The Quantum Multiverse
Some time ago, I argued in favor of the Many-Worlds interpretation of quantum physics, explaining how I had misunderstood it before, and why I changed my mind. I recommend reading that post for the full story, but the short version is, the Many-Worlds interpretation (or more accurately, the Universal Wave Function interpretation) is the straightforward interpretation of quantum physics, explaining the weird paradoxes without adding any extra assumptions to the theory. All other interpretations require adding something; either a collapse condition, where a measurement causes the wave function to immediately change shape all at once, ignoring the speed of light, or some kind of hidden variables we can’t measure. The Many-Worlds interpretation says the wave function described by the Schrodinger equation is correct, and that is all. Everything else, the multiple universes and such, are deduced from that single statement.
However, there is one thing that bugs me, one loose end Many-Worlds doesn’t explain. That is why today, I am going to continue the Best Arguments Against series, and put forth my best argument against the quantum multiverse.
Most people who argue against Many-Worlds don’t understand it. Some dismiss it out of hand as too weird. Others get Occam’s Razor backward and think Many-Worlds has branching universes as a postulate, rather than a deduction. These are not good arguments.
The best argument has to do with probabilities. If we look only at the math, it makes sense that the wave function can split into two worlds, one with an amplitude three times stronger than the other. Think of a pile of sand. Someone splits it into two, one of which is three times bigger than the other. Where there was one pile, there are now two of differing heights. That’s an apt analogy for the splitting of universes, and it’s quite easy to visualize. Nothing confusing or difficult to believe there.
However, when we do an experiment, the outcome with the higher amplitude has a 90% chance of happening, and the outcome with the lower amplitude has a 10% chance of happening. If the universe splits into two equally real branches, as Many-Worlds claims, where does this probability come from? Many-Worlds says there will be two universes, each of which will have an equally real version of us. Thus, before the experiment, it would be natural to think there would be a 50% chance of finding ourselves in each universe after the measurement. But it’s not; it’s 90-10. This is a contradiction, and a currently unresolved paradox in the Many-Worlds theory.
This is important, because we rely on quantum probabilities all the time. The nuclear fusion that powers the sun and gives the Earth energy uses quantum probability. The half-lives of unstable elements, used in various technologies and geological dating methods, rely on quantum probability. And most conspicuous of all, quantum computers work by minimizing the quantum probabilities of the wrong answers and maximizing the quantum probability of the right answer. If there were simply one universe for each possible outcome, our intuition says none of these should work!
There is one way to resolve this conceptually. Suppose instead of two worlds, there are a large number of them, and 90% of them go in the direction of higher amplitude, and 10% of them go in the direction of lower amplitude. Those 90% are completely identical to each other, as are the 10%. This would square off the probabilities. Instead of there being two of you, one in the low amplitude world and one in the high amplitude world, there would be many of you, split between the worlds 90-10.
However, this takes away the straightforward purity of Many-Worlds. The math does not say there will be lots of identical universes, it says there will only be one for each possibility. Resolving this conflict by proposing large numbers of identical universes adds extra fluff to the theory, taking away its advantage over the other interpretations!
This could mean Many-Worlds is the wrong interpretation of quantum physics. Or it could be that we just don’t understand quantum probability yet, and we will find a satisfactory resolution to the paradox. Many-Worlds is still the tidiest interpretation so far, since it explains all the other phenomena of quantum physics, like entanglement and the measurement problem, so neatly. For now, I still rank it as the most plausible, leaving the door open for something else to come in and explain it all.
I don’t like the quantum multiverse. I want there to be only one course of history. It feels cheap if everything that is physically possible happens in one timeline or another. But, as mentioned at the beginning of the post, the fact that it is weird is not valid evidence. I hope Many-Worlds is wrong, but as a truth seeker, I cannot let that hope influence what I believe. The truth is the truth, whatever it turns out to be. Maybe there are a near-infinite number of universes splitting off from one another every moment, or maybe there is just this one. All we can do is follow the logic and evidence where it leads us.
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