Sean Carroll on the Many-Worlds Interpretation of Quantum Mechanics and its Alternatives

This podcast episode features Sean Carroll discussing the philosophical and physical implications of quantum mechanics, particularly focusing on the Many-Worlds Interpretation (MWI). Carroll challenges the textbook notion that observation plays a fundamental, distinct role in physics, arguing instead that observers are merely quantum systems governed by the same laws. He explains that what appears as a 'measurement' or 'observation' is actually the observer's wave function becoming entangled with the observed quantum system, leading to a branching of the universe into separate, non-interacting worlds, each representing a different outcome. This interpretation, pioneered by Hugh Everett, posits that the universe's wave function always obeys the Schrodinger equation, eliminating the need for ad-hoc collapse postulates.

The discussion delves into the mechanics of world creation, clarifying that worlds split whenever a quantum system in superposition entangles with its environment, not necessarily requiring a conscious observer. Carroll emphasizes that MWI describes a pre-existing universe splitting into 'thinner' versions, rather than creating new copies, thus preserving energy conservation. He touches upon the profound cosmological implications, such as the finite dimensionality of Hilbert space within our accelerating universe's horizon, suggesting a limit to branching and an eventual 'emptying out' of the universe. This perspective offers a coherent, albeit counter-intuitive, mapping of the universe's wave function onto observed reality.

Carroll addresses why MWI, despite its mathematical elegance, remains controversial. He distinguishes between the simplicity of a theory's formalism and the complexity of mapping that formalism onto our everyday experience. MWI excels in the former but presents a significant challenge in the latter, leading to skepticism. He contrasts MWI with other leading interpretations: Hidden Variable Theories (which posit additional, unobserved variables alongside the wave function), Spontaneous Collapse Theories (where wave functions randomly or gravitationally collapse), and Epistemic Interpretations (which view the wave function merely as a tool for predicting outcomes, not a description of reality). Each alternative introduces additional rules or complexities to the fundamental quantum equations.

Ultimately, Carroll advocates for MWI due to its formal simplicity and its 'plug-and-play' compatibility with modern physics, especially quantum field theory and quantum gravity. He argues that other interpretations carry 'classical baggage' by privileging classical notions like specific locations in space, which become problematic when classical descriptions break down. MWI, by starting from the most quantum description possible, offers a more robust foundation for understanding the emergence of space-time and developing a unified 'theory of everything,' making it his preferred choice for its austerity and adaptability.

my personal feelings such as it is is that things like measurement and observers and stuff like that are not going to play a fundamental role in the ultimate laws of physics

you the observer are just a quantum system like anything else there's nothing special about you don't get so proud of yourself you know you're just a bunch of atoms you have a wavefunction you obey the Schrodinger equation like everything else

when you think you're measuring something or observing something what's really happening is you're becoming entangled with that thing

all of those different parts of the wavefunction once they come into being no longer talk to each other they no longer interact or influence each other it says if they are separate worlds

you don't need all these extra rules about looking at things just listen to what the Schrodinger equation is telling you

there's zero question about whether or not many-worlds violates conservation energy yes it does not

many worlds is the version of quantum mechanics where it is hardest to map on the underlying formalism to reality

all of the other versions of quantum mechanics prejudice or privilege some version of classical reality like locations in space

many worlds is plug-and-play you tell me the theory and I can give you as many worlds version