Scattered across the universe, black holes are objects with compelling gravity that nothing can escape their grasp. These cosmic voids can tear apart whole stars and planets. In studying the nature of black holes, physicists look at specific light features to figure out the closest one can get to a black hole without struggling to prevent disaster.
Over the Waterfall
The black hole has the event horizon region, the invisible line in the sand across which an object cannot return. Once a celestial object passes through this region, it can no longer return to the universe. This means that the black hole's gravity is too strong within the event horizon.
Outside the black hole, everything is just dandy. A typical black hole has a certain mass, while an orbiting black hole is like orbiting anything of an identical group. There are lots of objects in the universe that find themselves orbiting around black holes. Once these travelers get caught in the void's gravitational pull, they get squeezed into a razor-thin bank called an accretion disk. This disk spins and spins while being energized by friction, heat, and magnetic and electric forces. This motion can cause the material to glow brightly.
In the case of the most massive black holes, the accretion disks around them glow so intensely that they are called active galactic nuclei (AGN). Their intense brightness even allows them to outshine millions of individual galaxies.
In the accretion disk, individual pieces of materials rub against other pieces and drain them of rotational energy, driving them inward to the gaping maw of the event horizon. If not for those frictional forces, the material could orbit around the void in perpetuity, the same way the planet can orbit around the Sun for billions of years.
READ ALSO: What Is a Black Hole? How Big Can It Get?
A Call for Help
As an object gets closer to the center of the black hole, it reaches a certain point where all remaining hopes of stability are dashed against gravity. Just outside the cosmic void, but before going to the event horizon, the forces of gravity are so extreme that stable orbits become impossible. Once this region is reached, a traveling object cannot remain in a peaceful orbit. There are only two choices left for the traveler: They can propel themselves away to safety if they have rockets or some other energy source, or they are doomed to fall freely toward the abyss that waits below. This boundary, the innermost stable circular orbit (ISCO), is a firm prediction of Einstein's general theory of relativity. The same theory predicts the existence of black holes in the first place.
Despite the success of general relativity in predicting and describing the phenomena across the universe, together with the experts' sure knowledge that black holes are natural, they still have not been able to verify the existence of the ISCO and whether it conforms to the predictions of general relativity.
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