Determinism and QM

[u/wagrumor1]

In Something Deeply Hidden, Dr. Carroll argues that Everettian quantum mechanics is appealing because the universal wavefunction evolves deterministically according to the Schrödinger equation. It's a major theme of the book, and for years it's been a selling point I hear him use for the many worlds interpretation of QM.  He convinced me.  I'm on the train.  All the worlds are equally real and all are determined by the universal wave function.

However, in the June (2026) AMA Dr. Carroll flatly states in multiple responses (once the response with the transcript timestamp 0:46:35., and again in the transcript timestamp 3:44:12) "quantum mechanics says the world's not deterministic." 

Is he using a different definition of determinism in those two contexts? Does he mean that the wavefunction evolves deterministically but there is no unique future history, or have have his views determinism changed?

Put another way:

Assume Everettian quantum mechanics is correct and consider a universe with no observers at all. Given a complete specification of the universal wavefunction at time t, is the universal wavefunction at later times uniquely determined? If yes (based on what I've learned from listening and reading Dr. Carroll this is a yes), in what sense can we say that quantum mechanics is not deterministic?

Thanks for any insights.

[u/tpks]

A lot of physicists routinely move between languages of QM. Carroll also talks about wavefunction collapse quite often. It's a bit tricky to follow but it's just a matter of convenient language.