###### By Don Lincoln, Fermilab

## When it comes to black holes, we often hear how they devour things around them and have gravity so strong that not even light can escape. Terrifyingly though, there is a plausible theory that says that it is very much possible to make black holes in particle accelerators. Would these man-made subatomic black holes be stable?

### Three Dimensions and a Subatomic Ant

One of the mysteries of physics is just why gravity is so weak. We don’t know why it is so. However, an idea has been put forth that there are additional dimensions of space beyond the familiar three of left/right, up/down, and backward/forward. Now that is obviously a silly statement, because we quite clearly see that there are only three dimensions. But there’s an explanation, which is that, maybe the extra dimensions are very small. That’s kind of an odd idea, but we can kind of sketch it by analogy.

Let’s imagine a tightrope walker. They can go in one dimension. They can go forward or go backward. That’s it. Well, for a human. For an ant, things are different. For an ant, they can walk like humans can, but they can also walk around the rope. For ants, a rope is two dimensional, not one dimensional. There is the same long dimension that we inhabit, but there is also a much smaller dimension wrapped around the rope. And that’s the basic idea of extra dimensions—at each familiar point in space there could be one, two, three, or more tiny dimensions—too small for us to see, but maybe something that a subatomic ant, so to speak, could travel in.

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### Comparing One Versus Two Dimensions

Thus, extra dimensions are the first hypothesis. The second hypothesis is that maybe gravity can enter the small dimensions, but the other forces can’t. And if those two conjectures are true, then we have a possible explanation for why gravity is so weak.

Let’s just take a moment to think about that and compare something familiar, which is one dimension versus two, both of them big. In order to understand this better, let’s imagine a big empty and flat plane. Let’s suppose there’s a long and straight road and we bring helicopter load after helicopter load of people and have them disembark at the same place. If the people are forced to walk along the road and we’re standing alongside the road, we’ll see lots of people go by. There’s just no other place for them to go.

### A Similar Exercise for Gravity

But suppose the people can walk off in whatever direction they want. If we stand in the same spot by the side of the road, we might not see many people pass us at all. Those people will wander off this way, or that way but none of those paths might be near us. In both cases, there are the same amount of people brought in by helicopter, but the people simply have more places to go if they can move in two dimensions.

That’s a case of comparing one versus two dimensions, and it works roughly the same way for gravity and extra dimensions. If gravity can access more dimensions than the other forces, then gravity won’t have to pass our way, so to speak, and for us, gravity will seem weak, even though it isn’t.

### A Subatomic Black Hole

If gravity is really strong but just seems weak, it is because it can go into more dimensions. So then, what happens if we are able to be small enough to see the other dimensions? Well ,then, we’ll see gravity’s true nature, which is to be strong; and, if we see strong gravity, we could then see a microscopic manifestation of the strongest gravity we know—we could see a subatomic black hole.

And that’s what scares people. After all, they have heard that black holes suck up the matter around them and they grow. Thus, they imagine that if we make a subatomic black hole, it will eat nearby matter and eventually consume the entire Earth. And that, of course, would be terrifying.

### Hawking Radiation

Fortunately, there are counter arguments to these worries. For instance, Stephen Hawking realized that black holes can radiate via, what we now call, Hawking radiation. According to it, the bottom line is that small black holes evaporate very quickly from Hawking radiation. Big black holes evaporate much more slowly. So, according to Hawking, even if subatomic black holes are real, they evaporate away before becoming dangerous.

Thus, in conclusion, it’s worth remembering that the idea of subatomic black holes depends on a couple of hugely speculative conjectures, and these are that:

- extra dimensions exist
- only gravity can enter the extra dimensions

These are both pretty unlikely. Not impossible, of course, but unlikely. So, they’re probably not real. But if they’re possible, we do need to think about if there is any danger at all.

### Common Questions about a Subatomic Black Hole

**Q: What is the basic idea behind the theory of there being extra dimension**s?The basic idea of extra dimensions—at each familiar point in space there could be one, two, three, or more tiny dimensions—too small for us to see, but maybe something that a subatomic ant, so to speak, could travel in.

**Q: What would happen if we were small enough to see the other dimensions?**If we were small enough to see the other dimensions, we would see gravity’s true nature, which is to be strong; and, if we see strong gravity, we could then see a microscopic manifestation of the strongest gravity we know—we could see a subatomic black hole.

**Q: What does Hawking radiation**propose?According to it, the bottom line is that small black holes evaporate very quickly from Hawking radiation. Big black holes evaporate much more slowly. So, according to Hawking, even if subatomic black holes are real, they evaporate away before becoming dangerous.