IF black holes do have hair, it must be long â at least so say physicists at the National Independent University of Mexico in Mexico City. âBlack holes cannot have short hair,â says DarĂo NĂșñez, one of the team. By âhairâ, they mean anything that could stick out of the hole and reveal the details of how it formed.
Back in the 1960s, the Russian theorist Vitaly Ginzburg and others discovered that when a body collapses under its own gravity to form a black hole, all telltale features are either sucked into the black hole or radiated away. For example, the magnetic field of a shrinking star is sucked into the hole, while a mountain or other irregularity in a gravitational field is radiated into space as waves of gravity.
This meant, said the theorists, that whatever the shape of the body to begin with, its form as a black hole is specified only by properties that cannot be radiated away: its mass, its spin and its charge. Lacking any âhairâ, a black hole retains no âmemoryâ of its former existence.
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By 1990, however, particle physics had complicated the picture. Theorists realised that some black holes could have hair after all, not made out of magnetic fields or gravitational fields, but from the kind of fields that give rise to subatomic particles such as pions, or W and Z bosons.
âThe discovery came about when people put their favourite particle physics theories into space warped by gravity and looked for objects with the properties of black holes,â says Ian Moss of the University of Newcastle upon Tyne. NĂșñ and his colleagues claim that the hair can appear because of the peculiar nature of the gravitational field. A gravitational field contains energy; and energy, according to Einstein, has a gravitational effect. It is the interaction between the gravitational field sucked into the hole and the gravitational field radiated away from it that the Mexican physicists say âbindsâ the two fields together and gives rise to permanent features, or hair.
NĂșñez and his colleagues have used this insight to determine the extent of the âhairosphereâ of a hairy black hole. They conclude that it must extend beyond one-and-a-half times the radius of the holeâs âhorizonâ -the point of no return beyond which an object cannot escape the holeâs gravity. In other words, a black hole cannot have short hair (Physical Review Letters, vol 76, p 571).
Of course, no one knows for certain whether black holes exist at all. The definitive proof of their existence will be the detection of the unique gravitational wave signature produced at their birth.
But in the meantime, devotees of bald black holes need not worry. According to Moss, the result is merely a curiosity since the hair on black holes arises only in theories of particle physics â and so is very short in absolute terms. âOnly microscopic black holes â for instance, any that might have been created in the big bang â will have hair,â says Moss.
The new theorem is unlikely to have the significance of the original âno-hairâ theorem, says Moss, which demonstrated that black holes are âremarkably simple beastsâ. Big black holes therefore remain hairless. And even their microscopic cousins eventually lose their long hair and become bald because, in the end, all black holes simply evaporate away.