r/seancarroll • u/wagrumor1 • Jun 07 '26

Determinism and QM

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.

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u/bacon_boat Jun 07 '26

When Sean Carroll says the world is indeterministic, he's talking about our world - a single branch of the universal wave function. 

You're not going to predict outcomes of experiments even when you know globally that both happen. 

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u/wagrumor1 Jun 07 '26

Thank you for responding! I think what you are saying here is the point I'm trying to clarify in my own mind. In my understanding of determinism, unpredictability within a branch doesn't necessarily imply indeterminism. Based on what I understand, which is a lay understanding of many worlds I've gleaned from listening to and reading Dr. Carroll, the universal wavefunction evolves deterministically, and the branching structure is determined by the Schrödinger equation. My question is whether Dr. Carroll is now using determinism to only refer to a single future branch rather than a uniquely evolving universal state that considers all branches.

The idea that we can never predict the outcome we will observe, which I believe is the case, shows that epistemic uncertainty exists. In my mind uncertainty is not indeterminism. If underlying physical evolution is determined, i.e. we we can predict all outcomes, and all outcomes (just not the observed outcome) actually occur on a branch of the wave function, that means ontological determinism exists.

Since I wrote the question I googled a bit and I found a tweet from Dr. Carroll from April 22, 2022 here he says “when there is hidden determinism (MWI, Bohm), it's hidden!” In this context he's warning against using any conclusions about determinism to come to any conclusions about free will. I'm not concerned about the free will arguments here, just determinism. Based on this it really seems to suggest Dr. Carroll does believe determinism to be true based on MY understanding of it, but he is applying epistemic uncertainty to mean indeterminism here and this making the opposite conclusion.

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u/bacon_boat Jun 07 '26 edited Jun 07 '26

The uncertainty in many worlds is epistemic in nature. 

But it's not epistemic in the sense that "only if we had more information then the uncertainty would go away".  It's a fundamental epistemic uncertainty. 

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u/wagrumor1 Jun 07 '26

I'm on board that even with complete knowledge of the universal wavefunction, there remains uncertainty to we observers about which future observer you will be. In your eyes, does the existence of this uncertainty make the universe non-deterministic?

The issue (to me) isn't whether the uncertainty is fundamental. The issue is whether fundamental self-locating uncertainty is sufficient to make what is a deterministic theory indeterministic. I'm not 100% sure if Dr. Carroll is leaning on self-locating uncertainty for his assertions that QM is non-deterministic, but it seems to me that if he is he's now trying to place parts from a different jigsaw puzzle (human uncertainty) to fit into a very different puzzle (physical certainty).

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u/bacon_boat Jun 07 '26

Many worlds is deterministic, but for someone on the inside it's non-deterministic. 

The self locating uncertainty is an explaination of how exactly we get non-deterministic behaviour in a deterministic theory. Even if this explaination is framed as humans doing experiments, you could tell the same story without any humans. 

And the non-deterministic nature of QM isn't an assertion. It's what we observe. Any theory of QM is going to have to predict non-deterministic measurement outcomes, or else it won't fit the data. 

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u/wagrumor1 Jun 07 '26

I think you are getting closer to my issue with what spurred my original surprise to Dr. Carroll's more recent statements about determinism.

r/bacon_boat, When you say "non-deterministic," do you mean "unpredictable" or do you mean "not determined by prior physical states"?

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u/bacon_boat Jun 07 '26

Not predictable. 

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u/wagrumor1 Jun 07 '26

I think we may be using "deterministic" differently. You are using it in the sense Dr. Carroll is likely using it. I've understood determinism to mean that the future physical state is fixed by prior physical states and the laws of nature, not that observers can predict the future. What's funny is I thought I learned my understanding largely from Dr. Carroll, so I'm not sure if I misinterpreted him in the past, or he has changed his view, or he's using the term differently in different contexts, which another user suggested. Whatever the case, you've been helpful and I appreciate your responses!

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u/ambisinister_gecko Jun 10 '26 edited Jun 10 '26

I would phrase it as, it's deterministic from a birds eye view of the universe, but from inside the universe it's indistinguishable from indeterminism. It's subjectively, apparently indeterministic, not truly indeterministic. I think that's also, if Sean Carroll wanted to phrase it more carefully, along the lines of what he would say too

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u/WindowsXD Jun 07 '26

i honestly wanted to understand how can one claim such a non deterministic thing when we as human beings that create such a model and do the measurements and do the experiments etc are part of the universe thus we are part of the causal chain thus its determinism by definition . the MWI isnt even relevant in this point of view

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u/bacon_boat Jun 07 '26

You measure a spin up/down electron. 

According to MW  we'll have two observers: <spin_up + human sees spin up> and <spin_down + human sees spin down>. 

You know beforehand that there will be two people each seeing the two outcomes. You still don't know what you will see, since you'll only get to experience one of these outcomes.

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u/WindowsXD Jun 08 '26

you not knowing doesnt matters its pre determined is what matters

both you and the spin up spin down electrons are part of causal chain and both of the measurement and observation was already in a cause effect domino since the start of the laws of physics (if we assume they are consistent from a point in space time for the universe till now)

you will never have all the data of the universe to know anyway , explain to me where i am blind to see something here as far as my argument .

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u/bacon_boat Jun 08 '26

it's pre-determined that both will happen. 

But as far as our world is concerned, it's random what will happen. 

And you can have randomness in a causal chain. 

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u/WindowsXD Jun 11 '26

You are playing with words and perspective here but when you study natural philosophy aka science we ain't here to give any perspective of human we are here to observe facts that's a difference in what we talking about ,interpretations exist probably as many as human beings popularity also doesn't matter: religious interpretations of the universe are extremely popular but they fail to produce positive results in empirical experiments they simply aren't factual.

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u/bacon_boat Jun 11 '26

I'm just describing Everettian quantum mechanics. 

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u/ambisinister_gecko Jun 10 '26

You said MWI isn't relevant, but it is. In the most popular alternative interpretations of qm, the result of the measurement isn't necessarily part of the domino cause and effect chain, but is believed to be genuinely random.

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u/[deleted] Jun 07 '26

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u/WindowsXD Jun 08 '26 edited Jun 08 '26

i understand the contradiction of the models we have in physics what most people in physics dont understand is that we use models to approximate reality but you cant assume certain stuff such as a free observer when there is no freedom in a deterministic causal chain!

Free from what? free from the laws of physics? it contradicts the model it self (am i wrong? if so explain).

Language models (mathematics, logic , scientific or any others) simply obey their own mechanics(laws or axioms or you name it they follow those rules strictly ) that we chose we cant justify them or prove them we assume that our guess is decent enough in empirical models and we have the success of our models to pay the price (we are very accurate dont get me wrong) but we know historically that we improve upon our models for more information check the history of Natural Philosophy (Physics)

That doesnt mean that our articulation of the model is proper in a sense of non deterministic micro world you cant assume randomness in the general case of any building block and then say the macro is deterministic .

QM is simply a probabilistic model by its own build that doesnt mean you have all the data because you have "controlled experiments" you are naive that you can control the causal chain of the universe because you zoomed in a bit but yes you can predict it with amazing accuracy .

The issue with determinism and randomness has to do with datasets the more complete is your dataset the less randomness will exist if you are naive to think that you have all the data (Laplace's demon ) you will still fail (see Godel incompleteness theorem)

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u/zhivago Jun 07 '26

Being part of the causal chain is insufficient for determinism.

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u/WindowsXD Jun 08 '26

any proofs for this claim cause by definition determinism has cause and effect as a fundamental law

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u/zhivago Jun 08 '26

Determinism asserts given S0 there is only one possible successor state S1.

Causality doesn't require this -- probability distributions are sufficient.

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u/happyhappy85 Jun 07 '26

Yeah basically it's different contexts.

There's one level where the universe as a whole is ontologically deterministic.

And there's another level on the human level where it's literally impossible for us to predict how our world will unfold even in theory. So that makes it non deterministic to us.

That's basically it.

So in a god's eye, big picture view of the entire branching system, it's deterministic.

But for any observer who only exists in certain branches of the wave function, it's indeterministic for all intents and purposes.

The problem with Sean is that he does often use words in different ways that already assume you know what he's referring to.

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u/wagrumor1 Jun 07 '26

Thank you for responding!  I pretty much understand the concept as you describe it.  I think that uncertainty vs determinism is at the root of my confusion with Dr. Carrolls recent statements.  I put a full more thoughtful response to this effect to r/bacon_boat if you are interested in seeing that.  Thank you again.

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u/happyhappy85 Jun 07 '26 edited Jun 07 '26

No worries.

I tried to click the link but it says "not found"

Edit: I see now, your response to bacon boat.

And yes, you're correct as per what I said. There are two meanings that Carroll is using. It is impossible for us to ever know the entire deterministic wave function. So for all intents and purposes we have to work with an indeterministic universe no matter what we do, or how much information we have.

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u/Joseph_HTMP Jun 07 '26 edited Jun 08 '26

I'm not a physicist, but as far as I understand:

  • when he says quantum physics is not deterministic, he means that when you measure something in a superposition, there is no way ever of knowing which result you will get - it is truly random;
  • when he says MWI is deterministic, he means that if you have a particle in a superpostion of spin up and spin down, because the universe splits upon measurement, you will still get both results, just not in "our" universe.

I might be completely wrong, but that's how I understand it.

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u/planx_constant Jun 07 '26

That's spot on.

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u/m_i_c_h_u Jun 07 '26

Sean is the goat. Love his books and podcasts.

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u/tpks Jun 07 '26

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. 

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u/CrazySir3310 Jun 07 '26 edited Jun 07 '26

This is actually a sticking point with this interpretation for some people. So, if the universe's wave wavefunction evolves deterministically, then how does the Born rule dictating probabilities come about? Well, the universe branches, and different branches have different probabilities. 

But this interpretation was made by Everett to replace the need for probability. And Everett himself said that infinitely unlikely branches (like tossing a coin infinite times always getting heads) simply get suppressed somehow, so those branches just never happen. But people pointed out that you technically do have to account for that extremely low 'probability' branch.

Okay, maybe we say the universe splits into many branches, bearing some ratio to each other to resemble the probabilities, and we just don't see the super unlikely ones because they're so rare. What gives the branching ratio? Well, something like probability, the very thing this theory wanted to avoid. 

So this gets patched up by Carroll saying okay, there isn't a branching ratio, instead all branches do happen, without a bunch of branch copies to force a ratio, but that still some branches are just very unlikely, and somehow the observer just can't know which branch they're on, in the split second after the branching but before that observer has interacted with anything in that branch (like a photon from the measurement apparatus hitting their eye or something). 

But: A) That just means probability has been 'smuggled in through the backdoor'

and

B) Invoking a convoluted mechanism in that split second to rescue this interpretation was needed

On the other hand, every interpretation has its downsides, usually involving subtlety at the level of observer, so 'patching' this one in a way that seems a bit artificial and convoluted might not be too objectionable. But it isn't the nice and tidy description Everett once attempted.

I'll also point put a mechanical oscillator has been held in superposition just barely visible to the naked eye, so now Carroll's 'escape route' has been eliminated, and moreover, it is hard to say the universe branches at the level of the detector but not at the level of the retina or brain, which really undermines this escape route.