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 (opens a new tab) 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 (opens a new tab), a physicist at the Niels Bohr Institute in Copenhagen.
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