Published online 8 November 1999 | Nature | doi:10.1038/news991111-3


When holes collide

Some events in the cosmos are so awesomely violent that they make the very fabric of space and time shudder. Astronomers are currently building detectors that can detect these ripples over phenomenal distances. In Physical Review Letters1, an international team reports on how far this new form of observational astronomy should be able to see.

In 1915, Albert Einstein showed that, in effect, every blob of matter creates a dimple in space-time that diverts the path of a moving particle. The more massive the blob, the deeper the dimple. When one body is accelerated by the gravitational field of another a small fraction of its mass gets converted to energy according to his equation E=mc2, and this is radiated away as 'gravity waves' which spread outwards through space-time like ripples in a pond. If the bodies are large enough, these ripples might be detectable over vast stretches of space.

One of the best candidates for creating strong gravity waves is the collision of two black holes. Black holes can be likened to objects so massive and dense that the dimples they impress in space-time are bottomless wells from which nothing - not even light - can escape. They are formed by the runaway collapse of massive bodies under their own gravity into super-dense objects. It seems likely that there are huge - so-called 'supermassive' - black holes in the centre of many galaxies, including our own, where many stars have coalesced.

Because of their intense gravitational fields, two nearby black holes would attract each other like magnets. They would collide with great fury, and some small fraction of their masses would be converted to gravity waves. The detectability of these cosmic collisions from afar will depend on just how much energy is released. But this detection is bizarre in that it involves sensing distortions wherein space gets momentarily shorter in the direction of the wave.

A detector called the Laser Interferometer Gravitational-Wave Observatory (LIGO) is being constructed in the USA. The LIGO detector consists of two 'arms', each two and a half miles long, arranged in the shape of an L. When the waves pass over the detector, they will decrease the end-to-end distance in one arm of the L while increasing it in the other. Although these changes are extremely small - just a thousandth of a trillionth of a millimetre, or one-hundred-millionth of the diameter of a hydrogen atom - they can be sensed.

Now Gaurav Khanna from Pennsylvania State University, State College, Pennsylvania, and colleagues, have determined how energetic a typical black-hole collision would be, and so how far away such events should be 'visible' to these detectors.

Black holes range in size from those formed from individual stars to the supermassive black holes in the centres of galaxies. But the best source of gravity waves should be from collisions of intermediate-sized black holes, with several hundred times the mass of our Sun. Some galaxies are believed to contain these 'middleweight' objects.

Once captured within each other's gravitational field, two black holes collide after spiralling in towards one another. Some gravitational energy is radiated during this 'in-spiralling' process, but most is released in the final stages of coalescence. The researchers use the theory of relativity to calculate that one per cent of the total mass of the pair is converted into gravity waves in this final violent event.

That may not sound like much - but just a tiny mass is equivalent to a vast amount of energy. The researchers say that a collision event of this sort should therefore be detectable by the initial LIGO configuration from a distance of about seven hundred million light years - encompassing many thousands of other galaxies (the average distance between galaxies is about ten million light years). The planned refinements to LIGO should extend the range by a factor of about 20, making it far-sighted indeed. So the prospects look good that we should, within a few years, be able to see collisions that leave the Universe ringing. 

  • References

    1. Khanna,G., Baker, J. et al. Inspiraling Black Holes: The Close Limit. Physical Review Letters 83, 3581 - 3584 1999. | Article | ISI | ChemPort |