Reversed Centrifugal Force

Throughout the history of human scientific development, there had been no shortage of research into black holes, and naturally, plenty of research into Black Hole Photon Spheres as well. After all, a black hole was called a black hole because its Gravity was powerful enough to capture even light, making it difficult to observe in the starry sky.

Research into black holes was therefore inseparable from photons. Or rather, most physical research in the universe was related to light.

Black holes displayed only three physical properties: mass, electric charge, and angular momentum.

This black hole had evidently formed after ancient stars exploded into black holes and then merged. Over the passing ages, it had swallowed all the nearby dust clean, leaving not even an Accretion Disk behind. Such a black hole grew by devouring matter, and because that matter was electrically neutral, it also displayed a neutral charge.

Thus, if it were studied through direct observation, only its mass and angular momentum could be examined. However, its effects on surrounding objects were also one avenue of study, such as its Photon Sphere.

A black hole's Photon Sphere consisted of photons temporarily trapped by the black hole's powerful Gravity and unable to escape. Like a shell of light clinging to the black hole, it made the black hole appear, from close range, like a miraculous sphere of light whose back side could be seen directly from the front.

Humanity had long calculated the thickness of this shell of light through the principles explained by General Relativity: 0.5 times the Black Hole Radius. A black hole's Schwarzschild Radius was the radius of the sphere formed by its Event Horizon. In other words, the so-called thickness of the Photon Sphere extended from the black hole's Event Horizon to 1.5 times its Schwarzschild Radius.