NO ONE has ever seen a black hole. It is not even certain that they exist. But theorists have been enthusiastically predicting their existence ever since Einstein published his general theory of relativity in 1915, and after decades of searching astronomers are now finding an increasing number of positive signs. According to Einstein’s theory, a black hole forms when a massive object shrinks catastrophically under its own gravity. All that remains in space is a fantastically strong gravitational field. It is so strong that nothing, not even light, can escape, hence the “blackness”. Definitive proof of these bizarre objects, however, remains elusive.
Reasonable doubt
Not that you would always know this from the way some blackhole seekers, carried away by the excitement of finding slightly better evidence, talk about their findings. Last May, for example, NASA issued a press release announcing that the Hubble Space Telescope had confirmed the existence of a “supermassive” black hole in the centre of the nearby giant elliptical galaxy, M87. “Anything at all coming from Hubble gets lots of publicity,” says Astronomer Royal Martin Rees of the University of Cambridge. Many newspapers and magazines duly ran the story, backed up by extravagant claims from astronomers from NASA and elsewhere along the lines of “This is a conclusive evidence of a supermassive black hole” and “All reasonable astronomers will be convinced”.
Advertisement
Not surprisingly, this exasperated many astronomers, who insist that while there is considerable evidence that supports the existence of black holes, the case is by no means proved. And for the minority who do not accept the black hole theory at all, the statements by the M87 proponents were further evidence of a “black hole establishment” that sees them everywhere. Sir Fred Hoyle, one of the most dedicated of the “black hole sceptics”, says that the members of this establishment blithely talk about the objects as if they are “as certain as tomorrow’s sunrise”.
What Hubble actually found was that the gas in the heart of M87 was whirling unusually fast about the very centre of the galaxy. All that could be honestly deduced from this observation was that the gravity of a large concentration of mass was doing the whirling – and a giant black hole is one possible candidate for this concentration of mass. Moreover, it was not exactly news. Fifteen years earlier, Peter Young and his colleagues at the California Institute of Technology in Pasadena had come to much the same conclusion from observations showing that the stars were orbiting unusually fast about the centre of the same galaxy (Astrophysical Journal, vol 221, p 721, 1979).
The Caltech astronomers had estimated that the central object would have a mass of about 5 billion times the mass of the Sun, whereas the Hubble team, which saw significantly deeper into the heart of the galaxy, estimated a mass of 3 billion times that of the Sun. Hubble also saw a fuzzy structure 65 light years across which could conceivably be interpreted as an accretion disc of gas being sucked into the hypothetical black hole. But according to Rees, “the Hubble data were only slightly more convincing than that of the Caltech team in the early l980s”.
Body of evidence
Nevertheless, like the majority of astronomers, he believes there is still a considerable body of evidence – admittedly indirect – which supports the existence of black holes. But getting those all important hard data will not be easy. Whoever said that “black holes are out of sight” hit the nail squarely on the head. How do you see an object that is, by definition, invisible? And the fact that they pack a given mass into the smallest possible volume makes black holes extremely tiny. So actually “seeing” a dark object, blotting out the light from background stars, as the Moon blots out the Sun in a total eclipse, is out of the question. Instead, astronomers must infer the existence of black holes indirectly from the influence of their extraordinarily strong gravity on the visible bodies that surround them.
One place to look is star systems where the motion of a single visible star implies it is orbiting around a second, invisible companion which has a strong gravitational field – Cygnus X-1 in our Galaxy, for example, and LMC X-3 in the Large Magellanic Cloud. Because the mass of the invisible object in these binary star systems is estimated to be greater than the mass allowable for a neutron star (see Box “What is a black hole?”), the most likely candidate is considered to be a black hole.
There is often additional evidence that supports the black hole hypothesis. For instance, Cygnus X-1, like the handful of other binary star systems that are suspected of containing starsized black holes, emits copious X-rays. The source of the X-rays could be a superhot accretion disc made from material that has been torn off the companion star and is heated by internal friction as it spirals down into a hungry black hole.
Active galaxies
The fact that these X-rays fluctuate on timescales of very much less than a second indicates that there is an extremely compact object in the star system. Two points on opposite sides of an X-ray emitting region can brighten and fade in unison only if whatever is causing the fluctuation has had time to pass between them. Since nothing can travel faster than the speed of light, it follows that a region a light second across cannot brighten and fade in less than a second.
In “active galaxies”, the evidence for black holes seems to be even more compelling. An active galaxy is one where most of the energy it gives out comes not from starlight but from a mysterious “central engine” buried deep in its nucleus. They can be thousands of times as luminous as a galaxy such as the Milky Way, make up about 1 per cent of all galaxies in the Universe and include quasars, BL Lac objects, Seyfert galaxies and radio galaxies such as M87.
In M87, the fast orbital motion of stars and gas near the centre can be explained if they are being whirled around by the strong gravity of a supermassive black hole. In other active galaxies, twin jets of matter can sometimes be detected emitting radio waves. The jets stab out from the nucleus in opposite directions and stream across hundreds of thousands of light years (“Quasar jets and cosmic engines”, New Scientist, 16 March 1991). These jets create huge radio-emitting lobes where they slam into the intergalactic medium on either side of the galaxy. And in the case of objects such as quasar 3C273, astronomers have recorded material travelling along the first few light years of the jets at around ten times the speed of light. Such speeds are almost certainly an illusion: if the jet is pointing almost directly towards us, material travelling at close to the speed of light and emitting pulses of light as it moves will “catch up” its own pulses, making it seem as if it’s travelling faster than it really is. But they do show that the central engines of active galaxies are very powerful, since they are apparently capable of accelerating large amounts of matter – often many times the mass of the Sun each year – to within a whisker of the speed of light.
Most astronomers think the central engine is a rotating supermassive black hole, ranging in size from a few million times to a few billions times the mass of the Sun. The hole is surrounded by a searingly hot accretion disc of gas that is swirling inward and they believe that the hole somehow accelerates some of this infalling gas out again from its poles and along two jets. The tremendous amount of light produced by such active galaxies can be explained because matter swirling into a rotating black hole can heat up enough to release up to 43 per cent of its rest mass, compared with the mere 1 per cent released by the nuclear fusion reactions that power stars like the Sun.
Although this picture of active galaxies powered by rotating supermassive black holes is attractive, the evidence is still circumstantial. And sceptics tend to point out that such a concentration of mass can be explained without recourse to black holes. It is possible, they point out, that the mass concentration in M87 is a cluster of a billion or so dim stars such as neutron stars or white dwarfs.
In January this year, however, the existence of such a cluster was dealt a severe blow when a team led by Makoto Miyoshi of the National Astronomical Observatory in Japan observed a nearby galaxy called NGC 4258 with the extremely high-resolution possible with the Very Long Baseline Array, a network of radio telescopes stretching from Hawaii to Puerto Rico. The astronomers measured the speed of a ring of dust and gas which is rotating within a third of a light year of the centre of that galaxy. From its speed, they worked out that the core has a density equivalent to at least 100 million Suns per cubic light year, more than forty times the density estimated for any other active galaxy. According to Miyoshi, if NGC 4258 contained a star cluster, the density of stars would be so high that collisions between individual stars would have long ago torn the cluster apart. Moreover, a dense cluster of objects in the centre of active galaxies cannot explain other phenomena such as the powerful jets.
Fatal attraction
But some theorists argue for some other kind of dense object, not predicted by the known laws of physics, instead of black holes. Recently, Huseiyn Yilmaz of Hamamatsu Photonics in Japan and Carroll Alley of the University of Maryland have developed a modified version of general relativity which makes all the classic predictions of Einstein’s theory, but does not allow the formation of black holes, although it is not entirely clear what exactly would be formed instead. Yilmaz and Alley were motivated to modify general relativity because they discovered that it fails to predict an attractive gravitational force between two parallel infinite plates. Most theorists, however, feel that the case does not show general relativity to be wanting. Parallel infinite plates, they say, have nothing in common with any matter known to exist in the real world.
Two other sceptics are Neil Cornish and John Moffat at the University of Toronto. They have added a new sort of geometry to space-time – one with a natural twist or torsion. Their aim was to modify general relativity so that it would prevent the formation of the infinitely dense and infinitely dangerous “singularities” at the centre of black holes. Singularities would play havoc with the laws of physics if they ever broke free from the “event horizon” – the edge of the black hole.
Cornish and Moffat’s theory also relaxes the constraint that neutron stars must be less than three solar masses, raising the intriguing possibility that active galactic nuclei might contain supermassive neutron stars. But their justification for modifying Einstein’s theory, say most other theorists, remains weak – in the eighty years since Einstein proposed general relativity not one of its predictions has ever been found to conflict with observations.
A final alternative to supermassive black holes is the quasi-steady-state theory proposed by Hoyle, Geoffrey Burbidge of the University of California in San Diego and Jayant Narlikar of the Inter-University Center for Astronomy and Astrophysics at Poona in India. They argue that the Universe is infinite in space and time but that it undergoes “creation events” on all scales in which matter fountains out of the vacuum. Their theory views the big bang as a creation event that occurred 15 billion years ago in our corner of the Universe, and active galactic nuclei as somewhat smaller creation events. According to Hoyle and his colleagues, a “creation tap” is turned on when gravity is extremely intense. For these sceptics, the jets flowing out from active galactic nuclei are the most important evidence for their ideas.
Hoyle and his colleagues, and indeed Cornish, Moffat, Yilmaz and Alley are firmly in the minority with their assault on black holes. Even so, the enthusiasts will have to provide far stronger evidence than they have managed to date. So what would constitute the definitive evidence for the existence of a black holes?
Rees has his answer: Unambiguous evidence of the Kerr metric would prove the existence of black holes.” The Kerr metric – the tornado-like swirl that a spinning black hole produces in the space around it – affects the motion of particles falling into the hole in a very distinctive way. Astronomers might be able to observe this motion if a lump of matter close to a black hole could hold itself together for many orbits before plunging into the hole. The X-rays emitted by such matter would sweep around like a lighthouse beam, generating a series of pulses as the beam swept past us. The interval between pulses would be determined by the Kerr metric. “It would say, ‘I am a black hole'”, according to Kip Thorne of the California Institute of Technology in Pasadena.
There is, however, another way that the existence of black holes might be proved once and for all. When black holes form they should generate vast amounts of gravity waves, which spread out like wavelets on a pond. Such “ripples” in space-time should be detectable by gravity-wave detectors currently being built or planned (“Gravity’s secret signals”, New Scientist, 26 November 1994).
The gravity waves are produced by the violent vibrating of the event horizon whenever a black hole forms or two black holes coalesce to form a single black hole. “The event horizon rings like a bell,” says Ian Moss of the University of Newcastle. “And just like a bell, after a while only certain frequencies dominate. The frequencies are always the same and they decay in intensity in a characteristic way. It’s the unique and definitive signature of a black hole.”
Unique signature
Thorne calls it a symphony. “The symphony will contain a signature that says, ‘I come from a pair of black holes that are spiralling together and coalescing,”‘ he says. “This will be the kind of absolutely unequivocal black-hole signature that astronomers have so far searched for in vain using light and X-rays.”
Thibault Damour of the Institut Des Hautes Etudes Scientifiques near Paris agrees: “It’s unmistakable. Nothing else produces it.” The important thing, says Damour, is that the gravity waves are sensitive to the internal structure of a black hole, just as the character of the sound emitted by a bell depends on its shape and the material it is made from. “All other techniques for detecting a black hole are sensitive only to a distant dot-like mass. Gravity waves will allow us to actually ‘see’ the dot vibrating.” And Thorne believes that such a gravity-wave symphony would tell us how heavy each of the black holes was, how fast they were spinning and even whether the shape of their orbit was circular or elliptical.
Such a signature should be picked up within the next decade or two by one of the ground-based gravity-wave detectors currently being built or planned. In the US, LIGO will use two 4-kilometre detectors, one in Louisiana and one in Washington state. In Pisa, French and Italian teams are building Virgo, a 3-kilometre detector, while German and British scientists and engineers are planning a 600-metre detector known as Geo-600. The European Space Agency is even planning to launch a gravity-wave detector, known as LISA, into space by 2015.
With all this activity, and if all goes well, we should know some time in the first decade of the 21st century whether blacks holes really do exist.
What is a black hole?
A BLACK HOLE may form when a very massive star comes to the end of its life, having exhausted all its nuclear fuel. Deprived of the heat-generating nuclear reactions which have suported the star’s material against gravity, the star implodes. Under the relentless crushing force, all the electons and protons in the core form neutrons.
The stiffest material known to physics is one made of neutrons. Most times, the neutrons prove sufficient to oppose gravity and the collapse of the star is halted. The result is a neutron star. If, however, the neutron star has a mass of more than three times that of the Sun, not even the neutrons can stop its runaway collapse and the star continues shrinking remorselessly until it becomes a black hole.
The gravitational field of a black hole has a characteristic radius known as the Schwarzschild radius. If the Earth were to be crushed to form a black hole, the Schwarzschild radius would be barely a centimetre in size. For a solar mass black hole, the Schwarzschild radius is 3 kilometres, while a supermassive black hole of a billion solar masses has a Schwarzschild radius of 3 billion kilometres – equivalent to the distance between the Sun and Uranus.
Black Holes and Time Warps: Einstein’s Outrageous Legacy by Kip Thorne (Picador).