Just what is OUTER DARKNESS?
A study by: Timothy M. Youngblood
Copyright ©
The Master's Table
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Research Based on information in:
Science and Engineering Research Council
Royal Greenwich Observatory
Information Leaflet No. 9: `Blackholes'
webman@mail.ast.cam.ac.uk

 

I don't know if Outer Darkness referred to in scripture is in fact a black hole, however it is very interesting that what is described as Outer Darkness where even spirits such as demons or even Satan the Devil can't escape, is also described as light not being able to escape a black hole. Just food for thought!

What Is a Black Hole?

If a ball is thrown upwards from the surface of the Earth it reaches a certain height and then falls back. The harder it is thrown, the higher it goes. Laplace calculated the height it would reach for a given initial speed. He found that the height increased faster than the speed, so that the height became very large for a not very great speed. At a speed of 40000 km/h (25000 mph, only 20 times faster than Concorde) the height becomes very great indeed - it tends to infinity, as the mathematician would say. This speed is called the `escape velocity' from the surface of the Earth, and is the speed which must be achieved if a space craft is to reach the Moon or any of the planets. Being a mathematician, Laplace solved the problem for all round bodies, not just the Earth.

He found a very simple formula for the escape velocity. This formula says that small but massive objects have large escape velocities. For example if the Earth could be squeezed and made four times smaller, the escape velocity would need to be twice as large. This surprisingly simple derivation gives exactly the same answer as is obtained from the full theory of relativity.

Light travels at just over 1000 million km/h (670 million mph), and in 1905 Albert Einstein proved in the Special Theory of Relativity that nothing can travel faster than light. The above Laplace formula can be turned around to tell us what radius an object must have if the escape velocity from its surface is to be the speed of light. This particular radius is called the `Schwarzschild radius' in honour of the German astronomer who first derived it from Einstein's theory of gravity (General Theory of Relativity). The formula tells us that the Schwarzschild radius for the Earth is less than a centimetre, compared with its actual radius of 6357 km. Values for some other astronomical objects are given in a table in a following section.

Behavior of Light in Gravitational Fields

It might seem surprising that light can be thought of as behaving like rocket ships and cricket balls! It was Einstein who showed that light can be considered as a collection of particles, called photons, which have mass, or more correctly energy, by virtue of the famous formula E = Mc^2 relating energy E and mass M. Photons always travel at the same speed, i.e. the speed of light, but when moving away from a gravitating object they lose energy and, to an external observer, appear to be redder. It is this `red-shift' which means that photons from a black hole ultimately lose all their energy and become completely invisible. Indeed, light in the vicinity of such strong gravitational fields exhibits quite bizarre behavior. Here are links to some movies illustrating virtual trips to black holes and neutron stars.

Escape Velocities for Light

When light energy does not travel fast enough to escape (and nothing can travel faster), then no signals of any kind can escape and the object would be `black'. The only indication of the presence of such an object is the pull of its gravity. Away from the surface this is just the same as if an ordinary object of the same mass were there. The presence of gravity means that objects can fall into it, and hence `hole'.

So, a black hole is an object which is so compact that the escape velocity from its surface is greater than the speed of light. The following table lists escape velocities and Schwarzchild radii for some objects:

The speed of light is 299,800 km/sec (186,000 miles/sec). 11 km/sec is equivalent to 40,000 km/h (or 25,000 mph), 147,000,000 km is almost equal to the radius of the Earth's orbit round the Sun.

Where Might We Find Black Holes?

It is impossible to observe a black hole directly and so any black hole candidates have to be identified by their effect on the matter surrounding them. If no other explanation for the observed phenomena is valid then it is likely that a black hole is present. There are some objects that are good candidates for the presence of a black hole.

Any star shines and survives because the pull of gravity, which is trying to compress it, just balances the pressure generated by the nuclear furnace at its centre, which is trying to expand it. Once the furnace runs out of fuel, which must eventually happen, the pressure decreases, loses its battle with gravity, and the star collapses. Astronomers believe that one of only three things can happen to a star in this situation, depending on its mass. A star less massive than the Sun collapses until it forms a `white dwarf', with a radius of only a few thousand kilometers. If the star has between one and four times the mass of the Sun, it can produce a `neutron star', with a radius of just a few kilometers, and such a star might be recognised as a `pulsar'. The relatively few stars with greater than four times the mass of the Sun cannot avoid collapsing within their Schwarzschild radii and becoming black holes. So, black holes may be the corpses of massive stars.

Most astronomers believe that galaxies like the Milky Way were formed from a large cloud of gas which collapsed and broke up into individual stars. We now see the stars packed together most tightly in the centre, or nucleus. It is possible that at the very centre there was too much matter to form an ordinary star, or that the stars which did form were so close to each other that they coalesced to form a black hole. It is therefore argued that really massive black holes, equivalent to a hundred million stars like the Sun, could exist at the centre of some galaxies. (Evidence for galactic size black holes may be found in the section on active galaxies.)

How Might We See Black Holes?

Because black holes are small, and no signals escape from them, it might seem an impossible task to find them. However, the force of gravity remains, so if we detect gravity where there is no visible source of light then a black hole may be responsible. This type of argument, by itself, is not very convincing, and so we must look for other clues. If there is other material around a black hole which might fall into it, then it will. There is then a good chance that as it falls it will produce some detectable signal not from the black hole itself, but from just outside it.

Most stars are not single, like the Sun, but are found in pairs, small groups or large clusters. If a pair of stars have different masses then the more massive one will burn up its nuclear fuel and may become a black hole, whilst the other remains a normal star consuming its fuel more slowly. Gas can then be sucked from the star into the black hole. The gas becomes very hot, with a temperature of millions of degrees, and will shine not with visible light but with X-rays. These X-rays will have an observable effect on the light output from the ordinary star. Since the star and black hole go round each other every few days, we might expect to see regular variations in the brightness and X-ray output.

There are some X-ray sources which have all the properties described above. Unfortunately it is impossible to distinguish between a black hole and a neutron star unless we can prove that the mass of the unseen component is too great for a neutron star. Strong evidence was found by Royal Greenwich Observatory astronomers that one of these sources called Cyg X-1 (which means the first X-ray source discovered in the constellation of Cygnus) does indeed contain a black hole. Things are rather different if there is a massive black hole in the centre of a galaxy. It is possible there for a star to be swallowed by the black hole. The pull of gravity on such a star will be so strong as to break it up into its component atoms, and throw them out at high speed in all directions. Some of the fragments will fall into the hole, increasing its mass, whilst others could produce an outburst of radio waves, light and X-rays. Evidence for such behavior may be found in the section on Active Galaxies.

This is just the behaviour which is observed in galaxies of the type called `Quasars' and may well be happening in a milder way in the centre of our own Milky Way.

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