What does it mean if the universe has other dimensions?

In one of the most memorable conversations of my career so far, I was sitting at my desk with my head in my hands, talking on the phone with a physicist about other dimensions. I was trying to understand what it means that the entire dimension is small. After a few minutes of back and forth, I said, “Not as small as a jellyfish, right?” as I mentally blocked out the laughter of the colleague sitting across from me. response? Well, it’s complicated.

In physics, we invoke “extra” dimensions with some regularity, but very rarely talk about what that actually means. They often come into play when string theory is mentioned, a radical set of ideas that suggests that everything is made of incredibly small strings. As these strings vibrate, they create effects that we interpret as atoms, electrons, and quarks. I tend to be a little skeptical of string theory, mostly because the ideas in it range from difficult to impossible to test practically, which gives me pause. And it relies on extra dimensions to hide those coiled strings from our view, which is just hard to get out of our heads.

Sure, there are one or two familiar explanations, such as the satirical one flat areas novel from 1884 (written by Edwin Abbott under the pseudonym “A Square”) and very entertaining educational videos based on it. They are largely allegories and metaphors – yet they offer a way to understand what an encounter with another dimension might be like from the point of view of a person used to living with four, even if the other dimensions themselves are not really involved. Most of the time, when we really dig into what it might mean that there are other 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 major problems in physics and cosmology, so it’s worth trying to understand them. A classic example of one of these problems is gravity, which is much weaker than any other fundamental force. We do not know why this is, and it has been suggested that some gravity may escape into other dimensions and weaken its effects in our own perceptible universe. Recently, something similar has been proposed to explain new measurements of dark energy that suggest it may be weakening over time. If there are other dimensions that change in size over time, this trickles down into the dimensions we know and experience – three spatial and one temporal – and shifts the energy balance of the universe.

Plus, even though I have some questions in my mind about whether there are possibly other dimensions, the idea is interesting.

Probably the easiest type of extra dimension to understand is the type used in flat areasa story about geometric shapes living in a two-dimensional world. They inhabit a flat sail and run around it like pucks gliding on ice. From its flat perspective, looking at the edge of its shape, it sees every other shape as just a line.

But some kind of Lovecraftian extradimensional horror that has three dimensions (say, 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 what is hidden within them. When you live in a 3D world, you can snap one of the shapes out of its plane and turn it on its side; the other shapes still remain flat areas he would only see a strange cross-section through its interior intersecting their plane instead of the steady line of its edge.

If you extrapolate this 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 rip you out of your known spacetime into its dimension. If that happened, those of us left behind would see a shifting cross-section of your true five-dimensional form as your body moved.

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

In one version of this idea, the membrane bounding our universe is the edge between a higher dimensional space—called hyperspace—and nothing. That would put us right at the edge of the cosmic void. It is aptly called the gate of the end of the world. The fundamental particles we know would then be the very ends of the five-dimensional strings that live in hyperspace, but we would never be able to see the whole string, just like the triangle in flat areas would never be able to see any shape more complex than a line.

This brings us to five dimensions, but there could be many more, and not all of them would be as vast as the braneworld ones. In fact, they couldn’t even be cosmic; imagine that time can move sideways and not just forwards and backwards. (Let’s not get into the details.) And some dimensions could be as small as a jellyfish—or even much smaller.

You can think of the dimensions as a set of glass matryoshka dolls, each nested inside larger ones, where you only have access to a doll that represents the number of dimensions you inhabit—probably four—and those within it. Jello dimensions—those that would be physically small, not small in terms of a lower dimension—are like bubbles in glass. They may seem completely different, but just like one of the matryoshka dolls, one of these bubbles encloses space. It’s just a small space, like a kind of pocket universe.

Could you enter this pocket universe? Not unless you are as small as a jellyfish or rather as small as a photon as those dimensions are considered extremely small. The reason is that we didn’t see any – if they were big they would be much easier to detect. But it would not be entirely impossible to record a small dimension. Think about how to make your glass matryoshkas shine through with light. Any bubbles would cause distortion and reflections in the light. A real next dimension would do something similar.

So let’s say a gravitational wave went through one of these bubbles in our universe. It would emerge with a slightly distorted shape, and with a powerful enough detector we could measure this distortion. There are other ways to search, including tiny quantum effects and exotic particles that we think could only be produced in extra dimensions.

Researchers from gravitational wave detectors, particle accelerators and even ordinary telescopes are looking for these subtle hints, but so far none have been found. However, the very fact that it is possible to look for additional dimensions may undermine my earlier claim that string theory tends not to make testable predictions. If we do eventually find them, it could mark a radical shift in my views on string theory—and, of course, in our understanding of the universe.

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