A researcher stands by her arguments against black holes; other scientists are skeptical.
“Completely implausible”—a controversial paper exists, but so do black holes!
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.
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.
Observational evidence: “Yes, Virginia, there are black holes”
Mersini-Houghton’s work, even if accurate, leaves a massive, gaping question. If black holes really don’t exist, if they’re impossible, then what, exactly, have we been looking at this whole time?
Black hole skeptics might be quick to point out that black holes cannot be observed directly, due to their very definition. Black holes don’t reflect any light, they certainly don’t emit any, and any Hawking radiation would be far too weak to be observed, so there’s no way for us to directly observe a black hole.
“Yes, ‘black holes are observed,’” Jeffrey McClintock, an astrophysicist with Harvard University, told Ars, “but not in the way, e.g., that a planet, the Sun, or a neutron star is observed—all objects with material surfaces. There is no surface at all at the [event horizon], and anything found there is falling inward (relative to a local observer) at the speed of light. Moreover, according to [general relativity] the EH is a surface of infinite redshift, which means that distant observers like us cannot hope to see any radiation (excluding ultra-feeble and unobservable Hawking radiation) coming from the EH.”
That much is true (although some recent work suggests Hawking radiation might be amplified by the black hole enough to be observable). But despite the difficulty of direct observation, the indirect observational evidence is not ambiguous. We’re looking at something, and if it’s not black holes, it sure looks a lot like them.
For one thing, black holes are rarely observed in isolation. They tend to have matter falling in, or accreting onto, the black hole, often from a companion star. This matter forms an accretion disk, and those disks are very much observable. As it falls in, matter speeds up, much like an unfortunate boat caught in a whirlpool, swirling down faster and faster as it gets close to the bottom. The swiftly moving matter creates quite a bit of friction, which creates heat—and intense x-ray light, which our telescopes can easily detect. The black hole’s effect on the motion of their companions is also observable.
But how do we know that the objects we’re seeing are really black holes? According to McClintock, the answer is twofold. “The strongest evidence that the compact objects I study are black holes is (1) dynamical evidence (i.e., based on Newton’s Laws) that the masses of these objects exceed three sun masses, and (2) that any compact object (e.g., neutron star, quark star) this massive will collapse to a [black hole] according to [general relativity]. We and others have measured the masses of about two-dozen compact stars with typical values of 10 sun masses,” he explains.
In other words, the motions of other objects near the black hole can be observed, such as the black hole’s accretion disk material or a star co-orbiting the black hole. Based on the velocities of those objects, the black hole’s mass can be determined. And once scientists know the object’s mass and its size, they can determine, using general relativity, whether it’s compact enough to be a black hole.
It’s pretty hard to argue that any step in this process is wrong. Our understanding of dynamic motions of celestial bodies is incredibly powerful. It’s been honed by studying our own Solar System, where it consistently predicts the motions of the planets and moons in their complex interactions with each other as they orbit the Sun. It works just as effectively everywhere else in the Universe. And general relativity is also rock-solid as theories go. A huge body of evidence confirms its predictions. If general relativity were wrong, GPS systems would constantly get out of sync with their satellites—and they don’t.
Another astrophysicist, Brian Koberlein, responding to the recent headlines about Mersini-Houghton’s paper, summarizes the observations in his blog :
“Black holes absolutely exist. We know this observationally. We know by the orbits of stars in the center of our galaxy that there is a supermassive black hole in its center. We know of binary black hole systems. We’ve found the infrared signatures of more than a million black holes. We know of stellar mass black holes, and intermediate mass black holes. We can even see a gas cloud ripped apart by the intense gravity of a black hole. And we can take images of black holes, such as the one above. Yes, Virginia, there are black holes.”
Given that such strong observational evidence exists for black holes, what can be made of Mersini-Houghton’s work? Even if her methodology has no flaws, how might it be reconciled with these observations?
Part of the problem is that the authors do not address this issue in their paper.
“It would have been very helpful if the authors had spelled out how they view the many detailed observations that have been made of dozens of stellar-mass BHs,” says McClintock. “Are the authors really attempting to refute the existence of these compact stellar remnants? What do they say to observations that clearly show 100,000 times the power of the sun being emitted in X-ray radiation from a region around a stellar-mass BH the size of New York City? Likewise, how do they view other astounding phenomena observed for BHs, both stellar-mass and supermassive?”
Responding to this, Mersini-Houghton told Ars that these observed objects are “The same massive objects as we thought before, except that they do not have an event horizon and a singularity in the center. We have to find a suitable name for black holes without horizons and singularities. Obviously they won’t be black.”
Given that perspective, it’s not clear whether these objects can be experimentally differentiated from true black holes. If some future observation can show that these objects really aren’t black—whatever that would look like—Mersini-Houghton’s work would be vindicated.
(We sent her a follow-up question, asking if any such observation is possible, and haven’t received a reply as of this writing.)
Theoretical evaluations: “This model is incorrect”
If observational evidence shows that black holes really do exist, where did Mersini-Houghton and Pfeiffer go wrong?
One culprit is negative energy. This is one of the more confusing aspects of Hawking radiation. When the negative Hawking radiation goes inside the black hole, it causes the black hole to lose mass. But that phrase isn’t as straightforward as it sounds. For one thing, the phrase “negative energy” is fairly ambiguous in the first place.
“I do not find the proposal by Laura Mersini-Houghton & Harald P. Pfeiffer credible,” Andrew Hamilton of the University of Colorado told Ars in a response that later became a blog post. “They model Hawking radiation inside the horizon as a relativistic fluid with negative energy, which is gravitationally repulsive, and therefore causes the center of the black hole to bounce rather than collapsing to a singularity. However, this model is incorrect.”
If the negative Hawking radiation inside the black hole isn’t like a fluid that puts a gravitationally repulsive force on the black hole, as Mersini-Houghton presents it, then what is it?
“Hawking radiation has negative energy inside the [event] horizon because its gravitational binding energy exceeds its rest mass energy,” explains Hamilton. “This is something that happens inside black hole horizons: trajectories of particles can be so bound by gravity that it would take more than the rest of mass energy to transport the particle to infinity. But any real observer inside the horizon would see the Hawking radiation as having positive energy, contrary to the author’s simplistic assumption.”
That explains why the negative energy only exists inside the event horizon. Which is a problem for Mersini-Houghton’s model, since the bounce predicted in the paper occurs because of negative energies that appear before the horizon’s formation. “The negative energy particles though only exist inside the horizon,” confirmed Sabine Hossenfelder, a physicist with NORDITA (the Nordic Institute for Theoretical Physics), in a blog of her own post. “Now in Laura’s paper, the negative energy particles exist inside the collapsing matter, but outside the horizon.”
Hossenfelder also argues that the math shown in the paper isn’t set up correctly.
“She doesn’t integrate the mass loss over time and subtracts [sic] this from the initial mass, but integrates the negative energies over the inside of the mass and subtracts this integral from the initial mass,” she says. “There is no time-integration in these expressions, which puzzles me.”
This all goes back to the negative energy problem. Rather than calculating how much mass the star loses in this process and then subtracting it from the star’s original mass, Mersini-Houghton calculates how much negative energy has accumulated from Hawking radiation inside the star and subtracts that. The problem, of course, is whether this is a realistic picture of how negative energy works.
William Unruh, a theoretical physicist from the University of British Columbia who was cited by Mersini-Houghton in the paper, had an even harsher take on the authors’ math and reasoning.
“The [paper] is nonsense,” he told IFL Science in an e-mail. “Attempts like this to show that black holes never form have a very long history, and this is only the latest. They all misunderstand Hawking radiation, and assume that matter behaves in ways that are completely implausible.”
“it would take 10^53 (1 followed by 53 zeros) times the age of the Universe to evaporate,” he explains. “Unfortunately explicit calculations of the energy density near the horizon show it is really, really small instead of being large—those calculations were already done in the 1970s. To call bad speculation ‘has been proven mathematically’ is, shall we say, and overstatement.”
Another issue is that Mersini-Houghton’s work requires a specific model of something called a firewall. A firewall is a wall of Hawking radiation particles that purportedly forms at the event horizon, where it can prevent new matter from falling in. This contradicts a prediction of general relativity, wherein an observer should be able to pass through the event horizon without noticing they’d passed through anything. This is such a significant problem that it has become known as the “firewall paradox,” and it has no resolution as of yet. Quite a bit of theoretical work has attempted to resolve the problem, including Mersini-Houghton’s, but physicists haven’t reached a consensus on a solution yet.
The new paper offers what its authors think is a solution. In order to solve the firewall paradox, Mersini-Houghton wasn’t thinking small: she claims to have unified general relativity with quantum mechanics. “Physicists have been trying to merge these two theories—Einstein’s theory of gravity and quantum mechanics—for decades, but this scenario brings these two theories together, into harmony,” said Mersini-Houghton in a press release. “And that’s a big deal.”
Assuming that black holes exist, general relativity and quantum mechanics yield vastly differing predictions of what an observer entering a black hole would experience: a firewall, or nothing. But if black holes never form in the first place, there’s no longer a paradox—there are no event horizons for observers to pass through, so black holes no longer create a region of the Universe where the two theories make conflicting predictions. (Of course, there are other areas where the two are incompatible, so even getting rid of black holes wouldn’t be enough to get the theories “in harmony.”)
This model is appealing in some ways. It almost seems to make the Universe ‘make sense’ again. Black holes have arrogantly dared us to figure out what’s going on inside their event horizons. Under Mersini-Houghton’s model, none of that is necessary. General relativity has always worked out perfectly well with quantum mechanics, at least in the case of black holes.
“Since there are no singularities,” Mersini-Houghton told the Huffington Post, “we are back in the land of certainty as far as stars in our Universe are concerned. We can study them with physics we trust and can follow their evolution through all the stages, with no mystery of incomprehensible exotic objects such as singularities involved.”
It can be incredibly tempting to embrace such a model, which neatly removes some of the most perplexing issues in physics, solving a puzzle by removing it. But that’s all the more reason to investigate this claim with rigor. If her work really reflects the Universe, it’s all the more important for us to find out—by testing it.
As a larger scientific point, there’s no reason we should expect that our current theories necessarily work in every region of the Universe. Before Einstein and relativity, Newtonian laws were thought to account for objects moving at any speed. But if those laws of motion are applied to objects moving at a significant fraction of the speed of light, they break down—just like relativity and quantum mechanics break down at the event horizon of a black hole. We had to wait for a more fundamental theory (special relativity) to be developed before successful predictions could be made about speeds previously beyond the reach of our best theories. It could be that event horizons need a similar development before they’ll make sense.
Of course, the historical precedent might not apply in this case, but it does show that the Universe doesn’t always conform to our expectations, either intuitive or theoretical.
Although it gets rid of firewalls, Mersini-Houghton’s work depends on Hawking’s model of firewalls being correct—which hasn’t been demonstrated. Other physicists favor differing models. Hossenfelder and Koberlein, for example, belong to the camp that argues quantum mechanics doesn’t create a firewall at the event horizon in the first place.
“This is interesting theoretical work, and it raises questions about the formation of stellar-mass black holes,” says Koberlein. “To say that this work proves black holes don’t exist is disingenuous at best.”
The back-and-forth: “Your astrophysicist is confused”
Mersini-Houghton responded to Hamilton’s theoretical criticism: “I am afraid your astrophysicist made a mistake or is confused about Hawking radiation,” she told Ars. “Hawking radiation is pair creation of particle-antiparticle from spacetime vacuum outside the star. Vacuum means that total energy and momentum of this pair should add up to zero. If right outside the star you have positive energy particles flying off to infinity (this is what conventionally people call Hawking radiation) and also ‘positive energy particles’ going inside the star, then how do you add that up to zero?”
Hamilton was quick to rebut. “I am not at all confused,” he said. “There are negative energy geodesics inside the horizon of a black hole. That means there are geodesics in which the negative gravitational binding energy of a particle exceeds its rest mass—it would take more than the rest mass energy of the particle to transport it to infinity.”
A geodesic is the closest thing to a straight line in a curved space. For example, airplanes traveling to a destination due east may follow a curved path, turning north and then back south, rather than traveling straight east to west. Because of the curved surface of the Earth, the geodesic path is shorter than traveling straight. In the case of black holes, the space near the hole is curved due to gravity, and in cases where Hawking radiation forms, the negative particle falling in follows a geodesic in the curved space.
“Hawking radiation is in a sense made possible by the fact that negative energy geodesics exist,” he explained. “A particle pair can be created with zero total energy, with the negative energy particle being on a negative energy geodesic that falls inside the black hole, and a positive energy particle being on a positive energy geodesic that goes to infinity.”
“The key point here,” he continued, “is that a particle on a negative energy geodesic has positive proper energy, that is, it has positive energy when measured in any locally inertial frame.”
In other words, the negative particle is only negative when its total gravitational binding energy involved with being inside the black hole is taken into account. Locally (meaning to anything else inside the black hole), it’s positive and it doesn’t have an anti-gravitational force on its immediate surroundings, according to Hamilton. Therefore it shouldn’t act like it’s modeled in Mersini-Houghton’s work.
Lawrence Krauss, a theoretical physicist who’s done work on Hawking radiation, weighed in on the debate, telling Ars, “I am dubious about the Mersini-Houghton work… I would tend to agree with [Hamilton], though I didn’t go through the work in detail.”
Other kinds of black holes
Another issue is that the paper only addresses stellar-mass black holes. This is problematic because black holes of other masses may form by completely different processes.
While the paper seemingly rules out all singularities, it only examines what happens as a star collapses. It’s quite possible this can be extrapolated to other forms of singularity, such as black holes formed by other processes and perhaps even the singularity of the Big Bang. But it’s fair to note the paper doesn’t address them directly.
Theoretically, black holes of any mass can exist. You could turn an apple or any other object into a black hole if you could crush it down small enough. Even microscopic black holes could exist. But microscopic black holes have yet to be observed, and there is no known mechanism by which nature could produce them.
On the opposite end of the scale are supermassive black holes, which have at least 100,000 solar masses but can contain millions. Supermassive black holes are extremely mysterious objects, because it’s not known how they could have formed. No star could be big enough to have collapsed into them, not even the extremely massive stars in the very early Universe. As a result, Mersini-Houghton’s model wouldn’t apply to them.
Supermassive black holes are observed to exist in the core of every galaxy, including the Milky Way. Even tiny dwarves have supermassive black holes. And it’s just as impossible to make the case that these aren’t really black holes. Observations of the Milky Way’s black hole, Sagittarius A*, for example, reveal a number of stars orbiting the object in different trajectories, providing us with a lot of information available about the object’s mass and size. It clearly has an incredible amount of mass, all stuffed inside Uranus’ orbit.
Another class of black holes is intermediate black holes, which also lack an explanation. These can be hundreds of solar masses or more. Some proposed explanations are mergers of stellar-mass black holes, or the collapse of primordial material after the Big Bang. Intermediate mass black holes have not yet been conclusively observed, but there are some good candidates.
To all these cases, Mersini-Houghton’s work might not apply; even if she’s right, they provide a serious challenge to the proclamation that black holes are impossible. However, if it were true that stellar-mass black holes can’t form, it might imply that a similar mechanism prevents them from forming by other means.
The takeaway: “Very strong claims”
Black holes are firmly grounded in a mountain of observational evidence, and the new theoretical work doesn’t present a sufficient reason to abandon them from current models. In science, if there’s a conflict between a model and our observations, it’s the model that needs to be changed, provided the observations hold up to scrutiny.
Nonetheless, a new theoretical work that conflicts with observation can raise interesting questions. If the methodology was correct, why does the model fail to predict reality? Searching for answers to questions like that can help to build stronger models, rooting out the flaws in old ones. But that assumes the methodology is correct. Mersini-Houghton’s paper has yet to be peer-reviewed, but we’ve seen other knowledgeable physicists weigh in. And the ones we’ve heard from so far aren’t fans of Mersini-Houghton’s methodology.
Of course, this is not an official peer-review process, and it may still be that the larger scientific community will come to validate her work or come closer to reconciling it with observation. Until then, looking to other knowledgeable researchers for an informal review of her work seems like the best available way to get perspective.
Peter Edmonds, press scientist for NASA’s Chandra X-Ray Observatory, argues that in cases like this, a sort of “informal peer review” is an appropriate response. “These reviews of the new black hole paper aren’t formal ones conducted by a journal, but that doesn’t matter,” Edmonds told Ars. “One could argue that they might be more reliable than formal peer review, because they’re not anonymous. Also, because the authors and press office decided to do publicity before peer review, informal peer review by experts is a very appropriate response, to give a timely response to the very strong claims reported in the press.”
Edmonds has written previously about the dangers of publicizing one’s results before undergoing the peer review process. While there are advantages to doing so, and while in some cases it can be a good thing, it often has clear negative consequences, as he discusses in the case of the recent BICEP2 story.
The bottom line here. It’s fine to wait for peer review, but let’s not abandon black holes just yet.