Black Holes

Young Astronomers Blog, Volume 28, Number 22.

The 2020 Nobel prize in physics was recently awarded to three scientists, Roger Penrose, Reinhard Genzel, and Andrea Ghez, for their landmark work with Black Holes.

Black Holes are probably one of the strangest and most interesting objects in the cosmos. Their density is so great that gravity prevents light (and anything else) from escaping. A black hole is not just a massive object. Black Holes form when enough stuff (mass) is squeezed into a sufficiently small volume. As an object is compressed, its density increases giving rise to a stronger and stronger gravitational force. As a result, light passing near the object bends more and more. Eventually, the gravitational force is so great that light bends into the object never to be seen again.

John Michell, in a 1783 letter to Henry Cavendish, was the first to speculate about objects massive enough to prevent light from escaping. He thought they would be dark but could be detected by the gravitational influence on nearby objects. A few years later in 1796, Pierre Simon Laplace discussed a similar concept.

The edge of a black hole is known as the event horizon. In 1916, Karl Schwarzschild showed, using Einstein’s General Theory of Relativity, that a singularity could exist, warping space-time so much that light would bend in on itself. The point of no return, the event horizon, is defined by the Schwarzschild radius, which is R_s = 2GM/c², where G is the gravitational constant, M is the mass of the object, and c is the speed of light. For example, the Sun would theoretically become a black hole if it were crushed into a sphere with a radius of 3 km. The Earth would have to shrink to a radius of 8.9 mm (roughly the size of a large ¾ inch “shooter” marble) to become a black hole.

The term “black hole” was first used by Ann Ewing in a 1964  Science News Letter article. However, Physicist John Wheeler is generally given credit for the term. It was likely suggested to him by someone in the audience during a 1967 talk at the Goddard Institute for Space Studies in New York. Wheeler, who was using the phrase “gravitationally completely collapsed star”, adopted the term and it caught on.

Black holes remained just a theory until the early 1970s when the x-ray source Cygnus X-1, discovered in 1964, was generally accepted as being a black hole.

It was thought that black holes were totally black. However, in 1974, Stephen Hawking showed that over an extremely long period of time, black holes will evaporate by emitting what is now known as “Hawking Radiation.” This led to a long-standing debate over the “information paradox” of black holes. Information is conserved according to physical laws. However, Hawking argued that, because black holes evaporate, they lose information in violation of this law. Leonard Susskind and others countered Hawking’s arguments using something called the holographic principle (and the anti-de Sitter/conformal field theory or AdS/CFT) to show that information is still conserved. The arguments are extremely subtle, and, although Hawking conceded to Susskind’s point of view in 2004, the debate is still simmering.

There are two primary types of black holes.

• Stellar mass black holes, a few to 100 or so times the mass of the Sun, are formed after the collapse of a massive dying star as it explodes in a supernova.
• Supermassive black holes, millions to billions of times as massive as the Sun, form at the center of large galaxies.

There is also some speculation that there might be intermediate-mass black holes and diminutive primordial black holes.

Subrahmanyam Chandrasekhar determined, in 1930, that a dying star with a mass greater than 1.44 times that of the Sun (the Chandrasekhar limit) will collapse into an extremely dense object and not into a white dwarf. A few years later, in 1939, J. Robert Oppenheimer and Hartland Snyder showed that a massive star will continue to contract under its own gravity until light can no longer escape. In 1965, Roger Penrose generalized much of the theory of black holes showing that, according to Einstein’s General Theory of Relativity, they are the consequence of the collapse of a massive star.

Astronomers believe that stars will end their lives in one of three forms depending on their mass: a white dwarf, a neutron star, or a black hole. The latter two are the remnants after a star explodes as a supernova. For more on this, see Betelgeuse is dimming.

Donald Lynden-Bell and Martin Rees hypothesized in 1971 that black holes could be found in the center of galaxies. Three years later in 1974, Bruce Balick and Robert L. Brown first identified the black hole at the center of the Milky Way as a bright radio source. Brown eventually gave it the name Sagittarius A* (“Sagittarius A star”) in 1982 because of its location near the radio object Sagittarius A.

For twenty five years, two teams, one led by Reinhard Genzel (Max Plank Institute for Extraterrestrial Physics) and the other by Andrea Ghez (UCLA Galactic Center Group) have been measuring the movement of stars at the center of the Milky Way. Their measurements demonstrated that the object at the center of our galaxy is, in fact, a supermassive black hole and it is around four million times the mass of the Sun.

Today astronomers think that supermassive black holes can be found at the center of most galaxies. Of course, this leads to the question: which came first the black hole or the galaxy? There are some recent suggestions that the black hole forms first followed by the galactic structure (for example, see the Universe Today article and video).

Two of the most violent and energetic events in the cosmos are associated with black holes.

• Quasars (Quasi-Stellar Objects) result from “active” supermassive black holes where the accretion disk becomes so hot that it emits high energetic electromagnetic radiation.
• Gamma Ray Bursts (GRB) come in two types, both resulting in a black hole. Short GRB (< 2 seconds) are the result of the collision of two neutron stars or of a neutron star with a black hole. Long GRB (2 seconds to a few minutes) occur during the collapse of a large and rapidly rotating dying star.

The evidence for black holes keeps growing. In 2015, The LIGO observatories detected Gravitational Waves from two colliding black holes. In 2019, astronomers in the EHT collaboration linked eight telescopes from around the world, creating the “Event Horizon Telescope”, and producing the first ever image of a black hole. In October 2020, The European Southern Observatory observed a star being stretched and undergoing “spaghettification” as it is pulled into a black hole.