Black holes rank among the universe's most enigmatic objects. While their existence is firmly established in modern astrophysics, direct observation remains impossible, fueling debates about their precise form—especially their shape.
The concept of an object with escape velocity exceeding light speed emerged in the 18th century, but it gained rigor in the early 20th with Albert Einstein's general theory of relativity. Published in 1915, it prompted solutions like those from Karl Schwarzschild and Roy Kerr, later interpreted as black hole models (a term coined in France by 1973).
The first black hole detection came in 1971 via Cygnus X-1, a high-mass X-ray binary. Countless others followed. Since no light escapes, we observe indirectly: through accretion disk radiation, relativistic jets, gravitational perturbations, and waves.
Visible celestial bodies—absent collisions or unusual origins—tend toward sphericity from gravitational collapse or accretion. Gravity pulls equally in all directions, yielding spheres.
Imperfections arise from mass distribution, magnetic fields, or rotation, often producing oblate spheroids—flattened at poles, especially in fast spinners. Might black holes behave similarly?
This query fits: most form via massive star collapse. Could the progenitor's shape persist? Typically, we model black holes as spherical.
For non-spherical collapse? Physics offers two views: non-sphericity vanishes in the singularity, or it yields a naked singularity instead of a black hole.
In 1916, Karl Schwarzschild found an exact general relativity solution: the Schwarzschild metric for a static, spherical, uncharged, non-rotating mass. This describes the quintessential Schwarzschild black hole.
Key features: the event horizon (where escape velocity hits lightspeed); the singularity (infinite gravity core); and Schwarzschild radius (horizon size).
The radius sets formation: collapse must shrink below it, birthing a spherical horizon. Thus, the event horizon defines black holes, implying sphericity.
In 1917, Hans Reissner and Gunnar Nordström extended it to charged, non-rotating cases (Reissner-Nordström metric). Roy Kerr's 1963 solution added rotation for uncharged black holes (Kerr metric).
In 1965, Kerr and Ezra Newman included charge (Kerr-Newman metric). Rotation—via angular momentum—alters structure. Real astrophysical black holes are Kerr-type; zero rotation demands unrealistic progenitors.
Black holes can therefore be…(continued on next page)
