# 10 things you should know about black holes

And what makes them different — or not so different— from everything else in the Universe.

1. What is a black hole?

A black hole’s defining property is its horizon, which is the boundary of a region from which nothing, not even light, can ever escape. If the disconnected region remains disconnected forever, we speak of an “event horizon”. If it’s only disconnected temporarily, we speak of an “apparent horizon”. But temporarily could still mean the region remains disconnected for much longer than the present age of the universe! If the black hole horizon is temporary but very long lived, the difference between the both cases cannot be observed.

2. How large are black holes?

You can think of the black hole horizon as a sphere, and its diameter is directly proportional to the mass of the black hole. So the more mass falls into the black hole, the larger the black hole becomes. Compared to stellar objects though, black holes are tiny because the mass has been compressed into a very small volume by enormous gravitational pressure. The radius of a black hole with the approximate mass of planet Earth, for example, is only a few millimeters. Compare that to the actual radius of Earth, which is about 10,000,000,000 times larger!

The radius of a black hole is called the Schwarzschild radius, after Karl Schwarzschild who first derived black holes as a solution to Einstein’s General Relativity.

3. What happens at the horizon?

Somebody crossing the horizon doesn’t notice anything different in their immediate surroundings. This is a direct consequence of Einstein’s equivalence principle, which implies that one cannot tell the difference between acceleration in flat space and a gravitational field that causes the curvature of space. However, an observer far away from the black hole who watches somebody fall in would notice that the person seems to move slower and slower the closer they get towards the horizon. It appears like this because time close by the black hole horizon runs much slower than far away from the horizon. But it only takes a finite amount of time for the infalling observer to cross over that event horizon, and find themselves inside that Schwarzschild radius.

What you would experience at the horizon depends on the tidal forces of the gravitational field. The tidal forces at the horizon are inversely proportional to the square of the mass of the black hole. This means the larger and more massive the black hole, the smaller the forces are. If the black hole is only massive enough, you might cross the horizon before you even noticed anything was happening. The effect of these tidal forces is that you would get stretched: the technical term physicists use is “spaghettification”.

In the early days of General Relativity it was believed that there was a singularity at the horizon, but this turned out to be wrong.

4. What is inside a black hole?

Nobody really knows, but it is almost certainly not a bookshelf! General relativity predicts that inside the black hole is a singularity, a place at where tidal forces become infinitely large, and that once you cross the horizon, you cannot avoid crashing into the singularity. Alas, General Relativity is not good to use in this region because we know that the theory breaks down. To be able to tell what is inside a black hole we would need a theory of quantum gravity. It is generally believed that this theory would replace the singularity with something else.

5. How do black holes form?

We presently know of four different ways black holes may form. The best understood one is stellar collapse. A sufficiently large star will form a black hole after its nuclear fusion runs dry because everything that can be fused has been fused. When the pressure generated by the fusion stops, the matter starts falling towards its own gravitational center, becoming increasingly dense. Eventually it is so dense that nothing could overcome the gravitational pull on the stars’ surface: a black hole has been created. These black holes are called ‘solar mass black holes’ and are the most common ones.

The next common type of black holes are ‘supermassive black holes’ that can be found in the centers of many galaxies and have masses about a billion times that of solar mass black holes. Exactly how they form still isn’t entirely clear. It is believed that they once started out as solar mass black holes that in the densely populated galactic centers ate up a lot of other stars and grew. However, they seem to be eating stuff faster than this simple idea suggests, and exactly how they manage this is still subject of research.

A more controversial idea are primordial black holes, that might have been formed at pretty much any mass by large density fluctuations in the early universe. While this is possible, it is difficult to find a model that produces them without producing too many of them.

Finally, there is the very speculative idea that tiny black holes with masses similar to that of the Higgs boson could form at the LHC. This only works if our universe has additional dimensions. So far, there has not been any observation that this might be the case.

6. How do we know black holes exist?

We have a lot of observational evidence for very compact objects with large masses that do not emit light. These objects reveal themselves by their gravitational pull, for example by affecting the motion of other stars or gas clouds around them. They also cause gravitational lensing. We furthermore know that these objects do not have a hard surface. One can tell this from observations because matter falling onto an object with a surface would cause more emission of particles than matter falling through a horizon. An upcoming experiment, the “Event Horizon Telescope” will be looking for another hallmark of black holes, their photosphere. This is basically an extreme gravitational lensing event.

7. Why did Hawking say last year that black holes don’t exist?

He meant he thinks black holes do not have an eternal event horizon but only a temporary apparent horizon (see 1). In a very strict, and not common, use of terminology, only an event horizon counts as black hole.

8. How can black holes emit radiation?

Black holes emit radiation by quantum effects. It is important to note that these are quantum effects of matter and not quantum effects of gravity. What happens is that the dynamical space-time of the collapsing black hole changes the notion of what a particle is. Like the passage of time that gets distorted nearby the black hole, the notion of particles too depends on the observer. In particular, while an observer falling into the black hole thinks he is falling in vacuum, the observer far away from the black hole thinks that it’s not vacuum but full of particles. It is the stretching of the space-time itself that causes this effect.

First discovered by Stephen Hawking, the radiation that black holes emit is called “Hawking radiation”. This radiation has temperature which is inversely proportional to the black hole’s mass: the smaller the black hole the hotter. For the stellar and supermassive black holes that we know of, the temperature is well below that of the CMB and cannot be observed.

9. What is the information loss paradox?

The information loss paradox is caused by the emission of Hawking radiation. This radiation is purely thermal which means it is random except for having a specific temperature. The radiation in particular does not contain any information about what formed the black hole. But while the black hole emits radiation, it loses mass and shrinks. Eventually, the black hole will be entirely converted into random radiation and the remaining radiation depends only on the mass of the black hole. It does not at all depend on the details of the matter that formed it, or whatever fell in later. Therefore, if one only knows the final state of the evaporation, one cannot tell what formed the black hole. Such a process is called “irreversible” — and the trouble is that there are no such processes in quantum mechanics.

Black hole evaporation is therefore inconsistent with quantum theory as we know it and something has to give. Somehow this inconsistency has to be removed. Most physicists believe that the solution is that the Hawking radiation somehow must contain information after all.

10. What is Hawking’s recent proposal to solve the black hole information loss problem?

The idea is that black holes have a way to store information which has so far been neglected. This information is stored on the black hole horizon and can cause tiny shifts of the particles in the Hawking radiation. In these tiny shifts there could be the information about the infalling matter. Exactly how this is supposed to work is presently entirely unclear. Scientists are waiting for a more detailed technical paper of Stephen Hawking, in collaboration with Malcom Perry and Andrew Strominger. The paper is rumored to appear in late September.

At this point in time, we are certain that black holes exist, we know where they are, how they form, and how they’ll eventually, on timescales of 10^67 years and up, cease to exist. But the details of where the information that went into them goes are still up for grabs, and that’s one of the problems unique to black holes among all objects in the Universe.