- General Relativity is our theory of gravity, which relates the curvature of space to the gravitational acceleration of objects.
- This theory only works in certain regimes; it breaks down at the point of singularities.
- A black hole, as predicted by Schwarzschild, is a singularity.
- Therefore, since singularities are forbidden by General Relativity, there is no reason to think that black holes exist.
(You can watch his video here, or read his full argument here.) Therefore, he argues, astronomers are wasting their time looking for black holes, since their existence isn’t even a physical prediction.
Talk about not seeing a forest for the trees. The “singularity” is not essential for a black hole to exist. Honestly, it isn’t important at all whether there’s a singularity or not. All that matters, in the real world, is that something is both massive and compact enough so that, within a certain radius, light cannot escape from it. That is the astrophysical definition of a black hole.
So, do they exist? Definitely. Where do you look for incontrovertible proof? The center of the galaxy! There are no two ways around it; there is definitely a black hole there.
How am I so sure? The above image shows the center of our galaxy. There are many, many stars orbiting the central point where the arrows are pointing. We have tracked these orbits over more than a decade, thanks to the UCLA Galactic Center Group. Here’s a screenshot of their results.
From the motion of these orbits, we can figure out what the mass of the object they orbit around is. It turns out to be over 2 million times as massive as our Sun. And yet, we don’t see any light coming from that point. We don’t see a white dwarf, we don’t see a neutron star, we don’t see any object at all.
For a mass that large, you will have a black hole if that mass is confined to a sphere of a diameter of about ten million kilometers. That isn’t hard, considering we have many, many stars that we know of where an entire solar mass is confined to a diameter of about ten kilometers. (These are neutron stars.) If you up the mass, the neutrons at the core will eventually collapse under the tremendous pressures, and collapse farther. There’s a well-known upper limit to how massive a neutron star can be, and it’s less than three solar masses, much less two million.
So you can argue about whether singularities violate General Relativity or not until you’re blue in the face. It doesn’t have a damned thing to do with whether any light gets out of your ultra-dense, massive object. And that’s what we call a black hole, and it exists. Don’t believe it? Then tell me what’s going on at the galactic center.