'Moving sofa' puzzle made famous by 'Friends' pivot scene finally solved by mathematician after 60 years
A Korean mathematician has cracked an almost 60-year-old geometry problem popularized in the Friends TV series. Even more impressively, he did it without using a computer.
The so-called "moving sofa" problem, posed by Canadian mathematician Leo Moser in 1966, is misleadingly simple. It asks for the largest shape that can be turned around a right-angled corner in a narrow hallway without getting stuck.
The solution came after 58 years, with a 119-page paper published online by Dr. Baek Jin-eon, 31, a postdoctoral researcher at Yonsei University in Seoul and a research fellow at the June E Huh Center for Mathematical Challenges at the Korea Institute for Advanced Study.
His research rocked the math community so much so that Scientific American magazine included it in its Top 10 Mathematical Breakthroughs of 2025.
While mathematicians argued about the problem using figures and equations, on TV, Friends demonstrated the challenge in a scene featuring series regulars David Schwimmer, Jennifer Aniston, and Matthew Perry playing Ross Geller, Rachel Green, and Chandler Bing, respectively.
In a Season 5 episode, Ross, Rachel, and Chandler are attempting to maneuver Ross' straight single-piece couch around a 90-degree angle in his apartment building's staircase. Ross' constant reminder to "pivot" the couch eventually resulted in the couch being cut in half.
For 58 years, mathematicians attempted to solve the problem without resulting in a halved couch like Ross.
In 1968, according to Scientific American, British mathematician John Hammersley suggested stretching a semicircle to what looked like the shape of a landline phone's handset. That would create an even larger shape and, with the curve longer, it would be easier to slide it around a 90-degree corner.
Then in 1992, mathematician Joseph Gerver came up with a more complex shape with 18 distinct curves. While it was slightly larger than Hammersley's shape, Gerver couldn't prove that his own "sofa" was the largest possible size, which the problem originally asked for.
Researchers created comprehensive computer simulations to build on Gerver's sofa. Scientific American reported Baek to be the leading figure in this line of research.
His solution, however, required no computer work. In his paper, Baek used mere logic to definitively explain how Gerver's sofa is the largest shape that can squeeze through a 90-degree angle in a narrow corridor.
The research is still under peer review, but the response to Baek's findings has been positive. Finally, there will be no need to pivot.
