You might imagine this produced religions, superstitions, cults and would-be sorcerers. But magic is just another way of trying to understand the observed world. Your historical magicians, who you might scorn for their credulity, were deeply concerned with the rules and mechanics of how things work. In our world as in yours, their studies transformed seamlessly into those of the first scientists. We worked out how to use minute local variances of pressure, we harnessed electricity and the fundamental properties of matter as a source of power.

Exactly one hundred and seven years ago, give or take a few lunar months, our most visionary scientist of the day completed her great work—a unified theory of the substance of space–time. She theorized that in order to account for the phenomena we could perceive and measure, there must be other Earths parallel to our own. The earliest beliefs of our mystical forebears were true; the weak spots really were doorways to other worlds. Practical proof of this came some nineteen years later. We were able to open a door into another timeline for precisely two point eleven seconds.

Research and experiment proceeded at a rapid pace from there on in. Very few of us ever asked why these spots existed—this was the natural order of things, we thought. What we did not realize, at that point, was that other species had begun to test the bars of their cages too. What we did realize was that the branching of timelines was part of a process.

Imagine a tree trunk with many branches—and all the branches, wherever they arise on the stem, all reach up to the same height. That’s because they’re timelines and that height is the present moment, and that’s how time works. And throughout the history of time, as we know it, the tree has put out new branches. These are new timelines, each one growing off the trunk a little higher—thus diverging later in time. And all these worlds share the same roots. There was, we hypothesized, a prime Earth, somewhere far back in time: a point where all the timelines agreed, which formed the stem all these branches arose from. And we turned out to be exactly right.

But before we get to that—before we get to the important bit, really—I need to share something with you. We worked out something else, and to our knowledge no other species ever took this step. We realized that the process whereby timelines branched off from one another was not only scientifically explicable, but replicable.

Our technology, being based on the existence of the parallel Earths, was uniquely suited to such an act of hubris. And our scientists were so buoyed by their understanding of the cosmos that they did not examine the consequences of such an act. Drunk on our wisdom, we simply did it.

We made our own subordinate timeline.