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

The excerpt opens partway through Janna Levin's account of what happens when black holes move. Black holes curve space and time, and things fall along those curves. When black holes move, the curves have to follow them, but no faster than the speed of light. So two black holes orbiting each other create a wave in "the actual shape of space" that follows them as they undulate. Levin says the two will eventually merge. Despite time dilation, she says the merger does not take an infinite time, because both objects are so big and deform spacetime so strongly. She contrasts this with the usual picture of a small marble falling across a single horizon. Here there are two event horizons, and simulations show them bobble, merge and form one bigger black hole.

After the merger, the new black hole radiates away its "imperfections" in gravitational waves and settles into one quiescent, spinning black hole. Levin ties the energy accounting to E = mc²: the final black hole has less mass than the two starting black holes combined, and the difference is radiated away as a ringing of spacetime.

It’s really important to emphasize that it’s not light.

She is clear on this point. None of the energy comes out anywhere in the electromagnetic spectrum, which she lists as X-rays, gamma rays, infrared and optical light. Instruments that detect light would see nothing, and the event is "completely dark." The signal exists only as a rippling of the shape of space.

That is why the waves are often compared to sound. Levin goes further, but she qualifies the claim. She says "we’ve kind of argued" and adds "I haven’t done an anatomical calculation." Her suggestion is that if you were near enough to two colliding black holes, their frequency would fall in the human auditory range. Spacetime could then squeeze and stretch your eardrum even in a vacuum, and you could literally hear the ringing. She presents this as an informal argument, not a calculated result.

Lex Fridman introduces Levin as a theoretical physicist and cosmologist who works on black holes, the cosmology of extra dimensions, the topology of the universe, and gravitational waves. He lists her books: How the Universe Got Its Spots (on the shape and size of the universe), A Madman Dreams of Turing Machines (on genius, madness and the limits of knowledge), Black Hole Blues and Other Songs From Outer Space (on LIGO and the detection of gravitational waves), and Black Hole Survival Guide.

The conversation starts with an anecdote Levin sent Fridman. Asked to attend a 9:00 AM meeting, the physicist Sidney Coleman replied, "I can’t stay up that late." Fridman says the best thoughts, and maybe the worst ones too, come at night, perhaps because of the silence or the darkness. Levin calls the middle of the night "stolen hours," when gadgets aren't pinging and there is no pressure to do anything. She notes that the two of them had been exchanging messages at 4:00 in the morning.

Fridman asks what black holes are and how they form. Levin argues that the two questions are often confused. Dead, very massive stars are one way nature makes black holes, but she says black holes are "more general and more fundamental" than the end state of a star. She also says Einstein was surprised that such a solution turned up so quickly, but he "really thought nature would protect us from their formation."

Levin sets the history in 1915–1916, when Einstein arrived at the canonical formulation of relativity after a lot of back and forth, and during World War I. Karl Schwarzschild was an astronomer who had enlisted in his forties. He sent Einstein a solution to his equations from the eastern front, worked out in the trenches amid cannon fire. Levin passes on the joke that he was supposedly calculating ballistic trajectories. It was the first exact solution to Einstein's equations. Schwarzschild did not call it a black hole, and Levin notes the name didn't appear for decades.

What Levin values in Schwarzschild's work is that it was purely about the idea, not about observations or how nature might make such objects. Paraphrasing him, she describes a deliberately "untenable" setup: crush all the mass of a star to a point, don't ask how, and work out what spacetime does. The background logic, in her summary, is that matter and energy curve space and time, and the curved spacetime then tells matter and energy how to fall.

For Levin, the most important feature of Schwarzschild's solution is a boundary: the event horizon, the region beyond which not even light can escape. More than a hundred years later, she says, that boundary is what she would call the black hole.

The black hole is not the mass crushed to a point. The black hole is the event horizon.

She describes the horizon as a surface in spacetime that separates events, which is where the name comes from. The separation runs one way. Events outside can affect the inside, so you can throw a probe in and cause something to happen there. Nothing inside can affect events outside, so someone who fell in cannot send a probe out. Levin calls this one-way quality "what’s profound about the black hole."

This leads to her claim that black holes are "nothing." People tend to imagine a very dense dead star, but she says that if you go up to the event horizon you find an empty region of spacetime. It's "more of a place than it is a thing." Fridman calls this a "brain breaking idea" that cuts against the usual image of super-dense matter. Levin confirms it with a play on words: "black holes are no thing." She says this holds even when black holes form in messy astrophysical systems: "there’s still nothing at the end of the day left behind."

On how the solution was received, Levin says Einstein found it fascinating and helped get it published, but didn't think such objects would form in nature. She offers her own guess, "I doubt Karl Schwarzschild did either." In her view, both thought they were solving theoretical mathematical problems, not describing what turned out to be the end state of gravitational collapse.

Fridman suggests the point of such a thought experiment might be to push a theory to its extremes and find where it breaks down. Levin says the solution does several things at once. It describes the sun from far away and accounts well for Earth's orbit, which she calls "almost overkill," since that problem doesn't need relativity's precision. It predicts black holes, though it doesn't explain how nature would form them. And near the center it does signal a breakdown, a place where "this doesn’t sound right anymore."

She compares the singularity to "a dying man marking in the dirt that something’s gone wrong here": a sign that something in the theory is at fault. She cites Roger Penrose, whose general work on black hole formation through gravitational collapse found that once a black hole forms, a singularity is inevitable. In Levin's account, Penrose also thought this was probably a shortcoming caused by leaving out quantum mechanics, and that including it would change the picture.

Fridman restates this as: near the singularity quantum mechanics takes over, so there is no singularity but "something else." Levin agrees with the first half but frames it as a requirement of consistency. Close to the singularity you have to include quantum mechanics. Describing such small scales and such extreme curvatures and energies without it would be inconsistent with the world as we understand it. She doesn't say what replaces the singularity, and the excerpt doesn't resolve that question.

Coming back to how black holes form, Levin says it was "quite astounding" that real stars turned out to produce them, given that Einstein accepted the prediction but didn't expect nature to make them. She credits physicists like J. Robert Oppenheimer, who worked on nuclear physics and quantum mechanics, and calls this "probably Oppenheimer’s most important theoretical work." She describes their starting questions as "utopian" ones, such as why stars shine and why the sun is so radiant and hot. From there, people like Oppenheimer asked whether stars could collapse until they were so dense that not even light could escape.

Limitations of this excerpt: it starts mid-sentence, partway through the gravitational-wave explanation. One of Fridman's questions is cut off by an inaudible gap. The claim about hearing the waves is explicitly informal and uncalculated. The views attributed to Einstein, Schwarzschild and Penrose are Levin's characterizations, and in Schwarzschild's case her own guess ("I doubt"). The Oppenheimer story stops before the stellar-collapse result is explained.

Janna Levin starts by correcting a common picture. Black holes are often described as dense objects, but she frames them as the result of a process that happens inside very massive stars. Such a star burns thermonuclear fuel and gives off energy according to E equals MC squared. She calls it "a constantly going thermonuclear bomb". In the end it runs out of hydrogen and helium to fuse and reaches an iron core. Fusing beyond iron is "energetically expensive", so the star effectively has no fuel left.

Levin says collapse happens only if the star is "much more massive than our sun, maybe 20, 30 times the mass of our sun". The collapse is "incredibly fast and dramatic" and creates a shockwave. That is the supernova. Many of these stars rebound because the crunch takes them to a new critical point where they can reignite and fuse heavier elements, which "sets off a bomb essentially." She stresses that this explosion matters to us: stars send their material back out into space, and that is why people are made of carbon and oxygen, not just hydrogen.

What is left after the explosion is sometimes a neutron star. Levin describes it as super dense but bigger than a black hole, meaning it is "not compact enough to become a black hole." In her description it is like a giant neutron: electrons are jammed into protons, making a giant nucleus of strange superconducting matter. If the remaining core is heavier than that, which she puts at "heavier than twice the mass of the sun", it becomes a black hole.

She credits Oppenheimer and his student with a 1939 paper saying they believed the end state of gravitational collapse is a black hole. She calls this a stunning and "visionary conclusion." She also notes the timing: the paper came out the same day the Nazis advanced on Poland, so it got little newspaper attention. Lex Fridman points out how short the paper is ("It two pages. It’s like three pages.") and reads its opening. It says that "when all thermonuclear sources of energy are exhausted, a sufficiently heavy star will collapse". It adds that unless fission due to rotation, radiation of mass, or mass blown off by radiation reduces the star’s mass, "this contraction will continue indefinitely." Levin calls the paper "gorgeous."

Fridman is struck by the contrast between the paper and its timing: a physicist predicting how nature could form an object that "broke even Einstein’s brain" just as Hitler took the first steps of a global war. Levin adds that Christopher Nolan’s film includes a small, easy-to-miss nod to this. In it, someone jokes that nobody will pay attention to the paper now because of the advance on Poland.

Levin explains the collapse with numbers. The Sun is about a million and a half kilometers across. Picture a much larger and heavier star that goes supernova and leaves a core of perhaps 10 times the mass of the sun. That core keeps collapsing. When it reaches about 60 kilometers across, roughly the size of a city, a black hole has essentially begun to form. Spacetime is curved so strongly that not even light can escape.

In her account, the event horizon is "almost imprinted on the spacetime." The star cannot stay in that dense state, just as it cannot race outward at the speed of light, because even light is forced inward. So the star keeps falling, leaves the event horizon behind, and falls into the interior. Levin calls this "the magic part." She is clear about what remains unknown: where the star goes, "Nobody really knows, but it’s gone from sight."

The star, like the Cheshire cat fades from view one leaves behind only its grin, the other only its gravitational attraction.

Levin attributes this line to John Wheeler, whom she calls the granddaddy of American relativity. She presents it as approximate, saying it is "something to the effect" of what he said.

Levin says these objects were still not called black holes in 1939. Physicists described them in long-winded terms such as the catastrophic end state of gravitational collapse. She places the change in 1967, at a lecture Wheeler gave near Columbia in New York, in a room above Tom’s restaurant from Seinfeld. She reports it as a rumor: while Wheeler kept repeating that loaded phrase, someone reportedly shouted from the back row, "Well, how about black hole?" Wheeler then apparently pushed the term on the world.

Fridman says he likes plain names like big bang and black hole. To him they point out the elephant in the room and call it an elephant, and he calls "black hole" "a pretty accurate and deep description."

Levin complicates the naming story. Wheeler, who popularized the term, gave Oppenheimer "quite a terrible time" over the 1939 result and spent decades saying it was wrong. She is careful about how to describe the relationship. It has sometimes been called bitter, but she says she does not know if you would actually call it a feud, only that there were bad feelings. Wheeler's objection was that Oppenheimer's setup was too simple and too idealized. He believed a more realistic, more complicated model would make the black hole go away.

Wheeler changed his mind through his own early computer work. That work was tied to his efforts to understand nuclear weapons, and in peacetime it turned back to astrophysical questions. He ended up concluding that Oppenheimer had been right and that black holes were the likely end state of collapse for very heavy stars. Levin repeats a story that Oppenheimer was sitting outside the auditorium when Wheeler announced this. Asked about it, Oppenheimer reportedly said, "well, I’ve moved on to other things."

Levin draws a broader lesson from 1939: science is agnostic. Oppenheimer and others were asking "utopian questions" about nuclear physics and stars. Meanwhile, scientists were under pressure to put nuclear physics to use and join the race for a nuclear weapon. She cites Michael Frayn’s play Copenhagen, which attributes a line to Niels Bohr: he tells his wife that nobody has thought of a way to kill people using quantum mechanics. She notes that the nuclear bomb then followed.

Fridman points out that Oppenheimer was also central to building the bomb. He calls it "theory and experiment clashing together with the geopolitics." Levin agrees. Oppenheimer, "now known as the father of the atomic bomb", was working with "the same technology": overcoming a critical mass and setting off a nuclear reaction. She first speaks of thermonuclear fusion, then notes that the original bomb used fission.

In Levin's account, fission was first shown by Lise Meitner. She describes bombarding a certain uranium with protons, which broke it into smaller pieces whose total mass was less than the uranium's. That missing mass had escaped as energy, which Levin calls the first concrete demonstration of Einstein’s most famous equation. This is her characterization from the conversation. The episode offers no further detail on the experiment.

Fridman notes that he talks with world leaders for whom nuclear weapons are a tool on the chessboard of geopolitics, perhaps only implicitly. He asks Levin how she sees the moment when physicists built a weapon powerful enough to destroy human civilization. She calls it "an excruciating moment in the history of science."

Levin addresses Werner Heisenberg, who stayed in Germany and worked on the Nazi bomb effort. There was once "hopeful talk" that he deliberately derailed the program. She says, "I think that’s been largely discredited", and that he would have built the bomb if he could. In her view, what stopped him were fairly simple errors in his original estimates of how much material was needed or how to get over the energy barriers. She calls that "a terrifying thought." She also admits she cannot really imagine facing that choice: having to take the initiative in finding a way for quantum mechanics to kill people.

On the present, Levin speaks only for physicists in general. She thinks physicists today overwhelmingly feel nuclear weapons should not keep spreading, and that "Very few theoretical physicists want to see this continue." This is her impression, not a survey result.

Fridman then lays out a counterfactual. The Soviet Union, Nazi Germany, and the United States all had excellent scientists, and he puts the probability that the United States got the bomb first at 33%. He believes a Soviet bomb would have been used "in a much more terrifying way" in the European theater, possibly against the United States. He believes Hitler would have used it "to kill hundreds of millions of people". Both are his speculative judgments. Levin accepts the framing only partly. Within the game theory, she says, "This was the least harmful outcome." She then adds a limit: "there is no outcome with no bomb" that a game theorist would play, in her view.

Fridman ends the section on the gap between scientists and how their work is used. He suggests that without the geopolitics, ideology, and dictators, everyone involved was "just scientists" who may not even think about ideology. He leaves it as an open question whether scientists have any control over how their science is used, by politicians he calls "annoying, sometimes evil", for good or bad ends: "It’s hard."

The excerpt opens in the middle of a discussion about scientists' responsibility for the weapons they create. Janna Levin argues that scientists lose authority over a weapon once it exists. Decisions about using it or holding back are then made on grounds other than science.

Once it’s made, it’s no longer scientific reasoning that dictates the use or it’s restraint.

Levin then makes a historical argument: the work was more likely to succeed in America because "America was different." Part of her sentence is marked inaudible in the transcript, so her exact comparison cannot be fully reconstructed. Her evidence is the direction of migration: "So many scientists fled here," while "Americans weren’t fleeing to Nazi Germany." She describes these scientists' motivation as patriotism but also "more than that." In her account, it was a firsthand understanding of the threat in Europe and of "how quickly this free spirited Berlin culture was suddenly in this repressive and terrifying regime." She links this history explicitly to "this particular climate" today, saying it is "something we have to think about right now."

Lex Fridman extends the point to American institutions. He admits it is "cliche to say," but argues that individual freedoms make possible what he calls, "at its best, a very vibrant scientific community." In the next segment, which the transcript attributes to Levin, the speaker stresses maintaining "the vibrancy of the debate of the funding, those mechanisms." Levin then ends the exchange with a conditional warning.

The world flocked here. And that won’t be the case if we no longer have intellectual freedom.