Physicists at UC Santa Barbara have pushed the search for microscopic black holes at the Large Hadron Collider (LHC) at the European Center for Nuclear Research (CERN) into new territory - and, as is tradition in particle physics, found nothing. Which, they hasten to add, is still something.
These hypothetical black holes would be extraordinarily small and short-lived. If they could be produced at the LHC, their existence might help physicists address some of the deepest unanswered questions about spacetime and gravity. The search also gave researchers a chance to test a new technique for finding rare and previously unknown particles, because why waste a perfectly good null result?
"Had we found evidence, we could have begun to directly study quantum gravity," said Tamas Vami, a researcher in the Compact Muon Solenoid (CMS) experiment who is conducting his postdoctoral work under the guidance of UCSB physics professor Joe Incandela. "It's a step toward unifying all of the known fundamental forces, which has been a goal of physicists for more than a century." So, no pressure.
The search did not uncover evidence of quantum black holes. But in particle physics, failing to detect something can still provide important information by ruling out where it could exist - a discipline built almost entirely on the scientific equivalent of checking under the couch.
"It's not a dead-end," said Incandela Lab graduate student researcher Danyi Zhang. "The result is an exclusion limit, which is a real, publishable statement: 'If this thing existed with these properties, we'd have seen it. We didn't, so we can rule it out here.' That's genuine knowledge about how the universe works." Genuine knowledge, delivered in the form of a shrug at a very specific energy range.
One of the major puzzles in fundamental physics involves the enormous difference between the scale of the universe we experience and the Planck scale, the fundamental energy scale associated with quantum gravity. Some physicists have proposed that new physics or an undiscovered symmetry could explain this difference. Crucially, some of those effects might appear at energy levels the LHC can reach.
Years of experiments have already eliminated many theoretical possibilities, and the continued absence of clear signs of new physics at the LHC has become a major challenge for researchers. But similar situations have happened before. Periods in which existing theories struggled to explain observations have sometimes led to radically new frameworks, including Einstein's theory of relativity. For that reason, the researchers say null results are an important part of scientific progress. Each one reduces the number of viable possibilities and helps determine where future experiments should look. It is, essentially, the world's most expensive game of Marco Polo.
Vami's and Zhang's results are published in the journal Progress in High Energy Physics (PHEP).
The possibility of producing black holes at the LHC emerged roughly two decades ago. Physicists proposed that if enough energy were concentrated into an extremely small region, and if extra spatial dimensions (which are already required in string theory) exist, then quantum black holes might form during the trillions of proton-proton collisions created by the accelerator.
These objects would be nothing like the enormous astrophysical black holes found throughout the universe.
"They wouldn't stick around very long - if you made one, it would disintegrate immediately," said UCSB physics theorist Steven Giddings, an expert in the paradoxical implications of combining quantum mechanics with gravity, and one of a few scientists at the time who proposed that under certain conditions these tiny voids in spacetime could exist.
When scientists first discussed the possibility, the idea became widely misunderstood. Public concerns focused on the possibility that the LHC might create stable black holes, even though the quantum black holes being considered by physicists would disappear almost instantly.
"People were more focused on the classical behavior of black holes," said Giddings, referring to those massive voids in spacetime, areas of extreme gravity that can eat whole stars, grow, merge. The kind that would ruin your afternoon rather than evaporate before you noticed.
The hypothetical black holes produced at the LHC would instead arise from proton-proton collisions combined with the effects of extra spatial dimensions that have never been observed.
Hidden Dimensions Could Make Gravity Stronger
Creating any black hole requires squeezing a large amount of energy into an extremely small region.
"So what do you need to make a black hole? Well, you have to compress some energy into a really small volume," Giddings explained.
That "really small volume" might extend through two or more hypothetical spatial dimensions that are too small for humans to detect within our 3 + 1 dimension reality. Such extra dimensions have been proposed as one possible answer to the hierarchy problem, a longstanding question in physics that asks why gravity is dramatically weaker than the other fundamental forces.
One possibility is that gravity is not intrinsically as weak as it appears. Instead, some of its strength could be "leaking" into these extra dimensions. If that were true, the Planck scale could be much closer to the energy scales physicists can experimentally reach.
"Basically, the gravitational force gets stronger, faster, as you go to shorter distances," Giddings said.
But stronger gravity alone would not be enough. Scientists would also need to concentrate enormous energy into an exceptionally small volume. That is where the Large Hadron Collider becomes important.
The LHC accelerates protons to tremendous energies before smashing them together. These collisions give physicists access to extremely small distance scales.
"At the LHC, we're colliding particles at extremely high energy, which corresponds to tiny distance scales," Incandela said. "As with microscopy, higher energies mean smaller wavelengths, allowing one to probe smaller distances."
Researchers are probing scales as small as 10-20 meters at the LHC, a distance that is to an atom, what an atom is to a human.
"The extra dimensions wouldn't have to be that small," Incandela continued, "meaning that the LHC proton-proton collisions could be affected by them."
If gravity became sufficiently strong at those scales and enough energy were concentrated into a tiny enough region, spacetime could theoretically fold in on itself and produce a quantum black hole.
Safety concerns surrounding this idea were eventually addressed through detailed reports and comparisons with ultra-high-energy cosmic rays. These naturally occurring particles have been striking Earth's upper atmosphere and other astronomical objects at immense energies without producing dangerous effects. Those comparisons showed that high energy particle collisions do not pose a black hole threat. Any quantum black holes produced under the proposed models would evaporate essentially immediately. Even so, their extremely brief existence might leave detectable traces in the particles produced as they decay.
Earlier searches by the ATLAS and CMS experiments failed to find such evidence, but those studies had access to much smaller datasets. With far more collision data now available, researchers could search at higher energies and increase their chances of seeing an exceptionally rare quantum black hole event if such events occur.
The search is ultimately connected to one of the biggest unresolved problems in modern physics.
"We have two big theories that describe nature," Tamas Vami said. "If you want to describe things that are small, you go to quantum field theory. We have the Standard Model to describe all the particles, and it performs exceptionally well in practice. And when you go to the very, very big you have general relativity that would describe how big and massive objects behave."
Physicists have spent decades trying to combine these frameworks into a single description of nature. The difficulty is that quantum physics generally describes extremely small objects, while general relativity becomes most important for very massive ones.
The goal is to somehow merge the two theories into a single, unified theory, Vami said, "and that's really hard to do because you don't often have a situation which is really tiny but also extremely heavy." Microscopic black holes could provide exactly that combination. They would be small enough for quantum effects to become important while also concentrating enough mass and energy for gravity to matter. A rare cosmic two-for-one.
Searching for the Signature of a Black Hole
The researchers analyzed CMS detector data collected between 2016 and 2018 and used two approaches to look for evidence of quantum black holes.
One involved a property called sphericity.
"You form a black hole, and it immediately disintegrates. But it has a very spherical decay signature, lots of things going in all directions."
Another clue would be an unusually large amount of energy in the particles produced by a collision.
"We know that black holes are very high energy," Danyi Zhang said. "So we basically just take the energy of these particles that are decay products of whatever was created in the collision and sum them together. And if the sum is large enough, we can say that this is the region where we are likely to find the signal."
Those unusual event patterns also provided an opportunity to use a new analytical method known as "phase-space distance," developed by UCSB particle theorist Nathaniel Craig and collaborators. The method works with a machine learning system called a Support Vector Machine, which helps researchers distinguish possible signal events from the enormous background of conventional high energy particle collisions.
In particle physics, "phase space" is a multidimensional mathematical representation incorporating properties such as space, time, energy and momentum into a description of a particle system.
"We developed the idea of the phase space between events, which can be combined with SVM to help the search," Craig said.
The method converts the distances between events into a single measurement called an SVM score. Events with larger scores are more likely to resemble the signal scientists are searching for. This study marked the first time the phase space distance method had been used in a particle physics data analysis.
"We compared phase space distance with the sphericity variable and our conclusion is that phase space distance outperforms sphericity," Zhang said.
The approach also differs from some "black box" machine learning systems because it is supervised. Researchers can examine the mathematics that produced the result rather than simply accepting an unexplained output - a novel concept in an era of AI doing things for reasons nobody can quite articulate.
The search ultimately found no evidence for quantum black hole production. Based on the theoretical models examined, the result means quantum black holes are unlikely up to about 12 TeV (Tera-electron volts). It also constrains certain theories involving extra spatial dimensions. Those restrictions are scientifically valuable because they eliminate parts of the range in which these theoretical models could operate.
String theory, for example, assumes a total of 10 dimensions.
"But these measurements say that, assuming the parameters of the theories we considered, you cannot have more than two," Vami said.
"Theories don't predict one exact answer," Zhang added. "They predict a whole range of places a particle could be hiding. Each search clears out part of that range and says 'not here,' and over time the map of where new physics could still be, shrinks."
That process of elimination has repeatedly played an important role in particle physics. The Higgs boson was discovered in 2012 only after decades of experiments gradually excluded one energy region after another. By eliminating possibilities, physicists can improve their theories, develop new models and design better experiments and detectors.
For now, the hierarchy problem is still unresolved. Without extra dimensions, Giddings estimates that particle collisions would need to reach roughly a million billion times the energy currently achieved at the LHC to produce even the smallest black holes, which would have masses measured in micrograms. So, a project for the next collider. Or the one after that.