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I've always been fascinated by the notion of parallel worlds. As a child I read a lot of books by Andre Norton: she was my "gateway drug" into the SF genre. Her novel The Crossroads of Time introduced me to the idea that there existed different versions of the Earth, where history had taken different paths; and with a suitable technological gizmo, you could travel between all these alternate worlds.

It's a well-worn trope, of course. Authors are often drawn to gaudy, big-picture scenarios: Earths with radically different histories, based on different outcomes from major events such as the American Civil War, the extinction of the dinosaurs, etc.

However, if there are many parallel worlds, then some of them will be very similar to our own, having only recently diverged. And it seems plausible to me that if the technology for travelling to parallel worlds is ever invented, then these similar worlds will be much easier to reach than the radically different ones (e.g. Earths where the dinosaurs survived), because they're "nearer". After all, in the realm of space travel, it is much easier to land on the Moon than it is to reach distant stars and galaxies.

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