# Particle decay in Everett's many-worlds interpretation (MWI) - is it probabilistic?

I've watched Sean Carroll videos where he describes how to use Everett's many-worlds interpretation (MWI) for e.g. https://en.wikipedia.org/wiki/Schr%C3%B6dinger%27s_cat. Superposition of one particle's stats entangled with corresponding environment. No need to use probabilities for collapse, one function of the universe. Also I thought it supports Einstein's "God does not play dice with the universe".

Recently I recalled https://en.wikipedia.org/wiki/Particle_decay and got curious: does Everett still need to use probabilities for decay or it is explained in other way in his interpretation?

I could not find the answer via googling.

• Everett's many worlds interpretation, as the name implies, is an interpretation on the principle of quantum mechanics. When doing computation, everything is done in exactly the same that you would usually do with QM, regardless of the interpretation you subscribe to. As for the probabilities of decay, you still use those since you will observe either one or the other "universes" according to the probabilities computed in QM. Commented Jun 7, 2022 at 12:08
• @Frotaur, thanks. In such case favoring Everett's interpretation is not enough to support favoring "block universe" which IIRC Sean Carroll is fan of. Commented Jun 12, 2022 at 6:54

## 1 Answer

When a particle decays in quantum physics the state changes so that $$|undecayed\rangle\to\alpha|undecayed\rangle+\beta|decayed\rangle$$

The quantities $$|\alpha|^2$$ and $$|\beta|^2$$ obey the rules of probability when the states $$|undecayed\rangle$$ and $$|decayed\rangle$$ have undergone decoherence and can't interfere with one another obey the rules of probability and can be treated as decision theoretic probabilities. This isn't the same as the standard idea of probability, which claims that probability is a result of states being picked randomly, but it is testable. For another perspective on getting probabilities without collapse, see Zurek's work on envariance.

• Thanks. Rather technical for me, would take ages to comprehend, I'm started trying. Might be more appropriate a separate question, still: "do you see how does it relate to block universe idea which Sean Carroll favors (IIRC)?" Commented Jun 12, 2022 at 6:59
• Quantum mechanics without collapse implies the existence of other universes, which are required to explain single particle interference anyway, other times are special cases of other universes. See "The Fabric of Reality" by David Deutsch, especially Chapter 11 for a popular explanation of the quantum theory of time. Commented Jun 13, 2022 at 7:29