According to Albert Einstein’s general theory of relativity, the behavior of a black hole depends on two numbers: how heavy it is, and how fast it is rotating. And that’s it. Black holes are said to have “no hair” — no features that distinguish them from their fellows with the same mass and spin.
With new data, it has started to become possible to test this no-hair conjecture. Astronomers have detected hundreds of signals from colliding black holes over the past 10 years. In these dramatic events, two of the invisible, inescapable pits in the fabric of space-time circle one another faster and faster, and then merge into a single, massive black hole that jiggles like Jell-O as it settles down after the collision. The merging and jiggling sends ripples called gravitational waves cascading outward through the fabric of the universe and to detectors here on Earth. If general relativity is correct, those jiggles have a cookie-cutter form that depends only on each hole’s mass and spin. (In theory, there’s a third defining property: electric charge. But real, astrophysical black holes have negligible net charge.) If the theory is wrong, astronomers might observe something new — subtle distinctions that reveal the unique history and makeup of each black hole.
“As years went by and the events piled up, we realized that we could have stronger, more robust tests of the theory [of general relativity] — or alternatives,” said Vitor Cardoso, a physicist at the Niels Bohr Institute in Copenhagen.
Cardoso and dozens of other gravitational wave astronomers recently summed up the status of those tests. Their report, released at the end of May, covers a variety of methods and results, including an analysis of gravitational wave signals conducted by Cardoso and colleagues last fall. Assembling data from multiple black hole collisions, that group found that the data agreed with Einstein’s theory as best they could tell. Any deviation from what general relativity predicts for the shape of space-time around a black hole — any “hair” — would have to lie closer to the hole than 40 kilometers.
Vitor Cardoso led a recent analysis of black hole collisions that found that any “hair” would have to lie closer to a black hole than 40 kilometers. Ana Carvalho
So no hair has been seen, for now. But theorists continue to think through a multitude of possibilities, with cracks in Einstein’s theory suggesting that subtle, quantum “hair” ought to exist, even if it might be extremely challenging or even practically impossible to spot.
What Einstein Didn’t Predict
The question of hairy black holes is intimately connected to the greatest puzzle in modern physics: How can general relativity be merged with quantum theory?
Consider the situation where an object crosses a black hole’s point of no return, called the event horizon. According to general relativity, all outsiders will see is how the swallowed object contributes to the two numbers that describe the black hole: how much mass the object adds, and how much faster or slower it makes the black hole rotate.
Yet the disappearance of all other information about the engulfed object conflicts with one of the central tenets of quantum mechanics. Quantum theory requires that all information be preserved and that it remain, in theory, accessible — otherwise quantum probabilities won’t add up to 100%, as they must. Theoretical physicists have long been fascinated by this conflict between the predictions of general relativity and those of quantum mechanics, which is known as the information paradox.