Date: May 17, 2010

Title: What Are Black Holes?


Podcaster: Stuart Clark


Description: The most destructive forces in the Universe, black holes continue to defy our understanding.

Bio: Dr Stuart Clark is an award-winning astronomy author and journalist. His books include The Sun Kings, and the highly illustrated Deep Space, and Galaxy. His next book is Big Questions: Universe, from which this podcast is adapted. Stuart is a Fellow of the Royal Astronomical Society, a Visiting Fellow of the University of Hertfordshire, UK, and senior editor for space science at the European Space Agency. He is also a frequent contributor to newspapers, magazines, radio and television programmes. His website is and his Twitter account is @DrStuClark.

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Hello I’m Dr Stuart Clark, astronomy author and journalist.

Today I’d like to explore the question: What are black holes?

‘Black holes’ conjure up curiosity and confusion in equal amounts. The concept of a black hole sprang out of Einstein’s general relativity but has only recently attracted huge popular attention. Often black holes are portrayed as all-powerful destroyers that capture and crush everything around them. Thankfully for the Universe at large, that’s not quite true.

A black hole containing four or five times the mass of the Sun would occupy a spherical volume just a few kilometres across. It would curve the fabric of space so sharply that the strength of gravity would change even across the length of a human body stretching it like a medieval rack. It would eventually pull that body apart in a process known, with black humour, as spaghettification.

The third Astronomer Royal, James Bradley, was the first to calculate the currently accepted value for the speed of light. Working at Greenwich, London, in 1728 he detected a small movement in the position of the stars. He proposed that it was caused by the finite speed of light. The angle of his telescope, made to compensate for the Earth’s motion through space, allowed Bradley to calculate the speed of light in relationship to the speed of the Earth. He computed a figure of 186,000 miles per second (or around 300,000 kilometres per second).

A geologist, John Michell took Bradley’s figure and used Newtonian gravity to estimate the size of body needed to have an escape velocity equal to the speed of light. He came to an estimate of 500 times the mass of the Sun, so wrote to the Royal Society suggesting that not even light could escape from such a star. The idea sparked a debate that rumbled for years as astronomers mulled the possibility of such ‘dark stars’.

Afterwards, the matter rested for a couple of centuries until Einstein published his General Theory of Relativity in 1915. The German mathematician Karl Schwarzschild found that Einstein’s equations allowed celestial objects to become so dense that they create gravitational traps, the size of each depending on the mass inside. A black hole containing the mass of the Earth would have a Schwarzschild radius the size of a small coin, whereas a black hole containing billions of times more mass than the Sun would be as large as our Solar System. Once any object, even light, had passed beyond this Schwarzschild radius – eventually called the event horizon – it could never escape and astronomers wondered how they could possibly observe something that emitted no light.

In the early 1970s, the first X-ray telescopes were lofted into space and revealed an extraordinarily bright X-ray source in the constellation of Cygnus, some 8000 light years away. After much analysis, it was decided that the source of X-rays was a superheated cloud of gas spiralling into a black hole, dubbed Cygnus X-1.

Known as stellar black holes, such objects contain several times the mass of the Sun and they are formed when a very massive star explodes as a supernova, at the end of its life. The next size up is termed an intermediate mass black hole. These, contain a few hundred or a few thousand solar masses, and possibly result from several stellar black holes merging together.

The largest black holes are the supermassive black holes. A supermassive black hole, containing anything from millions to billions of times the mass of the Sun, is thought to sit at the centre of every galaxy, but taking up no more volume than an average solar system. In 90 percent of galaxies it is inactive but, in the other ten percent, it is constantly feeding from surrounding celestial objects driving vast quantities of radiation to be released before the doomed matter disappears forever. The most powerful active galaxies generate more energy per second than a trillion Suns, with the result that the active nucleus outshines the rest of the galaxy by a hundred times or more.

Thanks to technological advances astronomers anticipate being able to look for a black hole’s ‘silhouette’ against the background of bright stars within a decade. Sagittarius A*, our Galaxy’s central supermassive black hole, is estimated to contain 4.5 million solar masses, all squeezed into an event horizon only 27 million kilometres in diameter (that’s about half the distance of Mercury from the Sun). From our vantage point on Earth, the silhouette of Sagittarius A* would appear no larger than a football on the surface of the Moon.

There may be a fourth, as yet undetected, type of black hole: a primordial black hole. Theses are thought to have been created during the big bang when the space−time continuum was so crushed that minuscule regions could seal themselves off from the rest of the cosmos.

There may not seem to be anything in common between a black hole, which sucks things out of existence, and the big bang, which created the Universe and set it expanding, but, to a mathematician they share an identical feature: a point of infinite density and zero volume, known as a singularity.

Inside a black hole, the singularity is presumed to be the last resting place of matter, crushed into smaller and smaller volumes by the titanic force of gravity. But as the volume approaches zero, no theory can be used to study the resulting singularity and there is also the black hole ‘information loss’ problem. To the outside Universe, only three properties of the black hole are visible: its mass, its electric charge and its angular momentum. Once something crosses a black hole’s event horizon we cannot then discover what it was, let alone recover it. This goes against one of the deepest principles of physics: that of reversibility.

String theory may have the answer. It suggests that the volume from the centre of the black hole to the event horizon is not empty but a highly compressed ball of sub-atomic ‘strings’, the fundamental building blocks of nature that give us particles of matter. The strings would store the information about the objects that have fallen into the black hole, so no information will actually be lost.

If this description is right, matter does not pass through the event horizon on its way to the singularity; instead it compresses itself onto the surface of the ‘fuzz ball’ and merges with the other strings and the black hole’s Schwarzschild radius grows a little larger to make room for the latest arrivals. In relativity this is explained as the curvature of space becoming a little steeper, because the black hole has swallowed more mass. In string theory, the black hole simply expands a little to accommodate the new information.

Black holes are the most extreme celestial objects that we know and, despite thirty years of strong observational evidence, they continue to challenge our understanding of the most fundamental laws of physics. Even if a black hole is not a hole but a fuzzy ball of quantum string, everyone agrees about one thing: you definitely would not want to fall into one.

End of podcast:

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