Mathematician Solves 50-Year-Old Möbius Strip Puzzle

Möbius strips are curious mathematical objects. To assemble one among these single-sided surfaces, take a strip of paper, twist it as soon as after which tape the ends collectively. Making one among these beauties is so easy that even younger youngsters can do it, but the shapes’ properties are complicated sufficient to seize mathematicians’ enduring curiosity.  

The 1858 discovery of Möbius bands is credited to 2 German mathematicians—August Ferdinand Möbius and Johann Benedict Itemizing—although proof means that mathematical large Carl Friedrich Gauss was additionally conscious of the shapes presently, says Moira Chas, a mathematician at Stony Brook College. No matter who first thought of them, till lately, researchers had been stumped by one seemingly simple query about Möbius bands: What’s the shortest strip of paper wanted to make one? Particularly, this downside was unsolved for easy Möbius strips which are “embedded” as a substitute of “immersed,” which means they “do not interpenetrate themselves,” or self-intersect, says Richard Evan Schwartz, a mathematician at Brown College. Think about that “the Möbius strip was really a hologram, a sort of ghostly graphical projection into three-dimensional area,” Schwartz says. For an immersed Möbius band, “a number of sheets of the factor may overlap with one another, form of like a ghost strolling by a wall,” however for an embedded band, “there are not any overlaps like this.”

In 1977 mathematicians Charles Sidney Weaver and Benjamin Rigler Halpern posed this query concerning the minimal dimension and famous that “their downside turns into simple should you permit the Möbius band you’re making to have self-intersections,” says Dmitry Fuchs, a mathematician on the College of California, Davis. The remaining query, he provides, “was to find out, informally talking, how a lot room you should keep away from self-intersections.” Halpern and Weaver proposed a minimal dimension, however they couldn’t show this concept, referred to as the Halpern-Weaver conjecture.

Schwartz first realized about the issue about 4 years in the past, when Sergei Tabachnikov, a mathematician at Pennsylvania State College, talked about it to him, and Schwartz learn a chapter on the topic in a book Tabachnikov and Fuchs had written. “I learn the chapter, and I used to be hooked,” he says. Now his curiosity has paid off with an answer to the issue eventually. In a preprint paper posted on arXiv.org on August 24, Schwartz proved the Halpern-Weaver conjecture. He confirmed that embedded Möbius strips made out of paper can solely be constructed with a side ratio better than √3, which is about 1.73. For example, if the strip is one centimeter large, it should be longer than √3 cm.

Fixing the quandary required mathematical creativity. When one makes use of a typical strategy to this sort of downside, “it’s at all times tough to differentiate, by way of formulation, between self-intersecting and non-self-intersecting surfaces,” Fuchs says. “To beat this issue, you should have [Schwartz’s] geometric imaginative and prescient. However it’s so uncommon!” 

In Schwartz’s proof, “Wealthy managed to dissect the issue into manageable items, every of which primarily necessitated solely fundamental geometry to be solved,” says Max Wardetzky, a mathematician on the College of Göttingen in Germany. “This strategy to proofs embodies one of many purest types of magnificence and sweetness.” 

Earlier than arriving on the profitable technique, nonetheless, Schwartz tried different ways on and off once more over just a few years. He lately determined to revisit the issue due to a nagging sensation that the strategy he had utilized in a 2021 paper ought to have labored.

In a manner, his intestine feeling was appropriate. When he resumed investigating the issue, he observed a mistake in a “lemma”—an intermediate consequence—involving a “T-pattern” in his earlier paper. By correcting the error, Schwartz rapidly and simply proved the Halpern-Weaver conjecture. If not for that mistake, “I might have solved this factor three years in the past!” Schwartz says. 

In Schwartz’s answer to the Halpern-Weaver conjecture, the T-pattern lemma is a crucial element. The lemma begins with one fundamental concept: “Möbius bands, they’ve these straight traces on them. They’re [what are] referred to as ‘dominated surfaces,’” he says. (Different paper objects share this property. “At any time when you could have paper in area, even when it’s in some difficult place, nonetheless, at each level, there’s a straight line by it,” Schwartz notes.) You’ll be able to think about drawing these straight traces in order that they reduce throughout the Möbius band and hit the boundary at both finish. 

In his earlier work, Schwartz recognized two straight traces which are perpendicular to one another and likewise in the identical aircraft, forming a T-pattern on each Möbius strip. “It’s not in any respect apparent that this stuff exist,” Schwartz says. Exhibiting that they do was the primary a part of proving the lemma, nonetheless.

The following step was to arrange and remedy an optimization downside that entailed slicing open a Möbius band at an angle (quite than perpendicular to the boundary) alongside a line phase that stretched throughout the width of the band and contemplating the ensuing form. For this step, in Schwartz’s 2021 paper, he incorrectly concluded that this form was a parallelogram. It’s really a trapezoid.

This summer time, Schwartz determined to attempt a unique tactic. He began experimenting with squishing paper Möbius bands flat. He thought, “Possibly if I can present which you can press them into the aircraft, I can simplify it to a better downside the place you’re simply considering of planar objects.” 

Throughout these experiments, Schwartz reduce open a Möbius band and realized, “Oh, my God, it’s not the parallelogram. It’s a trapezoid.” Discovering his mistake, Schwartz was first aggravated (“I hate making errors,” he says) however then pushed to make use of the brand new info to rerun different calculations. “The corrected calculation gave me the quantity that was the conjecture,” he says. “I used to be gobsmacked…. I spent, like, the subsequent three days hardly sleeping, simply scripting this factor up.” 

Lastly, the 50-year-old query was answered. “It takes braveness to attempt to remedy an issue that remained open for a very long time,” Tabachnikov says. “It’s attribute of Richard Schwartz’s strategy to arithmetic: He likes attacking issues which are comparatively simple to state and which are identified to be arduous. And usually he sees new facets of those issues that the earlier researchers didn’t discover.” 

“I see math as a joint work of humanity,” Chas says. “I want we may inform Möbius, Itemizing and Gauss, ‘You began, and now take a look at this….’ Possibly in some mathematical sky, they’re there, taking a look at us and considering, ‘Oh, gosh!’” 

As for associated questions, mathematicians already know that there isn’t a restrict on how lengthy embedded Möbius strips could be (though bodily developing them would turn into cumbersome in some unspecified time in the future). Nobody, nonetheless, is aware of how quick a strip of paper could be if it’s going for use to make a Möbius band with three twists in it as a substitute of 1, Schwartz notes. Extra usually, “one can ask concerning the optimum sizes of Möbius bands that make an odd variety of twists,” Tabachnikov says. “I count on somebody to resolve this extra common downside within the close to future.”

Editor’s Notice (9/14/23): This text was edited after posting to appropriate the descriptions of what a Möbius strip’s size is when it’s wider than √3 centimeters and the way the 2 traces that Richard Evan Schwartz recognized type a T-pattern on each strip.