Other meanings of black hole
Astrophysics
A black hole is a region of spacetime where gravity is so strong that nothing—no particles or even electromagnetic radiation such as light—can escape from it. The theory of general relativity predicts that a sufficiently compact mass can deform spacetime to form a black hole. The boundary of the region from which no escape is possible is called the event horizon. Although the event horizon has an enormous effect on the fate and circumstances of an object crossing it, no locally detectable features appear to be observed. In many ways, a black hole acts like an ideal black body, as it reflects no light. Moreover, quantum field theory in curved spacetime predicts that event horizons emit Hawking radiation, with the same spectrum as a black body of a temperature inversely proportional to its mass. This temperature is on the order of billionths of a kelvin for black holes of stellar mass, making it essentially impossible to observe directly.
A black hole is defined by its event horizon, a boundary in spacetime beyond which events cannot affect an outside observer. According to the no-hair theorem, a stationary black hole is completely described by only three externally observable quantities: mass, electric charge, and angular momentum (spin). This theorem, formulated by Werner Israel, Brandon Carter, and Stephen Hawking in the late 1960s and early 1970s, implies that all other information about the matter that formed the black hole is lost except for these three parameters.1
The simplest black hole is the Schwarzschild solution, which describes a non-rotating, uncharged black hole. Its event horizon is a sphere with radius equal to the Schwarzschild radius, given by rs = 2GM/c2. For a black hole with the mass of the Sun, this radius is about 3 kilometers. The Kerr solution describes a rotating black hole, which possesses an ergosphere—a region outside the event horizon where spacetime is dragged along with the rotation, allowing energy extraction via the Penrose process.2
At the center of a black hole lies a gravitational singularity, where spacetime curvature becomes infinite. The singularity is hidden behind the event horizon, so it cannot be observed directly. The cosmic censorship hypothesis, proposed by Roger Penrose, suggests that singularities are always hidden, preserving predictability in the universe.3
Black holes are formed when massive stars collapse at the end of their life cycles. When a star with more than about 20 solar masses exhausts its nuclear fuel, its core collapses under gravity, and if the core's mass exceeds the Tolman–Oppenheimer–Volkoff limit (about 2–3 solar masses), no known force can halt the collapse, and a black hole is formed. The collapse may be accompanied by a supernova or a gamma-ray burst.4
Black holes can also form through other processes, such as the merger of two neutron stars or the direct collapse of a massive gas cloud in the early universe, producing so-called primordial black holes. Primordial black holes, if they exist, could have formed shortly after the Big Bang from density fluctuations and could have masses ranging from tiny fractions of a gram to thousands of solar masses.5
Once formed, black holes can grow by accreting matter from their surroundings, including gas, dust, and even other black holes. Supermassive black holes, with masses from millions to billions of solar masses, are found at the centers of most galaxies, including the Milky Way's Sagittarius A*. Their formation remains an active area of research, with theories including the merger of smaller black holes and the direct collapse of massive gas clouds in the early universe.6
Although black holes themselves emit no light, they can be detected through their gravitational influence on nearby matter and through radiation emitted by infalling material. The first strong candidate for a black hole was Cygnus X-1, discovered in 1964, which is a binary system with a visible star orbiting an invisible companion of about 15 solar masses, likely a black hole.7
In 2019, the Event Horizon Telescope (EHT) collaboration released the first direct image of a black hole, the supermassive black hole at the center of galaxy M87. The image shows a bright ring of emission surrounding a dark central shadow, consistent with predictions of general relativity. In 2022, the EHT released an image of Sagittarius A*, the Milky Way's central black hole.8
Gravitational waves provide another observational window. In 2015, the Laser Interferometer Gravitational-Wave Observatory (LIGO) detected the first gravitational wave signal from a merger of two stellar-mass black holes, confirming a major prediction of general relativity and opening a new era of astronomy.9
In 1974, Stephen Hawking showed that quantum effects near the event horizon cause black holes to emit thermal radiation, now called Hawking radiation. This radiation arises from virtual particle pairs near the horizon, where one particle falls in and the other escapes, carrying away energy. As a result, black holes lose mass over time and eventually evaporate completely, though the timescale for stellar-mass black holes is vastly longer than the current age of the universe.10
Hawking's work led to the formulation of black hole thermodynamics, which draws an analogy between black hole mechanics and the laws of thermodynamics. The area of the event horizon is analogous to entropy, and the surface gravity is analogous to temperature. This framework has profound implications for fundamental physics, including the black hole information paradox, which questions whether information that falls into a black hole is lost or preserved.11
Beyond the well-known facts, black holes have many surprising and niche aspects. For instance, the concept of a black hole was anticipated in the 18th century by John Michell and Pierre-Simon Laplace, who speculated about "dark stars" from which light could not escape, based on Newtonian gravity.12
Black holes can have electric charge, described by the Reissner–Nordström solution. Charged black holes have two event horizons, and their thermodynamics is more complex. However, astrophysical black holes are expected to be nearly neutral because any charge would quickly attract opposite charges from the surrounding plasma.
Another intriguing possibility is the existence of black holes in extra dimensions, as proposed in some string theory models. These could have observable signatures in gravitational wave signals or cosmic rays, but none have been detected.
Black holes also play a role in the Fermi paradox: some have speculated that advanced civilizations might use black holes as energy sources (via the Penrose process) or as time machines, though these ideas remain speculative.
In popular culture, black holes have been depicted in films like Interstellar, which consulted physicist Kip Thorne to create a realistic visual representation of a black hole and its accretion disk, including gravitational lensing effects.
Black holes remain among the most mysterious objects in the universe, challenging our understanding of physics at the intersection of general relativity and quantum mechanics.
Served from cache
Help improve the encyclopedia. Reports go straight to the site manager.