Black holes seem to be the stuff of science fiction (and, in fact, have starred in many sci-fi books and movies), so it's not uncommon for people to wonder, are black holes real? As it turns out, the answer is yes, though for a long time most scientists were convinced that black holes were purely theoretical objects.
The Idea of a Black Hole
The concept of a black hole was first conceived by an English amateur astronomer named John Michell in 1783. Michell was laboring under the Newtonian assumption that light particles had mass. So, using Newton’s equation for gravity, Michell suggested that if there were an object with 500 times the radius of the sun, but with the sun’s average density, then its escape velocity would be faster than the speed of light. A few years later, the French mathematician and astronomer Simon Pierre Laplace came to a similar conclusion.
Unfortunately, the speculations of Michell and Laplace were not taken seriously in the scientific community because there was simply no evidence to suggest that such exotic objects existed in the universe. Moreover, Thomas Young’s double slit experiment in 1803 confirmed the wave nature of light, and it seemed impossible that gravity could have any influence on massless waves.
But in 1905, Einstein used the photoelectric effect to demonstrate that light is composed of massless particles called photons. Moreover, his theory of general relativity, published in 1915, proved that gravity could affect these particles even though they had no mass. According to relativity, the force of gravity results from massive objects warping the surrounding spacetime (the three spatial dimensions and time combined in a four-dimensional continuum). Since even massless particles such as photons should obey the curvature of spacetime, gravity can indeed influence light.
In 1916, Karl Schwarzschild solved Einstein’s general relativity equations to determine the radius of an object whose escape velocity would exceed the speed of light. However, Einstein himself claimed that the possibility of a black hole was nothing more than a mathematical curiosity—an interesting prediction of general relativity, but not an accurate depiction of reality. Not until the mid-twentieth century, when neutron stars were discovered, did astrophysicists begin to seriously consider whether objects as compact as black holes could actually exist.
Evidence for Black Holes
In recent decades, scientists have gathered much observational evidence for the existence of black holes. As the name suggests, we cannot see black holes themselves, but we can observe the effect a black hole has on its environment. As black holes devour the matter around them, this material forms an accretion disk, which radiates primarily in the X-ray band of the electromagnetic spectrum.
Furthermore, when black holes occur in multiple star systems, we can witness their gravitational effects on the visible companion stars. In fact, the first true black hole candidate, Cygnus X-1, was discovered because of its strong X-ray emission and its gravitational effect on a tightly orbiting massive star. Similarly, the supermassive black hole in the center of the Milky Way, Sagittarius A*, was confirmed by astronomers monitoring the orbit of star S0-2 (abbreviated S2) over its 15 ½ year period.