Black holes have captured our imaginations for years: the idea of a near-invisible giant vacuum cleaner drifting through space, ready to devour the Earth in one gulp, is enough to keep anyone awake at night.
But now, despite their mystical past, we understand more about black holes and how they work than we ever have.
Black holes are immensely massive objects, with gravity so unbelievably strong that not even light can escape from them. They are thought to be incredibly common, scattered around the galaxy, perhaps even closer than we think.
“They’re just these like prisons for light matter and everything,” astrophysicist and science communicator Dr. Becky Smethurst, awning newsweek.
Some black holes are thought to have been around since the universe itself was born, referred to as primordial black holes, while others are essentially the corpses of old stars, having collapsed in on themselves after they die in an immense explosion known as a supernova.
“In the same way that there are lots of stars hanging around the galaxy, so there are lots of black holes hanging around the galaxy, as well,” Smethurst said.
Black holes behave in the same way that any other massive object does: if a black hole with the same mass as the sun spontaneously replaced it, then the Earth’s orbit wouldn’t change at all.
According to Smethurst, there may be black holes roaming around the universe, with one lurking even in the outskirts of our solar system.
“A lot of stars formed in clusters,” she said. “With a black hole, what can happen, either from a supernova or interaction with other stars afterwards, is that you can get ‘slingshotted’ out of those clusters by gravity, essentially, in the same way that we do gravitational slingshots around planets [for spacecraft].”
This would then result in a rogue interstellar black hole just roaming through space.
“There could also be a black hole on the edge of the solar system,” Smethurst said. “Like there’s this idea that there’s another planet out there that we haven’t discovered yet, that could be shepherding some of the objects out there into these weird orbits that we’ve seen, and we haven’t found it.”
This is how Neptune was discovered: the orbit of Uranus was irregular, leading scientists to investigate if there was another mass in the same area of the solar system. They eventually found Neptune.
“[Some people] raise the hypothesis that the reason we haven’t found [the planet] could be because it’s a black hole, 10 times the mass of the Earth or something, that would be classed as one of these primordial black holes, that could just be hanging out on the edge of the solar system causing a bit of chaos,” Smethurst said.
Despite their name, black holes can be detected if you know where to look.
“It would perhaps have this big halo of matter around it that it collected, and there could be some antimatter in there,” Smethurst said.
“And if that encountered matter, it would create a big burst of energy with annihilation, until we might be able to see some gamma-ray flashes from that direction from wherever it was.
“So it’s fun to think that the solar system could have its own little pet black holes.”
This wouldn’t be as scary as it sounds, however. “They’re not just vacuum cleaners in space,” Smethurst says. “They are more like couch cushions, I like to say. They’re just sitting there in your living room, not pulling anything towards them. But if you lose something down the side, it’s gone forever.”
It is thought that there are supermassive black holes at the center of every galaxy: Sagittarius A*, the example in the Milky Way, the spiral galaxy containing the sun, was photographed for the first time in May.
“We are orbiting a black hole already,” Smethurst said. “The sun orbits the black hole in the center of the Milky Way. We don’t have to worry about the solar system falling into the black hole at the center.
“But what’s really interesting is that the galaxy of stars has enough of what’s called ‘self-gravity’ to hold itself together and still orbit the center, even if the black hole wasn’t there.”
Smethurst said: “That’s the reason that all the gas in the galaxy hasn’t just spiraled into the black hole. Because there are other things further out that it is more gravitationally attracted to, including itself.”
Dr. Becky Smethurst is the author of A Brief History of Black Holes: And Why Nearly Everything You Know About Them is Wrong.