The aliens of classic science fiction work because they are other than us. They have impossible biologies, motives we cannot follow, bodies that repel us. We look at them and we know where we end and where the threat begins.
Then the multiverse arrives and that boundary disappears.
In parallel universe fiction the threat has our face. It has our childhood, our siblings, the same scar on the chin. At some point in its life it chose differently, and now it wants to destroy us. The unease this produces in the reader has a particular quality, because it has nothing to do with the unknown. It has to do with how little it took for that person to turn out this way.
The double before quantum physics
The idea is far older than quantum mechanics. The doppelgänger of the German tradition, The Picture of Dorian Gray, Poe’s William Wilson: literature has always known that the most effective horror is the reflected one.
Science fiction did one thing more. It took the psychological intuition and gave it a physical foundation. Hugh Everett’s many-worlds interpretation, formulated in 1957, holds that a quantum measurement does not collapse into a single outcome: every outcome occurs, on different branches of the wave function. Nobody has ever observed one of those branches, and physicists remain divided on its standing. As a narrative engine, though, it is perfect, because it strips the double of anything supernatural. Your alter ego is not a ghost. It is a consequence.
Where the theme works best
Dark (Netflix, 2017) remains the most rigorous example of how to build a multiverse without cheating. The two worlds of Winden are not decorative variants: they have an internal logic that holds to the final episode, and the fatalism of the series comes from there. People return to their state, always.
The Man in the High Castle by Philip K. Dick works on the political level. One different outcome of the Second World War, and the book spends its time asking which of the two Americas is the real one. Dick does not answer.
The Dark Tower by Stephen King uses parallel worlds as the geography of obsession. Roland crosses versions of the same reality and every passage costs him something he never gets back.
Dark Matter by Blake Crouch brings the theme onto the ground of domestic regret: a man meets the lives he could have had and has to decide which one truly belongs to him. It is the multiverse applied to a personal biography.
The Three-Body Problem by Liu Cixin does not use parallel universes in the strict sense, but it shares with this strand the structure of the conflict: two fronts separated by a physical barrier, each convinced the other is the aggressor. The mutual misunderstanding is the real engine.
The mutual misunderstanding
Here is the knot that the best novels on the theme recognise. When two worlds open onto each other, each sees the opening as an invasion. Neither is wrong from its own side of the portal. Both arm themselves in self-defence and the result is a war nobody wanted.
It is the logic of the security dilemma applied to cosmology. It works in fiction because the reader sees both sides and understands that the catastrophe is avoidable, while the characters cannot see it. This asymmetry of information produces a tension the hostile alien never produces.
How I handled it in the H.O.M.S. trilogy
When I started writing the second volume I had an idea that seemed sufficient: the crew of the Blue Whale crosses the singularity opened by Aldon Eqok and ends up in a mirror world. Then I realised the real subject lay elsewhere.
In that universe there is another Aldon Eqok. Same genome, divergent biography: in place of the obsessed billionaire of the first volume, a professor of theoretical physics with two Nobel prizes. He sees a ship arrive out of nowhere and concludes the only reasonable thing, that it is the vanguard of an invasion. From that moment on the plot almost writes itself, because the Professor’s fear is legitimate.
In the third volume I pushed the thing where it had to go. Blondie, who in the trilogy is the most instinctive and most alive voice, meets her own version of that world: Director Marek. Same origin, five brothers, one brother killed in a robbery when they were children. Then the two biographies separate, and one of them reaches the top of the International Space Force determined to invade an entire universe. When they come face to face in the chapter titled “The Mirror”, the Director alludes to a trauma that made her what she is, and does not tell it. I chose not to let her say it because the explanation would have made the character easier to judge.
The physical principle that holds the ending together I called conservation of frequency: matter can be altered in its vibration only for as long as the device stays on, then it returns to its natural state. It was there as a technical basis for the sabotage of the fleet. Rereading it months later I notice it applies to people too, and that is probably why I wrote it.
The price of the crossing
One detail that fiction about parallel worlds often neglects: crossing costs something. In H.O.M.S. 3 passing through the singularity brings a temporal shift, and the characters discover it only on their return. Twenty-two years. The world has moved on, the children are adults, some people are gone.
Anyone who has seen Interstellar will recognise that kind of blow. It interested me because it shifts the closing question of the book. No longer “will you manage to stop the invasion”, but “what does it mean to come home when home did not wait for you”.
The H.O.M.S. trilogy is complete. All three volumes are available on Amazon in Kindle format, free for Kindle Unlimited subscribers, and in hardcover.
If you are drawn to science fiction that starts from a scientific premise and follows it all the way through, here are other titles working the same ground.
