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16th February, 2008

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Cosmic quicksands and the puny weakling paradox

The force of gravity is so weak that even you can mock it. The downward pull on a brick exerted by the gravitational field of planet earth is no match for the forces generated in your arm by the energy contained in today's breakfast; you can pick up the brick. The energy contained in a few tons of fossil fuel would be sufficient to carry that brick past the extreme reaches of earth's gravity.

But don't be fooled. As with everything that comes from the infinite mind of God, gravity has some neat tricks up its sleeve. Compress planet earth down to the size of a peanut and something remarkable would happen. At that point an unstoppable process of collapse would occur that would not cease until the entire mass has become a singularity - a pinprick of near infinite density, "so dense that it requires new laws of physics to describe" (Pankaj Joshi February 2009, Naked Singularities, Scientific American ). The vanishingly puny force of gravity is now "concentrated" in a much smaller volume of space and so becomes irresistible over a very small distance - in this case, about 9 mm. Anything, including light, that unwisely strayed that close to the microscopic speck would be helplessly sucked into the cosmic quicksand. Escape would be impossible.

This scenario never plays out for planets. But astronomers universally believe that it does when stars between roughly two times and fifteen times the mass of our sun run out of fuel. (The fates of exhausted stars on either side of those parameters varies.) In such cases, the compression required to reach the unstoppable "collapse threshold" is brought about by the force of their own gravity, while the threshold itself is reached way before they shrink to peanut size. For a median-sized candidate star, it occurs when the radius of the collapsing body is roughly three miles (four km). The result (well, usually, anyway) is - you guessed it -a black hole, long known about only by mathematical theory but now also well-attested by observation.

A black hole has two parts. At its core is a singularity, the infinitesimal point into which all the matter of the star gets crushed. Surrounding the singularity is the region of space from which escape is impossible, the perimeter of which is called the event horizon. Once something enters the event horizon, it loses all hope of exiting. Whatever light the falling body gives off

is trapped, too, so an outside observer never sees it again (Joshi).

The event horizon is the same distance from the singularity as the collapse threshold - roughly three miles. Outside the event horizon, the gravitational field drops off dramatically in accordance with the universal law of gravitation. Inside the singularity itself, gravity attains almost infinite levels so that anything that hits it gets destroyed.

Supermassive black holes - up to eighteen billion times the mass of our sun! - are often found in the centre of galaxies. Astronomers believe they are the result of long-term coalescence of individual black holes or the incessant build-up of mass through slurping in nearby gas and stars. Alternatively, they arose "in the beginning, [when] God created the heavens and the earth" (Gen. 1:1). (This author finds it hard to believe that any proto-galaxy that was dynamically stable before such a supermassive black hole formed could also be stable under dramatically changed conditions. But I'm willing to be convinced otherwise. The ageing process built into the universe itself testifies to the infinite wisdom and everlasting power of God.)

Because event horizons make black holes invisible, they have been dubbed "cosmic censors". But - oh, the mind of God - physicists are beginning to suspect that a phenomenon Stephen Hawking wagered in 1993 doesn't exist does in fact exist. Hawking has conceded defeat and the race is on to find one. Known as a "naked singularity" because the singularity lacks an event horizon cloak, this "troublesome sibling" of black holes lets light out and so is visible! How could such a beast possibly exist? Possibly the conditions of formation of the naked singularity imparted a hyper-rapid spin to the singularity that renders it capable of throwing off material. Alternatively, Joshi suggests that, "When the star is inhomogeneous, its gravity might never intensify enough to bend light rays back on themselves". Got that? Fact is, of course, the physics and math involved would be accessible to only a tiny proportion of people. To God, it's "elementary, dear Watson".

What sort of mind is capable of contriving such bizarre phenomena as black holes and naked singularities? An infinitely powerful and imaginative mind, a mind worthy of worship.


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