How to Understand Parallel Universes

They once used to be the exclusive domain of science fiction, but today, many physicists believe in parallel universes: that is, alternate realities that can be either inconceivably different from the world as we know it (say, with different elementary particles or forces) or nearly identical in every respect. Here’s how to wrap your mind around the idea of an alternate “you” somewhere out there in the cosmos.

Instructions

Difficulty: Moderate

1. Understand that there are many different types of parallel universe. The parallel universes most folks are familiar with stem from quantum physics: according to one interpretation of this discipline, all possible realities exist simultaneously, but our consciousnesses split off at each measurement point (say, when a particle registers at a detector). But parallel universes may also exist on the fringes of our own spacetime, trillions of trillions of light-years away, or in other universes spawned by Big Bangs.
2. Get used to the idea—it’s practically a fact. As you read this, scientists are busy trying to build quantum computers, which harness the “many universes” interpretation of quantum physics to perform trillions of operations simultaneously (the vast majority of them transpiring in parallel universes). According to the quantum theoretician David Deutsch, if a sophisticated quantum computer is built, that will prove that parallel universes exist in the quantum sense.
3. Picture a world far, far away. As far as scientists know, our universe is infinite in extent, with the farther reaches only cut off from view because their light waves haven’t had time to reach us yet. If this is true, then there may be a parallel Earth an inconceivable distance away (say, 1 followed by a trillion zeros light years), or even an entire mini-universe that resembles our own in every respect.
4. Imagine a bunch of Big Bangs. One popular cosmological theory posits that baby universes are constantly bubbling up out of primordial chaos, with our universe residing inside one such bubble and others inside other bubbles that reside outside our own spacetime. Since this process presumably has been going on forever, there should be an infinite number of such universes, and an infinite number of parallel "you"'s.
5. Don’t plan on visiting anytime soon. According to most theoretical models, there’s no way to visit a parallel universe in the quantum or chaotic Big Bang sense (though if you had a fast rocketship and an infinity of time, you could conceivably reach an island universe inside our own universe). But even if there’s no way to meet an alternate version of yourself, isn’t it kind of comforting to know he’s out there somewhere?