Modern WisdomSEAN CARROLL | The Problem With Quantum Mechanics | Modern Wisdom Podcast 126
CHAPTERS
- 0:00 – 0:59
Why quantum foundations stalled: war, distance, and a focus on applications
Carroll explains why foundational questions in quantum mechanics (especially the measurement problem) were deprioritized after the 1920s. World events, reduced collaboration, and the rise of practical/experimental physics shifted attention away from interpretational progress.
- •Early quantum pioneers could debate in person; later geopolitics and lack of communication slowed progress
- •WWII and practical goals (e.g., weapons) redirected physics priorities
- •Foundations felt hard to test compared with smash-and-measure particle experiments
- •Interpretations seemed like ‘literary’ speculation without clear empirical discriminators
- 0:59 – 3:59
Mindscape, writing goals, and why Carroll wrote 'Something Deeply Hidden'
Chris introduces Carroll and asks about his Mindscape podcast and writing direction. Carroll positions his quantum book as a corrective to both woo-woo ‘quantum’ branding and the posture that quantum mechanics is inherently beyond understanding.
- •Mindscape’s broad guest range reflects Carroll’s interest beyond physics
- •Carroll alternates between broad philosophy-infused books and pure physics projects
- •He critiques ‘quantum’ self-help publishing as content-free
- •Core aim: quantum mechanics is understandable, even if not yet fully understood
- 3:59 – 5:06
‘Understandable vs. understood’: quantum’s unusual taboo against deep questions
Carroll distinguishes ordinary scientific unknowns from quantum’s culturally enforced “don’t ask” zone. He argues physicists became adept at using quantum theory instrumentally while avoiding what it says about reality.
- •Many scientific mysteries are open problems, but presumed solvable
- •Quantum was historically ring-fenced: ‘don’t ask what’s really going on’
- •Physicists can compute outcomes without a coherent ontology (smartphone analogy)
- •Carroll calls this a major wrong turn that current work is trying to fix
- 5:06 – 6:05
The measurement problem: two rulebooks for one universe
Carroll lays out the measurement problem: standard quantum teaching uses one set of rules for unobserved evolution and another for measurement outcomes. The Copenhagen move introduces ‘collapse’ without a precise definition of what counts as a measurement.
- •Quantum theory adds special ‘measurement’ rules unlike any other physical theory
- •Wavefunction collapse is postulated but not mechanically defined
- •Key ambiguities: what counts as measuring, how fast, how accurate, whether consciousness matters
- •Probability of outcomes is fundamental in the textbook story
- 6:05 – 8:39
Wave vs. particle behavior: electrons, atoms, and collapse as a patch
Using atomic stability, Carroll shows why electrons can’t be little orbiting balls and motivates the wave description via Schrödinger’s equation. He then contrasts this with particle-like detection tracks, illustrating why the textbook account invokes collapse.
- •Classical orbits would radiate energy and collapse atoms
- •Quantum waves allow stable ‘allowed’ modes (standing-wave analogy)
- •Detectors reveal localized hits/trajectories that look particle-like
- •Textbook response: collapse to a definite outcome upon observation
- 8:39 – 10:39
Who should fix it: experiments are fine; theory is behind
Chris asks whether this is an experimental or theoretical shortfall. Carroll argues the measurement problem is not due to inadequate experiments but to conceptual/theoretical mismatch and decades of neglect.
- •Foundational experiments have long been performed successfully
- •The problem is reconciling formal predictions with observed definiteness
- •Einstein and Schrödinger objected; Bohr/Heisenberg urged ‘move on’
- •Foundational work was discouraged, leaving theorists ‘behind’
- 10:39 – 15:51
Where does the classical world come from? The ‘cheat’ and why quantum is universal
They explore the quantum-to-classical transition and why physicists often assume classicality rather than derive it. Carroll emphasizes quantum mechanics applies at all scales; classical mechanics is an approximation that emerges under certain conditions.
- •We take tables/trajectories for granted instead of deriving them from quantum theory
- •Quantum isn’t ‘for small things’; it’s universally true, including for observers
- •Everett’s complaint: treating observers classically is inconsistent
- •Open question: principled account of classical emergence remains incomplete
- 15:51 – 26:24
Sociology of ideas: reputation, gatekeeping, and foundations getting sidelined
Carroll discusses how influence, credibility filters, and historical contingencies shaped what physicists worked on. He shares examples of foundational work being ignored or professionally risky, even into modern grant-writing incentives.
- •Disagreement without decisive experiments invites claims of bias/groupthink
- •Famous scientists’ ideas travel farther than unknown students’ (sometimes for good reason)
- •Historical examples: journal neglect of foundations; Bohm’s political persecution; Bell working in secret
- •Even today, some are advised not to emphasize quantum foundations in funding proposals
- 26:24 – 30:00
Entanglement explained via spin and Higgs decay
Carroll introduces entanglement with a concrete conservation-law setup: a spinless particle decays into two spinning particles that must be correlated. The key shift is that the pair shares a joint quantum state rather than separate independent ones.
- •Spin/angular momentum conservation constrains decay outcomes
- •Each particle individually can be 50/50 up/down, but outcomes are perfectly anti-correlated
- •Entanglement means one wavefunction describes the composite system
- •The correlation is real even when the individual outcomes are not predetermined
- 30:00 – 32:05
Einstein’s ‘spooky action’ and Bell’s theorem: why hidden certainty fails
Chris presses on whether the particles ‘really had’ definite spins all along. Carroll explains Einstein’s worry about instantaneous correlations over vast distances and summarizes Bell’s result: local hidden-variable certainty can’t reproduce quantum predictions.
- •Entanglement correlations persist regardless of separation distance
- •Einstein’s intuition: outcomes should be predetermined to avoid nonlocality
- •Bell’s theorem rules out local hidden variables matching quantum statistics
- •Nonlocality (in some form) is unavoidable given experimental results
- 32:05 – 33:38
Faster-than-light influence—but no faster-than-light messaging
They clarify how entanglement seems to update the distant system instantly while still respecting no-signaling. The distant observer cannot know the nearby result without classical communication, so information can’t be transmitted superluminally.
- •The description of the remote system changes ‘outside the light cone’
- •But the remote observer still sees a 50/50 distribution locally
- •No controllable signal is sent via entanglement alone
- •Relativity’s speed limit survives operationally, if not intuitively
- 33:38 – 36:47
Many-Worlds: Everett removes collapse by taking Schrödinger evolution literally
Carroll presents Everett’s move: drop special measurement postulates and apply unitary quantum evolution universally to observers and systems. The result is branching: observer+system evolve into non-interacting components corresponding to different outcomes.
- •Observers are quantum systems; measurement is just interaction + entanglement
- •Schrödinger equation predicts an entangled superposition of ‘observer saw up’ and ‘observer saw down’
- •Everett’s claim: we don’t feel superposed because there are effectively separate copies
- •Many-Worlds is motivated by simplifying the rules, not by adding worlds ad hoc
- 36:47 – 44:16
Branching, decoherence, and why you don’t see other worlds
Chris asks why multiple worlds don’t appear overlaid. Carroll explains decoherence using Schrödinger’s cat and environmental interactions: different branches rapidly become dynamically isolated, preventing interference and cross-observation.
- •Branching occurs when superpositions entangle with the environment (records like dots on a screen)
- •Decoherence rapidly separates branches via interactions with photons/air molecules, etc.
- •Everyday branching happens continually (e.g., radioactive decays in the body)
- •Branches are effectively non-interacting—‘perpendicular’ in wavefunction space
- 44:16 – 51:13
Quantum woo, observers, and why consciousness isn’t needed
They address why quantum language gets misused in spirituality and ‘manifestation’ culture. Carroll argues the confusion was enabled by sloppy foundational messaging about observers, but modern approaches don’t assign a fundamental role to consciousness.
- •Wishful thinking + ‘mystery’ invites bad analogies and pseudoscience
- •Misread ‘observer effect’ suggests mind creates reality; Carroll rejects this as nonsense
- •Respectable quantum theories don’t require the word ‘observer’ at all
- •Some consciousness-quantum claims persist, but are less common than before
- 51:13 – 53:27
Determinism, Laplace’s demon, and what ‘predicting everything’ means in Many-Worlds
Carroll contrasts classical determinism (positions/velocities fix past and future) with quantum interpretations. In Many-Worlds, the universal wavefunction evolves deterministically, but predicting a single experienced branch requires information about the entire branching structure.
- •Laplace’s demon: classical state + infinite compute implies fixed past/future
- •Quantum implications depend on interpretation; not all are deterministic
- •In Many-Worlds, global wavefunction evolution is deterministic
- •But practical prediction is impossible: you’d need the full universal wavefunction
- 53:27 – 1:01:30
What Carroll works on now: classical emergence, and why quantum gravity is hard
Carroll closes by describing his research focus: deriving classical reality from quantum foundations and using that perspective to tackle quantum gravity. He outlines technical problems (infinities, renormalization, string theory dimensions) and deeper conceptual issues (spacetime itself in superposition).
- •Goal: derive classicality rather than assume it ‘by hand’
- •Quantizing classical GR leads to uncontrolled infinities; string theory helps but introduces 10D requirement
- •Conceptual challenge: if spacetime geometry is quantum-superposed, ‘location’ becomes ill-defined
- •Hope: starting quantum-first may reveal how curved spacetime emerges as an effective classical limit
