A Closer Look at Black Holes

When a heavy neutron star collapses, the gravitational force of attraction on its surface raises up to a point where the escape velocity is as high as light speed (approx. 300,000 km/sec) itself. At that point nothing, not even light, can escape from the collapsed star. The dead sun becomes a cosmic vacuum cleaner and attempts to swallow everything in its range.

John Michell (1724-1793) and Pierre Simon Laplace (1749-1827) have already thought about such phenomena. Can gravitation on a star grow enough as to not let any light escape anymore? They only could rely on Isaac Newton's (1643-1727) gravitational law which could barely describe such phenomena.

A brief time before his death Karl Schwarzschild (1873-1916) was the first to have calculated - with the help of Albert Einstein's (1879-1955) General Theory of Relativity of 1915 - the radius of a bowl at which the escape velocity equals light speed. The border where both velocities are equal is called "event horizon". Since, as Einstein has claimed, nothing can travel faster than light, nothing can escape from there.

If a mass is compressed to a bowl with Schwarzschild radius, it collapses and becomes a singularity, a place with an infinitely small size where our laws of physics are not valid anymore. There the whole mass is gathered within the form of a point. In the singularity the dimensions of space and time, the four-dimensional space-time (a term Hermann Minkowsky, a teacher of Einstein, introduced in 1908), are curved infinitely strong.

The phenomenon was known as "Schwarzschild singularity" until 1967 when it was given a fresh impetus by researchers such as John Wheeler, Roger Penrose, and Stephen Hawking since the mid 60s. Wheeler coined the term "black hole" in 1967 which has been used ever since. By exclaiming "a black hole has no hair" he and other scientists asserted that, if describing a black hole, only information about mass, electric charge and angular momentum could be gathered. When speaking of "hair" Wheeler was thinking of every property getting lost by the emission of radiation, e.g. magnet fields and curves of the horizon.

When collapsing, the developing black hole keeps the star's magnet field, but then emits it in the form of electro-magnetic waves.

Schwarzschild's calculations however did consider neither a black hole's angular momentum nor its charge. But as black holes develop from dying stars and as they turn, a different way of finding a solution had to be chosen. Roy Kerr was the first to succeed. The New-Zealandian mathematician was checking Einstein's field equations for solutions which were to describe the curvature of space-time in the proximity of a rotating star. Brandon Carter, a physicist from down under, has developed Kerr's solutions further and has found out that these were actually describing turning black holes. In contrast to Schwarzschild's holes, rotating black holes set the space-time in their vicinity moving whirlpool-like, comparable to water that goes down a bathtub drain.

Ted Newman has finally delivered the formulas for charged rotating black holes. Although the charge of most black holes is neglectably small, those to be found in nature obey the laws of the so-called Kerr-Newman geometry. The Schwarzschild geometry can describe really existing collapsars only insufficiently. It applies for singularities whose event horizons are ideally round, hence do not possess any angular momentum and are not charged electrically - and can hardly be found in nature.

Rotating black holes have a different anatomy: the rotation creates a region of space wherein the force of attraction is not strong enough to drag light into the event horizon but as great as not to let it escape from there: the ergosphere. In this area any particle, e.g. a photon, would be dragged around in the same direction the hole turns. Its outer border is called stationary limit. Only beyond this limit a still, stationary state is possible. In addition to that, the event horizon bulges. The centrifugal forces caused by the hole's rotation expand the hole's diameter at the equator and flatten it at the pole regions.

Rotating Black Hole

The event horizon forms the border between two regions of space whose dark side mankind will never be able to explore completely. If an astronaut considered to cross the boundary to the dark world, he would agree to a voyage without return. An exterior observer who watches an astronaut flying towards a non-rotating black hole's event horizon will never live the moment when that one passes the horizon. The Doppler effect, a phenomenon causes the length of a wave become longer or shorter when it moves away from the observer or towards him, can be also be applied to waves in gravitational fields. Emitted light or radio waves are more expanded the nearer they are situated to the event horizon - gravitation can jolt or expand waves.