The idea of parallel worlds or alternate timelines is a compelling source of endless entertaining science fiction stories. It is also taken seriously in theoretical physics as a possible interpretation of quantum mechanics, that bizarre empirical phenomenon in which matter loses its substantiality at sub-atomic scales, particles become waves, and reality becomes probabilistic – until the moment when an observation is made. An observation precipitates the so-called “wavefunction collapse” and restores the familiar objective world of classical physics. In the many worlds interpretation of quantum mechanics, it is assumed that the wavefunction simply describes all the possible versions of the Universe, and when we make an observation, we don’t collapse the wavefunction, we just determine which version of reality we are currently in. It’s an appealingly simple interpretation, which saves us from having to resolve the paradoxes of wavefunction collapse; it is a theory that has been jokingly described as “short on assumptions, long on Universes.” And now it has been updated with the many interacting worlds version, with its tantalizing possibility of being verifiable experimentally.

I remember reading a short story by Larry Niven, All The Myriad Ways, in which a corporation had figured out a way to travel among the many worlds, discovering all the possible timelines, and demonstrating to all that, indeed, the Universe does split into different versions every time a choice is made. This ends up prompting a wave of casual crimes – including suicides, murders, and rapes – as people process this knowledge to the extreme logical conclusion that there is no reason to consider the consequences of actions. After all, if I kill someone, what does it matter, since there is some other Universe in existence in which I didn’t kill them?

I wouldn’t hold out for a dimension-hopping corporation to come along and sell us souvenirs from alternate historical timelines, though. The best interpretation of quantum mechanics is the Von Neumann–Wigner interpretation – consciousness causes collapse. This interpretation neatly resolves the paradox of wavefunction collapse by accepting its salient property – it occurs at the moment of observation. Through the vehicle of consciousness, the Universe comes into being, one timeline unfolding according to our choices. This is why our actions are rife with moral significance: each choice fatefully fixes the storyline, and dispenses with the alternatives.

So Niven’s story can be seen as a parable about the nature of moral choice – it is meaningful because there is one world, limiting us with its physical laws, shaping our destiny as we travel through time. Parallel realities make for compelling television entertainment, but they are not actually going to let us escape this reality. This life is your one chance to make the right choices, and the stakes are very real.