Black hole system H1743-322 with "bullets" of ionized gas ejecting from right outside the event horizon. Photo: NASA Goddard
Steven Hawking has sent a bit of a shock through the astrophysics community, arguing in a new paper uploaded to arXiv that “there are no black holes.” At face value, this goes against so much of what we know about the universe, where these massive gravitational pockets are the focal point around which most galaxies orbit. So has Hawking just turned everything we know on its head? No, though he has managed to redefine black holes rather significantly in just a few words.
In the late 18th century, more than 100 years after Isaac Newton showed that all objects in the universe are attracted to each other through gravity, John Michell used Newton’s laws of gravity to posit the existence of dark stars, an early name for what became known as black holes Pierre-Simon Laplace came up with the idea of dark stars a few years later, independent of Michell, and their calculations are the first modern records to suggest the universe could be filled with unseen black holes.
Our understanding of black holes has developed a lot in the last half century. We know that what makes black holes black is their event horizon, a level around a black hole’s perimeter from this nothing can escape its gravitational pull. And if not even light can’t escape, these holes must be black. But as Hawking showed in 1975, that’s not entirely true. Quantum theory suggests that black holes can leak particles, known as “Hawking Radiation,” over time. That means that, theoretically, a black hole could disappear, but only over incredibly long time scales.
So what does that mean for, say, an astronaut unlucky enough to fall into a black hole? (Incidentally, this happens to a character in the children’s book Hawking co-wrote with his daughter Lucy, called George’s Secret Key to the Universe.) Well, there are two main options, as Hawking's research team explains.
This delightful video takes you on a tour of a standard model of a black hole, before Hawking called them grey.
If you base your black hole assessment on general relativity, an astronaut wouldn’t notice anything until he or she passed the event horizon. Only once inside the black hole’s strong gravitational field would he or she be pulled apart.
But the same quirk of the quantum universe that allows for Hawking radiation means an astronaut’s journey across the event horizon wouldn’t be peaceful at all. According to quantum theory, the event horizon must be a highly energetic region called the firewall, meaning that passing the event horizon would be a fiery event. Passing into a black hole can’t be both a calm and fiery event, which created the paradox.
Hawking’s latest paper offers a third option and a solution to the paradox: keep both quantum and relativity theories intact, but take away the fire around the event horizon. In fact, take away the event horizon altogether. Hawking’s claim centers around the idea that the quantum effects around a black hole cause space-time to fluctuate too wildly for such a distinct boundary to exist. Instead of an event horizon, Hawking suggests an apparent horizon, a surface where light trying to escape the black hole will be stalled, but information can theoretically get out.
It gets weirder. An event horizon-less black hole could, in theory, not have a singularity at its core. In this case, whatever goes into the black hole wouldn’t crunch down to nothingness. It would just exist inside the black hole. (If it managed to get back out again, it wouldn’t be in good shape.)
Hawking’s latest works adds a big grey area to the already complicated world of black holes, and will certainly be a hot topic among physicists for a while. (It's important to note, as usual, that the arXiv preprint server isn't peer-reviewed.) Hopefully whatever information comes out of these discussions makes more sense than anything that manages to escape from a black hole.