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## The Quantum Multiverse

We know through a century of testing that quantum physics describes the universe more exactly than any other theory.

In classical mechanics, to calculate what a particle will do, you measure all of the forces that are acting on it, plug those into a formula, and the formula will tell you exactly what happens next. This works well for all large-scale actions, such as planets orbiting a sun, balls hitting into each other on a pool table, heat propagating through a piece of metal.

But when it comes to measuring very small things, such as where photons will hit a wall when fired through two slits, we find that we cannot predict the individual photons exactly. Instead, we calculate how probable the photons are likely to hit parts of the wall, etc.

This is a very important difference. In the classical world, we knew exactly how things behaved, and could predict the entire future of the universe if we could know the current positions, etc, of all objects in it. But in the quantum world, we cannot do this anymore. Instead, we can only say that some things are more probable than others. For large collections of particles, such as the total number of atoms in a ball, when we drop the ball, the probability that the ball as a whole will fall straight down is so close to certain that we might as well just call it certain, but if we consider each particle of the ball individually, there is always a chance that a particle may just go up instead of down. But on the whole, the ball goes down, so Classical Mechanics works even though Quantum Mechanics works as well.

The math of quantum mechanics is very precise and makes predictions that are the most accurate descriptions of reality that we have, but there is a big philosophical question about them – the math says something that can be interpreted in two very different ways. The interpretations are different, but the results are the same whichever interpretation you use. Because the results are the same, a lot of physicists don’t think it’s important which interpretation is right. But it’s important for us, because one of these interpretations says that there are infinite universes that all exist at the same time.

In quantum mechanics, to calculate what a particle will do, you use the current state of every particle in the system (the group of all particles than can possibly affect this one) to generate a wave equation. This wave equation predicts all possible outcomes of the system, along with a probability for each outcome.

In the “Copenhagen Interpretation”, the wave equation then “collapses” to choose one possible solution, and that result is what we interpret as reality.

In the “Many Worlds Interpretation” (MWI), all results of the equation are considered equally real. There is no collapse. Reality is simply one among a crowd of possible results, each of which is also reality. You, being a construct of your reality/universe, obviously only experience the reality you are in. But, the MWI says that almost every other result also has a “you” in it that considers that result to be reality and wonders if the others (including the one where you are reading this) are real.

*Illustration 9: in a quantum multiverse, all possible outcomes of the wave equation are equally real. Schrödinger's cat really is both alive and dead.*

Some people misinterpret the MWI to say that every moment, the entire universe literally splits into infinite pieces, each of which will then also split infinitely, etc. That’s not true, though. The MWI does not say that the universe splits; just that there are infinite possible results of the equation. The universe does not actively come apart particle by particle and clone itself. We’ll think more about this in the next section, when we talk about the Mathematical Universe.

The point here is that the MWI says that everything that can possibly happen, does happen. We may not experience it directly here, but there exist universes where a light photon hit the wall 1cm to the left, or a Geiger counter blipped one time more in the last second, or even that you stayed home instead of going to work.