Black Holes that are neither Black nor Holes

Stephen Hawking showed some time ago that black holes might actually emit light known as
Hawking radiation
, not from their bottomless interiors of course, but from the event horizon that marks the point of no return as you approach one of these monsters. In other words, black holes probably aren’t black.

Now it seems collapsing masses that aren’t black holes quite yet can bend space much as true black holes do, and give off the signature black hole Hawking radiation.

From a distance, such a thing would look just like a black hole, except that it wouldn’t have a hole at the middle.

OK, OK, I can guess what you’re thinking. Doesn’t the lack of a singular point mass at the center mean it’s not a black hole?

Hey, it still passes the duck test. After all, whether there is a hole at the center or not, I don’t think any person or probe will ever be able to visit one and get back intact to tell us about it.

Carlos Barceló, Stefano Liberati, Sebastiano Sonego, and Matt Visser of the Astrophysics Institute of Andalucía in Granada, Spain predicted the unholey black holes. Because they can’t actually check a gravitational black hole, they confirmed the mathematics of their theory by studying a simulation of fluid flowing down a narrow pipe. Once the speed of the fluid going down the drain passes the speed of sound in the fluid, the system acts just like a black hole, at least as far as sound getting sucked inside goes. The fluid analog even emits acoustical Hawking radiation from its acoustical event horizon.

The thing is, the fluid analog emits Hawking radiation slightly before it becomes a true acoustic black hole. The researchers believe a collapsing mass that is almost, but not quite, a black hole should give off Hawking radiation too.

Check out the APS Physical Review Focus story to get more information about theory that will be published soon in Physical Review Letters.

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