Carroll, Sean

theoretical physicist at Caltech and the Santa Fe Institute; host of the Mindscape podcast; author of "Something Deeply Hidden"

Sean Carroll: Quantum Mechanics and the Many-Worlds Interpretation

Lex Fridman Podcast · Nov 1, 2019

• Sean Carroll
• quantum mechanics
• Many-Worlds Interpretation
• wave function
• entanglement
• Hilbert space
• measurement problem
• emergent spacetime
• arrow of time
• conservation of momentum

Core ideas

Carroll opens by revisiting Newton's own discomfort with "action at a distance" in gravity — a problem Newton explicitly left unsolved, later resolved not by Einstein but by Laplace around 1800, who showed Newtonian gravity could be rewritten as a field theory (a gravitational field permeating space) with identical predictions but no instantaneous action at a distance; general relativity's real addition was making gravitational influence propagate at the speed of light rather than instantaneously. He uses this to make a broader epistemological point: our intuitions about how physics "should" look (locality, contact-based causation) are historically contingent and trainable rather than fixed, and pushes back on the idea that human cognition has hard biological ceilings on scientific understanding — arguing that once minds cross a threshold into abstract, symbolic (effectively Turing-machine-like) reasoning, there's no clear evidence we're near any limit, comparing our current position to asking 500-years-ago thinkers whether they were near the limits of understanding nature (they were nowhere close, and had no way of knowing it).

Carroll identifies conservation of momentum as physics' most beautiful idea, not for its math but for the philosophical shift it represents: Aristotle's physics required "natures" and teleological purposes (objects "wanting" to be somewhere) to explain why pushed things stop moving, whereas Newton's insight — that objects left alone move in a straight line forever, and only friction and air resistance make them stop — replaced purpose-driven explanation with impersonal, pattern-based, instantaneous determinism (a "Laplace's demon" view where the current state alone determines all future and past states). He traces this insight's surprisingly long gestation (Avicenna partially anticipated it around 1000 CE but doubted a true vacuum could exist; full articulation took another 600 years) as evidence of how difficult it was to escape everyday intuition. On the broader relationship between math and physics, he distinguishes them cleanly — math describes the logical structure of all possible worlds, physics describes our one actual world — and argues the truly surprising fact isn't that math is useful for describing reality (any coherent theory could be described mathematically, however messy), but that our universe's mathematical description is astonishingly simple, compressible enough to "write on a T-shirt," a fact he suspects may have no further explanation beyond being a brute fact about our particular world.

Carroll then works through core quantum mechanics concepts: the atom (nucleus of protons/neutrons carrying nearly all the mass, surrounded by much lighter electrons that do essentially all of the chemical and electrical "work"); why classical planetary-orbit models of atoms fail (an orbiting electron would radiate away and spiral into the nucleus in about 10⁻¹¹ seconds); and the resulting "wave function" — a value assigned to every point in space (or, for multiple particles, one single joint wave function for the whole system, ultimately for the entire universe) whose squared magnitude gives the probability of observing the system at that location. He explains entanglement as a feature of these joint wave functions with no classical analogue: a state where you don't know what either of two particles will do individually, but you do know their outcomes are correlated (e.g., "if I observe electron A here, I'll certainly observe electron B there") — and notes that at the level of quantum fields (the more fundamental modern picture, in which even the vacuum is filled with vibrating fields), nearby field regions are naturally strongly entangled while distant ones are not, contrary to popular oversimplifications about entanglement's independence from distance. He also clarifies Hilbert space (the abstract space of all possible quantum wave functions for a system, whose dimensionality reflects how much information is needed to fully specify the system's state, with no direct relationship to ordinary 3D physical space) and entropy (roughly, a measure of how much is unknown about a system's exact microscopic state relative to what's known about its macroscopic state).

The heart of the conversation is quantum mechanics' "measurement problem" and Carroll's preferred resolution, the Many-Worlds Interpretation (developed by Hugh Everett III in the 1950s). He explains that the textbook (Copenhagen) formulation gives special, seemingly fundamental status to the act of observation/measurement — a system's spread-out wave function abruptly "collapses" to a definite value only when observed — which Carroll considers philosophically unsatisfying and unsupported by any real evidence that consciousness or measurement plays a fundamental physical role. Many-Worlds instead insists an observer is just another quantum system obeying the same Schrödinger equation as everything else; "measuring" a particle really means becoming quantum-mechanically entangled with it, producing a combined wave function containing branches (electron-here / observer-saw-it-here, electron-there / observer-saw-it-there, etc.) that, once formed, no longer interact — effectively separate "worlds." Carroll stresses these worlds are best understood as the universe's single wave-function splitting into thinner branches (analogous to decomposing one vector into a sum of two shorter vectors) rather than as new energy-creating "copies," which is why Many-Worlds does not violate conservation of energy — a point he considers settled and non-controversial, unlike genuinely open questions such as where quantum probability comes from. He notes an intriguing, still-speculative connection between Many-Worlds and cosmology: because our universe's accelerating expansion creates a cosmic horizon beyond which nothing can ever be observed again, there may be only a finite-dimensional Hilbert space available (estimated around 10^(10^122) dimensions) within our observable patch — implying branching must eventually exhaust available "room" as the universe empties out and cools.

Carroll candidly explains why Many-Worlds is controversial despite being, in his view, the mathematically simplest interpretation: the difficulty isn't in the theory's own internal simplicity but in the much harder work of mapping that austere formalism (a universal wave function obeying the Schrödinger equation, nothing else) onto everyday classical-feeling experience — compared to rival interpretations (hidden-variable theories like pilot-wave theory, which add literal particles pushed around by the wave; and spontaneous/induced collapse theories, including Roger Penrose's gravity-triggered collapse proposal) that build classical-looking features directly into their formalism at the cost of additional theoretical complexity. He also flags a fourth family, epistemic interpretations, which treat the wave function as purely a bookkeeping tool for predicting experimental outcomes rather than a description of physical reality — a view he rejects because it declines to say what, if anything, is actually being predicted about. His personal preference for Many-Worlds rests on two grounds: its formalism is "plug-and-play" (extend any quantum theory to a many-worlds version without added assumptions), and — more importantly for his own research — every rival interpretation smuggles in privileged classical concepts (like definite particle locations) that become a liability when trying to build a quantum theory of gravity and emergent spacetime, an area where locality itself appears to break down (as suggested by black hole information and Hawking radiation) and where starting from a "maximally quantum" theory like Many-Worlds seems more promising than trying to quantize a classical starting point.

Carroll closes on time, space, and consciousness: he distinguishes the (still-debated, but likely emergent rather than fundamental) arrow of time — tied to the Second Law of Thermodynamics and, in Many-Worlds, to the universe's steadily increasing branching from a single low-entropy initial state — from time itself, which is not necessarily emergent even if the arrow is; and separately argues that space itself likely is emergent from a more fundamental description involving only a wave function in Hilbert space, with no privileged three-dimensional structure built in at the ground level. He states firmly that he does not think quantum phenomena like entanglement play any special role in human consciousness or cognition (he identifies as a thoroughgoing physicalist who thinks mental states, like spacetime, are emergent rather than fundamental) and distinguishes genuinely falsifiable open questions (ongoing experiments testing for spontaneous wave-function collapse, which would rule out Many-Worlds if detected) from the still-too-immature state of quantum-gravity/emergent-spacetime research to generate clear testable predictions yet.

Excerpts