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String theory began over 50 years ago as a way to understand strong nuclear forces. Since then, it has grown into a theory that can explain the properties of every particle, every force, every fundamental constant, and the existence of the universe itself. However, despite decades of effort, they were unable to deliver on that promise.
What went wrong and where are we going?
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Like most revolutions, string theory had humble origins. It began in the 1960s as an attempt to understand the workings of the recently discovered powerful nuclear forces. Quantum field theory, which had been successfully used to explain electromagnetism and the weak nuclear force, had failed.
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A group of physicists took and extended a mathematical technique developed by Werner Heisenberg, the godfather of quantum, (later abandoned). In that extension, they discovered the first string, a mathematical structure that repeats in spacetime. Unfortunately, this string primordial theory made erroneous predictions about the nature of strong forces and also had various nasty artifacts (tachyons, the presence of particles that only travel faster than light, etc.). When another theory was developed to explain the strong forces, the theory we use today was developed, based on quarks and gluons, and string theory faded from the scene.
But like most revolutions, the whispers lingered through the years, keeping hope alive. In the 1970s, physicists revealed some remarkable properties of string theory. The first is that this theory can support more than just strong nuclear forces. The strings in string theory had great tension and were wound up around the smallest possible volume, the Planck scale. Once in place, the string can support a wide range of vibrations, much like a taut guitar string. Different vibrations led to different manifestations of force: one sound a powerful nucleus, another sound an electromagnetic force, and so on.
One of the possible vibrations of the string acted like a massless spin 2 particle. Because this is a very special particle, the quantum force carrier of gravity, the holy grail of quantized gravitational theory. Theorists of the time couldn’t believe their blackboard: string theory naturally and elegantly included quantum gravity, but they hadn’t even tried!
The second major problem published in the 1970s was the introduction of supersymmetry, in which all force-carrying particles (called bosons, a category that includes photons and gluons) have supersymmetry from a collection of constructing particles. Claimed to be linked to a partner. Things (called fermions, like electrons and quarks), and vice versa.
This symmetry is not seen in everyday environments. It appears only at very high energies. So if we had enough money to go back to the earliest moments of the Big Bang, or build a particle collider along Jupiter’s orbit, we wouldn’t just look at the usual menagerie of particles we’re all familiar with. . All supersymmetric partners are also shown. These were given appropriately silly names like selectrons, sneutrinos, squarks, photinos, and my personal (least) favourite, wino boson.
By establishing this connection, string theory can build a bridge from bosons to fermions, leaping from just a theory of forces to a theory of all particles in existence. The introduction of supersymmetry also solved the thorny problem of tachyons by replacing pesky particles with supersymmetric partners.
By the end of the 1970s, string theory had the potential to explain all particles and all the interactions between them, providing a quantum solution to gravity.
One theory to rule them all, one theory to find them, one theory to bring them all, and string them together.
