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Every revolutionary idea seems to evoke three stages of reaction.
(1) It’s completely impossible.
(2) It’s possible, but it’s not worth doing.
(3) I said it was a good idea all along.
Arthur C. Clarke

As is so often the case, string theory arose from a collection of discredited ideas. Over a period of 50 years vital pieces of the puzzle came to light, only to be ignored in favour of more fashionable topics. When string theory became mainstream, physicists realised that these early insights were extraordinarily prescient.

The story begins in 1919 with a little known Polish mathematician, Theodor Kaluza. Inspired by Einstein’s revolutionary ideas, he attempted to overthrow a central tenet of physics. “What if there are extra dimensions we just can’t see?” he asked.

Working alone, he attempted to incorporate a hidden dimension into Einstein’s model for gravity. Unsurprisingly, his five-dimensional theory had more equations than the usual four-dimensional approach. Looking closely at the extra equations he had found, Kaluza spotted something remarkable. They were precisely Maxwell’s equations governing the electromagnetic field!

This unification of electromagnetism and gravity was completely unprecedented. Even Einstein had praise for the achievement, commenting, “I like your idea enormously”. But Kaluza didn’t have an explanation for why we don’t notice the fifth dimension. This discovery fell to another outsider, Oskar Klein.

Klein realised that the extra dimension could curl up to form a circle. If the circle were small enough we’d never realise it was there. Between them, Kaluza and Klein had taken the first baby steps towards a higher dimensional reality.

Sadly Kaluza-Klein theory was almost immediately shown to be wrong: its predictions differed vastly from experimental data. Alongside the birth of quantum mechanics, nobody had any time for a broken toy model of reality. And so Kaluza and Klein were forgotten. Forgotten until string theory, that is.

The second foundational fragment fared somewhat better. By 1943 quantum mechanics was established at the forefront of physics. Experiments had shown that electrons could be thought of as pointlike particles, but the atomic nucleus was causing some problems: protons and neutrons seemed to behave more like spheres than points.

Werner Heisenberg was among the first to tackle the issue. He proposed that our usual notions of smooth spacetime break down at subatomic scales, due to the uncertainly principle. This allows protons and neutrons to have spatial extent from the quantum perspective, but appear pointlike for general relativity.

Although this solved the problem of protons and neutrons, it raised yet bigger problems. Heisenberg was claiming that at the quantum scale, space and time are unreliable. It’s pointless trying to keep track of exactly how particles interact if you can’t rely on a fixed backdrop. “So”, said Heisenberg, “just calculate the probabilities of different interactions taking place”. In other words, don’t worry about exactly how things happen.

This bizarre new viewpoint became known as S-matrix theory. Unfortunately it was rather hard to do calculations in Heisenberg’s model. Although it had some success, it was eventually supplanted by the ideas of quantum field theory.

Nevertheless some researches persevered. One of these was Gabriele Veneziano. Using tools inspired by S-matrix theory, he kickstarted string theory. Heisenberg’s ideas of quantum spacetime breakdown eventually reached full fruition during the 1980s. These neglected ideas, and others, found joyous vindication in the humble string.

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