History of Black Holes

• Theoretical History
• The first suggestion of a black hole was by Michell in 1783 (he called the objects "dark stars"). The idea is based on Newton's law of gravity and the escape velocity, and it was logical to ask whether an object could have enough mass that nothing could escape. He even calculated the radius, which is close to the Schwarzschild radius for a black hole.
• This idea was also published by Laplace although a few years later. However, no one believed that these objects would actually exists.
• In 1915, Einstein presented his theory of General Relativity.
• In 1916, Schwarzschild solved the equations and determined the radius of a black hole, now called the "Schwarzschild Radius". It is the same equation as found by Laplace and Michell. However, this theory contained a central singularity which caused concern from many (including Einstein).
• From the 1920s to the 1960s there was a great amount of work on GR, and specifically on black holes. Including rotation in a black hole proved to difficult, and it was finally solved by Kerr in 1963. A rotating black hole is called a "Kerr Black Hole". There is no direct observational evidence yet for the spin of a black hole, although the indirect evidence is becoming compelling.
• The idea of the event horizon leads to the No Hair Theorem, which states that black holes can be described by three numbers: mass, spin, and charge. All other information is thought to be lost (e.g., what type of matter the black hole is made of). This causes problems for some conservation laws.
• John Wheeler coined the phrase "black hole" in 1968 (it was apparently suggested to him during a talk).
• A complete theoretical understanding of black holes is still a ways off. It will likely involve understanding String Theory first, or at least quantum gravity.

• Observational History
• We need to distinguish between normal black holes and supermassive black holes. Normal black holes (if there is such a thing) are believed to be the result of stellar evolution. End products of a star are white dwarfs, neutron stars (and pulsars), and black holes. The difference has to do with the amount of mass that was originally there in the star. Stars that have masses greater than around 5 times the mass of the Sun may end up as a black hole. Supermassive black holes are those that are thought to be a million solar masses or greater, and they have only been found in the middle of a galaxy (more massive things fall in to the center due to gravitational friction).

• An Outline of Stellar Evolution
• Normal Star: burning H in core (H -> He)
• Giant Star: ran out of H in core and started to burn He
• White Dwarf core and Planetary Nebula : ran out of He in core and blew off outer shell. If the mass is less than 5 Suns then the white dwarf can sustain the pressure from gravity due to its DEGENERATE ELECTRONS, and this become the final state.
• Neutron Star: if mass is greater than 5 Suns, gravity becomes strong enough to force proton and electrons together to become neutrons. We then end up with DEGENERATE NEUTRONS, which can sustain very strong pressures. This will become an end-state for some stars.
• If the mass is still greater (maybe 10 or more Suns?), then even degenerate neutrons can't fight off gravity. This is what we call a BLACK HOLE.

• In terms of observations, not many scientists believed that black holes could actually exists in nature. However, in 1963, Maarten Schmidt discovered Quasars. And the search for black holes was born.
• Quasars are extremely bright, small objects. They are considered bright since they are extremely distant from us, yet we can easily see them. The average quasar is seen at a distance when the Universe was 1/4 of its present age (when it was only 3 billion years old). They are considered to be small because of their rapid variability. An object cannot vary faster than it takes light to travel across it---this determines the size of the quasar.
• One of the strongest arguements for a quasar is that it is a black hole that is actively feeding. This is based on measuring the energy output from the quasar and its small size.
• Quasars are more abundant in the past --- we see very few quasars locally. The idea is that as a galaxy forms, there is more material around for a black hole to eat. After it is gone, then it becomes quiet. Thus, there should lots of dead quasars around, and the numbers suggest that most galaxies may have gone through a quasar phase. Therefore, every galaxy might have a black hole.
• Here are some HST images of quasar hosts. Cygnus A shows huge radio lobes from an active galactic nucleus.
• In the 1980s, astronomers began searching for supermassive black holes in earnest. The first suggestion was in the galaxy M87 -- the giant central galaxy in the Virgo cluster of galaxies. Remarkably, the black hole mass estimate (based on a very crude approximation) is very close to the present-day measurement.
• However, the big boost for black hole searches came when the Hubble Space Telescope was launched (after it was fixed).

• The search for stellar mass black holes started around the same time as that for supermassive ones. The best candidate was Cyg X-1, a binary system. Today, there are about 20 stellar mass black hole candidates that range in mass from 3 to 15 solar masses.