These hypothetical entities, if they could ever be produced in a laboratory setting, would be extraordinarily small and remarkably short-lived. Yet, their potential creation could provide physicists with the empirical keys needed to address some of the most profound, long-standing questions regarding the fundamental nature of spacetime and gravity. Beyond the search for miniature black holes, this extensive investigation also offered researchers a valuable opportunity to test and validate a novel analytical technique designed to uncover rare and previously unknown particles.
"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."
Although the exhaustive search ultimately did not uncover empirical evidence of quantum black holes, the endeavor is far from a failure. In the rigorous domain of particle physics, failing to detect a predicted phenomenon still yields vital scientific information by systematically ruling out regions where it could theoretically exist.
"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."
Why Missing Black Holes Still Matter
One of the most perplexing puzzles in fundamental physics involves the enormous, seemingly inexplicable difference between the macroscopic scale of the universe we experience daily and the Planck scale, the fundamental energy scale intrinsically associated with quantum gravity.
To bridge this massive conceptual gap, some theoretical physicists have proposed that new physics or an undiscovered symmetry could account for the discrepancy. Crucially, some of those theoretical effects might operate at energy levels that the LHC is actually capable of reaching.
Over years of continuous experiments, scientists have already systematically eliminated a wide array of theoretical possibilities. The ongoing absence of clear, unmistakable signs of new physics at the LHC has undeniably become a major hurdle for contemporary researchers. However, similar historical precedents have occurred before in the history of science. Periods during which existing theories struggled to reconcile with new observations have frequently catalyzed the development of radically new frameworks, most notably including Albert Einstein’s theory of relativity.
For that exact reason, the researchers emphasize that null results are a vital, indispensable component of scientific progress. Each exclusion effectively reduces the total number of viable possibilities, helping to guide and determine precisely where future experiments should focus their efforts.
The findings compiled by Vami and Zhang have been formally published in the academic journal Progress in High Energy Physics (PHEP).
Could the LHC Create Tiny Black Holes?
The provocative possibility of producing microscopic black holes at the LHC first emerged roughly two decades ago. At the time, physicists proposed a compelling hypothesis: if a sufficient amount of energy could be concentrated into an extremely localized region, and if extra spatial dimensions—which are already heavily required by string theory—actually exist, then quantum black holes might briefly form during the trillions of high-speed proton-proton collisions generated inside the particle accelerator.
These microscopic objects would bear no resemblance whatsoever to the enormous, highly destructive astrophysical black holes scattered throughout the cosmos.
"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. Giddings was among the small group of scientists who initially proposed that, under very specific conditions, these tiny voids in the fabric of spacetime could theoretically exist.
When scientists first discussed the concept publicly two decades ago, the idea became widely misunderstood. Public concerns frequently centered on unfounded fears that the LHC might inadvertently create stable, runaway black holes, completely overlooking the fact that the quantum black holes considered by physicists would vanish almost instantaneously.
"People were more focused on the classical behavior of black holes," said Giddings, referring to those massive cosmic voids characterized by extreme gravitational pull capable of consuming entire stars, growing exponentially, and merging with other objects.
Instead, the hypothetical black holes produced at the LHC would arise directly from ultra-high-energy proton-proton collisions, facilitated by the presence of extra spatial dimensions that have remained entirely hidden from human observation.
Hidden Dimensions Could Make Gravity Stronger
Creating any form of black hole fundamentally requires compressing a massive amount of energy into an exceptionally minute volume.
"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 extremely small volume might extend outward through two or more hypothetical spatial dimensions that are far too small for human instruments or perception to detect within our standard three-plus-one dimensional reality.
Such extra dimensions have long been proposed as a potential solution to the hierarchy problem, a longstanding conundrum in physics asking why gravity is dramatically weaker than all other fundamental forces in nature.
One leading hypothesis suggests that gravity is not intrinsically as weak as it appears in our everyday lives. Instead, a portion of its true strength could be steadily "leaking" into these hidden extra dimensions. If that hypothesis holds true, the fundamental Planck scale could actually be much closer to the energy scales that modern physicists can experimentally access in a laboratory.
"Basically, the gravitational force gets stronger, faster, as you go to shorter distances," Giddings noted.
Yet, possessing stronger gravity alone would not be sufficient. Scientists would simultaneously need to concentrate enormous amounts of energy into an exceptionally small volume of space. This precise requirement highlights the irreplaceable scientific value of the Large Hadron Collider.
Colliding Particles at Extreme Energies
The LHC operates by accelerating beams of protons to tremendous, near-light speeds before violently smashing them together. These high-energy collisions grant physicists direct access to extraordinarily 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."
Today, researchers are probing physical scales as small as $10^-20$ meters at the LHC—a distance ratio roughly comparable to comparing the size of an individual atom to the scale of an entire human being.
"The extra dimensions wouldn’t have to be that small," Incandela continued, meaning that the high-energy proton-proton collisions inside the LHC could realistically be influenced by them.
If gravity indeed became sufficiently strong at those minute scales, and if enough energy were successfully concentrated into a localized region, spacetime could theoretically fold in on itself and generate a quantum black hole.
Early safety concerns regarding this bold theoretical concept were rigorously addressed and thoroughly dismissed through detailed scientific reports and direct comparisons with ultra-high-energy cosmic rays. These naturally occurring particles have continuously struck Earth’s upper atmosphere and other astronomical bodies at immense energies throughout cosmic history without causing any hazardous or catastrophic effects.
Those comprehensive comparisons conclusively demonstrated that high-energy particle collisions pose no genuine black hole threat. Any quantum black holes produced under these theoretical models would evaporate almost immediately upon formation. Even so, their extraordinarily brief existence might still leave behind distinct, detectable signatures in the secondary particles produced as they decay.
While earlier searches conducted by the ATLAS and CMS collaborations failed to unearth such evidence, those initial studies were limited by much smaller datasets. With far more collision data now successfully accumulated, contemporary researchers have been able to search at significantly higher energy levels, thereby dramatically increasing their chances of identifying an exceptionally rare quantum black hole event if such phenomena actually occur.
Where Quantum Physics Meets Gravity
This high-stakes search is ultimately tied to one of the most significant, enduring unresolved problems in contemporary 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."
For decades, physicists have endeavored to merge these two distinct theoretical frameworks into a single, cohesive description of nature. The fundamental difficulty lies in the fact that quantum mechanics is specifically designed to describe extremely small objects, whereas general relativity governs massive, cosmic bodies.
The ultimate goal is to somehow unify the two theories into a single framework, Vami noted, adding that "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 potentially provide precisely that elusive combination. They would be diminutive enough for quantum mechanical effects to become dominant, while simultaneously concentrating enough mass and energy for gravity to play a critical role.
Searching for the Signature of a Black Hole
To investigate these theories, the research team meticulously analyzed CMS detector data collected between 2016 and 2018, employing two distinct methodological approaches to search for evidence of quantum black holes.
One primary approach relied on measuring a geometric property known as 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," researchers explained.
Another critical clue would involve detecting an unusually large concentration of energy among the particles generated by a single collision event.
"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."
These unusual event patterns also provided a unique opportunity to deploy a brand-new analytical technique known as "phase-space distance," which was recently developed by UCSB particle theorist Nathaniel Craig and his close collaborators.
This innovative method operates in tandem with an advanced machine learning architecture called a Support Vector Machine (SVM). Together, these tools assist researchers in effectively distinguishing potential signal events from the massive, overwhelming background noise of conventional high-energy particle collisions. In particle physics, "phase space" represents a multidimensional mathematical formulation incorporating physical properties such as space, time, energy, and momentum to comprehensively describe a system of particles.
Machine Learning Joins the Search
"We developed the idea of the phase space between events, which can be combined with SVM to help the search," Craig said.
This analytical method systematically converts the calculated distances between individual events into a single quantitative measurement referred to as an SVM score. Events yielding higher scores are significantly more likely to closely resemble the specific theoretical signal scientists are striving to find.
This recent study marked the historic first application of the phase space distance method in an active particle physics data analysis.
"We compared phase space distance with the sphericity variable and our conclusion is that phase space distance outperforms sphericity," Zhang noted.
Furthermore, this machine learning approach differs fundamentally from traditional "black box" algorithms because it remains strictly supervised. Researchers retain the ability to thoroughly inspect the underlying mathematics that generated a specific result, rather than simply accepting an unexplained, automated output.
New Limits on Quantum Black Holes
Ultimately, the exhaustive data analysis uncovered no empirical evidence confirming the production of quantum black holes.
Based on the specific theoretical models examined in the study, the negative result indicates that quantum black holes are highly unlikely to exist up to an energy threshold of approximately 12 Tera-electron volts (TeV). Additionally, the findings successfully constrain certain theoretical frameworks that incorporate extra spatial dimensions.
These strict limitations hold immense scientific value because they effectively eliminate broad ranges where these speculative theoretical models could otherwise operate. String theory, for instance, typically presumes the existence of a total of ten physical dimensions.
"Pero 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."
This continuous process of elimination has repeatedly played a foundational role in the advancement of particle physics. Notably, the historic discovery of the Higgs boson in 2012 occurred only after decades of rigorous experiments systematically excluded one energy region after another until the particle had nowhere left to hide. By eliminating incorrect possibilities, physicists are able to refine their core theories, construct more accurate models, and design vastly superior experimental detectors.
The Mystery of Weak Gravity Remains
For the time being, the broader hierarchy problem remains completely unresolved.
Without the existence of extra dimensions to alleviate the mathematical strain, Giddings estimates that particle collisions would need to achieve approximately a million billion times the energy currently attainable at the LHC to produce even the absolute smallest black holes—objects that would possess microscopic masses measured merely in micrograms.
"Theorists will continue to generate ideas and maybe we will do better in figuring things out without experimental data, but it will be difficult," Giddings said.
"The best guide is experimental data, and that’s what we’d really like to have," he added, emphasizing the need to further investigate quantum gravity, which he regards as "the most profound problem in theoretical physics."
By testing particle collisions at some of the highest energy thresholds currently available to humanity, Vami and Zhang have successfully pushed the established Standard Model toward its absolute limits. Their findings provide fresh, essential guidance for future searches targeting quantum black holes as a potential solution to the persistent hierarchy problem. Equally important, the research demonstrates that the phase space distance method can be successfully deployed on a broader scale to hunt for unfamiliar particles, unusual interactions, and other exceedingly rare physical phenomena.
Searching for Another Exotic Phenomenon
As part of their comprehensive study, the researchers also leveraged the same extensive dataset to hunt for another theoretical entity known as a sphaleron.
Sphalerons are not physical particles in the traditional sense; rather, they represent theoretical, highly unstable configurations of particle fields that—much like quantum black holes—would be anticipated to generate relatively spherical energy decay patterns upon formation.
Crucially, sphalerons could potentially help resolve another massive cosmic mystery: why the observable universe is overwhelmingly dominated by matter.
According to our current physical understanding, the primordial Big Bang should have generated equal quantities of matter and antimatter. Consequently, those two opposing forms of matter ought to have completely annihilated one another in the early universe, leaving behind nothing but pure energy rather than the matter-filled cosmos we inhabit today. This glaring imbalance is widely recognized as the matter-antimatter asymmetry problem.
Despite a thorough examination, the researchers found no empirical evidence of sphaleron processes within the collision data. The absence of these signatures allowed the team to establish strict new limits on how frequently particle interactions might involve sphaleron transitions.
A More Powerful LHC Is Coming
Future experimental campaigns promise to push these exploratory boundaries considerably further.
"We will be putting constraints on what theories can be true," said Zhang, who is eagerly anticipating the acquisition of fresh data once the Large Hadron Collider completes its current, extended operational shutdown.
The accelerator is presently undergoing an ambitious set of upgrades. The upcoming High Luminosity Large Hadron Collider (HL-LHC) will equip scientists with vastly larger datasets and significantly enhanced luminosity, providing unprecedented opportunities to detect extremely rare physical events.
These future experiments will ultimately empower researchers to study the fundamental components of matter in vastly greater detail, potentially uncovering complex processes that illuminate how our universe evolved in its earliest moments.
