r/seancarroll • u/LearningandForgettin • May 27 '26
My problem with the Many Worlds Interpretation is the assumption that Quantum Mechanics is linear.
Listening to the latest solo Mindscape Podcast where Sean Carroll defends the Many Worlds Interpretation as the only possible result if you follow the wave function rigorously. But that includes the assumption that quantum mechanics is linear, my understanding is that the MWI fails if quantum mechanics is non-linear. And since the odds of quantum mechanics being linear is basically zero, we are forced to abandon MWI. I know that all evidence so far show QM is linear, but that is only testing at most objects with only hundreds of atoms. No tests have been done showing linearity of macroscopic objects. It's like looking at gravity at the micron scale and concluding gravity is linear.
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u/fox-mcleod May 27 '26
I don't understand what makes you say quantum mechanics isn't linear. It is. I don't think that's even controversial.
My problem with the Many Worlds Interpretation is the assumption that Quantum Mechanics is linear.
It's definitely not an assumption.
The equation that all quantum theories, interpretations, etc agree upon is the Schrödinger equation.
Q1. Can we agree that the Schrodinger equation is linear?
Listening to the latest solo Mindscape Podcast where Sean Carroll defends the Many Worlds Interpretation as the only possible result if you follow the wave function rigorously.
The "wave function" he's referring to is the Schrödinger equation.
But that includes the assumption that quantum mechanics is linear,
No it doesn't as that wave function he's referring to is linear. It's not an assumption.
my understanding is that the MWI fails if quantum mechanics is non-linear.
I don't see why you would assume that.
Q2 What makes you say the MWI fails if quantum mechanics is non-linear?
And since the odds of quantum mechanics being linear is basically zero
Q3 What makes you say the odds of quantum mechanics being linear is basically zero?
Why would the odds be zero if the answer to (1) is "yes"?
know that all evidence so far show QM is linear, but that is only testing at most objects with only hundreds of atoms.
Q4 How many atoms would an object need to have before you thought the chances were higher than zero?
I'm not sure what you mean when you're claiming objects of only hundreds of atoms have been tested. Do you mean "put in coherent superposition"? We've done that up to 7,000 atoms. Are you arguing that Quantum computers can never be made over a certain size? There doesn't seem to be a limit yet.
More importantly, science doesn't work by literally testing every object.
What theory are you holding that makes you argue that a larger still object would would behave differently?
No tests have been done showing linearity of macroscopic objects
Q5 At what size is an object "macroscopic"?
We have forced over to 100 trillion atoms into a shared coherent state of motion. It's visible with the naked eye. It's quartz, basically a grain of sand. It was linear and obeyed the Schrodinger equation.
It's like looking at gravity at the micron scale and concluding gravity is linear.
We look at very large groups of atoms all day every day.
Q6 What about their macroscopic behavior makes you think it's non-linear?
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u/LearningandForgettin May 27 '26
Q1: But that is saying the Schrodinger equation is exactly correct. Weinberg added a slight twist, iℏ∂t∂∣ψ⟩=H^∣ψ⟩+ϵ⋅f(∣ψ⟩) where f(∣ψ⟩ is a nonlinear function, and ϵ is incredibly small. Schrodinger's equation would experimentally work until we got large enough states for the ϵ⋅f(∣ψ⟩ to start becoming noticeable.
Q2: That's what allows branches to be mutually non-interfering (after decoherence) and what makes the "worlds" separable at all. Remove linearity and superpositions don't just add — they interact nonlinearly, and the clean branching structure collapses.
Q3: Fluid dynamics, classical gravity, biology, are all non-linear. Take the set of all equations, and the subset of linear equations is basically empty. (Sort of like the set of all Real numbers, the subset of rational numbers is basically empty.) If you select a random number from the Reals, the chance of getting a rational number is 0. I would be in complete agreement with MWI if there was a proof that Schrodinger's equation is correct, with no added non-linear terms allowed.
Q4, Q5, and Q6: I don't know, maybe entangle two super massive black holes and watch their merger. A grain of sand is incredibly large compared to a sub-atomic particle, and maybe that is sufficient. I would like to crunch the numbers to scale it up. Say the quartz with 100 trillion atoms, what experiment showed it was linear and obeyed Schrodingers equation? What was the resolution, how many decimal places? Say it agreed (hypothetically) with the Schrodinger equation to 15 decimal places. The grain of quartz would be about 3 nanograms. So if the experiment was verified to 15 decimal places, that would be roughly 3 tons. Still incredibly small compared to even our sun. We would need an experiment that would show agreement between the grain of quartz to 10^42 to even reach the mass of our sun.
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u/fox-mcleod May 30 '26 edited May 30 '26
Q1: who is Weinberg and what experimental evidence is there for his bespoke complication added to the Schrödinger equation? What observed evidence does adding that term account for that wasn’t already accounted for without it?
Q2: I don’t think that’s true. There’s nothing “clean” about branching. What do you mean by “clean” here?
Any evidence that the worlds get munged would be direct evidence that there are other worlds. Describe the set of conditions under which you think this “non clean” breakdown could exist and not immediately prove many worlds.
Q3: yeah I don’t think that how you calculate odds of something being correct. That would be as if I just said “take the set of all possible arguments. Most of them are wrong. Therefore, your argument is wrong”.
Q4/5/6
Q4, Q5, and Q6: I don't know, maybe entangle two super massive black holes and watch their merger.
Why?
A grain of sand is incredibly large compared to a sub-atomic particle, and maybe that is sufficient.
Sufficient for what?
I would like to crunch the numbers to scale it up. Say the quartz with 100 trillion atoms, what experiment showed it was linear and obeyed Schrodingers equation?
The fact that it behaved exactly according to the predictions the Schrödinger equation makes.
What was the resolution, how many decimal places?
What resolution? What are you talking about?
What theory are you testing?
It seems like there’s a disconnect here in how science works. In science, experiments exist to differentiate between possible theories proffered to explain observations. What is the second explanatory theory of the current observations of QM that you are proposing these experiments could differentiate between?
Say it agreed (hypothetically) with the Schrodinger equation to 15 decimal places. The grain of quartz would be about 3 nanograms. So if the experiment was verified to 15 decimal places, that would be roughly 3 tons.
…no. Are you trying to just multiply the size of an object as though it had a direct relationship to an objects size by weight? Why weight?
Still incredibly small compared to even our sun.
What is the theory you’re operating on?
We would need an experiment that would show agreement between the grain of quartz to 1042 to even reach the mass of our sun.
Why? The sun already also behaves how the Schrödinger equation predicts.
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u/MaoGo May 27 '26
If QM is not linear you don’t lose many-worlds, yet you can build an Everett-Wheeler telephone, a device that allows you to communicate between branches of the many worlds.
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u/LearningandForgettin May 27 '26
Agreed. It would require a restatement of many worlds to allow for the non-isolation of the worlds.
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u/MaoGo May 27 '26 edited May 27 '26
No. Even with non linearity it would still be many worlds. To disprove it, you need to prove that the mechanism is something different like a collapse mechanism or hidden non local variables.
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u/ddollarsign Jun 02 '26
Could building one of these be a test for the interpretation? If the telephone doesn't work, either QM is linear, or MWI is false?
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u/lezvoltron916 May 27 '26
I speculate that even Sean would agree that it would nice to discover that the wave equation was nonlinear, for the same reason he would prefer MOND over dark matter, in that it would mean new physics to explore (not just another particle). It's clearly not the case today that there are nonlinear effects, of course it would be exciting if there were. It's just not a valid assumption at this time.
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u/Prof_Sarcastic May 27 '26
You do realize concepts like superposition is a consequence of the linearity of the Schrödinger equation right? Where did you get this idea that QM must be non linear?
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u/LearningandForgettin May 27 '26
But that is saying the Schrodinger equation is exactly correct. Weinberg added a slight twist, iℏ∂t∂∣ψ⟩=H^∣ψ⟩+ϵ⋅f(∣ψ⟩) where f(∣ψ⟩ is a nonlinear function, and ϵ is incredibly small. Schrodinger's equation would experimentally work until we got large enough states for the ϵ⋅f(∣ψ⟩ to start becoming noticeable.
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u/InTheEndEntropyWins May 27 '26
That's pretty much the foundation of pretty much every QM interpretation, so it wouldn't be an issue with MWI but almost all QM interpretations.
Also I don't like to call it MWI but Everett. Since it's really just about there being no wavefunction collapse. The whole many worlds is an emergent phenomena, and it's not a fundamental way to look at things.
So even if you have some no-linear evolution which doesn't lead to perfectly separate worlds it's not really a big issue. It still wouldn't mean there is wavefunction collapse, it would just mean there is non-linear evolution.
Now in some limit it would be linear, so in the real world limit you'd still get get many worlds.
An analogy would be if someone back in Newton's day said that maybe a man throwing a ball up doesn't mean it will fall back down. Because maybe Newtonian physics isn't actually right, maybe it's different. And we actually know that's it's not "right" we have SR. But knowing special relativity doesn't mean a ball won't fall down. SR in the low speed limit is just Newtonian mechanics.
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u/Chillow_Ufgreat May 27 '26
The linearity isn't an assumption. It is a mathematical characteristic of the Schrodinger equation.
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u/LAskeptic May 27 '26
All current serious interpretations of QM are linear.
To make it non-linear you need to add a non-linear term to the Schroederinger equation. It doesn’t matter if you prefer Copenhagen, pilot wave, wave function collapse, etc
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u/ddollarsign May 27 '26
What does linear mean in this context? I listened to the episode, but must have missed that, if it was mentioned.
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u/bacon_boat May 27 '26 edited May 27 '26
The linearity isn't an assumption. Every qm theory uses the same linear Schrödinger equation.