Science › Black holes & quantum gravity
Black holes
1916· Discovery / experiment
ProvenTested many times; part of working physics.
Regions where matter is packed so densely that gravity lets nothing, not even light, climb back out.
See it on the Science map →How it works
Squeeze enough mass into a small enough space and the escape speed exceeds the speed of light. The boundary of no escape is the event horizon. Most known black holes form when a massive star burns out and its core collapses; giant ones, millions to billions of times the Sun's mass, sit at galaxy centers. A black hole is described by just three numbers: mass, spin and electric charge. The name was popularized by John Wheeler in 1967.
Why it matters
They are where gravity is at its most extreme, so they are the best test lab for general relativity and the place where it collides with quantum theory.
Think of it like
A one-way lobby door: tenants can walk in, but there is no handle on the inside.
How we know
X-ray source Cygnus X-1 (early 1970s); stars orbiting the Milky Way's center (Nobel 2020); LIGO's gravitational waves from merging pairs (2015); Event Horizon Telescope images (2019, 2022).
Still a mystery
What really happens at the center, and whether information that falls in is ever released, are unknown.
Connections (11)
▸General relativity: gravity is curved spacetime predicts Black holesProven
The relationship
Einstein's 1915 equations say mass curves spacetime. Put enough mass in a small enough region and the curvature becomes so steep that every path, even light's, bends back inward. The equations allow this outcome directly: Schwarzschild's 1916 solution, Oppenheimer and Snyder's 1939 collapsing star, and Roy Kerr's 1963 spinning version all describe black holes.
How the link was found
Karl Schwarzschild found the first solution in 1916; Oppenheimer and Snyder showed in 1939 that a collapsing star forms one; Penrose proved in 1965 that formation is generic.
Why it's proven
The math is exact. Observations confirm it: Cygnus X-1 (1970s), star orbits around the Milky Way's center, LIGO's 2015 merger signals, and Event Horizon Telescope images (2019, 2022).
Open questions and mysteries
Relativity also predicts a singularity at the center, which signals its own breakdown. What truly lies inside is unknown.
▸Gravitational waves (LIGO, 2015) were first caught from merging Black holesProven
The relationship
When two black holes orbit each other, they shake spacetime, sending out ripples that carry away energy. The pair spirals closer, faster, then merges, producing a characteristic rising "chirp" followed by a ringing-down as the new black hole settles. LIGO's detectors measure stretches of space smaller than a thousandth of a proton's width, revealing the masses and spins of the merging holes.
How the link was found
LIGO detected the first signal, GW150914, on September 14, 2015: two black holes of about 36 and 29 solar masses, 1.3 billion light-years away. Weiss, Barish and Thorne shared the 2017 Nobel Prize.
Why it's proven
Two detectors 3,000 km apart saw the same signal 7 milliseconds apart, matching relativity's predicted waveform. Hundreds of mergers have been detected since, with Virgo and KAGRA.
Open questions and mysteries
Some detected black holes have masses that stars should not be able to produce. How such pairs form and meet is debated.
▸Karl Schwarzschild found the first solution for Black holesProven
The relationship
Schwarzschild solved Einstein's equations for the space around a single round mass. His solution describes ordinary stars from outside, but if the mass is squeezed small enough, it describes a region light cannot leave. Every non-spinning, uncharged black hole is described by his solution, though it took decades for physicists to accept what it implied.
How the link was found
Karl Schwarzschild, December 1915, while serving in the German army; published in early 1916. The black hole reading came later, from Lemaître (1933), Oppenheimer and Snyder (1939), and Finkelstein (1958).
Why it's proven
The solution is mathematically exact and matches observation: planetary orbits, light bending, and the shadow sizes of black holes imaged in 2019 and 2022.
Open questions and mysteries
Real black holes spin, so they follow Kerr's 1963 solution instead. What lies at the center of either is unknown.
▸Black holes are wrapped in Event horizon: the point of no returnProven
The relationship
Every black hole is surrounded by an event horizon, the boundary inside which escape is impossible. Its size is set by the mass: about 3 km in radius per Sun's worth. Spin shrinks it somewhat. The horizon is what makes a black hole black: no light from inside can reach us, so we see only its effect on surroundings.
How the link was found
Implicit in Schwarzschild's 1916 solution; David Finkelstein in 1958 showed it is a one-way boundary rather than a math glitch. Wheeler popularized the term black hole in 1967.
Why it's proven
Event Horizon Telescope images show the dark shadow expected from a horizon. Gas falling into black holes shows no flash from hitting a surface, unlike gas hitting neutron stars.
Open questions and mysteries
Is the horizon truly smooth for someone falling in, or is there a "firewall" or other quantum structure there?
▸Black holes hide at their center Singularities: where physics breaksOpen question
The relationship
Inside the horizon, relativity says all matter keeps falling inward with no force able to stop it. The result is a point (or, for spinning black holes, a ring) of infinite density. Because it is hidden behind the horizon, it cannot affect the outside world. Most physicists read it as the place where relativity stops working, not as a real infinite thing.
How the link was found
Present in Schwarzschild's 1916 solution; Penrose proved in 1965 that it is unavoidable, not a product of perfect symmetry. Penrose also proposed (1969) that singularities are always hidden.
Is it proven?
The prediction is a proven feature of relativity. Whether it describes reality is untested, since nothing inside a horizon can be observed.
Open questions and mysteries
What actually replaces the singularity: a dense core, a bounce into a new region, or something only quantum gravity can describe?
▸Black holes were photographed in First pictures of a black hole (2019)Proven
The relationship
A black hole itself emits no light, but gas swirling around it glows brightly. Gravity bends that light, forming a bright ring around a dark "shadow" about two and a half times wider than the horizon. A planet-sized network of radio telescopes, the Event Horizon Telescope, was sharp enough to resolve this ring for the giant black holes in M87 and the Milky Way.
How the link was found
The Event Horizon Telescope collaboration, more than 200 scientists, observed in April 2017 and released the M87* image on April 10, 2019; Sagittarius A* followed on May 12, 2022.
Why it's proven
Separate teams processing the data independently, with different methods, produced the same ring. Later observations in 2018 confirmed it persists, with the bright spot shifting as gas moves.
Open questions and mysteries
Finer structure, such as the thin "photon ring" of light orbiting near the hole, is not yet resolved.
▸Black holes joined in pairs would be WormholesSpeculative
The relationship
The complete mathematical version of Schwarzschild's solution includes a second universe-like region joined to ours by a throat, now called an Einstein–Rosen bridge. In principle, two black-hole mouths could be connected. But the throat pinches shut faster than anything, even light, could pass through. Real black holes formed from collapsing stars do not have the second side at all.
How the link was found
Einstein and Nathan Rosen described the bridge in 1935. Wheeler coined "wormhole" in 1957; Fuller and Wheeler showed in 1962 that it collapses too fast to cross.
Is it proven?
The bridge is exact math, but no wormhole has ever been observed. A traversable one would need "exotic" negative-energy matter (Morris and Thorne, 1988) never seen in bulk.
Open questions and mysteries
The 2013 "ER = EPR" idea suggests entangled particles might be joined by tiny wormholes. It is unproven but influential.
▸John Wheeler gave the name to Black holesProven
The relationship
Collapsed stars were called "frozen stars" or "gravitationally completely collapsed objects." Wheeler's short, vivid name "black hole" stuck and helped turn them from a mathematical oddity into a serious research subject.
How the link was found
Wheeler popularized the term in a late-1967 lecture after an audience member suggested it. It had appeared in print in a 1964 news report, so he did not strictly invent it.
Why it's proven
Black holes are now confirmed: Cygnus X-1 in the early 1970s, LIGO's gravitational waves in 2015, and the Event Horizon Telescope images in 2019 and 2022.
Open questions and mysteries
What happens at a black hole's center, and how information escapes, are still unknown.
▸Pauli exclusion: why matter is solid holds dead stars up until they become Black holesProven
The relationship
Pauli's exclusion rule says no two electrons (or neutrons) can share the same state, so squeezing them creates outward pressure. This pressure holds up white dwarfs and neutron stars after a star stops burning fuel. But it has a limit. Above about 1.4 solar masses, electrons give way; above roughly two to three, neutrons do. Then nothing stops collapse into a black hole.
How the link was found
Pauli stated exclusion in 1925; Fowler (1926) applied it to white dwarfs; Chandrasekhar found the 1.4 limit in 1930–31; Oppenheimer and Volkoff (1939) the neutron-star limit; Oppenheimer and Snyder (1939) the collapse.
Why it's proven
No white dwarf above the Chandrasekhar limit has been found; heaviest neutron stars measured are about 2 to 2.3 solar masses; heavier collapsed objects are black holes.
Open questions and mysteries
The exact neutron-star maximum mass is uncertain, because nuclear matter at such densities is not fully understood.
▸Roger Penrose showed they really form (Nobel 2020) Black holesProven
The relationship
Penrose proved that black holes are not a fragile result of perfect symmetry but the general fate of enough collapsing mass. Before his work, many, including Einstein earlier, doubted nature would ever make them. After it, astronomers had strong reason to search for black holes, and observations from the 1970s onward found them in X-ray binaries and galaxy centers.
How the link was found
Roger Penrose's 1965 theorem. He received half the 2020 Nobel Prize in Physics; Reinhard Genzel and Andrea Ghez shared the other half for the Milky Way's central black hole.
Why it's proven
The theorem is mathematical proof. Observations since have shown black holes to be common, from star-sized ones in binary systems to giants in nearly every large galaxy.
Open questions and mysteries
Penrose also conjectured that every singularity is hidden by a horizon ("cosmic censorship"). That remains unproven.
▸A universe born inside a black hole says our universe began inside one of the Black holesSpeculative
The relationship
Collapse inside a black hole might stop at a maximum density, as in bounce models, and rebound into a new expanding region. That region would grow into its own spacetime, cut off from the parent behind the horizon. From outside, nothing looks unusual: just a black hole. From inside, the rebound would look like a Big Bang.
How the link was found
R. K. Pathria proposed in Nature (1972) that our universe could be a black hole's interior; Lee Smolin (1992) and Nikodem Popławski (2010) developed bouncing versions.
Is it proven?
Unproven. An often-noted coincidence, that the mass within about 14 billion light-years (where expansion reaches light speed) would form a black hole about that size, holds automatically in a flat universe, so it is not evidence.
Open questions and mysteries
Can black-hole interiors bounce at all? Would a baby universe grow large enough to make galaxies? Nothing inside a horizon can be observed.