Physicists at the University of California, Santa Barbara (UCSB), working within the international collaboration at the Large Hadron Collider (LHC) at the European Center for Nuclear Research (CERN), have pushed the boundaries of particle physics into new territory. Their latest research, centered on the hunt for microscopic, quantum-scale black holes, has provided some of the most rigorous constraints to date on the existence of these elusive, hypothetical objects.
These proposed black holes would be radically different from the gargantuan, star-consuming giants that populate the cosmos. They would be extraordinarily small, possessing life spans that end almost as soon as they begin. Yet, despite their fleeting existence, their potential discovery at the LHC could offer a gateway to addressing some of the most profound, unanswered questions about the fabric of spacetime, the nature of gravity, and the fundamental structure of the universe.
“Had we found evidence, we could have begun to directly study quantum gravity,” explained Tamas Vami, a researcher in the Compact Muon Solenoid (CMS) experiment. Vami, who is conducting his postdoctoral research under the guidance of UCSB physics professor Joe Incandela, emphasized the stakes of the endeavor. “It is a vital step toward the grand ambition of unifying all known fundamental forces—a goal that has remained the holy grail of physicists for more than a century.”
While the search did not uncover direct evidence of these quantum black holes, the study serves as a masterclass in the scientific process. In the high-stakes realm of particle physics, a null result is far from a failure; it is a significant contribution to the collective understanding of the universe. By failing to detect the signatures of these objects, researchers have successfully "excluded" them from specific energy ranges, effectively mapping out where they cannot exist.
“It is not a dead-end,” noted Danyi Zhang, a graduate student researcher in the Incandela Lab. “The result is an exclusion limit, which is a real, publishable statement: ‘If this thing existed with these properties, we would have seen it. We didn’t, so we can rule it out here.’ That is genuine knowledge about how the universe works.”
Why Missing Black Holes Still Matter
One of the most persistent enigmas in fundamental physics is the staggering discrepancy between the scale of the world we experience and the Planck scale—the fundamental energy level where gravity is expected to behave according to the laws of quantum mechanics. Some theorists have long proposed that the existence of "new physics" or an undiscovered symmetry might bridge this gap, potentially manifesting at energy levels reachable by the LHC.
Years of experimentation have steadily narrowed the field, eliminating many theoretical possibilities. The continued absence of clear signals for new physics at the LHC has presented a challenge, yet history suggests this is part of a recurring cycle. Periods where existing theories struggle to account for observations have historically served as the catalyst for revolutionary frameworks, such as Albert Einstein’s theory of general relativity. For the UCSB team, each null result is a necessary piece of the puzzle, reducing the number of viable possibilities and guiding the scientific community toward where future, more powerful experiments should look. The results of this specific inquiry, led by Vami and Zhang, have been published in the journal Progress in High Energy Physics (PHEP).
Could the LHC Create Tiny Black Holes?
The concept of generating black holes at the LHC first gained traction approximately two decades ago. Physicists hypothesized that if a sufficient amount of energy could be compressed into an incredibly small region—and if the extra spatial dimensions suggested by string theory were a reality—then quantum black holes might form during the trillions of proton-proton collisions occurring within the particle accelerator.
These objects would bear no resemblance to the astrophysical black holes that anchor galaxies. “They wouldn’t stick around very long—if you made one, it would disintegrate immediately,” said UCSB physics theorist Steven Giddings, an expert on the implications of combining quantum mechanics with gravity. Giddings was among the few scientists who originally proposed that such tiny voids in spacetime could theoretically exist under specific conditions.
When the concept was first introduced, it was frequently misunderstood by the general public. Concerns were raised that the LHC might create stable black holes, ignoring the reality that the quantum-scale black holes being studied by physicists would evaporate almost instantaneously. “People were more focused on the classical behavior of black holes,” Giddings said, referring to the massive, gravitational voids that consume stars and merge over cosmic timescales. In contrast, the hypothetical LHC black holes would be a byproduct of high-energy collisions coupled with the potential existence of hidden spatial dimensions.
Hidden Dimensions and the Strength of Gravity
To create a black hole, one must concentrate a massive amount of energy into a minuscule volume. Giddings explains that this "really small volume" might, in a higher-dimensional universe, extend through two or more spatial dimensions that remain invisible to human perception.
These extra dimensions are a leading candidate to solve the "hierarchy problem," a long-standing question regarding why gravity is so dramatically weaker than the other fundamental forces. One hypothesis suggests that gravity is not inherently weak; rather, its influence is "leaking" into these extra dimensions. If this theory holds, the Planck scale could be far closer to the energy scales currently reachable by experimental technology. “Basically, the gravitational force gets stronger, faster, as you go to shorter distances,” Giddings said.
Colliding Particles at Extreme Energies
The LHC serves as the ultimate tool for this investigation because it accelerates protons to massive energies, allowing scientists to probe matter at incredibly small distance scales. “At the LHC, we are colliding particles at extremely high energy, which corresponds to tiny distance scales,” Incandela explained. “As with microscopy, higher energies mean smaller wavelengths, allowing one to probe smaller distances.”
Researchers are currently probing scales as small as $10^-20$ meters. To put this in perspective, the ratio of this distance to an atom is comparable to the ratio of an atom to a human. If extra dimensions exist, they do not necessarily need to be that small for LHC collisions to be impacted by them. Should gravity become sufficiently strong at those scales, and enough energy be packed into a tiny enough region, spacetime could theoretically fold, producing a quantum black hole.
Safety concerns, which once dominated the public discourse, were addressed by comprehensive reports and empirical data from ultra-high-energy cosmic rays. These particles have been striking Earth’s atmosphere for billions of years at energies far exceeding those of the LHC without causing any negative effects. These observations confirm that any quantum black holes produced by the LHC would dissipate essentially the moment they were created, leaving behind only the particles produced as they decay.
Where Quantum Physics Meets Gravity
The search for these signatures is deeply tied to the effort to unify the two pillars of modern physics: quantum field theory, which governs the subatomic world, and general relativity, which explains the behavior of the macro-universe. The fundamental challenge for physicists is that these two frameworks rely on different mathematical languages. “The goal is to somehow merge the two theories into a single, unified theory,” Vami said, “and that is really hard to do because you don’t often have a situation which is really tiny but also extremely heavy.” Microscopic black holes represent the exact intersection where quantum effects and extreme gravity would be forced to coexist.
To search for these signatures, the team analyzed data from the CMS detector collected between 2016 and 2018. They employed two distinct approaches, one of which focused on "sphericity." Because a black hole would theoretically disintegrate in a spherical pattern, looking for particles flying off in all directions is a key indicator. The second approach involved summing the energy of all particles resulting from a collision. “We know that black holes are very high energy,” Zhang noted. “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.”
Furthermore, the team utilized a new analytical method known as "phase-space distance," developed by UCSB particle theorist Nathaniel Craig and his collaborators. This method uses a machine learning system called a Support Vector Machine (SVM) to distinguish signal events from the "noise" of conventional high-energy collisions. This study marked the first time this method was applied to such an analysis. “We compared phase-space distance with the sphericity variable, and our conclusion is that phase-space distance outperforms sphericity,” Zhang said.
The Mystery Remains
Ultimately, the search revealed no evidence of quantum black holes. The data suggests that such phenomena are unlikely to occur up to the 12 TeV (Tera-electron volts) range. While this narrows the field of possibilities, the mystery of weak gravity remains. Without the existence of extra dimensions, Giddings estimates that particle collisions would need to reach energies a million billion times higher than those currently achieved at the LHC to produce even the smallest black holes.
In addition to the search for black holes, the researchers also hunted for "sphalerons"—theoretical, unstable configurations of particle fields that might help explain why the universe is dominated by matter rather than antimatter. Similar to the black hole search, no evidence was found, allowing for new, tighter constraints on these processes.
As the LHC undergoes upgrades to become the High Luminosity Large Hadron Collider (HL-LHC), the team is preparing for a new era of discovery. The future facility will provide vastly larger datasets, offering researchers even more opportunities to scrutinize the fundamental components of matter. For now, the work of Vami, Zhang, and their colleagues continues to sharpen the focus of modern physics, clearing away the fog of uncertainty to reveal where the answers to our universe’s deepest questions might still be hiding.
