When Leonard Susskind came up with the idea for string theory in 1969, he wasn’t searching for a theory of everything or trying to provoke a fundamental crisis in physics. His ambition was much more modest, at least by the standards of his field: Susskind was trying to explain the “strong” force binding protons and neutrons inside atoms.

Susskind, then a young particle physicist at New York’s Yeshiva University, realized that the mathematical formula that explains what happens when particle pairs collide made more sense if one imagined the particles as individual loops of string that combine and oscillate together for a little while before parting ways.

Susskind describes his joy in that moment in his book The Cosmic Landscape. “You say to yourself, ‘Here I am, the only one on the planet who knows this thing. Soon the rest of the world will know, but for the moment I am the only one.’”

Susskind’s elation lasted for all of two days, until he learned that two other physicists, Yoichiro Nambu at the University of Chicago and Holger Nielsen at the University of Copenhagen, had converged on the idea at exactly the same time.

A unifying theory

Susskind’s excitement began spreading to the rest of the physics community in 1984, when physicists John Schwarz and Michael Green, now at Caltech and the University of Cambridge, respectively, published a paper suggesting that string theory could describe not only the strong force – the one Susskind sought to explain – but the weak and electromagnetic forces as well. Most intriguing of all, it also predicted the existence of a massless particle called a graviton. Gravitons are the presumed quantum messengers of gravity, the slippery fourth force that refuses to be corralled into the Standard Model, the theoretical framework devised by physicists to explain how the basic building blocks of matter interact.

Leonard Susskind, professor of physics, is one of the fathers of string theory. (Image credit: L.A. Cicero)

Exuberant theorists everywhere soon felt that they were on the verge of reconciling the mathematical discord between general relativity, which explains gravity, and quantum mechanics, which describes the interactions of the other fundamental forces. “There appear to be no insuperable obstacles to deriving all of known physics,” declared one group of supremely confident string theorists.

It wasn’t just string theory’s explanatory power that physicists found bewitching. They were also wooed by its mathematical beauty. Those who plumbed the depths of its rigorous equations compared the quest to spiritual enlightenment.

But the theory demanded a lot from its votaries, such as the blind acceptance of at least 10 dimensions – the four that we are familiar with (up-down, left-right, front-back, and time) – plus another six or more that are invisible because they are curled up, or “compactified,” like origami folded from the fabric of reality.

What’s more, these extra dimensions could have different shapes and sizes and be shuffled in myriad ways. Since the geometry of the dimensions determined how the strings vibrate and thus the particles and forces that they can manifest as, the theory allowed for many combinations of physical laws and constants. “In string theory, the same framework that makes it clear that you can get the same qualitative ingredients of gravity, quantum mechanics and the Standard Model together also makes it clear that you can do that in many, many ways,” Shamit Kachru said.

Hints of a multiverse

The situation grew more complex in the 1990s, when theorists realized that in addition to one-dimensional strings, the theory must also include higher-dimensional membranes, or “branes,” and other ingredients such as fluxes. Physicists had clung to a thread of hope that string theory would reduce down to one inevitable universe whose physics resembled our own. Instead, it seemed to describe an astronomical number of possible universes, a value that was, as Susskind put it, “measured not in the millions or billions but in googols or googolplexes.”

“In string theory, the same framework that makes it clear that you can get the same qualitative ingredients of gravity, quantum mechanics and the Standard Model together also makes it clear that you can do that in many, many ways.”

—Shamit Kachru

Professor of Physics

String theory had reached an impasse, and many physicists were disillusioned. Their best bet for the theory of everything contained so many solutions that it might as well be the theory of nothing at all.

Soon, however, a simple but radical proposition emerged: If the theory’s equations did not point to any particular geometry for the extra dimensions, maybe there was no right geometry. Perhaps each mathematical possibility in string theory is realized in nature as a physical reality, and our universe is merely one of a boundless variety in the multiverse.

But even though string theory allowed for an incredible diversity of worlds, it provided no mechanism for creating them. As Susskind put it, “Mathematical existence is not the same as physical existence. Discovering that string theory had 10⁵⁰⁰ solutions explains nothing about our world unless we also understand how the corresponding environments came into being.”

For that, physicists would need to go back to the very first moments of the universe – to the beginning of time itself.