This was the office of the transEarth Institute, an arm of the Black Corporation. The faceless office, all plasterboard and chrome, was dominated by a huge logo, a chesspiece knight. This wasn’t Joshua’s world. None of it was his world. In fact, when you got right down to it, he didn’t have a world; he had all of them.

All of the Long Earth.

Earths, untold Earths. More Earths than could be counted, some said. And all you had to do was walk sideways into them, one after the next, an unending chain.

This was a source of immense irritation for experts such as Professor Wotan Ulm of Oxford University. ‘All these parallel Earths,’ he told the BBC, ‘are identical on all but the detailed level. Oh, save that they are empty. Well, actually they are full, mainly of forests and swamps. Big, dark, silent forests, deep, clinging, lethal swamps. But empty of people. The Earth is crowded, but the Long Earth is empty. This is tough luck on Adolf Hitler, who hasn’t been allowed to win his war anywhere!

‘It is hard for scientists even to talk about the Long Earth without babbling about m-brane manifolds and quantum multiverses. Look: perhaps the universe bifurcates every time a leaf falls, a billion new branches every instant. That’s what quantum physics seems to tell us. Oh, it is not a question of a billion realities to be experienced; the quantum states superpose, like harmonics on a single violin string. But perhaps there are times — when a volcano stirs, a comet kisses, a true love is betrayed — when you can get a separate experiential reality, a braid of quantum threads. And perhaps these braids are then drawn together through some higher dimension by similarity, and a chain of worlds self-organizes. Or something! Maybe it is all a dream, a collective imagining of mankind.