Black holes

A black hole, is a region of spacetime, exhibiting extreme gravitational attraction. It is a legitimate prediction of general relativity that can be produced by a sufficiently compact mass.

This image shows that the distortion of spacetime near the Sun becomes more intense as the mass of the star is concentrated into a smaller volume. Near a white dwarf and a neutron star, the curvature becomes that much more intense. The black hole would arise from a singularity, a point of infinite curvature. This singularity would be inevitable at a certain point (when the star grows small enough to be contained in it's own Schwarzschild radius).

According to the no-hair theorem, a black hole only has 3 real physical properties:

Mass
Charge
Angular momentum

This is what we can see from the outside of the black hole itself.

The birth of the idea, for black holes, is fascinating, and I will go into the critical figures below:

John Michell

Pierre-Simon Laplace

John Michell and Pierre-Simon Laplace were the first figures to consider fields of gravitation that were so strong, that not even light could escape. This was the 18th century. Michell, in an 1783 paper proposed what he called "dark stars". Michell was a believer in Newton's theory that viewed light as composed of "corpuscles". These are minuscule particles. Michell reasoned that as a star emits these particles, they could be slowed down by the gravitational pull of the star itself. Perhaps the mass of the star could be determined by this reduction in speed? Could the gravitational attraction of a star be so strong, that light itself could not even escape? Or in other words: could the escape velocity of a star, exceed the speed of light? According to Michell's calculations, these effects would arise for a star that has about 500 times the mass as our Sun. They were called dark stars, since light could not escape from them. Hence, they were invisible. Laplace came to a similar conclusion in 1796 for objects whose gravity is so strong that light cannot even escape.

In 1916, Karl Schwarzschild found the first real solution to Albert Einstein’s general theory of relativity, that would characterize a black hole. His solution characterized a point mass and a sphere mass and their gravitational field.

In 1958, David Finkelstein interpreted the work of Schwarzschild as a region of space, that nothing could escape from (not even light). Finkelstein identified the event horizon as a unidirectional membrane. It was an extension of Schwarzschild's solution for an observer falling into a black hole.

Karl Schwarzschild

David Finkelstein

There are 4 different kinds of black holes, determined by whether they have spin or charge (they all have mass):

Schwarzschild black holes - Do not have charge or spin.

Reissner-Nordstrom black holes - Have charge but do not have spin.

Kerr black holes - Do not have charge but do spin.

Kerr-Newman black holes - Have charge and spin.

Schwarzschild black holes, do not possess angular momentum.

Kerr black holes, do possess angular momentum.

4 different kinds of black hole:

1. Schwarzschild

This kind of black hole has no electric charge, nor, does it have angular momentum. The only way to distinguish Schwarzschild black holes from each other is by their mass. They are named for Karl Schwarzschild.

2. Reissner-Nordstrom

This is a black hole that has electric charge, however, no angular momentum. They are named for Hans Reissner and Gunnar Nordstrom.

3. Kerr

This is one of the two kinds of rotating black holes (any kind of black hole that possess angular momentum). The difference between the Kerr and the Kerr-Newman black holes is that the Kerr black holes, don’t carry electric charge, while the the Kerr-Newman black holes, do. They are named for Roy Kerr.

4. Kerr-Newman

The Kerr-Newman black hole is both rotating and electrically charged. They are named for Roy Kerr and Ted Newman.

Hans Reissner

Gunnar Nordstrom

Roy Kerr

Ted Newman

There are two defining features of the black hole that I want to discuss with some detail. These are the event horizon and the singularity:

Event horizon: The event horizon of the black hole is sometimes referred to as the “point of no return”. This is the spherical spacetime boundary that defines a black hole. Matter, light and physical information can only pass into the event horizon. It can not go the other way around. Hence, it is the point of no return. Indeed, not even electromagnetic radiation, such as light, can escape from the other side of the event horizon of a black hole. From the outside of the event horizon, we have no idea what is happening on the inside, since the light from events on the inside, cannot even escape the gravitational attraction and hence the event horizon. In a way, the shape of the black hole event horizon, depends on the kind of black hole. If the black hole is static, or, does not rotate, than, the black hole will take on a spherical geometry. However, if a black is rotating, the spherical geometry will become oblate, or, flattened at the poles.

Singularity: At the center of the black hole, lies the region of infinite spacetime curvature. This is the gravitational singularity. Again, as for the event horizon, the shape of the singularity, depends on the kind of black hole. For static black holes, the singularity, is a single point region. However, if the black hole, is indeed, rotating: It will take on a ring singularity geometry. Despite these geometric variations, the gravitational singularity at the center of a black hole has no volume. However, though hard to intuitively visualize, it has infinite density.

There is also a general consensus that supermassive black holes exist in the center of most galaxies. Supermassive black holes are formed when more than one black hole merge and absorbs other stars.

It is expected that black holes form when massive stars, at the end of their lives, undergo gravitational collapse. This would mark the end of the stellar evolution life cycle for these massive stars.

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