New experiment utterly alters what we know about black holes

Date:21 January 2021

Wait, are black holes fuzzballs, or are they hairless? The big quest to understand black holes continues in the form of new research about the fastest-spinning examples.

Scientists have found that while most black holes follow a particular theorem about what falls inside, a black hole spinning fast enough can extend “hairs” all the way back into regular space.

Recently, scientists released new observations about aging black holes, a special case in which the black holes appear to slacken and begin belching information back over the event horizon. Now, a second special case could do kind of the same thing, but from the opposite end: Black holes that spin fast enough end up creating a kind of vortex of hairs that link it across the event horizon as well.

Think of a black hole like, well, a black box. This idea from engineering and programming posits that a closed system could have almost anything inside, and our only way to scrutinize it is by studying the input and output. In the case of a black hole, what we put in is literally anything, and what we get out is a mere measure of mass only.

Universe Today’s Brian Koberlein explains: “You could make a black hole out of a Sun’s worth of hydrogen, chairs, or those old copies of National Geographic from Grandma’s attic, and there would be no difference. Mass is mass as far as general relativity is concerned.”

This is the “no hair theorem”—that what you throw into the black hole is totally despecified and impossible to call back out. It’s like those tea bags with no strings, and your mug is the black hole. Or, more helpfully, it’s like a wet dog at rest. The new research from Italian, French, and British scientists studies the wet dog in motion: “[R]apid rotation can induce instability. This instability, which is the hallmark of spontaneous scalarization, is expected to endow the black hole with scalar hair. Hence, our results demonstrate a broad class of theories that share the same stationary black hole solutions with general relativity at low spins, but which exhibit black hole hair at sufficiently high spins.”

Basically, black holes at low spin speeds have one set of qualities, but when you shift gears and spin faster, they acquire new qualities that go against what is widely believed about black holes.

And it doesn’t stop there: “There is no obvious reason to believe that this instability is restricted to [black holes],” the researchers write, “and it could well affect rapidly rotating stars as well. Hence, our results demonstrate that there is a broad class of theories where rotation might control deviations from [general relativity].”

Like almost all research into black holes and other less-understood cosmic objects, the goal is to continue to chip away at a fundamental mismatch at the heart of physics. How can things that violate general relativity be understood within the same single physics framework as things that do observe relativity? We still don’t know, but every step is important.

This article was written by Caroline Delbert and published in Popular Mechanics on 14 December 2020.

Picture: Pixabay

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