Earlier this month we all celebrated the observation of gravitational waves by LIGO. Among the many rasons why this is a huge scientific and technological achievement is that this is practically a direct observation black holes.
The `black holes’ that we always hear mentioned in the news and popular media are almost always Kerr black holes. These are black holes that rotate. (That is, carrying mass and angular momentum.)
The reason why only these black holes are mentioned is that these are the ones that commonly exist in space, being created when a star collapses and dies. The mass and rotation of the black hole was simply inherited from the mass and rotation of the dead star.
Of course, General Relativity is more than just about what happens after a star dies. In the 100 years since General Relativity was formulated, theoretical physicists have found mathematical solutions describing many more types of black holes.
Here are 10 more black holes you’ve probably haven’t heard about.
1. Schwarzschild black hole
Well, if you’ve taken an undergraduate course in physics, you’ve probably did hear about this one. The Schwarzschild black hole is basically a black hole that doesn’t rotate. It was one of the earliest exact solutions to Einstein’s equations, found by Karl Schwarzschild just months after Einstein published his theory of General Relativity.
2. Reissner-Nordström black hole
The RN black hole is a non-rotating, electrically charged black hole. This is what you would get if you throw in electrical charges (like electrons or protons) into a Schwarzschild black hole. This solution was found by Hans Reissner and Gunnar Nordström when they attempted to combine Einstein’s theory of gravity with Maxwell’s theory of electromagnetism.
3. Kerr-Newman black hole
This an electrically charged black hole that is also rotating. So, this is what you’d get if you charge up a Kerr black hole, or spin up a Reissner-Nordström.black hole.
4. The C-metric
The C-metric describes an accelerating black hole. Its shape is deformed by the acceleration. As we know from Newton’s law, objects undergoing acceleration must be experiencing a force exerted upon it. The equations of General Relativity tells us that one way to accelerate a black hole is to attach a `cosmic string’ to the black hole and pull!
5. Ernst spacetime
This solution was found by Frederick J. Ernst in 1975 which depicts a black hole immersed in an external magnetic field. This one has some relevance to astrophysics since most black holes in the center of galaxies are found inside strongly charged gases and plasma which carry strong magnetic fields.
6. Hyperbolic black holes
In more general cases, Einstein’s equation might include an extra cosmological constant. When this constant is positive, the solutions are used to describe our expanding and accelerating universe, which was observed in 1998. On the other hand, negative cosmological constant is typically used in relation to other physical theories, such as the AdS/CFT correspondence. Hyperbolic black holes do not have spherical shapes like all the other black holes we’ve discussed previously. Instead, it is `saddle shaped’, and its horizon extends all the way to infinity.
So far, everything we’ve discussed so far are four-dimensional (3 space+1 time) black holes. Things get more interesting if we go beyond four dimensions. Theories like string theory and braneworlds have inspired lots of research in higher-dimensional gravity. And as a result, many more black holes are found in higher dimensional theories.
7. Black string
This is a basically a higher-dimensional (hyper-)tube. This can be explained by analogy to a three-dimensional tube/cylinder familiar to our everyday lives.
If we take a thin slice out of a tube, we’d get a circular disk. So we say a cross-section of a three-dimensional tube is a circle. Or, putting it the other way around, a tube is a stack of many circles.
Carrying this idea to one higher dimension, a cross-section of a black string is a Schwarzschild black hole. Or of you `stack’ infinitely many Schwarzschild black holes into the fifth dimension, you’d get a black string.
8. Black ring
Take two ends of a black string and join them together – you’d get a black ring. This solution was found by Roberto Emparan and Harvey Reall in 2002. The interesting thing about the black ring is that a static black ring is unstable and will just collapse into intself like a dying star. To avoid this it needs to be rotating so that it’s centrifugal force resists its collapse.
9. Black saturn
Put a spherical Schwarzschild black hole at the center of the black ring – so it basically looks like a planet with a ring around it. Hence the name.
10. Black di-rings
Instead of putting Schwarzshild black hole in the middle of a black ring, you put an even larger ring around another black ring. To quote Beyoncé, if you like it then you better –