In Praise of Black Holes

First time I came to work on black holes was a funny story. I went to one of the senior profs at the institute asking for a topic for my master's thesis. The focus area of the institute was heavy ion and nuclear physics, so the prof suggested two topics from this field. Since I wasn't too excited about that, he added a third that was to show that black holes can't exist by working out an argument that he briefly sketched. Faced with that selection, I went away with the task of showing black holes can't exist.

I was an undergraduate at that time, I had a bachelor's degree in math, not physics, and knew next to nothing about black holes and quantum effects in their vicinity. Still, it didn't take me more than 3 weeks to figure out that his idea wouldn't work for what I thought were quite obvious reasons. I went to some other prof asking for advice, upon which I learned that the topic had previously been given to two other students who, unfortunate for the prof, came to the same conclusion as I. I was then luckily handed over to a very bright postdoc who worked in astrophysics with whom I deviated the topic to one in which we could arrive at results and not start with the result.

Knowing what we know today, the topic suggested might seem odd to you. However, one has to see this in historical context. This anecdote happened in the mid 90s and by then it had been discussed for decades whether black holes are merely mathematically possible solutions of Einstein's theory of General Relativity, or if they are physical reality. It was only in the mid to late 90s that observational evidence became accurate enough to convince most scientists of the existence of black holes. At this time, evidence meant that a black hole is the most straightforward and most plausible, minimalistic, and generally accepted explanation of the data. As I recall one seminar speaker saying: If it looks like a duck and quacks like a duck, then it probably is a black hole. Though I might have garbled that up. The senior prof's argument against their existence was that god would not disconnect himself from part of his universe. Can't make that stuff up.

In any case, for some while there have been more or less artificial alternative explanations around for super-compact objects that resemble black holes but have no horizon. These alternative solutions typically are very implausible on the theoretical side. In addition, since a few years we also have experimental evidence that allows us to distinguish a solid surface from a horizon, see earlier post on Evidence for the Black Hole Horizon. So that duck clearly is a black hole. If you need any more evidence for the existence of black holes: they've made it into pop music. Muse, Supermassive Black Hole. Love that song, sorry, urgent need to embed:



Yeah.

Oohm, back to the physics, let me be very clear here: a black hole is characterized by its horizon, not by its quack. The full solution of Einstein's field equations does not only have a horizon, it also has a singularity at the center of the black hole. First thing to note is that we can't see the singularity because it's surrounded by the horizon and thus hidden from all observational efforts. (That's known as Cosmic Censorship.) Maybe more important, close by the singularity we come into a regime of very strong curvature and it is doubtful that General Relativity is still the correct theory to apply. Close by the singularity, quantum effects of gravity should become important. We do not yet know what the right theory is, but the correct solution in this regime looks probably nothing like the classical solution.

If a singularity appears in your theory, it usually means you've used your theory out of its range of applicability. A nice example for that are singularities that appear in hydrodynamics. Stefan wrote about this in his post Singularities in your Kitchen. Needless to say, these singularities are an artifact of still using a theory on scales where instead a more precise theory of the microscopic structure of the fluid would have become relevant. Most people working on quantum gravity believe that the singularity in the black hole solution is a similar artifact of applying a theory out of its range of applicability.

What is so fascinating about black holes is how many different areas of physics they combine. There is, first of all, of course the astrophysical aspects, both in experiment and theory. These differ greatly between supermassive black holes (like at the center of galaxies) and stellar black holes (formed from collapse of stars). An interesting topic is also the transition from the latter to the former. There's further so-called 'primordial black holes,' that might have been formed from density fluctuations in the early universe. These are usually thought to be of much smaller mass than stellar black holes. These primordial black holes come back every now and then as dark matter candidates or as explanation for gamma ray sources etc, but so far none of the models, both for their formation and signature, has been really convincing. And then there's the quantum black holes that might be possible to produce in colliders in scenarios where the Planck scale is lowered; for more details see my post on Micro Black Holes.

Since black holes inevitably imply the existence of regimes where quantum gravitational effects are relevant, it is not surprising that they have received a lot of attention in the quantum gravity community. The study of black holes has thus lead us to discover and explore the relation between General Relativity and thermodynamics, a topic that's been going strong since almost 30 years now and most recently resulted in Erik Verlinde's claim that General Relativity actually is thermodynamics. Black hole physics has given rise to 't Hooft's conjecture of the Holographic Principle, probably one of the most surprising and also intriguing thoughts that physics has heard of since Einstein, and a thought that has spurred a rather dramatic amount of research. A lot of this research has been dedicated to find a solution to the Black Hole Information Loss Problem. The need to connect black hole physics (both emission and accretion) to the standard model further brings in particle physics and there are also interesting analogies between black holes and some condensed matter systems. You can find a nice review of the latter in Ralf Schützhold's recent paper Emergent Horizons in the Laboratory.

The workshop on black holes that I was attending 2 weeks ago in Bonn covered a good part of these topics, though the astrophysics was clearly dominant and other aspects like holography were entirely missing. In any case, it reminded me of my initial fascination with the topic. If you are a student, I can warmly recommend to work on a topic related to black hole physics because it gives you the opportunity to learn about a broad range of interesting concepts.