22 January 2015
Yesterday’s post on black holes stirred up things a bit, particularly among those who truly think black holes don’t exist. Some of the counter-arguments centered on the strangeness of event horizons, but most of it centered on the fact that black holes have singularities, which clearly defy all things science. Ergo, black holes don’t exist. Here we go again.
To begin with, let’s talk a little bit about what a singularity is and what it has to do with black holes. In simple terms, as the weight of an object tries to cause it to collapse, the pressure of the material pushes back until an equilibrium is reached. In the case of a star, it is the pressure of heated plasma that balances the weight, while for a white dwarf or neutron star it is the quantum degeneracy pressure of electrons or neutrons respectively. But at ever higher densities, the weight becomes harder to overcome. What’s worse, since mass and energy are related in relativity, there comes a point where the pressure of a material actually increases the weight, so the very thing used to counter gravity becomes part of the problem. It’s kind of like putting a fire out with water. Pour water on a small fire and it quickly goes out. Pour water on the Sun, and the weight of the water actually increases the Sun’s mass, which causes the Sun to heat up.
So the basic idea is that for things more dense than a neutron star, you get to the point where things are too dense to reach equilibrium. Even trying to create enough pressure to counter gravity would only help gravity, and so collapse is inevitable. The material of a star would simple collapse until there is literally no smaller volume to occupy. It would become a point of infinite density and zero volume, which is known as a singularity. At this point some of you are probably thinking “see, this is the kind of model-dependent nonsense I’m talking about.” Clearly singularities are nonsense, so clearly black holes don’t exist.
Interestingly, this is exactly the type of argument many astrophysicists made when black holes were first proposed. Even Einstein doubted black holes were possible. So you’re in good company if you doubt black holes, but you’re also about a century behind the times. Despite the way black holes are often presented, it wasn’t the model that convinced astrophysicists of black holes, it was the evidence.
The first “black hole” solution to Einstein’s general relativity equations was found by Karl Schwarzschild in 1916. At the time Schwarzschild himself showed that strange nonsensical things happened in the solution, which we now call the event horizon and the singularity. Interestingly, it was the event horizon that was seen as more problematic, because the “singularity” was just a mathematical concept, just like treating objects as point masses in Newtonian gravity. But soon more sophisticated models showed that matter within the event horizon of a black hole would most certainly collapse into a singularity. So for decades it was thought that black holes simply wouldn’t form. Surely the dynamics of material would prevent anything that dense from actually happening.
But in the 1960s neutron stars were discovered. It became clear that you could have several solar masses compressed into the volume the size of a small city. Neutron stars are fairly close to the critical density of a black hole, so it wasn’t unreasonable to imagine a collision or accretion of mass from another star triggering the formation a black hole. By the 1970s and 1980s there was growing evidence of black holes, both from x-ray binaries and the like for stellar mass black holes, and quasars and galaxy jets for supermassive black holes. In our own galaxy we now have orbital evidence from stars that show a supermassive black hole in our own galaxy. It is now decidedly clear that black holes exist.
But what about that pesky singularity? That’s actually a matter of some debate. Some argue that Hawking radiation will prevent singularities from forming. Some argue that things like dark energy might prevent their formation. Others argue that “singularities” might actually be a mechanism for forming other universes. It’s all pretty speculative, and they should all be considered a bit speculative.
But none of that disputes the existence of black holes. They are just discussions about one strange aspect of black holes. Models are a good way to understand things, but it’s the evidence that wins the day.