Levin, Janna

theoretical physicist and cosmologist; author of Black Hole Blues, A Madman Dreams of Turing Machines, How the Universe Got Its Spots

Janna Levin: Black Holes, Wormholes, Aliens, Paradoxes & Extra Dimensions

Lex Fridman Podcast · May 5, 2025

• Janna Levin
• black holes
• event horizon
• Hawking radiation
• information paradox
• ER=EPR
• wormholes
• extra dimensions
• gravitational waves
• Alan Turing
• Kurt Gödel

Core ideas

Levin's central reframing is that a black hole is best understood not as a super-dense object but as "no thing" — the event horizon is an empty region of spacetime marking a one-way causal boundary, not a physical surface, and the collapsed matter that formed it has, in a real sense, already fallen out of relevance to any outside observer. She traces this back to Karl Schwarzschild's 1916 solution to Einstein's equations (found from the trenches of WWI) as a pure thought experiment — crushing a star's mass to a point — that Einstein himself didn't believe nature would ever realize. The bridge from thought experiment to physical prediction came via Oppenheimer's 1939 paper (published the same day Germany invaded Poland, and largely eclipsed by it) arguing that sufficiently massive collapsing stars — those whose iron core exceeds roughly twice the sun's mass after supernova — have no way to halt collapse and must become black holes; John Wheeler, who coined the term "black hole" in 1967 after decades of disputing Oppenheimer's conclusion, eventually came around to it via his own computational work. Levin uses this history to draw out a recurring theme: the same physicists (Oppenheimer, Heisenberg, Lise Meitner's fission work) moved fluidly between "utopian" theoretical questions and the geopolitically weaponized version of the same physics, and she argues America's open, competitive scientific culture — which drew refugee physicists (many Nobel laureates came from the Bronx, not the Soviet Bloc) — made it more likely, not certain, that the US reached the bomb first, a distinction she considers worth remembering amid current tensions over scientific openness.

On the experience of falling into a black hole, Levin describes the counterintuitive result that a falling observer notices nothing special crossing the event horizon (there is no local marker), while a distant observer sees the infalling person's clock appear to freeze, stretching across "millennia" from the outside. Inside, space and time effectively swap roles: what looks like a spatial direction (toward the center) from outside becomes, for the infalling observer, a direction in time — the singularity lies in their future and is as unavoidable as time itself, arriving within microseconds for a stellar-mass black hole. Larger black holes are, paradoxically, less dramatic to fall into (curvature is less noticeable the larger the radius, just as Earth's curvature is imperceptible locally), and an infalling observer might actually see a bright flash — all of the light from the outside universe's history focusing inward — making the fall more like a literal "near-death experience," but a total one. She also stresses that supermassive black holes (in the centers of nearly every galaxy) likely didn't form from stellar mergers, since there hasn't been enough cosmic time for that; they're thought to have formed via direct gravitational collapse very early in the universe's history, tightly coupled to galaxy formation in ways still not fully understood.

The information paradox forms the episode's theoretical core. Levin explains Hawking radiation as arising from vacuum fluctuations near the event horizon: virtual particle pairs are normally created and annihilate, but if a horizon separates the pair before they can recombine, one particle escapes (appearing as radiation) while its negative-energy partner falls in, causing the black hole to lose mass and eventually evaporate. Hawking's calculation showed this radiation is purely thermal — featureless, encoding no information about what fell in — which would mean quantum information is permanently destroyed, violating unitarity, a principle physicists (she cites Leonard Susskind and Gerard 't Hooft's "black hole wars" against Hawking) consider close to sacred. Levin walks through the field's competing resolutions: information loss (which she personally rejects), string-theory "fuzzballs" (horizonless tangles of strings replacing the interior, so nothing ever really falls in), "soft hair" (subtle quantum hair on black holes encoding information, which she's also skeptical of), and firewalls (the AMPS proposal that the horizon must be a violent, incinerating boundary) — which she frames not as a real prediction but as a deliberately provocative paper that exposed how much prior "solutions" had been glossing over, spurring real progress. Her favored resolution is ER=EPR (Maldacena and Susskind's conjecture that entangled particles are connected by non-traversable microscopic wormholes), built on Maldacena's holographic AdS/CFT correspondence — showing a gravity-filled "universe in a box" is mathematically equivalent to a boundary theory with no gravity and no information loss — which suggests that the black hole's interior/exterior split is "embroidered" by quantum entanglement rather than truly separated, potentially implying gravity itself is an emergent phenomenon arising from more fundamental quantum degrees of freedom, rather than a candidate for direct quantization.

On extra dimensions and aliens, Levin discusses string theory's requirement of extra spatial dimensions (compactified since the Big Bang, with open questions about why only three grew large) and entertains brane-world scenarios where other "sheets" of reality could exist nearby in a higher-dimensional bulk, communicable in principle only via gravitational waves (expensive and hard to localize) rather than light. She pushes back on anthropocentric alien speculation, arguing life elsewhere need not resemble technology-manipulating humans at all — pointing to Earth's own jellyfish (no centralized brain, evidence of biological "immortality" via reversion to earlier life stages) as an example of radically different, long-lived, low-impact life, and suggesting the Fermi Paradox may partly reflect humanity's narrow assumption that intelligent life must be expansionist and detectable. On wormholes, she considers them mathematically legitimate curved-spacetime solutions (constructible in principle by reverse-engineering Einstein's equations to find the matter/energy distribution needed), with Kip Thorne's key finding being that traversable wormholes require exotic negative-energy matter — not obviously impossible, since quantum effects like the Casimir effect do produce negative energy, but far from an engineering problem today. Dark energy remains harder to pin down than dark matter (which she considers essentially confirmed via phenomena like the Bullet Cluster's separation of luminous and gravitational mass, and via known dark-matter-like particles such as neutrinos): the known candidate quantum vacuum energy comes out wildly wrong in magnitude compared to the tiny observed value, and she'd be delighted if dark matter and dark energy turned out to be connected, ideally via extra-dimensional physics.

The conversation closes on the human side of science: Levin's book on Alan Turing and Kurt Gödel explores how Gödel's incompleteness theorems (some true mathematical statements are unprovable) and Turing's uncomputable numbers both trace to the same crisis in the foundations of mathematics, yet the two men diverged completely — Gödel a Platonist who came to distrust physical reality and ultimately starved himself amid paranoid delusions; Turing a pragmatist who concluded humans might just be machines, helped break the Enigma code, and was chemically castrated after reporting a burglary that exposed his homosexuality, dying by apparent cyanide-laced-apple suicide referencing Snow White. Levin resists romanticizing madness as a precondition for genius, but suggests the same traits (refusal of ordinary social validation, total absorption in elusive problems) that produce singular achievement can also produce instability — a "tragic flaw" she compares to Terence Tao's discipline in walking away from problems that resist him versus Andrew Wiles's seven-year solitary confinement with Fermat's Last Theorem (including the terror of a flaw found after his announced proof) versus Grigori Perelman's total withdrawal from mathematics and public life after solving the Poincaré Conjecture. She discusses her role as chief science officer of Pioneer Works (a Brooklyn arts-and-science space founded by artist Dustin Yellin) as an explicit rejection of "outreach" or "education" framing in favor of treating science as culture in its own right, and cites literary favorites (Ishiguro, Cormac McCarthy's The Road, Orwell's Animal Farm, Martin Amis's Time's Arrow) as works where formal or structural experimentation illuminates fundamentally human stakes, closing with the thought that even all human discovery will eventually be erased from the universe — which she takes not as bleak but as a reason to focus on what we contribute to each other now.

Excerpts