*This initially appeared in the July/August challenge of *Discover *journal as “Message in a Clam.” Help our science journalism by turning out to be a **subscriber**.*

The human mind is both a marvel and a mystery of evolution: Packed into a volume about a person-quarter that of an inflated soccer ball, somewhere around 86 billion neurons type networks that help us to do anything from mindlessly scrolling by way of Instagram to safely sending men and women into space. But a further being familiar with of the composition of those networks is even now an open up query.

Notion continues to be particularly vexing: How does the human mind convert the deluge of incoming indicators — photons, odor molecules, audio waves, sensations on our skin — into an correct mental simulation? What neural community could stand for, say, the odor of chocolate?

New reports counsel that math may possibly assistance us type out these queries. To better approximate the complex networks included in perception and other cognitive jobs, some researchers convert to hyperbolic geometry. Like other geometries, it’s a established of regulations about space, length and connections. But as opposed to Euclidean geometry, which is what most men and women understand (or loathe) in high faculty, hyperbolic geometry describes the way space fits with each other — if space curved absent from by itself, all over the place.

The “Father of Geometry,” Euclid. (Credit rating: BilwissEdition Ltd. & Co. KG/Alamy)

“Hyperbolic geometry has very long been underappreciated in biology,” suggests Tatyana Sharpee of the Salk Institute for Biological Studies in La Jolla, California. In the final couple of decades, her investigation of the composition of the olfactory method has led her to hyperbolic geometry. But our perception of odor is only the starting she thinks the same methods can be generalized to other senses and procedures.

If researchers like Sharpee are suitable, then to recognize the intellect, we have to get ready to embrace hyperbolic geometry’s tenets — which, when they 1st appeared, bordered on heresy for the math planet.

## The Starting of the ‘Bottomless Night’

We usually regard the planet as adhering to the regulations place with each other much more than 2,000 decades in the past by the “Father of Geometry,” Greek mathematician Euclid, in his treatise, *Things. *These regulations approximate the flat, functional, actual physical planet, and are helpful on the scale of daily life. Euclidean geometry has enabled us to cross seas, build skyscrapers, race Ferraris.

But a dilemma arises with Euclid’s Fifth Postulate. In its initial type, it suggests that if a straight line crosses two other straight traces, and those intersections type interior angles on the same facet that incorporate up to significantly less than one hundred eighty degrees, then at some place those two other straight traces have to meet somewhere. (Most of us know the Fifth by easier terms: “Parallel traces under no circumstances meet.”) It is mainly because of the Fifth Postulate that we get the Pythagorean Theorem and proof that a triangle’s angles sum to one hundred eighty degrees.

A postulate is intended to be self-apparent, but this enterprise about parallel traces struck a nerve with mathematicians. It did not appear to be as intuitively convincing — Euclid did not even invoke the Fifth for most of the propositions in *Things.* These troubled scholars expended millennia wrestling with it, and last but not least, in the early 19th century, they commenced to talk to: What if the Fifth doesn’t have to keep?

That query adjusted anything. Violating Euclid’s Fifth was not just an discomfort, they understood. It was a gateway to unique new geometries that ended up even now self-regular.

Hungarian mathematician János Bolyai challenged the regulations Euclid experienced outlined much more than 2,000 decades before. (Credit rating: Science Record Illustrations or photos/Alamy)

The thought of breaking Euclid’s Fifth captivated huge thinkers of the time, which includes Carl Friedrich Gauss and Nikolai Lobachevsky. A person of the most exceptional figures was János Bolyai, a younger, aspiring mathematician from Hungary who was a person of the 1st to forge the regulations of this new geometry. In 1820, he undertook a radical strategy to thwart Euclid. János understood that comforting Euclid’s Fifth Postulate opened new windows to stranger, non-Euclidean geometries.

His father, Farkas, was not delighted, applying language we really don’t often listen to from mathematicians. Or fathers, for that matter.

“For God’s sake, you should give it up,” Farkas wrote to János.

“Detest it as lewd intercourse,” his letter ongoing. “It can deprive you of all your leisure, your overall health, your relaxation, and the full pleasure of your life.” Farkas, himself a mathematician and a lifelong close friend of Gauss, observed that he, as well, experienced the moment challenged Euclid. “I have calculated that bottomless night, and all the gentle and all the joy of my life went out there.”

So a lot for parental encouragement. Undeterred, János went on to hammer out regulations for what we now call non-Euclidean geometries. In Euclidean geometry, the angles of a triangle incorporate to one hundred eighty degrees and parallel traces under no circumstances meet. Not so in non-Euclidean geometries. Spherical geometry presents a person instance — if you attract a triangle on a sphere (like, say, connecting the North Pole to Honolulu to Miami), the angles sum to a quantity greater than one hundred eighty.

Hyperbolic geometry is a different properly-regarded kind of non-Euclidean geometry. A hyperbolic aircraft floating in 3 dimensions doesn’t search flat it appears to be like much more like a Pringles chip or a saddle. It is curved all over the place. If you’re standing on a hyperbolic aircraft and get a phase in a person path, you are going to rise if you convert ninety degrees and get a phase, you are going to go down. In hyperbolic space, the angles of a triangle incorporate up to significantly less than one hundred eighty degrees.

## Curving Corridors and Non-Euclidean Eyesight

Near-forgotten reports courting back again much more than one hundred decades instructed that hyperbolic geometry could assistance describe visual perception. In 1902, German scientist F. Hillebrand performed darkroom experiments — and about 10 decades afterwards, W. Blumenfeld recurring them — in which volunteers ended up asked to search straight in advance with their heads fixed. Then they ended up asked to organize a quantity of glowing lights in parallel traces, moving outward from their heads, so that every line was the same length absent from their line of sight. At the end of the experiment, it ought to have appeared that they ended up staring down the middle of an alley.

In the 20th century, hyperbolic geometry grew to become a supply of creative inspiration: Functions by Dutch artist M.C. Escher, for instance, depict models of hyperbolic geometry. Right now, mathematician Daina Taimina at Cornell University follows in that inventive custom by crocheting models of hyperbolic space that a person can keep and stretch. (Credit rating: Daina Taimina)

But these experiments revealed a paradox: The traces that the volunteers perceived as remaining straight and parallel ended up neither. In its place, the traces consistently adopted curves. In the nineteen forties, operate by German-born mathematician Rudolf Luneburg, doing work at the Dartmouth Eye Institute, assisted describe why parallel traces differ in perception and truth. He recognized that by way of binocular eyesight, we perceive a 3-dimensional map of our surroundings that contains both the designs and places of the matters around us. He established out to derive a metric, a way to translate concerning actual physical truth and what we see.

Luneburg and his collaborators concluded that the regulations of perception are not only non-Euclidean, but also are better represented by hyperbolic geometry. Many years afterwards, in 1983, thinker of science Patrick Heelan likewise argued for the existence of hyperbolic visual space Heelan also pointed out painters like Paul Cézanne, Vincent van Gogh and Joseph Mallord William Turner depicted hyperbolic structures in their operate.

## The Geometry of Odor

Right now, the affair isn’t settled. Scientists continue on to probe the composition of perception networks, and some new experiments give proof that visual space is without a doubt non-Euclidean. In a 2018 examine, researchers reported that men and women rank photographs designed applying non-Euclidean geometries as remaining much more practical than those applying the kind of Euclidean viewpoint we faithfully evaluate in high faculty.

Sharpee suggests the night sky presents compelling proof for hyperbolic perception. We see the dim cosmos as a dome, but astronomical distances are distorted. Small children achieve for the moon mainly because it appears to be like near adequate to contact, she notes, but “distances are compressed.”

And that may possibly be the vital to unlocking the hyperbolic insider secrets of perception: Its attributes only arise on big, encompassing scales. “Any curved geometry on a compact scale is Euclidean,” she suggests. The triangle shaped by Newark, New Jersey New York Town and Nyack, New York, follows Euclidean regulations. “It’s regular with a flat-Earth speculation,” she suggests. “But if we speak about the distances from New York to London to Melbourne, it’s not.”

That is a central thought in her olfactory map, which is big in conditions of its complexity. It is tempting to believe that odor molecules with similar molecular structures would be perceived likewise. (This is analogous to considering that we perceive parallel traces as parallel.) But that’s not what Sharpee found. She analyzed chemical structures of prevalent odors and the success of experiments in which volunteers ended up asked to team similar smells with each other.

Her findings counsel that the human mind groups odors according to how often they take place with each other, relatively than according to their molecular makeup. When she designed a map of these odor clusters, Sharpee found that the length concerning likewise structured molecules was most effective represented applying ideas of length from hyperbolic geometry, relatively than Euclidean. Her operate suggests that we might understand much more about how the mind organizes perception data if we tactic its organizational composition as a kind of curved space.

Hyperbolic geometry, that math of scoundrels, has also turn into helpful over and above perception in modeling the complicated structures of the mind. In a forthcoming paper, physicists in Barcelona modeled mind networks across a selection of animal species. They found that neurons really don’t necessarily talk with the neurons closest to them in space — which you might be expecting if you ended up searching for Euclid — but rather type relay networks that follow the regulations of a various, much more unique geometry. Hyperbolic space “provides pretty much flawlessly navigable maps” of the web of connections in brains in a range of species, they reported. Hyperbolic geometry, they say, indicates “a new cartography of the mind.” In the same way, some computer scientists take note that hyperbolic geometry presents an pleasing way to arrange the huge datasets essential for device understanding.

“Hyperbolic geometry is a extremely all-natural way to stand for the structural complexity of the mind,” suggests physicist Antoine Allard at Laval University in Quebec, who labored on the cross-species examine while a postdoctoral researcher at the University of Barcelona.

Not bad for a renegade math industry, born of a bottomless night.

*Stephen Ornes** is a science and math writer who life in Nashville, Tennessee. His reserve *Math Artwork: Reality, Magnificence, and Equations* contains artists for whom non-Euclidean geometry is a inventive muse.*