IT WAS a cryptic email that Juan Maldacena pinged across the US to fellow physicist Leonard Susskind back in 2013. At its heart lay a single equation: “ER = EPR”. The message clicked with its recipient. “I instantly knew what he was getting at,” says Susskind. “We both got quite excited.”

Excited, because that one equation promises to forge a connection between two very different bits of physics first investigated by Albert Einstein almost 80 years ago. Excited, because it could help resolve paradoxes swirling around those most befuddling of cosmic objects, black holes, and perhaps provide a route to a unified theory of physics. Excited, because it might even answer one of the most fundamental questions of all: what is reality made of?

The origins of the story lie precisely a century ago. In November 1915, Einstein presented the final form of his revolutionary theory of gravity .