Quantum mechanics, though weird, is fantastically accurate. The accuracy, though, is of a strange kind. Instead of giving values, quantum mechanics gives probabilities of finding values. This creates a unique problem for Many Worlds.

The situation is simple when the probabilities are 50/50. Either a photon passes through a half-silvered mirror, or else it bounces off. Simple, in one possible world possibility A occurs, in the other it’s B. No problem.

But what if most of the silver is scraped off the mirror, so that the probability of transmission is 90%. Does that mean that, instead of dividing into two worlds, the division is into ten, with 9 photons passing through and only 1 being reflected back? What if the probability is 88.6273%? You see the problem. How can many worlds reproduce the fantastic accuracy of quantum mechanics if its main tool is the splitting of worlds? Are we to believe that every pane of glass splits the world not just in two, but into an essentially infinite set of universes?

Yes, says David Deutsch. In Schrodinger’s Rabbits, Colin Bruce discusses the measure problem and describes Deutsch’s approach, constructed along with another Oxford researcher named David Wallace. Deutsch and Wallace have created the mathematical foundations for many worlds-style probability, and have derived the basic probability that emerges from traditional quantum mechanics from their new approach. It’s a stunning achievement, one mentioned by Brian Greene in The Hidden Reality, as well.

Though there are certainly still skeptics, it certainly appears that convincing progress has been made on the measure problem in many worlds. The cost, however, is high. Rather than an ordinary universe that might occasionally split into two when a quantum “decision” is made, we now see that many worlds describes a universe that is constantly dividing, not just into two alternate worlds, but into essentially infinite worlds. Many worlds is not just a necessary add-on; it is, in fact, the fabric from which reality is stitched. And this changes everything, as we will see.

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