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In 2008, there was a lot of excitement when the Large Hadron Collider (the LHC) was first turned on in Switzerland. The particle accelerator was the largest ever made, representing decades of work by thousands of people, and it promised to unravel some of the deepest mysteries of the universe.

But some people weren't so excited. In fact, they were afraid, because they worried the LHC would make a black hole and destroy the Earth. I mean, that wasn't true.

The fears were the result of some bad science reporting, and the LHC was never going to do that… but there was actually a kernel of truth to this idea. After all, there are some theoretical physicists who believed the LHC could make a black hole. It would just be a sub-microscopic one that would fizzle out instantly.

But while the accelerator hasn't found any of these micro black holes yet, it's still thought that maybe the next generation of particle accelerators will be able to. And if they can, that would provide evidence for some fascinating ideas in theoretical physics. We're talking about stuff like extra spatial dimensions.

The foundation of this idea is that black holes aren't just things out in space; they're a natural consequence of Einstein's theory of general relativity. The theory tells us that matter warps the fabric of spacetime, and that the more matter there is in a region of space, the more it warps its surroundings and draws nearby objects closer. If there's a lot of matter stuffed into a very small volume, then space becomes so warped that it becomes impossible to move away from the matter if you get too close.

That is a black hole. And while they usually form when huge stars collapse, they can technically form any time there's enough matter in a small area — so it's possible to get some really tiny ones. Weirdly enough, though, making a tiny black hole doesn't involve something really heavy, but something really light.

And that's where the Large Hadron Collider comes in. In many of its experiments, the LHC smashes extremely light particles, protons, together really fast. Like, “just a hair short of the speed of light” fast.

That allows us to learn all kinds of things about what happens when particles collide. Like, we've seen some brand-new particles emerge from these collisions. But the important thing to know here is that those super-fast protons have a lot of energy for their size.

And that's a big deal when it comes to black holes. See, we almost always talk about black holes forming because of some amount of mass. But in the high-energy collisions of the LHC, mass and energy actually become interchangeable.

That's the point of Einstein's famous “E equals m c-squared.” It says that mass is proportional to energy. So, theoretically, if you get some super-fast-moving particles really close together, then the energy of all that motion in one place can act like a lot of mass and be enough to form a black hole. Just, y'know, a very tiny one.

The problem is… with our current understanding of physics, that cannot happen at the LHC. To make the lightest possible black hole, the particles would each need about 10 quintillion electron volts of energy. That's a one with 18 zeros after it.

And while the LHC is good — it can get particles up to 14 trillion electron volts — it's not that good. But! There's a reason some physicists are still thinking about all of this.

And it's because there's a catch here:. These calculations all assume that there are no problems with general relativity. And, well, we already know the theory has problems.

I mean, it is very good and it makes lots of good predictions, but technically, it also predicts it should be possible for places to have infinite density. And we know that's just not a thing. If infinities start popping up in your physics results, you know you probably pushed your theory past its limits.

So, what is likely happening is that general relativity is only approximately true, and there's a more correct theory we don't know about. That theory would produce different results when you have a lot of energy in a really small space, which would solve this “infinite density” problem. Right now, scientists think this new theory will probably be some sort of quantum gravity — a combination of quantum mechanics and relativity.

And there are already a few candidates. But because there is no direct evidence for them yet, scientists spend a lot of time probing the mathematics of these theories, looking for predictions they make that can be tested. And one prediction some of these theories make is that the space we live in actually has more than three spatial dimensions.

That would mean that in addition to being able to go forwards, up, and to the side, it'd be possible for things to go in a direction that's at right angles to all of those directions at once. If you're having a hard time visualizing that… well, yes. Because to our eyes, that's clearly not the world we live in.

There is no direction that we can see that's at right angles to all three at once! So if extra dimensions do exist, they have to be extremely thin. This is similar to how a piece of paper is a 3D object, but if you don't look closely, it's basically 2D.

Only when you zoom in can you see that very thin extra dimension. So, in some quantum gravity theories, the extra dimensions can be up to about a millimeter in size — but in those cases, only gravity can interact with them. That means that if you measured anything except gravity at those scales, like, say, the strength of a magnet, it would behave as it does in a 3D world.

But if you measured the strength of gravity at those scales, it would behave as it does in a higher-dimensional world, and that behavior would be different. Why this “gravity only” rule? Well, if anything else interacted with this extra dimension, we would already know about it!

On that kind of tiny scale, we've seen every other force of nature work as-predicted, but we have not been able to test gravity — that's just because it's so weak compared to the other forces, not because of anything to do with relativity. So we're not sure what gravity is like in those situations — and it could very well act differently. Now, the nice thing is, if gravity were interacting with other dimensions like this, it'd be pretty easy to tell.

See, in the 3D world we live in, gravity follows what's called the inverse square law: If you decrease the distance between two objects by half, the gravitational force between them increases by a factor of four. But in a world with more dimensions, it's no longer the inverse square law. In, say, a 9D world, cutting the distance in half might mean increasing the force by a factor of 256.

So if there are extra dimensions smaller than one millimeter, then below that scale, the gravity between colliding objects gets a lot stronger a lot more quickly. And that means good things for black holes in the LHC. It can smash particles together with really high energy, which means they get pushed really close together.

And if the pull of gravity were stronger on tiny scales, it would be easier to push lots of energy really close together, which is what you need to make a black hole. In fact, depending on which theory you use, it's thought that if enough extra dimensions exist, you may “only” need to smash protons together at around 10 trillion electron volts to get a black hole. And that's an energy that's reachable with the LHC!

So far, though, the search hasn't been promising. One paper from 2016 reviewed LHC data with collisions at up to 13 trillion electron volts, and found no evidence that black holes were being made that way. Then, building on that, a 2018 paper found that, at those levels, gravity wasn't behaving differently due to extra spatial dimensions, either.

But there is a lot we don't know about these theories. It could be that the energy we need is just a bit higher than what we have now, meaning the LHC or a future replacement could reach it. That could give us the first experimental evidence for quantum gravity!

And, as a very nice bonus, it could also teach us something about black holes themselves. See, in the 1970s, Stephen Hawking predicted that all black holes — from big ones to tiny ones you'd see in the LHC — should release energy in what's called Hawking radiation. And while most physicists are completely confident that this is true, no one has been able to observe it.

That's because more massive black holes, like the ones you'd study in space, should emit radiation that's way too faint to see with telescopes. But according to Hawking, less massive black holes should release hotter radiation that's easier to detect. So, if you had an itty-bitty black hole pop up in your particle accelerator… you would be able to measure it.

If the LHC made a black hole, it would be so small that it would radiate away all its energy in about an octillionth of a nanosecond. But there would still be enough radiation to detect with our instruments — and thus, provide the first definitive proof that Hawking radiation is real. So, even if the evidence isn't promising so far, there's a reason people are looking into this.

Because if there's any chance that the LHC could make a black hole, we would probably want to try to make it. Unfortunately, though, the evidence keeps piling up that it might not be possible — even with those extra spatial dimensions. And some of that evidence doesn't come from our big, fancy particle accelerator: It comes from nature.

All the time, high-energy particles from deep space, called cosmic rays, are hitting our atmosphere. And lots of them have energies even higher than the particles in the LHC. So if these black holes can be made, they should be forming in the upper atmosphere.

But we don't see them there. Or from collisions anywhere else in space. Based on these observations and some modeling assumptions, one 2019 paper predicted that extra dimensions can be ruled out up to the exa-electronvolt range.

That's almost a million times higher than the energies in the LHC, which means the prospects of seeing these micro black holes do not look so good. But other people have used different assumptions to say that a near-future LHC replacement could maybe see micro black holes. So while things aren't looking great, that doesn't mean the case is closed.

There are a ton of quantum gravity models out there, so there may be a way to test our hypotheses and theories down the road. And even if it doesn't work out in the end… Well, black holes or not, when the LHC is turned back on in 2021 after some maintenance and upgrades, people are hoping that it will discover all sorts of new things, so there are still plenty of reasons to get excited. Thanks for watching this episode of SciShow!

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