Physicists say the multiverse imposes a paradox on us. Multiverse cosmology is based on cosmic inflation, the idea that the universe underwent a short burst of rapid expansion in its early stages. Inflation theory has relied on observation for some time, but it has the unfortunate tendency to generate not one but many universes. And because it doesn’t say which one we should be in – it misses that information – the theory loses much of its ability to predict what we should see. It is a paradox. On the one hand, our best early universe theory suggests that we live in a multiverse. At the same time, the multiverse destroys much of the predictive power of this theory.

In fact, it wasn’t the first time that Stephen [Hawking] was confronted with a mystifying paradox. In 1977, he hit upon a similar enigma regarding the fate of black holes. Einstein’s theory of general relativity predicts that almost all information about anything that falls into a black hole remains hidden inside it forever. But Stephen discovered that quantum theory adds a paradoxical twist to this story. He discovered that quantum processes near the surface of a black hole cause the hole to emit a light but steady stream of particles, including particles of light. This radiation – now known as Hawking radiation – is too faint to physically detect, but even its mere existence is inherently problematic.

The reason is that if black holes emit energy, then they must shrink and eventually disappear. What happens to the huge amount of information hidden inside when a black hole emits its last ounce of mass? Stephen’s calculations indicated that this information would be lost forever. Black holes, he argued, are the ultimate garbage can. However, this scenario contradicts a basic tenet of quantum theory which dictates that physical processes can transform and scramble information, but never irreversibly erase information. Once again we come to a paradox: quantum processes cause black holes to radiate and lose information, but quantum theory says this is impossible.

Paradoxes related to the life cycle of black holes and our place in the multiverse have become two of the thorniest and most debated physics puzzles of recent decades. They are interested in the nature and fate of information in physics and thus touch the heart of the question of what physical theories are ultimately about. Both paradoxes emerge in the context of so-called semi-classical gravity, a theoretical description of gravity pioneered by Stephen and his Cambridge gang in the mid-1970s, based on an amalgamation of classical and quantum thought.

The paradoxes arise when applying such semi-classical thinking either over extremely long timescales (in the case of black holes) or over extremely large distances (in the case of the multiverse). Together they embody the profound difficulties that arise when we try to make the two pillars of 20th century physics, relativity and quantum theory, work in harmony. In this role, they served as mind-bending thought experiments, with which theorists extrapolated their semi-classical thinking about gravity to extremes to see where and how exactly it would collapse.

Thought experiments have always been Stephen’s favorite. Having given up on philosophy, Stephen enjoyed experimenting with some of the deeper philosophical questions – whether time had a beginning, whether causality was fundamental and, most ambitious of all, how we as “observers” fit into the cosmic scheme. And he did it by presenting these questions as clever experiments in theoretical physics. Three of Stephen’s landmark discoveries all resulted from ingenious and carefully configured thought experiments. The first of these was his series of big bang singularity theorems in classical gravity; second, his 1974 discovery in semi-classical gravity that black holes radiate; and third, his boundaryless proposition, also in semi-classical gravity, for the origin of the universe.

Now, while one might argue that the black hole paradox is only of academic interest – the fine details of Hawking radiation will probably never be measurable – the multiverse paradox bears directly on our cosmology observations. At the heart of the paradox is the strained relationship in modern cosmology between the living world and observation, and the physical universe. The multiverse paradox became a beacon in Hawking’s quest to rethink this relationship by developing an entirely quantum perspective on the cosmos. His latest, resolutely quantum theory of the universe redraws the fundamental foundations of cosmology and constitutes Hawking’s fourth major contribution to physics. The grand thought experiment behind the theory had kind of been in the making for five centuries. Realizing it would be our journey.