It’s the simplistic way of explaining the multiverse theory. Whenever we make a decision we simultaneously travel down each and every path, and the universe divides like a pair of trousers with versions of us travelling down on leg each. I don’t like it… it has several huge issues. I prefer universal spaghetti. Here’s why:
We are living components in the most complex Rube Goldberg machine ever, but replace marbles for particles and elaborate conjoining mechanics with fundamental forces. The trajectory of every fragment from the very instant of expansion sets into motion a pattern that would be predictable if you can observe every part. A mass gathers in one location rather than another, a planet forms around a sun that’s astonishingly similar to ours, life begins, crawls onto land, and after several false starts forms a creature that raises a civilisation with a calendar, a complex social structure, and a biology that relies use of tools to survive the environmental phenomena.
The biggest difference between that universe and ours? A meteor strike that would have gone into Jupiter’s outer orbit instead collides with Earth, causing a temperature shift that alters the weather patterns for a few hours just enough to change cloud density, and I decide to wear a coat. In one world I’m too cold and feel like an idiot for leaving it behind, in the other I’m too warm and feel like an idiot for not grabbing a hoodie instead.
Did I still make a decision? Did I still divide universes in two? No, I don’t have that kind of agency. If the meteorological circumstances had been identical and yet I’d made two different choices, then there is an inherent difference between each version of me caused by genetic or environmental differences. Maybe that me has a thinner coat, it looks the same but somewhere in the design process in the manufacture of the coat made a thinner fabric, that affects my decision to wear a coat or not. Or circumstances involving a surprising number of complicated changes of local behaviour meant that I couldn’t buy the coat I currently own and ended up with a different one.
We simply do not have the agency to divide universes, that kind of power in theory only comes about when we manage to observe or navigate between universes, and personally I think that’s when things get more interesting, even time travel starts to make sense and a little extradimensional.
So, at the moment we have a sphere of infinite size that comprises the universe of which we know anything. We have an infinite sphere of universes that comprise all possible outcomes that could have arisen from the initial expansion, every possible permutation of probability. Now imagine that an individual manages to travel through time. The universe in which he arrives is functionally identical except that he has appeared and altered the course of all events, and if he can return to his own time he can never return to his own universe* because he just goes to a future which is changed.
The theoretical existence of time travelling anything be it people, particles, or arthropods with weird defensive measures, creates another layer of infinite possibility, spheres within spheres, and then if you start layering up that’s when things start looking like a Tool video. Tool videos rarely resemble trousers, and vice versa.
So here’s the world we live in. Things happen. You can’t change them. You couldn’t have chosen otherwise, your genetics, circumstances, and passing meteors all change the way you behave, and no matter how powerful you think you are you are a slave to the sloshy chemicals in your head. If you regret anything in your life, then… well stop it. The universe is the way it is, nothing could be anything other than the way it is, and right now these words are either making you feel better about your lot in life, or worse, or neither, and you can’t control that, or how that will affect your behaviour in the future.
In short, this is what there is, go and attend to it. Or learn how to travel between universes, that would also be awesome, actually that would be amazing, an escalation to four-dimensional existence, possible biological changes allowing for… this is a tangent that I’m not going on. Read The Long Earth, watch Sliders, something. Go have fun.
*Unless he already occupies the universe in which he arrives, that’s “perfect” time travel, and is highly unlikely to have happened in our universe. That sucks but we might still have a few universe-hoppers.