What does it mean if the universe has extra dimensions?

In one of the most memorable interviews of my career so far, I sat at my desk, head in hands, talking on the phone with a physicist about extra dimensions. I was trying to understand what it means that a whole dimension is small. After several minutes of back and forth, I said, “Not as small as a jellybean, huh?”, mentally blocking 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 suggest that everything is made of incredibly tiny strings. When these strings vibrate, they will produce effects we interpret as atoms, electrons and quarks. I tend to be a bit skeptical of string theory, mostly because the ideas in it range from difficult to impossible to practical to test, which gives me pause. And it relies on extra dimensions to hide the curly strands from our view, which is hard to get our head around.

Sure, there are one or two familiar explanations out there, such as the satirical one Flatland short story from 1884 (written by Edwin Abbott under the pseudonym “A Square”) and the very funny educational videos based on it. They are largely allegories and metaphors—yet they offer a way to understand what encountering another dimension might feel like from a perspective accustomed to living with four, even if they aren’t really about the extra dimensions themselves. Most of the time, when we actually dig into what it might mean that there are extra dimensions, there’s a lot of hand-waving, maybe a little confusion, and then we move on.

But extra dimensions, if real, could legitimately solve some big 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 suggested that some gravity may leak into extra dimensions, weakening its effects in our own perceptible universe. Recently, something similar has been proposed to explain new measurements of dark energy that indicate that it can weaken over time. If there are extra dimensions that change in size over time, it will trickle down into the dimensions we know and experience – three of space and one of time – and change the universe’s energy balance.

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

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

But some kind of Lovecraftian extradimensional horror that has three dimensions (eg a human) can see them from above or below – not only to see that they are actually shapes and not lines, but also to see whatever is inside them. When you live in a 3D world, you can pick one of the figures out of the plane and turn it on its side; the other forms still remain Flatland would only see a strange cross-section of its interior intersecting with their plane instead of the smooth line of its rim.

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 maybe even pluck you out of your familiar space-time into its dimension. If that were to happen, those of us left behind would see a shifting cross-section of your true fifth-dimensional form as your body moved.

A variant of this type 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 returned from relative obscurity in recent years as one of very few plausible ways to break our reality into the constraints of string theory.

In one version of this idea, the membrane that bounds our universe is the edge between a higher dimensional space – called hyperspace – and nothingness. That would put us right on the edge of a cosmic void. Appropriately enough, this is called an end-of-the-world bran. The fundamental particles we are familiar with would then be the very ends of five-dimensional strings living in hyperspace, but we would never be able to see the whole string, just like a triangle in Flatland would never be able to see a shape more complicated than a line.

That brings us to five dimensions, but there could be many more, and they don’t all have to be as vast as the brane world. In fact, they may 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 of that.) And some dimensions can be, well, as small as a jellybean—or even much smaller than that.

You can think of dimensions as a set of glass matryoshka dolls, each nested inside the larger ones, where you only have access to the doll that represents the number of dimensions you live in—presumably four—and those in it. The jellybean dimensions – those 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 small room, like a kind of pocket universe.

Could you enter this pocket universe? Not unless you are as small as a jellybean, or rather as small as a photon, as these dimensions are assumed to be extremely small. The reason for this is that we haven’t seen any – if they were big, they would be much easier to spot. But it would not be completely impossible to spot a small dimension. Consider shining a light through your glass matryoshka dolls. Any bubbles will 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 these bubbles in our universe. It would appear with a slightly distorted shape, and with a powerful enough detector we could measure that distortion. There are also other ways to search, involving tiny quantum effects and exotic particles that we believe can only be produced in extra dimensions.

Scientists at gravitational wave detectors, particle colliders and even ordinary telescopes are hunting for these subtle hints, but so far none have been found. Still, 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 views on string theory – and, of course, in our understanding of the universe writ large.