Greater than 50 years in the past Polish-American mathematician Mark Kac popularized a zany however mathematically deep query in his 1966 paper “Can One Hear the Form of a Drum?” In different phrases, in the event you hear somebody beat a drum, and the frequencies of the sounds it makes, can you’re employed backward to determine the form of the drum that created these sounds? Or can a couple of drum form create the very same set of frequencies?

Kac wasn’t the primary particular person to pose this or associated questions, however he garnered appreciable consideration for the topic. In 1968 he received the Mathematical Affiliation of America’s Chauvenet Prize, which is concentrated on mathematical exposition, for his 1966 paper. “It’s rather well written and really broadly accessible,” says Julie Rowlett, a mathematician at Chalmers College of Expertise in Sweden.

Kac’s work pushed these issues, which fall right into a mathematical subject known as isospectral geometry, additional into the limelight, inspiring researchers to ask related questions for various shapes and surfaces. Their work ignited an space of analysis that’s nonetheless energetic and rising at the moment.

## Listening to Drums

Greater than 20 years after Kac’s paper, three mathematicians proved that you could’t truly hear the form of a drum. The workforce was capable of produce a number of examples of drums with completely different geometries that created the identical frequencies of sounds.

The researchers’ findings started to crystallize in a brand new approach whereas one of many mathematicians—Carolyn Gordon, now an emeritus professor at Dartmouth School—was on a brief go to to Europe. She had traveled to Germany’s Mathematical Analysis Institute of Oberwolfach, nestled within the Black Forest. Regardless of all the benefits of sojourning to an “idyllic place the place you’re very removed from all the things,” Gordon says, her time at Oberwolfach “simply occurred to be the week when issues had been falling into place” for the workforce’s analysis on listening to shapes.

She had been engaged on associated issues for years. Gordon’s doctoral thesis concerned finding out how you can discern “whether or not two shapes which can be introduced in a form of summary method” are the identical, she says. By way of this different analysis query, she “slipped into” engaged on the drum drawback.

However the institute wasn’t arrange for guests to achieve the surface world simply. “There was a telephone that you can use at sure hours at night time, however you needed to wait in line,” Gordon says. “It was difficult to attach, however it was an thrilling time.” One of many different mathematicians on her workforce, David Webb, who’s married to Gordon, shares related recollections. “We had been attempting to settle the query as rapidly and effectively as we might, as a result of the query had been open for fairly a while, and we had been desirous to get one thing written up in print,” says Webb, now a mathematician at Dartmouth.

A turning level occurred when the researchers realized that an instance Gordon beforehand thought wasn’t going to work was simply what they wanted to point out two otherwise formed drums that sound an identical. “We acquired concepts for different pairs that had been way more sophisticated. We had been making these large paper constructions” to symbolize drums of various shapes, after which “attempting to smash them,” she says. After creating these paper “monstrosities,” as Webb known as them, the mathematicians discovered they didn’t work. “After which we went again to the unique pair and realized it was advantageous,” Gordon says.

Successfully, their work had answered a query that earlier researchers thought-about intractable. In 1882 Arthur Schuster, a German-born British physicist, wrote, “To search out out the completely different tunes despatched out by a vibrating system is an issue which can or will not be solvable in sure particular instances, however it might baffle probably the most skilful [sic] mathematician to resolve the inverse drawback and to seek out out the form of a bell by way of the sounds which it’s able to sending out.”

The invention was a serious step however nonetheless left many questions unanswered.

## The Rule or the Exception

Up to now a number of a long time, researchers have solved a number of issues about “listening to” the sounds of shapes.

It seems you *can* hear the form of a triangle, a consequence first proved in Catherine Durso’s 1988 doctoral thesis for the Massachusetts Institute of Expertise. You may as well hear the form of parallelograms and acute trapezoids, based on a 2015 paper by Rowlett and Zhiqin Lu, a mathematician on the College of California, Irvine. Each shapes produce distinctive sounds. And that paper yielded further attention-grabbing findings, Rowlett explains.

“Let’s say you’re making quadrilateral drums, so 4 straight edges,” she says. “You’d have the ability to hear a sq. one. It might sound particular. And the identical factor for triangle drums: an equilateral triangle drum would sound particular, not like several of the others.” Furthermore, for any common polygonal drum—a form with equal facet lengths and equal inside angles—“you’ll at all times have the ability to hear it amongst the others. And I prefer to assume it might sound notably good,” Rowlett says.

You may as well hear the form of a truncated cone—that’s, a cone that has its pointy tip minimize off, researchers reported within the December 2021 problem of *Bodily Assessment E. *Additionally in 2021 Rowlett and her colleagues confirmed that you could discern the form of a trapezoid from sounds if it isn’t obtuse.

But amongst all the person outcomes about listening to shapes, a special workforce of researchers identified a gaping unsettled thought: it stays to be seen whether or not it’s typically true that it is possible for you to to discern the define of a given sort of form or floor from its sounds.

The query of the connection between a form and its related set of frequencies “is much from being closed, from each theoretical and sensible views,” researchers wrote in a 2018 paper introduced on the IEEE/CVF Convention on Pc Imaginative and prescient and Sample Recognition. “Particularly, it isn’t but sure whether or not the counterexamples,” such because the case of the drum, “are the rule or the exception. Up to now, all the things factors in direction of the latter.”

A number of the questions regarding “listening to” shapes have taken researchers to locations which can be difficult to even image: larger dimensions.

## Visiting Fantastical Dimensions

One among Rowlett’s current preprint papers connects to an issue that was solved approach again in 1964 by mathematician John Milnor, now at Stony Brook College. It includes journeying past the acquainted three dimensions of area to a difficult-to-envision mathematical realm of 16 dimensions.

“We’re fascinated about [flat] tori,” Rowlett says. In a single dimension, a torus “is only a circle,” she notes. In three dimensions, mathematicians usually describe tori as having the form of a glazed doughnut, although they’re often solely referring to the floor of the sugary delight, not its doughy innards.

However Milnor thought-about what occurs when one listens to the shapes of much more mysterious and summary surfaces: 16-dimensional tori. He discovered, mainly, that one can’t hear the form of tori in 16 dimensions.

It might sound odd to leap to the sixteenth dimension, however there are surprisingly sensible causes for doing so. “The extra dimensions you have got, the extra methods there are for issues to be geometrically completely different,” Rowlett says. Thus, this case was truly “a easy instance the place it was simple to see” these variations, she notes.

Milnor’s paper, which is only one web page lengthy, “impressed Kac to a terrific extent. In order that was a basic contribution to getting this subject rising,” Rowlett says. However Milnor’s work left open the query of whether or not one can hear the form of lower-dimensional flat tori. “What about 15-dimensional—or 14?” Rowlett asks.

Rowlett’s current preprint paper, which she co-authored with two researchers who had been then her college students, was motivated by her want to find “the tipping level” between when you possibly can and might’t hear the form of a flat torus. “Three is the magic quantity,” that means one can’t hear the form of tori in 4 or extra dimensions, she says.

However reaching that reply required Rowlett’s workforce to take a circuitous path. Surprisingly, her then college students Erik Nilsson and Felix Rydell found that the query had already been answered. However the issue’s resolution was buried in work from the Nineteen Nineties by mathematician Alexander Schiemann.

The connection between Schiemann’s work and the query Rowlett was pondering was so muffled by mathematical variations that it had escaped wider recognition. That’s largely as a result of the reply to the query “was printed fully utilizing quantity concept language,” she says. Key phrases corresponding to “isospectral” weren’t talked about. “The paper that proves this mathematically by no means even mentions the phrase ‘torus,’” she notes.

Subsequently, of their not but printed paper, Rowlett, Nilsson and Rydell present three mathematical views—analytic, geometric and quantity theoretical—on the issue Schiemann studied, constructing bridges that join the technical facets of understanding his outcomes from the three mathematical viewpoints.

“People who find themselves thinking about a lot of these issues then have entry, as properly, to the instruments from the completely different fields,” Rowlett says. Possibly now, when a special workforce wants to drag out a associated consequence, they received’t need to dig so deep in search of it, she says.

## Amplifying Arithmetic

Within the late 1800s, when Schuster mused concerning the immense problem of figuring out the form of a bell by the sounds it emits, microphones had been a brand new expertise. Greater than 130 years later a workforce of researchers used microphones in a approach that may have shocked Schuster. They employed them to point out that you could, in a way, hear the shapes of rooms—particularly convex, polyhedral ones.

Utilizing a couple of microphones organized in an arbitrary setup, the researchers’ laptop algorithm “reconstructs the total 3D geometry of the room from a single sound emission,” they wrote in a 2013 paper. The scientists famous that their findings might be utilized to issues in architectural acoustics, digital actuality, audio forensics, and extra.

The panorama of analysis surrounding listening to completely different shapes and surfaces has shifted significantly since Schuster’s time. With the continued assembly of mathematical minds from completely different fields and extra advances in expertise, who is aware of what new sounds and shapes mathematicians will discover in coming a long time.