Evidence upholds the all-or-none hypothesis of black holes’ event horizons

From a safe distance, a black hole looks like a large patch of blackness in space. Like a giant pocket of darkness, no light passes through from more distant stars or escapes from the huge collection of mass inside its perimeter. Without any energy or objects coming from a black hole, it’s been hard to make any direct observations of these foreboding objects. However, by tracking the course of neighboring stars, researchers were able to confirm the nature of a black hole’s border, indirectly revealing information on the structure of the hole’s inner core that nobody can ever see.

The outer perimeter of a black hole, where the blackness seems to start, has been hypothesized to be a gravitational point-of-no-return. Dubbed the “event horizon,” this is the distance where the gravity of the mass at the center of the black hole can take hold of nearby objects, irreversibly pulling them towards its center. The event horizon was thought to be an instantaneous, one-way line in space, with even light being unable to resist the immense gravitational pull felt inside this border.

However, this model depends on a specific structure for the mass inside the center of the black hole, which we can’t directly confirm because of the whole “never coming out again” issue. For gravity to have this instant and complete grip on nearby objects, the mass creating it is thought to exist as a singularity— an object so collapsed on itself that it no long has measurable volume or surface area. Immense mass crushed into immeasurably small space is a weird idea, and it wasn’t the only possibility for how a black hole could be built.

Surface to smash into

The alternative is that the mass in the center of a black hole wasn’t completely crushed into itself, and exists more like a dense sphere that is voluminous enough to have a hard outer surface. In this model, the event horizon wouldn’t be quite so binary an experience, and incoming stars would instead grind against the dark sphere, emitting a bright layer of gas in the process. Neither model of a black hole allows for direct observations of escaping light, but stars getting brighter as they got pulled in would be detectable, giving researchers from the Harvard-Smithsonian Center for Astrophysics a target to search for.

Combing through years of observations from a telescope in Hawaii, the researchers saw… nothing. Over the course of three-and-a-half years, they would have expected to see at least 10 stars light up as they were ground into a spherical black hole, but no such activity was found. Researchers plan to keep looking for traces of this activity using a more sensitive telescope, but for now the hypothesized model of an all-or-none event horizon is holding true. Any stars that cross that line instead just disappear, essentially falling out of the sky and into a dark speck of infinite density. At least until we have a chance to see one doing otherwise.

My second grader, who was surprisingly fine with the idea of a singularity, asked: So wait, our solar system is orbiting a black hole?

It’s not so much our solar system as much as our whole galaxy. At the center of the Milky Way, as well as many other galaxies, is a super-massive black hole. That galactic center is very far away though. The closest black hole we know about is V616 Monocerotis, around 3,000 light years away. There might be others, but since they don’t really show themselves, they’re hard to spot.

Source: Do stars fall quietly into black holes, or crash into something utterly unknown? by Rebecca Johnson, Scienmag