How Do We Know That Black Holes Are Really Real?

There is no “black hole theory”.

What in your opinion is wrong with black hole theory? originally appeared on Quora, the place to gain and share knowledge, empowering people to learn from others and better understand the world. You can follow Quora on Twitter, Facebook, and Google Plus.

Black holes are predictions of a theory, namely general relativity. The existence of black holes is evident already from the very first solution of the equations of general relativity written down by Karl Schwarzschild in 1916 (though it was not recognized until much later that the solution does, in fact, harbor a black hole). An explicit mathematical description of how matter can collapse and form a black hole was given by Oppenheimer and Snyder in 1939.

We have very little doubt that black holes exist. We have seen stars moving in the gravitational field of an unseen, massive, compact object (including stars that are believed to be orbiting the supermassive black hole near the center of our Milky Way, Sagittarius A*). We have seen precursors to black holes or things that are almost black holes, such as supernova explosions and neutron stars. We have recently seen gravitational waves from the merger of black holes. We will soon see the “shadow” cast by Sagittarius A* by a radio telescope as a result of an ongoing effort to image this black hole’s event horizon.

A black hole is known, in addition to having an event horizon, to harbor a singularity inside that horizon. Near the singularity, physical variables become divergent. The gravitational field, in particular, becomes extreme, leading many to believe that in this region, Einstein’s gravity does not suffice; we need an (as yet non-existent) quantum theory of gravity to be able to accurately describe this region. So to the extent that something is “wrong” with the prediction of black holes, it’s that the theory may be incomplete. That said, this affects only the extreme environment near the singularity, which we cannot observe anyway, not unless we are foolish enough to jump into the black hole, and even then, the singularity is not a place that you can visit but an unavoidable moment in your future… So not exactly something that you can observe and tell others about. So perhaps we will never need a “complete” theory, since the incomplete theory fully and accurately describes everything we observe.

Then again, this incompleteness also affects our ability to describe the extreme early universe, which might have aftereffects that can be detected by detailed cosmological observations. (Not too long ago, just such a detection was announced, the so-called BICEP2 result of polarization in the microwave background due to gravitational waves originating from those very early moments; later, however, this discovery was retracted, as the analysis was flawed and there really was no signal detected.)

But none of these mean that the black hole predictions are wrong. They simply mean that, as so many times in the history of science, some predictions are valid across a wide range of parameters, but for extreme parameter values, the theory must be refined or amended.

This question originally appeared on Quora. More questions on Quora:

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