Black Hole Apocalypse: What's Inside a Black Hole? | Full Documentary | NOVA | PBS

Summary

The documentary explains how black holes went from mathematical predictions to objects supported by multiple lines of astronomical evidence. It also explores their formation, effects on nearby matter, role in galaxies, and the limits of what scientists can know about their interiors.

Gravity, relativity, and black-hole formation

• In Einstein’s general theory of relativity, mass curves spacetime, and objects—including light—follow paths through that curved spacetime. This provides the framework for understanding gravity and black holes.
• Karl Schwarzschild found a solution to Einstein’s equations that describes a boundary around a sufficiently concentrated mass. Today, this boundary is called the event horizon: once something crosses it, it cannot escape, not even light.
• Massive stars can end their lives in supernova explosions. When fusion can no longer provide enough outward pressure to resist gravity, the stellar core collapses. If the remnant is sufficiently massive, it may become a black hole.
• The documentary describes black holes as regions of spacetime rather than ordinary solid objects. The event horizon marks the point of no return; the nature of the interior and the predicted singularity remain unresolved because current physics cannot fully describe them.

What a journey near a black hole might involve

The program uses visualizations to illustrate predicted effects:

• Gravitational lensing: A black hole’s gravity bends light, distorting the appearance of stars and material behind it.
• Time dilation: To a distant observer, an object approaching the event horizon appears to slow. For the falling object, its own time passes normally.
• Spaghettification: Near a black hole, gravity can pull more strongly on one part of an object than another, stretching it apart.
• After crossing the event horizon, an object cannot send information back to the outside. What happens at the center remains unknown.

How astronomers detect black holes

Because black holes emit no light from within the event horizon, researchers look for their effects on nearby objects and light.

• Cygnus X-1: Astronomers identified an X-ray source in a binary star system. The visible star’s changing spectrum showed it was orbiting an unseen companion every 5.6 days. Parallax measurements helped establish the system’s distance, allowing researchers to estimate the companion’s mass at about 15 times the Sun’s—strong evidence that it is a black hole.
• Accretion disks and jets: Gas pulled toward a black hole can form a rapidly rotating disk. Collisions heat the gas to millions of degrees, producing X-rays. Some systems also produce powerful jets of particles and radiation from their poles.
• Quasars: Their extreme brightness and redshift showed that they are distant, energetic objects, not ordinary stars. The documentary presents supermassive black holes and their accretion disks as the likely power sources.
• The Milky Way’s center: Andrea Ghez and colleagues tracked stars moving rapidly around an invisible object. Their orbits imply a mass of roughly four million Suns concentrated at the galactic center.
• Other galaxies: Astronomers used Hubble observations and computer models of stellar motion. The models generally required a massive, invisible central object to match the observations, supporting the idea that supermassive black holes are common in galaxy centers.
• Tidal disruption: If a star passes too close to a black hole, tidal forces can tear it apart. Some of the star’s material may escape, while some falls toward the black hole and can produce a prolonged X-ray flare.

Supermassive black holes and galaxies

The documentary examines how black holes may grow and influence their host galaxies:

• Black holes can gain mass by accreting gas and dust, disrupting stars, and merging with other black holes.
• The Eddington limit describes a constraint on how rapidly a black hole can accrete: radiation from the hot material around it can push outward and impede further inflow.
• To explain how very massive black holes appeared early in cosmic history, researchers consider direct collapse: under some conditions, a large gas cloud might collapse into a massive black-hole seed without first forming ordinary stars.
• The mass of a galaxy’s central black hole appears related to the galaxy’s properties. The documentary describes a possible feedback relationship: energy from a growing black hole heats surrounding gas, potentially reducing the formation of new stars.

Gravitational waves and new observing methods

• General relativity predicts that accelerating massive objects can produce gravitational waves—ripples that stretch and squeeze spacetime.
• LIGO detects these tiny changes using laser interferometers. A laser beam is split along two perpendicular arms, reflected by mirrors, and recombined. A passing gravitational wave changes the arms’ relative lengths and alters the returning light.
• On September 14, 2015, LIGO recorded a signal from two merging black holes more than a billion light-years away. The event provided direct evidence of black-hole mergers and opened a new way to observe the universe.
• In 2017, LIGO detected gravitational waves from colliding neutron stars. Telescopes then observed light from the same event, demonstrating how gravitational-wave observations can be combined with conventional astronomy.
• The documentary also describes the Event Horizon Telescope’s effort to image a black hole’s shadow against surrounding light, and the anticipated use of the James Webb Space Telescope to study the early universe.

Methods and tools featured

• Observe effects on nearby stars, gas, and radiation rather than trying to see a black hole directly.
• Use Doppler shifts in starlight to identify orbital motion.
• Use parallax to estimate distance and improve mass estimates.
• Use X-ray observations to find hot accretion flows and feeding events.
• Use adaptive optics and long-term imaging to track stars near galactic centers.
• Compare observed galaxy motions with computer models.
• Use laser interferometry to detect gravitational waves.
• Combine radio telescopes around the world to improve imaging resolution.

Researchers and sources featured

Researchers and scientists named in the subtitles: Isaac Newton; Albert Einstein; Karl Schwarzschild; Subrahmanyan Chandrasekhar; J. Robert Oppenheimer; John Wheeler; Jocelyn Bell (Burnell); Arthur Eddington; Carl Jansky; Martin Schmidt; Paul Murdin; Louise Webster; Richard Woolley; Kip Thorne; Stephen Hawking; Johannes Kepler; Mark Reid; Martin Schwarzschild; Andrea Ghez; Todd Lauer; James Guillochon; Shep Doeleman; Ray Weiss; Barry Barish; Robert Schofield; and a colleague identified only as Lin.

Institutions, observatories, and projects featured: LIGO; the Chandra X-ray Observatory; Keck Observatory; the Hubble Space Telescope; the Event Horizon Telescope; the planned James Webb Space Telescope; Bell Labs; and the Royal Observatory.