Black Holes Across a Decade of Research: Binary Pairs, Overtones, Spin, and Analog Experiments

The host opens by describing black holes as some of the strangest objects in the universe and also some of the sharpest tests of how reality actually works. The objects push physics to its limits, touching both the warping of space and time and the mysteries of quantum mechanics.

The episode is a compilation that returns to a decade of earlier content, going back into the archives for a deeper look at how these cosmic objects form, evolve, and challenge the most basic theories.

The host starts with a general rule: when two astrophysical objects collide, one usually wins. Red giant stars can destroy planetary systems, neutron stars can consume red giant neighbors, and stellar-mass black holes can tear neutron stars apart. Supermassive black holes, the host says, eat all of these for breakfast.

The episode then turns to a paper published in Nature Astronomy by researchers from India's National Centre for Radio Astrophysics and the Rochester Institute of Technology, reporting a pair of supermassive black holes orbiting only one light-year apart. The host says such tight binaries were expected to exist for a long time, but this is the first one spotted.

The host explains that supermassive black holes sit at the centers of nearly every decent-sized galaxy. Stellar black holes from dying massive stars start at up to about 10 Suns, while those in galactic cores hold between a million and a billion Suns. The largest have event horizons that could envelop most of the solar system.

The system is in the galaxy Markarian 533, about 400 million light-years away. Resolving a separation of around one light-year at that distance requires resolution about 100 times better than the Hubble Space Telescope provides.

The host explains that galaxies grow by merging, so their supermassive black holes must eventually fall toward the new combined core. This happens through dynamical friction: black holes slingshot stars outward through gravitational interactions, losing orbital energy and sinking deeper into the gravitational well.

By the time the black holes are only a few light-years apart, the host says, there should be almost no stars left between them. They then stall and settle into stable binary orbits. This is known as the central parsec problem: researchers know the black holes must merge but do not yet know how.

One possibility is that gas can supply the friction needed beyond that point. The host notes that the newly discovered binary has a reservoir of gas, since that is what powers its jets, so it may help provide the answer.

The host also addresses gravitational waves. The system is producing them, but the host says it would take many billions of years to lose enough angular momentum to merge that way. The waves also have an extremely low frequency, around 1 to 10 trillionths of a hertz, while LIGO is sensitive to frequencies from 10 to 10,000 hertz, so this binary is too slow for LIGO to detect.

The host says that resolving the event horizon of the M87 black hole is almost impossible with a single telescope, and that interferometry is the only realistic route to the required resolution, and even then it remains extremely difficult.

Interferometry combines the light received by two or more telescopes separated by some distance. Light from a distant point arrives as nearly plane waves, reaching one telescope slightly before the other, so the two see different parts of the wave cycle. The resulting phase difference depends on the angular position of the source, and measuring it precisely reveals angular separations far finer than a single telescope can resolve.

The host explains that an interferometer resolves two points when their separation produces a relative phase shift of about one wave cycle. The resolution is set by the ratio of the observed wavelength to the baseline, the distance between the telescopes. A longer baseline and shorter wavelength give better resolution, and this is the same diffraction limit that applies to any telescope.

Applied to M87, the host says the team needed to resolve about one millionth of one percent of a degree. An array spanning Earth observing at roughly one millimeter wavelength makes that possible.

The team simulated the blurring of the image caused by the full observational process, and ran a wide range of parameters for black hole mass and spin rate. The direction of the rotation axis was constrained by the jet visible leaving the galaxy, which allowed the researchers to identify the configuration that fit the actual image.

The narrator reports that the simulations found the black hole has a mass of over 6 billion solar masses, with an event horizon about one-fifth the size of the bright ring. The narrator calls this very large even by supermassive black hole standards.

The black hole is spinning almost as fast as it can. The asymmetry in the ring indicates the rotation direction of the surrounding plasma vortex. Combined with the jet's axis, this gives a rotation direction roughly clockwise from our perspective, with an axis fairly close to our line of sight.

The narrator notes that, like the gravitational-wave signals reported earlier, the black hole's appearance matches predictions from Einstein's general theory of relativity. The narrator adds that black holes emerged from Karl Schwarzschild's solution to Einstein's equations, first seeming too outlandish to be real, and that the image gives a visceral picture of a prediction once thought too hard to verify.

The narrator describes a black hole ringdown as similar to a bell struck by a hammer, with gravitational waves trapped near the event horizon that gradually leak away. Many scientists had assumed that overtones could only be seen at the tail end of the ringdown, when the black hole was closer to spherical, because right after merger the oscillations would be too chaotic and nonlinear.

Matthew Giesler, Maximiliano Isi, Mark Scheel, and Saul Teukolsky of Caltech and MIT challenged that assumption. They looked for overtones starting right at the moment of merger, first in a simulated merger from the SXS project, where the mass and spin inputs were known in advance.

The narrator says the researchers found that the waveform is well described by oscillations in spherical harmonics from the moment of merger, and that at the start of the ringdown some overtones are stronger than the fundamental mode, although they decay faster. Using these overtones, the team could pinpoint the simulated black holes' mass and spin more precisely than from the pre-merger signal alone.

The narrator adds that the overtones may also be detectable in real merger signals from LIGO and Virgo, which the host says has exciting implications.

The narrator says that the frequency analysis of gravitational waves is now called gravitational wave spectroscopy, by analogy with the frequency analysis of light in astronomy. For this to be useful, the team analyzed a real merger in a follow-up paper with Will Farr.

They studied GW150914, the first event LIGO reported, in which a pair of black holes each about 30 times the mass of the Sun spiraled together from about 1.5 billion light-years away. The team analyzed the harmonics in the ringdown and claimed a likely detection of at least one overtone at a confidence of 3.6 sigma.

The narrator says that this makes a real detection quite likely, but that more observations would be needed to effectively eliminate doubt. From the harmonics, the team estimated the final black hole's mass at 68.5 solar masses and its dimensionless spin magnitude at 0.69, which the narrator describes as a rapidly rotating black hole, unsurprising since it absorbed the orbital angular momentum of two black holes.

The mass and spin derived from the ringdown are consistent with earlier estimates from analyzing the full waveform while ignoring overtones. The narrator stresses this matters because the overtone analysis used only the ringdown.

The narrator says that Yang and colleagues predict a particular distribution of black hole spins that could be tested with more LIGO observations. The narrator then turns to a more direct test in a paper by McCernan and collaborators.

If black holes merge in empty space, the event should be invisible and emit no electromagnetic radiation. The narrator says the situation differs inside an accretion disc, where several things could happen right after the merger to produce a bright burst of light alongside the gravitational waves.

The narrator explains the proposed sequence: the binary is surrounded by its own mini vortices of gas, and at merger it releases gravitational radiation powerful enough to carry away up to several percent of the original mass of the two black holes. Gas that was orbiting the binary suddenly moves too quickly for the weaker gravitational field of the final black hole, creating an expanding shock front that collides with the surrounding disc gas.

The narrator adds that the kick from the gravitational waves, like a gun's recoil, drives the black hole and its gas through the disc, producing more shocks and a flash of ultraviolet radiation that might be visible from Earth shortly after the gravitational waves arrive. The narrator notes that spotting these flashes will be a challenge, since the combined LIGO and Virgo localization covers a large patch of sky containing hundreds of active galactic nuclei and many thousands of ordinary galaxies.

The host explains that the Kerr solution describes the spacetime outside a rotating black hole and can be used to calculate the path of any body moving near, or even within, a rotating black hole or any rotating mass. The host says the deeper interior details are left for another episode.

The host asks what is actually rotating. Since nothing can escape the event horizon, it seems internal rotation could not affect the outside. The host's answer is that both the gravitational field and its rotation can be thought of as properties of spacetime itself, and that black holes are self-sustaining holes in its fabric.

Space at the event horizon, the host says, is dragged downward and drags more space behind it, somewhat like water dragged near the edge of a waterfall. In a Kerr black hole, space above the horizon is dragged in a circle, making it more of a whirlpool than a waterfall. The host notes that a black hole without mass can be constructed in general relativity, with warped spacetime persisting.

The flow of space around a rotating black hole is known as frame dragging. The host also mentions that Gravity Probe B measured frame dragging around Earth, a weak effect matching Einstein's prediction. Stable circular orbits around a non-rotating hole extend down to three Schwarzschild radii, while for a maximally spinning hole co-rotating orbits can approach the event horizon. The innermost stable circular orbit has not been directly detected, though tentative evidence comes from gravitational lensing studies of quasars.

The host describes the ergosphere, a region just above the event horizon where frame dragging carries space around the black hole faster than light. Everything, even light, must move in the direction of the black hole's spin.

The host says the mathematics represents this flow in an unusual way: space and time switch roles, so that the angular coordinate becomes timelike, making it as impossible to resist orbiting the black hole as to travel backward in time. The ergosphere extends down to the event horizon, and both the horizon and the ergosphere are flattened at the poles, more so for faster spin.

Roger Penrose, the host says, showed in the early 1970s how to extract energy from this region. An object is dropped into the ergosphere on a carefully tuned trajectory and split into two pieces at the right point. One half falls into the black hole and the other escapes with more kinetic energy than it had on arrival, gaining up to 20 percent of the energy bound up in the original object.

The host also mentions that light routed through the ergosphere can be amplified by superradiance, and that mirrors placed around the hole could make a black hole bomb.

The host explains that the usual maps break down inside a black hole, so new coordinate systems are needed to trace paths into its interior. Those maps reveal something unexpected: the paths do not simply end inside the black hole but continue beyond it. The host says that in these maps black holes become wormholes, with new universes on the other side.

To explain coordinate singularities, the host uses the Earth's latitude and longitude, which converge at the poles so that every direction becomes south. Such a singularity can be removed by changing the coordinates, for example by spreading the longitude lines near the pole so they never converge, which produces the Mercator projection. The host adds that this projection is useful for plotting a route as long as one remembers Greenland is not really larger than South America.

Mapping spacetime requires three spatial dimensions plus time, and maps of black holes also have coordinate singularities. The first map of black hole spacetime was the Schwarzschild metric, a relatively simple result derived by Karl Schwarzschild just months after Einstein published general relativity. The host says it describes the path of an object approaching a black hole and even works inside the event horizon, though it cannot plot a trajectory that actually crosses the horizon.

The host explains that a Penrose diagram of an eternal, non-rotating Schwarzschild black hole is geodesically incomplete. Light rays can come in from far away and fall into the black hole, but some light rays going the other way have no sensible point of origin, meaning their origins are undefined.

The host says that tracing the Schwarzschild coordinates to their full extent gives a maximally extended solution, which reveals new regions on the diagram. Tracing light rays backward from our universe leads into a region that looks like the black hole with time reversed, which is the white hole.

The host then focuses on a stranger region reached by tracing right-moving light rays backward from within the black hole. In these coordinates it resembles a mirror-reflected version of our universe, at least in terms of space and time coordinates. The host asks whether this parallel universe is real and whether it can be reached.

The host notes that this map applies to an eternal black hole whose coordinates do not change, implying it always existed, and says a black hole formed from a collapsing star will be treated later. Passage between the universes requires faster-than-light travel; only paths shallower than 45 degrees can cross between them, and even infinite speed would be needed to take those paths.

The host says black holes are very real, but that understanding them remains highly theoretical. Direct study is difficult for two reasons: the interior is hidden beneath the event horizon, and real black holes are very far away and tiny on cosmic scales. Imaging the nearest large one required a telescope the size of the planet.

The host lists the evidence for black holes: stars orbiting in tight slingshot paths around an empty point at the center of the galaxy, superheated gas discs in quasars and X-ray binaries, gravitational waves matching theoretical predictions for black hole mergers, and the image of the black hole at the heart of a galaxy more than 50 million light-years away.

Making a real black hole in a lab is far beyond current technology and potentially dangerous, the host says. Instead, researchers study analog black holes, physical systems that behave in similar ways and may reveal real behaviors. The host says the idea was started in 1972 by Bill Unruh, known for Unruh radiation, with a thought experiment about a blind fish in a river that cannot resist a flow faster than its swimming speed.

The host says the fluid dynamics equations can be written in a form close to those of general relativity, so a vortex can resemble a black hole down to Hawking emission. In a water tank, a current flowing over a slope speeds up as the depth decreases, passing the wave speed and forming an analog horizon; the reverse flow gives an analog white hole. A bathtub vortex that reaches ripple speed produces an analog event horizon.

Silke Weinfurtner demonstrated this with a giant tank of fluorescent water holding 2,000 liters. Water flows in from two sides, forming a whirlpool, and a wave generator sends waves past the vortex, analogous to incoming particles. The host says the waves are only 1 millimeter high but superradiance can increase their height by as much as 10 percent.

Because Hawking radiation is ultimately quantum mechanical, the host says deeper insights may require an analog quantum black hole, which is where Bose-Einstein condensates come in. These form when gases are cooled to nearly absolute zero, making normally microscopic quantum effects macroscopic. Jeff Steinhauer used a laser to create a flow in supercooled rubidium gas, with the laser's edge acting as the event horizon.

The host says that in this system some atoms escape as Hawking radiation, and both the existence and the temperature of that radiation can be measured. The narrator states that taking the temperature of the evaporating particles from a Bose-Einstein condensate provides the strongest direct experimental evidence for Hawking radiation of a black hole.

The narrator says the black hole analog is the best physical experiment available until researchers can travel to the stars or control real black holes in a lab. Theorists will continue to theorize, but a new breed of black hole experimentalist is emerging, and the narrator suggests their results may yield the next insight into black holes, Hawking radiation, and spacetime.

The narrator then raises the cosmic censorship hypothesis, which holds that every singularity is surrounded by an event horizon. Many physicists now suspect naked singularities may exist, since strange things happen inside black holes, including the switching of space and time roles and the possibility of time travel, which threatens causality.

The narrator explains that the event horizon is a blessing: without it, the singularity's madness would be exposed to the outside universe. Roger Penrose proposed the cosmic censorship hypothesis to prevent this, and the narrator says a future episode will examine why many physicists believe it must be true and why others doubt it.

The narrator explains that the amount of spin or charge a black hole can hold before becoming extremal depends on its mass, since more mass means more inward gravity. A more massive black hole can therefore hold more spin and charge before reaching the limit. Adding enough spin or charge to an existing black hole can make it extremal or perhaps a naked singularity.

The narrator describes a second route for charged black holes. Ordinary black holes lose mass through Hawking radiation, but for the most massive ones the radiation is mostly photons, because the lower temperature limits the energy available for other particles. A massive charged black hole therefore leaks mass while keeping its charge, so its outer event horizon shrinks until it meets the inner horizon, again producing an extremal black hole. Rotating black holes cannot follow this route because they lose angular momentum as well as mass.

Once formed, the narrator says an extremal black hole may last for a very long time, if not forever. Naked singularities do not emit Hawking radiation, and extremal black holes radiate only very slowly, so in the far future even if all particles decay, only radiation and naked charged singularities might remain.

The narrator notes that it appears possible to add a little more spin or charge to produce true naked singularities, which the cosmic censorship hypothesis would forbid, and that resolving this would be a major problem for physics.

The narrator describes how the latest notable event seemed unremarkable at first: two compact bodies spiraling together. From the waveform and calculations based on Einstein's general theory of relativity, the masses were determined. One body was 23 times the mass of the Sun, clearly a black hole similar to other LIGO mergers.

The other body was just 2.6 solar masses, which the narrator says is pushing the limit of what was thought possible for a neutron star and is lighter than what was thought possible for a black hole. The narrator frames this as one of the most informative and surprising events so far.

The narrator reports the event was detected on August 14, 2019, and that the masses were calculated as 23.2 and 2.59 solar masses. The host plans to work out what the smaller object could be.

The narrator says a neutron star's surface gravity is about 100 billion times stronger than Earth's, and its escape velocity at the surface reaches up to half the speed of light. Neutron stars are on the verge of being black holes, which by definition have an escape velocity at the event horizon equal to the speed of light.

Adding normal matter to an object would increase its radius, weakening the effect, but neutron stars are quantum mechanical objects in which adding mass does not necessarily increase size. At the highest masses the size actually shrinks, so more mass means higher surface gravity and escape velocity. The narrator describes a phantom event horizon: for a given mass there is a size below which the object would be a black hole, about a centimeter for Earth and several kilometers for a neutron star. When the phantom horizon and the actual surface overlap, a black hole forms.

The narrator says this picture is well accepted, but it is not known exactly how massive a neutron star can be before collapsing. Theoretical models predict maximum masses roughly two to three times the mass of the Sun. The uncertainty lies in how neutrons behave at the core, where they may break down into quark matter, including strange quarks, and the narrator says better measurements are now possible.

The narrator says black holes are scary because they are so final: nothing that crosses the event horizon can emerge, so classical general relativity implies they are black and eternal, growing but never shrinking. The narrator says this is the picture from the mathematics of Einstein's general theory of relativity.

General relativity is not the whole picture. In 1974, Stephen Hawking showed that bringing quantum mechanics into the calculation means a black hole is neither black nor eternal. He showed black holes must radiate, slowly leaking away their mass in what is now called Hawking radiation.

The narrator says quantum mechanics may save us from the eternal black hole, but perhaps not completely. If the evaporation is calculated to the very last instant, quantum mechanics may also save the final speck with the smallest conceivable size. These remnants, sometimes called Planck relics, may be everywhere. The narrator says Hawking's discovery came from examining how black holes interact with quantum fields, from which elementary particles arise.

The narrator says that if black holes never fully evaporate, the information they swallow would be trapped inside the tiny Planck relic, which would solve the information paradox. Pure thermal radiation would delete quantum information and break a conservation rule, while remnants could store that information, although this conflicts with the Bekenstein bound on how much information a region can contain.

The narrator describes a proposed way around this: space inside black holes could expand to a region larger than the event horizon, or a new inflation could be triggered at the singularity, giving the information room. The narrator frames this as far-out speculation.

The narrator then asks whether Planck relics could account for dark matter. A quick calculation suggests there might be about one Planck relic per 30 km cube in an average city, enough to make up most of the mass in the universe because most of space is empty. The narrator invites viewers to check the math and ends by noting that a tiny black hole could be somewhere nearby.