariance. But most crucially was, perhaps, the fact that the theory only involved bosons.
It was not until the second string revolution that supersymmetry was imposed, alleviating 16 of the dimensions, and producing Fermions. However, the newfangled superstring theory had five different flavors. Taking those five theories as different facets of the same overarching set of rules, M-theory was developed, but with an extra eleventh dimension. F-theory fixes other problems but adds an additional dimension, making a total of twelve spacetime dimensions.
All in all, string theory has gotten quite a bit of attention, compared to its competitors such as Loop Quantum Gravity and Causal Sets, but whether or not this attention is warranted is yet to be determined. It has been said before, and I will say it again: the true nature of the universe, if ever fully realized, is likely to be far beyond anything we can imagine.