One of the most striking interviews of my career began with me sitting at my desk, head in my hands, discussing extra dimensions with a physicist over the phone. I sought to grasp the implications of dimensions being “small.” Amidst the conversation, I tuned out the laughter of a colleague and asked, “They’re not as small as jellybeans, are they?” The answer? It’s a complex one.

While extra dimensions are routinely referenced in physics, their true significance is often overlooked. They frequently arise in discussions regarding string theory—a revolutionary concept proposing that everything stems from minuscule, vibrating strings. These vibrations create particles, from atoms to electrons to quarks. My skepticism about string theory stems from its ideas ranging from the profoundly challenging to the outright untestable, which can be quite daunting. Additionally, these theories usually depend on an extra dimension to conceal the curled strings, a notion that I find difficult to wrap my head around.

Some established explanations, like the Flatland novella, provide entertaining yet enlightening allegories—helping us understand the experience of encountering another dimension while accustomed to four. However, most discussions devolve into ambiguity before we move on.

If extra dimensions are indeed real, they could resolve significant issues in both physics and cosmology, making it imperative to explore them. A notable challenge is gravity: paradoxically weaker than other fundamental forces. This anomaly might occur because gravity “leaks” into other dimensions, reducing its force in our observable universe. Recent hypotheses suggest that dark energy might similarly diminish over time due to an evolving extra dimension, affecting the energy balance of our familiar four-dimensional setup: three spatial dimensions and one of time.

Moreover, this concept is captivating, even as I grapple with the likelihood of extra dimensions existing alongside our own.

One of the most comprehensible kinds of additional dimensions can be found in Flatland, a narrative about geometric entities inhabiting a two-dimensional realm. They navigate a flat surface, much like a puck on ice, and perceive other shapes merely as lines from their limited viewpoint.

Conversely, beings with additional dimensions (humans, for example) see these entities from above or below, recognizing them as shapes rather than mere lines. In our three-dimensional world, we can extract shapes from this plane and rotate them. The remaining forms in Flatland maintain their flatness; instead of seeing stable lines, we’d view an intriguing cross-section where the shape intersects our dimension.

When applied to our universe — with three spatial dimensions and one temporal — even higher-dimensional entities could peer within our world, potentially drawing us into their dimensional space. Observers left behind would witness shifting cross-sections of our likenesses as we traverse this five-dimensional reality.

A variation of this scenario is the brane-world hypothesis, suggesting that our universe exists as the boundary of a higher-dimensional space. Originally proposed in 1999, this concept has recently gained traction as a feasible integration of our universe with the principles of string theory.

In one interpretation, our universe resides at the precipice between a higher-dimensional construct known as hyperspace and the void. Essentially, we occupy the very edge of existence, intriguingly termed the End of the World Brain. The fundamental particles we recognize correspond to the terminals of five-dimensional strings within hyperspace — yet, like the shapes in Flatland, we can never perceive the entirety of these strings.

This theory introduces five dimensions, but there could be countless others, most not resembling our universe at all. Imagine time not merely progressing forward and backward but also moving sideways (details omitted). Some dimensions could possess sizes akin to jellybeans, or even minuscule.

Consider a dimension as a collection of glass matryoshka dolls, each nestled within a larger one, accessible depending on the dimensional level one inhabits (likely four) and the doll representing the inner dimensions. The dimensions comparable to a jellybean may seem physically minute but represent expansive realities, akin to bubbles in glass. Each of these bubbles encapsulates a small realm, a kind of pocket universe.

Wondering about entry into this pocket world? These dimensions are often extremely diminutive, making it improbable for anyone larger than a jellybean—or perhaps a photon—to encounter them. Their minuscule nature is partly why they remain elusive. More sizeable dimensions would certainly attract attention. However, discovering smaller dimensions is not entirely out of the question. Think of light passing through a glass matryoshka doll. Air bubbles distort and reflect light. A parallel phenomenon occurs in actual additional dimensions.

Imagine a gravitational wave traversing one of our universe’s bubbles. It could emerge distorted, and with a potent enough detector, such distortions could be measured. Other investigative methods might include subtle quantum effects and exotic particles believed to originate exclusively from extra dimensions.

Researchers utilizing gravitational wave detectors, particle colliders, and traditional telescopes are diligently searching for these faint signs. However, no concrete evidence has been unearthed yet. Nonetheless, the very endeavor of seeking out extra dimensions could undermine my initial assertion that string theory lacks testable predictions. Should we eventually uncover such dimensions, it could significantly reshape my perspective on string theory — and our overarching understanding of the universe.