Scientists Hunt for Microscopic Black Holes at the LHC

Welcome to the Quarkside Quantum Physics Podcast, an exploration of the fundamental structure of reality, where quantum laws govern matter, energy, and information. Here, uncertainty is a feature, not a flaw, and understanding begins at the smallest scales.

You know, usually when, uh, particle physics comes up in conversation, it feels just incredibly abstract.

Oh, absolutely.

Yeah. Like it's all invisible fields and, you know, subatomic math, theoretical particles that really don't seem to touch our day-to-day lives at all. Right.

It feels very detached from reality.

Exactly. But then you look at the actual physical experiments happening right now, like at the Large Hadron Collider, and the reality sounds straight out of a Well, a sci-fi blockbuster.

It really does.

We're talking about scientists intentionally trying to create black holes right here on Earth.

Yeah, and phrasing it like that definitely captures people's attention and usually spikes their anxiety pretty significantly.

I mean, I can imagine. Which makes it the perfect place to start. So our mission today is to figure out the reality behind that crazy science fiction concept.

It's a fun one.

We are going to explore exactly how physicists are actively hunting for for these microscopic quantum black holes, why they are so determined to create them in the first place. And here's the real twist for you, how discovering absolutely nothing at all in these experiments is actually a massive scientific breakthrough.

Yeah, that's the part that really throws people.

Right.

So by the time we wrap this up, you're going to have a completely new perspective on how science actually progresses. You'll see how systematically ruling things out is just as powerful as a sudden eureka moment.

It's a profound shift in how we think about discovery, really. I mean, in the popular imagination, we always expect the dramatic finding.

The big headline.

Exactly. The new particle, the literal spark. But the absence of a spark, it can fundamentally rewrite our understanding of reality, forcing us to rethink the universe from the ground up.

Well, before we get into the mechanics of how scientists are actually looking for these black holes, we really have to address the giant, terrifying elephant in the room. Yes. Safety.

Yeah.

Because if I go tell my neighbor, hey, physicists are building a circular track to compress energy and make a black hole. The immediate logical response is, don't we all die? Right.

Right.

Want to just swallow the earth? Because of a classical black hole is essentially this like cosmic vacuum cleaners, swallowing stars and light and everything around it. What on earth is a quantum black hole?

Yeah, it's a bit of a public relations nightmare, honestly.

I bet.

But it's born out of a fundamental misunderstanding of the physics involved. So physicist Stephen Giddey laid out the framework for this distinction decades ago. He explained that to make a black hole, any black hole, you essentially just need to compress a certain amount of energy into a phenomenally small volume. But the black holes they're theorizing at the Large Hadron Collider, or the LHC, they're quantum black holes.

But they're different.

Very different. They're born from proton collisions and they are so infinitesimally tiny that they just do not behave like those cosmic vacuum cleaners you find at the center of a galaxy. Right.

So I imagine they don't have the gravitational pull to start sucking in the laboratory furniture or anything.

Thankfully, no.

But infinitesimally tiny doesn't sound entirely harmless either. I mean, a virus is tiny and it could do a whole lot of damage.

Sure. But the key difference here is their lifespan. If an accelerator manages to make a quantum black hole, it disintegrates the very instant it's created.

Like instantly, instantly.

Instantly. It evaporates in a flash of particles before it can consume any surrounding matter or grow at all. It's just a fleeting instantaneous blip that's gone before it even truly registers.

Okay, so it evaporates instantly. But I can already hear people saying, well, what if the math is wrong?

Naturally.

Like what if it doesn't evaporate? We're talking about unprecedented energies here.

Mm-hmm.

I would hope physicists didn't just, you know, do some math on a chalkboard and decide to whisk the entire planet out of theory.

They most certainly did not. The safety of the LHC was definitively proven by looking at high energy phenomena that have been happening since the universe began. Every single day, our upper atmosphere is bombarded by ultra high energy cosmic rays. These are shot out from supernovas and active galaxies.

Oh, wow.

Yeah. And they hit the Earth's atmosphere with energies far, far greater than anything the LHC could ever dream of producing.

So basically nature has been running this exact same high energy collision experiment on our atmosphere for billions of years.

Exactly. And the atmosphere hasn't been swallowed by a black hole yet.

Fair point.

Right.

Any quantum black holes created by these natural collisions evaporate instantly and And the exact same physical laws apply inside the artificial environment of the accelerator.

Okay, so the safety check is cleared. The universe itself is basically our proof of concept here.

Exactly.

But that brings up an even more confusing point, I think. If these quantum black holes literally blip out of existence the exact millisecond they're created, and we can't even keep them in a jar on a desk to study them, what is the point?

Why bother?

Yeah. Why are physicists so obsessed with making them?

Because creating one could solve one of the most agonizing mysteries in all of physics. It revolves around something called the hierarchy problem.

The hierarchy problem. Okay, I've heard physicists complain about this before. Is that basically their frustration over the fact that a tiny fridge magnet can overpower the gravitational pull of the entire Earth.

Yes, exactly.

Because the Earth is huge, but a little piece of metal on my fridge... easily defies it just to hold up a piece of paper. It seems wrong.

That is a brilliant illustration of the core issue. Gravity is unbelievably weak compared to the universe's other fundamental forces, like electromagnetism, which is what the magnet uses. Right. So the hierarchy problem specifically asks why there is such a massive gap between the energy scale of everything we see and interact with and the fundamental energy scale of the universe, which is known as the Planck scale.

Let's pause on the Planck scale for a second. If we think about energy scales, a AA battery is low energy, a lightning bolt is high energy. Where does the Planck scale fit into that spectrum?

I assume it's at the very top.

Yeah, it has to be, right?

It is.

Think of the Planck scale as the universe's ultimate maximum limit.

Okay.

It is the point where energy is so densely packed that the laws of physics, as we currently know them, just completely break down.

Wow.

The math literally stops working. So the enduring mystery is why the everyday forces we experience operate at such incredibly low energies compared to that ultimate maximum Planck scale.

So going back to my fridge magnet, gravity just seems ridiculously weak. I want to try an analogy here to see if I'm grasping this.

Go for it.

It's like hearing a massive booming rock concert from 10 miles away. Okay. To us, standing in our living room, the music sounds really quiet, barely a whisper. But the concert itself is actually deafening. What if gravity isn't actually weak, we just aren't standing in the right dimension to hear how loud the universe really is?

That captures the theoretical framework beautifully. That's exactly it. The leading idea is that gravity is not weak at all. It might be incredibly strong, but it's leaking into undiscovered extraspatial dimensions.

Extra dimensions, like beyond the 3D space we walk around in plus time.

Yes.

String theory requires these extraspatial dimensions to exist in order for its complex math to balance out.

Right.

Often, string theory assumes a totality of 10 dimensions. Now, it can be hard to visualize, but think of a telephone wire stretched between two poles.

Okay, I'm picturing.

So from far away, it looks like a one-dimensional line. You can only go left or right. Right. But if you are a microscopic ant walking on that wire, you realize it's actually a cylinder. You can walk forwards, backwards, and loop entirely around it. The extra dimension is curled up so tightly that you can only experience it if you are small enough.

Oh, that makes sense. So we don't notice these extra dimensions because we are way too big, just like someone staring at the telephone wire from a mile away. Yeah. But gravity notices them.

Exactly. As you probe shorter and shorter distances, gravity gets stronger. If gravity is leaking into those curled up extra dimensions, its effect grows dramatically stronger at those incredibly microscopic scales. It implies that the massive Planck scale could actually be much closer to our everyday energy scale than we realize, just hidden away in the microscopic geometry of the universe.

And this is where the Large Hadron Collider comes back into the picture. Because the LHC is basically a giant microscope.

Precisely.

If we want to look at those tiny distances, we need to smash things together incredibly hard.

Yeah. Physics professor Joe Incandela, who guides a lot of this research, he points out a fundamental rule of particle physics. Higher energies means smaller wavelengths.

Okay.

And smaller wavelengths allow you to probe smaller and smaller distances. The LHC is colliding particles at such extreme energies that they're probing distances down to 10 to the power of negative 20 meters.

10 to the negative 20. That number is so small it almost loses all meaning. Is there any way to even visualize a space that incredibly tiny?

To put it in perspective, that distance is to a single atom what a single atom is to a human being.

Oh man, my brain hurts. But that does put it in perspective. So we're taking an enormous amount of high energy mass and packing it into a space... that makes an atom look like a giant. Yes.

And at that infinitesimally small scale, the extra dimensions wouldn't even need to be that tiny for the proton collisions to be affected by them.

Wow.

So you combine that incredibly dense energy with the theoretically super strong concentrated gravity in that tiny environment.

And boom, space-time folds in on itself and a quantum black hole is born.

Exactly.

Which means we would finally find the bridge between classical physics, the heavy stuff governed by gravity and relativity, and the tiny stuff governed by quantum mechanics.

And unifying those two domains has been the ultimate goal of physicists for over a century. Quantum mechanics currently relies on the standard model, which you can think of as the ultimate instruction manual for all known subatomic particles.

Right, but the standard model notoriously leaves out gravity entirely.

It does. Creating a quantum black hole would finally force gravity and quantum mechanics onto the same stage, proving they can interact.

The stakes are unbelievably high. We're talking about unifying the fundamental forces of nature so we know why we want to find them. But here's the logistical nightmare. How do you actually hunt for something invisible that evaporates the exact millisecond it's born?

It's very difficult.

It's like trying to investigate a massive fireworks display that you didn't actually get to see. And your only option is to try and map out exactly where every single spark landed on the ground afterward to prove the firework ever existed.

That's a great way to look at it. Since you cannot see the black hole itself, you have to detect those exact sparks, its decay products. When that quantum black hole disintegrates, it shatters into a burst of particles. Researchers like Tomas Vami and Denny Zhang, working with data from the CMS detector, have been analyzing trillions of collisions recorded between 2016 and 2018 to find that specific burst pattern.

I would imagine a black hole's firework burst looks pretty unique compared to a standard particle collision. How do they separate it out from all the noise?

Traditionally, physicists look for two specific markers. The first is sphericity.

Sphericity. OK. Yeah.

When a quantum black hole disintegrates, it creates a highly spherical decay signature. The particles explode outward equally in absolutely all directions rather than spraying in a single cone or a straight line.

Which is what standard particle collisions often do, right? They spray in a line. So we're looking for a perfect 3D sphere of debris. And the second marker is... It has to be heat or energy, right?

High energy.

Yeah.

The researchers take all the particles resulting from the collision and sum their energies together.

Just add them all up. Right.

If the total sum of that energy is extremely large and the physical shape of the debris is highly spherical, you are looking at a region where a black hole signal is very likely.

But digging through trillions of proton collisions looking for spherical high energy bursts... That sounds like an impossible computing task. It's way worse than a needle in a haystack.

It is a monumental challenge. I mean, the complexity of it led particle theorist Nathaniel Craig and his collaborators to develop a revolutionary new method. They call it the phase-space distance method.

Phase-space. It sounds like a sci-fi work drive. What does phase-space actually mean in this context?

Phase-space is a mathematical multidimensional map. It doesn't just plot the physical location of a particle in three dimensions.

Okay, what else does it include?

It incorporates every dimension of space and time, plus the energy and the momentum of an entire system of particles, all into a single, highly complex mathematical point.

Wow. So it's not just mapping where the sparks landed on the ground. It's mapping the weight of the sparks, how fast they were falling, the trajectory they took, and exactly when they hit the ground, all combined into one giant equation.

Yes. They then take this vast phase space data and feed it into a machine learning tool called a support vector machine or an SVM.

I want to guess how this SVM works.

Sure, try it.

If they have all this complex data, is the vector machine basically drawing a literal line on a graph to separate the normal boring collisions on one side from the weird black hole looking ones on the other?

That is a highly accurate way to picture it, actually. In a simple 2D graph, you draw a line. But in face space, which has many dimensions, the SVM calculates a complex multidimensional boundary called a hyperplane.

A hyperplane.

It analyzes the distances between all these events and converts them into a single variable, an SVM score.

So like a grading system.

Basically. The larger the score, the further the event is across that boundary line, making it significantly more likely to be a true signal of a black hole rather than just background noise.

You know, whenever I hear machine learning or AI, my mind immediately goes to the kind of software where you feed data into a black box. It spits out an answer and nobody, not even the programmers, actually knows how the computer figured it out. Right. Is that what they're doing here?

No. And that is a crucial distinction. The support vector machine they use is a supervised machine learning tool. It is not a black box. The researchers can look under the hood at the underlying math and see the exact criteria the algorithm is using to draw that boundary line. It's fully transparent. And honestly, this project was the very first time this specific method was applied to particle physics data analysis.

That's awesome. Did this transparent AI beat the old way of just looking for spherical energy?

Oh, absolutely. The phase-space distance method heavily outperformed the traditional sphericity variable. It proved to be a highly sensitive, incredibly precise tool for sifting through all that noise.

Okay, so the stage is completely set. We have the LHC generating unimaginable energies. We have trillions of collisions. We have extra dimensions theoretically amplifying gravity. And now we have this cutting-edge transparent AI perfectly tuned to spot the exact decay signature of a quantum black hole.

So what did they actually find when they read the numbers?

They found nothing.

Wait, nothing?

Nothing. Up to about 12 teraelectron volts, or TV, they found absolutely no telltale signs of quantum black hole production.

Let's put 12 TV in perspective for a second. How much energy is that? One TV is roughly the kinetic energy of a single flying mosquito.

A mosquito. That does not sound like much energy at all.

It doesn't until you remember that they are compressing that mosquito's energy into into a space a trillion times smaller than a single grain of sand.

Oh, right.

The energy density is mind-boggling. But even up to 12 TV of that massively compressed energy, they saw no black holes.

I have to ask the exact question you're probably screaming at your device right now. We just went through all this mind-bending theory about dimensions and gravity. We built the ultimate phase-space AI detector just to get a null result.

I know. It sounds disappointing.

How on earth is finding nothing a win?

The frustration is completely valid, but in fundamental physics, null results are the true drivers of progress. Dania Zhang explained that this result is an exclusion limit.

An exclusion limit.

Yeah. An exclusion limit is a rigorous, publishable statement. It essentially declares, if this phenomenon existed with these specific properties, our tools are so precise, we definitely would have seen it. We didn't, so we can definitively rule it out right here.

So it's not a failure, it's genuine knowledge about how the universe actually works.

Exactly.

It feels a bit like playing a game of cosmic battleship. Every time you call out a coordinate at 12 TV and get a miss, you get to put a white peg on the board. You haven't found the ship yet, but you have fundamentally shrunk the ocean of where it can legally hide.

The battleship analogy is spot on. By placing those white pegs, you are aggressively narrowing down the possibilities. This systematic process of elimination is exactly how the Higgs boson was discovered in 2012.

Wait, really? It wasn't just a sudden discovery?

No, it was not a sudden lucky guess. It was the result of decades of painstakingly excluding one energy region after another until there was only one coordinate left on the board where it could possibly be.

So finding nothing here actually corners the remaining possibilities for everything else. Does this null result affect the theories we talked about earlier, like string theory and its 10 dimensions?

It puts very concrete limits on them. Because we didn't see black holes at these energies, it means gravity isn't ramping up as fast as some string theory parameters predicted.

Okay, so what does that mean for the dimensions?

Based on these measurements, you cannot have more than two extra dimensions at these specific scales. The researchers are literally forcing theoretical models of the universe to shrink their footprint.

Wow. We didn't find a black hole, but we forced string theory into a tighter box.

We did.

And they didn't just use this phase space method to look for black holes either, right? They did a sort of bonus hunt while they were sweeping the data.

Yeah, they also used these techniques to look for something called sphalerons.

Sphalerons. Let me guess, another subatomic particle that refuses to be found.

Actually, they are not particles. They're theoretically unstable configurations of particle fields. They are important because they would produce fairly spherical energy signatures, very similar to a quantum black hole.

Okay, so the AI could spot them easily. Right.

And if they exist, they could explain another massive mystery, the matter-antimatter asymmetry problem.

Oh, the question of why we have matter at all. Because the Big Bang should have created equal amounts of matter and antimatter, and they should have, well, completely annihilated each other.

Leaving absolutely nothing but pure energy behind. But matter survived, and we are here.

Thankfully.

Sphaleron transitions might explain why that imbalance happened. However, just like with the black holes, the researchers found a null result for sphalerons in this data, too.

So they put white pegs on those coordinates as well. They set strict limits on those theories, relentlessly shrinking the map of the unknown. You know, this whole process, it feels deeply applicable to all of us, well beyond the realm of high-level physics.

How so?

In our daily lives, you and I are constantly bombarded by information overload, wild theories and endless claims of what is supposedly true.

Oh, absolutely.

Finding out what isn't true or what doesn't work is just as valuable as finding the truth. right answer. Knowing how to systematically rule things out, that is practically a superpower for critical thinking.

Knowing what is false brings you one step closer to what is true. In physics, when you place enough white pegs on the board and exclude enough possibilities, you are eventually forced into a completely new paradigm.

Right.

I mean, Einstein's theory of relativity was born out of analogous conundrums, where existing physics could not explain the observations, and expected answers kept turning up null. Null results force us to think differently.

Where does this leave the hierarchy problem, then? We still don't know why gravity is so weak. We still haven't found the black holes.

The mystery remains entirely intact. But Stephen Giddings points out something incredibly provocative.

What's that?

He estimated that if there are absolutely no extra dimensions, if the universe is just the three plus one dimensions we experience, it would take particle collisions a million billion times the energy currently produced at the LHC to create even the smallest microgram size black hole.

A million billion times. Yeah. That is a leap in energy that almost defies imagination.

It is an astronomical hurdle. But experimentalists like Vami and Zhang have pushed the standard model to the extreme with the current data, and the search is not stopping. The LHC is currently shut down to undergo an ambitious set of upgrades.

Oh.

When it reopens, it will be the High Luminosity Large Hadron Collider, or the HL-LHC.

High Luminosity. So they are going to be able to see even deeper into the data.

Much deeper. The upgrades will allow researchers to study the fundamental components of matter in unprecedented detail, probing even further into those uncharted regions of phase space. The map is going to keep shrinking.

Which leaves you with an incredible thought to chew on. We are hunting for the fundamental nature of reality by systematically proving where it isn't. But what happens when the map of where new physics can hide shrinks all the way down to absolute zero? When we have ruled out every single current theory, every dimension... and every particle field we can currently imagine, what completely unimaginable new paradigm will we be forced to invent next?

Scientists Hunt for Microscopic Black Holes at the LHC

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