Yesterday, I had an exchange with my colleague about a black hole. His theory was that black holes were merely an imaginary construction that is purported to have some mathematical meaning. Is what he said true? If not, how do we know about black holes' existence and what properties do we know about them?
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We mostly know about black holes from theorizing But here’s an image of one proving its existence. https://physicsworld.com/a/first-images-of-a-black-hole-unveiled-by-astronomers-in-landmark-discovery/
Bill Alsept
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We know that there are things that behave like a black hole should behave. As an example of this, in the center of most or all galaxies there is a massive object that seems to have the mass of thousands or millions of times the mass of our sun. We don't know of any other objects that could have that much mass. Likewise, these objects behave like we think that black holes should behave with things such as massive accreation disks, etc. We don't believe that much mass can accumulate that way except for it being a black hole.
Itsme2003
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Your colleague was right a few years ago but since the detection of gravitational waves he isn’t any more. Here is why.
When Ptolemy proposed his geocentric model he still thought that the planetary orbits where perfect circles. Circles where the mathematical model of that time for the orbits.
When the measurements of the orbits became more precise (by Tycho Brahe I think) it was noticed that the circle model is not valid. Kepler introduced ellipses as the new model and Newton delivered the explanation, deriving the ellipses model from the inverse square law of gravity.
Newton’s model kept being valid until it was noticed that Mercury’s orbit does not quite match it. Einstein’s new model of general relativity explained Mercury’s orbit very well. In addition, it offered the possibility of the existence of black holes.
No physicist thinks that Einstein’s model is the final answer because it has some major flaws, e.g. mathematical singularities in black holes, which are meaningless for physical reality. Quantum Gravity is currently considered the way out but no complete model exists yet.
However, concerning the existence of black holes one crucial question was open: does the model of general relativity break down before black holes can form or not? This is one of the big deals about the discovery of black hole mergers with LIGO: it proved that indeed the mathematical model of general relativity matches real observation in the sense that black holes do exist.
So the answer is: our current mathematical model is consistent with the observations; black holes do exist.
Hartmut Braun
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