Building on this unprecedented track record of success, most astrophysicists fully believe that general relativity’s description of the nature of black holes is the correct one. Lingering questions attempt to use our knowledge of black holes to improve our understanding of how gas, magnetic fields, and X-rays behave in the presence of such a tremendous gravitational force. This is the messy part of black hole research — Astronomer Royal Martin Rees famously described it as “mud wrestling” — and one where observation has been far ahead of theory for decades.

The first puzzle came right on the heels of the first detection of Cyg X-1. In 1973, from the most basic laws of conservation of energy and angular momentum, Igor Novikov and Kip Thorne derived a brilliant and elegant description of how gas slowly spirals in toward a black hole, releasing its gravitational potential energy as heat and radiation at temperatures of millions of degrees.

There are only two problems with the Novikov-Thorne model: It doesn’t work in theory, and it doesn’t work in practice. It doesn’t work in theory because it doesn’t explain how exactly the gas loses angular momentum. It doesn’t work in practice because it doesn’t agree with observations of high-energy X-rays coming from billion-degree gas.

Hot ionized gas experiences almost no friction or viscosity, so it should simply go around and around on perfectly circular orbits forever, never getting any closer to the event horizon. Novikov and Thorne fully appreciated this problem, and they absorbed it into their theory with a simple fudge factor, leaving the details to later work. In the end, it took almost 20 years to find the answer. In 1991, Steve Balbus and John Hawley discovered a powerful instability that comes from the twisting and pulling of magnetic field lines embedded in an accretion disk. Ionized gas is an excellent electrical conductor, which means it also can generate powerful magnetic fields. These fields, in turn, can pull back on the gas, slowing it down and allowing it to spiral in toward the black hole.

By 2001, supercomputers had become powerful enough to adequately simulate the Balbus-Hawley instability in accretion disks around realistic black holes, fully confirming their predictions. It took yet another decade before the simulations were sophisticated enough to include the effects of radiation, and study the interplay between the disk and corona. In doing so, we have finally reached the point where, starting from the most fundamental laws of nature, we can explain how the high-energy X-rays, first seen in 1971, are actually generated around real black holes.

In exactly 100 years, black holes have progressed from being a mathematical curiosity, to the subject of purely theoretical physics, to a central area of astronomy research, where theory and computer simulations confront experiments and observations on a daily basis. With the recent opening of the gravitational-wave window on the universe, in the coming years we fully expect to learn even more about the birth, life, and death of these remarkable objects. One thing we can say for certain: We will continue to be surprised by nature’s exotic imagination!

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