There once was a time when the existence of black holes was very much in doubt. Even Albert Einstein was an early doubter despite the fact that black holes were predicted using his equations of general relativity—he simply believed nature wouldn’t permit them. At best, Einstein thought black holes might be theoretically possible, but they weren't the sort of thing that would, or even could, ever be observed.
Today, that era is long past. Black holes remained theoretical for some time, but they have become a standard feature of astrophysics, a practical reality. While some physical details of black holes are still very much in question, currently a strong scientific consensus exists that black holes are real, and new black holes are regularly discovered.
In light of that, things have gotten peculiar recently. Headlines all over the Internet have declared that black holes don’t exist after all—see, for instance, "There are no such thing as black holes"—and some argue their lack of existence has been mathematically proven.
The headlines are reporting on a recent paper submitted to arXiv, which has not, as of this writing, emerged from peer-review, although it builds on the author’s earlier work which has. The paper’s primary author, Laura Mersini-Houghton, a theoretical physicist at the University of North Carolina at Chapel Hill, claims her work proves that black holes cannot form in the first place. "I'm still not over the shock," she said in a written statement issued by the university. "We've been studying this problem for more than 50 years and this solution gives us a lot to think about."
Mersini-Houghton is not the first one to claim that black holes don’t exist. Stephen J. Crothers has been claiming to have disproven their existence for quite some time, and even Stephen Hawking has issued a statement that “there are no black holes" (although he didn’t mean that literally).
Mersini-Houghton’s claims are even more extraordinary, however. (And we all know what extraordinary claims require.) In order to conclude that black holes don’t exist, she claims to have united general relativity with quantum mechanics, a feat which has been a sort of “holy grail” of modern physics. A unifying theory of this sort has thus far proved elusive despite the best efforts of the physics community.
So if Mersini-Houghton’s work is correct, much of physics and astrophysics might need to be rethought.
Is it time to consign black holes to the dustbin of scientific history, along with the luminiferous aether and the geocentric model of the universe? TL;DR version—no, it isn’t. But it does seem like time to explore what we do know about black holes and to outline how we know they exist. To that end, we spoke to some physicists with expertise in the relevant areas on both the theoretical and the observational side of things. Some of them took a look at Mersini-Houghton’s paper and were happy to respond.
These and other physicists have already discussed the issue elsewhere on the Internet (including on their blogs in some cases), raising objections to the work or defending the observational evidence for black holes. We’ll take you through some of their counter-arguments to see why, in the minds of these physicists, at least, the case is pretty strong that black holes do exist.
But first... we need to understand the reason black holes supposedly don't exist.
The claim: “Fireworks, not Firewalls"
In 1974, Stephen Hawking introduced the concept of Hawking radiation, something black holes can produce despite their definition as objects from which not even light can escape.
Thanks to quantum mechanics, unstable particles called 'virtual particles’ are created at random all the time. A virtual particle consists of two particles that are anti-particles of each other and have a combined energy of zero. In other words: normally, the two particles immediately annihilate each other. But if they form at the event horizon of a black hole, one of the two gets sucked in, while the other, being outside the event horizon, can escape.
The escaping particles can carry away some of the black hole’s mass with it. This may seem counter-intuitive, since there's a particle left inside of the black hole. But nonetheless, the black hole does lose mass. This is because the particle that's trapped inside can be thought of as having negative energy—instead of adding to the black hole’s mass, it subtracts. Given a long enough time, black holes can evaporate this way provided they’re not taking in more mass than they lose from Hawking radiation.
The arXiv paper relies on Hawking radiation starting to appear before the black hole itself has finished forming. Most black holes that we’re familiar with (the stellar-mass ones, anyway) form from a collapsing star. So as the star collapses, Hawking radiation would appear, injecting negative energy into the star’s core, reducing its mass. Mersini-Houghton and her co-author, Harald P. Pfeiffer, wanted to understand what effect this has on the star as it collapses.
What they found in their simulations is that the repulsive, anti-gravitational force from the negative energy builds up as the star collapses. Right before the star's core becomes a black hole, it bounces. Rather than continuing to collapse, the star stops before it gets dense enough to form an event horizon, then it rapidly expands.
What happens to the star next is uncertain, since the authors’ simulation consistently breaks down at this point. Regardless, their conclusion about black holes is certain: they can’t exist. They never form in the first place.
If correct, this applies everywhere, according to Mersini-Houghton. “Just like Hawking radiation which is universal,” the authors write in their paper, “we discover that this behavior of the collapsing stars bouncing and exploding before the horizon and singularity would have formed, is universal, i.e independent of their characteristics such as mass and size.”
That conclusion has a wide range of consequences. If stellar-mass black holes—singularities surrounded by event horizons—can’t exist, then it raises the question of what this means for other models involving singularities formed by other mechanisms, such as the supermassive black holes at the center of galaxies.
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