The parallel worlds of quantum mechanics

Modern physics is rich with speculation about multiverses and parallel realities. But there are very different ways that multiple universes might come about, and one of the most mind-blowing – the Many-Worlds formulation of quantum physics – is also one of the most plausible.

The English language rounded into shape long before modern physics came on the scene, so it’s no surprise that words like “world” and “universe” have ambiguous meanings. When you hear physicists talk about “the multiverse”, chances are they are thinking of the cosmological multiverse. That sounds pretty grand, and it is, but it’s not really a collection of distinct universes. Rather, it refers to a collection of regions of space, so far away that they are unobservable to us, where conditions of very different. There may be different particles, different forces, even a different number of dimensions of space from what we see around us.

The cosmological multiverse wasn’t invented because physicists thought it would be cool to have a bunch of universes out there. It arises naturally as a consequence of other speculative ideas, including string theory and cosmological inflation. But exactly because those ideas are themselves speculative, the cosmological multiverse should be thought of as speculative-squared. It may very well exist, but the only thing to say right now is that we don’t really know.

The multiple “worlds” of quantum mechanics are something else entirely. They are not far away – but only because they aren’t “located” anywhere at all. And they arise naturally from the simplest version of our most solidly tested physical theory, quantum mechanics. The many worlds of quantum mechanics, I would argue, are probably there. (Not everyone agrees with me about this.)

To see why, we have to think about how quantum mechanics works. Consider an electron, which is an elementary particle that has a certain fixed amount of a quantity called spin. When we measure its spin, we get only one of two possible answers: it’s spinning up or down, with respect to whatever axis we used to measure it.

That would be weird enough as it is – why only two possible answers? But even weirder is that we can’t always predict what that measurement outcome is going to be. We can prepare the electron in a “superposition” of spin-up and spin-down, such that there will be some probability of observing each outcome. Physicists describe the state of the electron in terms of a “wave function,” which tells us how much of the state of the electron is spin-up, and how much is spin-down. We can use the wave function to calculate the probability of each measurement outcome.

It’s natural to think that there really is some answer to how the electron is spinning, but we just don’t know what it is, and the wave function encapsulates our ignorance. That was the original hope of people like Albert Einstein. But it hasn’t worked out that way; the more we do experiments, and the more we understand the inner workings of quantum mechanics, the more it seems like the wave function really exists. It doesn’t just characterise our knowledge, it’s the real physical state of the electron.

There are many objections to Everett’s Many-Worlds formulation of quantum mechanics, of varying degrees of respectability. Some argue that it’s too extravagant, what with all those worlds. But the potential for the worlds is there in any version of quantum theory for which the wave function represents reality. Many-Worlds is actually the simplest and least elaborate version of quantum mechanics we can imagine. It’s just wave functions obeying the Schrödinger equation, nothing more or less.

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