What does it mean if the universe has extra dimensions?

In one of the most memorable interviews of my career thus far, I was at my desk, head in my hands, talking on the phone to a physicist about extra dimensions. I was trying to understand what it means for an entire dimension to be small. After several minutes of back and forth, I said, “Not small like a jellybean, though, right?”, as I mentally blocked out the laughter of the colleague sitting across from me. The answer? Well, it’s complicated.

We invoke “extra” dimensions in physics with some regularity, but very rarely do we talk about what that actually means. They often come into play when string theory is mentioned, a radical set of ideas that suggests everything is made of unbelievably tiny strings. As those strings vibrate, they would produce effects we interpret as atoms, electrons and quarks. I tend to be a little leery of string theory, mostly because the ideas within it range from difficult to impossible to practically test, which gives me pause. And it relies on extra dimensions to hide those curled up strings from our view, which is just hard to get my head around.

Sure, there are one or two well-known explanations out there, such as the satirical Flatland novella from 1884 (written by Edwin Abbott under the pseudonym “A Square”) and the very fun educational videos based on it. Those are largely allegory and metaphor – still, they offer a way to comprehend what encountering another dimension might feel like from a perspective used to living with four, even if they’re not really about the additional dimensions in themselves. Most of the time, when we actually dig into what it might mean for there to be extra dimensions, there’s a whole lot of hand waving, perhaps a little confusion, and then we move on.

But extra dimensions, if they are real, could legitimately solve some major problems in physics and cosmology, so it’s worth trying to understand them. The classic example of one of these problems is gravity, which is far weaker than any of the other fundamental forces. We don’t know why that is, and it has been proposed that some gravity may be leaking away into extra dimensions, weakening its effects in our own perceivable universe. More recently, something similar has been proposed to explain new measurements of dark energy that indicate it may be weakening over time. If there are extra dimensions that are changing in size over time, that would trickle down into the dimensions we know and experience – three of space and one of time – shifting the energy balance of the universe.

Plus, even if there are some questions in my mind about whether extra dimensions are likely to exist, the idea is interesting.

Probably the most easily understood kind of extra dimension is the type used in Flatland, a story about geometric shapes living in a two-dimensional world. They inhabit a flat sheet and scoot around it like pucks sliding on ice. From their flat perspective looking out the edge of their shape, they see every other shape as simply a line.

But some sort of Lovecraftian extra-dimensional horror that has three dimensions (say, a human) can view them from above or below – not only to see that they are, in fact, shapes and not lines, but also to see whatever innards lie within them. Living in a 3D world, you could pluck one of the shapes out of its plane and turn it on its side; the other shapes still left in Flatland would just see a strange cross-section of its interior that intersects with their plane instead of the steady line of its edge.

If you extrapolate that to the real world, with its three dimensions of space and one of time, an even higher-dimensional horror would be able to see inside you and perhaps even pluck you out of your familiar space-time into its dimension. Were that to happen, those of us left behind would see a shifting cross-section of your true five-dimensional shape as your body moved.

One variant on this kind of extra dimension is the braneworld hypothesis, which is the idea that our own universe is the edge – the membrane – of a higher-dimensional cosmos. First proposed in 1999, the idea has come back from relative obscurity in the past several years as one of very few plausible ways to wrestle our reality into the constraints of string theory.

In one version of this idea, the membrane constraining our universe is the edge between a higher-dimensional space – called hyperspace – and nothing. That would put us right on the border of a cosmic void. Fittingly, this is called an end-of-the-world brane. The fundamental particles that we’re familiar with would then be the very ends of five-dimensional strings that live in hyperspace, but we’d never be able to see the entire string, just like a triangle in Flatland would never be able to see any shape more complicated than a line.

That brings us to five dimensions, but there could be many more, and they wouldn’t all have to be as expansive as those of the braneworld. In fact, they could very well not even be space-like; imagine that time can move sideways and not just forwards and backwards. (Let’s not get into the details on that.) And some dimensions could be, well, small like a jellybean – or even much smaller than that.

You can think of dimensions as a set of glass matryoshka dolls, each nesting inside the larger ones, where you can only access the doll that represents the number of dimensions you inhabit – presumably, four – and those within it. The jellybean dimensions – the ones that would be physically small, not small in terms of being lower dimensional – are like bubbles in the glass. They may seem quite different, but, just like one of the matryoshka dolls, one of these bubbles encloses a space. It’s just a tiny space, like a sort of pocket universe.

Could you enter this pocket universe? Not unless you’re small like a jellybean, or rather small like a photon, as these dimensions are thought to be extremely minuscule. The reason for this is that we haven’t seen any – if they were large, they’d be much easier to detect. But it wouldn’t be completely impossible to spot a small dimension. Think of shining a light through your glass matryoshka dolls. Any bubbles would create distortions and reflections in the light. An actual extra dimension would do something similar.

So, say a gravitational wave were to travel through one of those bubbles in our universe. It would emerge with a slightly distorted shape and, with a powerful enough detector, we could measure that distortion. There are other ways to search as well, involving tiny quantum effects and exotic particles that we think could only be produced in extra dimensions.

Researchers at gravitational wave detectors, particle colliders and even regular telescopes are hunting for these subtle hints, but so far none has been found. Nevertheless, the very fact that it is possible to look for extra dimensions may weaken my earlier statement that string theory tends not to make testable predictions. If we end up finding them, it could mark a radical shift in my opinions about string theory – and, of course, in our understanding of the universe writ large.