Do Black Holes Have Hair?
October 3, 2013

Do Black Holes Have Hair?

Black holes are fascinating beasts. Not only do they maintain densities that we can’t possibly simulate on Earth, (meaning we have no way to accurately probe the physics that governs the internal dynamics of black holes) but they possess characteristics that make them nearly impossible to distinguish from one another. Or so we think.

Arising out of work by Roy Kerr in 1963, physicists have developed the No-Hair Theorem (or No-Hair Conjecture by some, as a rigorous mathematics justification has yet to appear) for black holes, suggesting that all black holes can be defined by three quantities: their mass, angular momentum (a measure of the black hole’s spin) and its charge.

Essentially, two black holes with the same mass, angular momentum, and charge would be completely indistinguishable, even if they maintained entirely different compositions. Since all other information about the black hole is lost once the progenitor collapses, there remains no distinguishing features, or “hairs,” related to its previous existence or its composition. To which the famed physicist John Archibald Wheeler quipped, “black holes have no hair”, meaning that all black holes are “bald” and therefore look the same.

But new research is challenging this view. The team, led by Thomas Sotiriou, a physicist at the International School for Advanced Studies (SISSA) of Trieste, has proposed that the “charge” quantity of black holes may need to be re-imagined, and that it may not be an “indistinguishable feature” after all. In fact, the specific charge composition may leave unique signatures that would allow astronomers to differentiate between black holes. Eventually such observations would allow scientists to probe the specific compositions and structures of black holes.

“Black holes, according to our calculations, may have hair”, explains Sotiriou. “Although Kerr’s ‘bald’ model is consistent with General Relativity, it might not be consistent with some well-known extensions of Einstein’s theory, called tensor-scalar theories. This is why we have carried out a series of new calculations that enabled us to focus on the matter that normally surrounds realistic black holes, those observed by astrophysicists. This matter forces the pure and simple black hole hypothesized by Kerr to develop a new ‘charge’ (the hair, as we call it) which anchors it to the surrounding matter, and probably to the entire Universe.”

Future experiments using advanced interferometers (instruments used to measure gravitational waves) may be able to detect the subtle differences caused by the charge composition of black holes. “According to our calculations, the growth of the black hole’s hair,” concludes Sotiriou “is accompanied by the emission of distinctive gravitational waves. In the future, the recordings by the instrument may challenge Kerr’s model and broaden our knowledge of the origins of gravity.”

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