Outside In
Why Hacker News recommends it
Linked as a classic visualization when sphere eversion or counterintuitive mathematics comes up.
What it is
A 21-minute animation showing how a sphere can be turned inside out without creases or tears, known as sphere eversion.
Recommended by 0 HN commenters across 0 different months, 2021 to 2025.
What Hacker News said
Cites st_goliath · 2021-10 · on “Conterintuitive facts in mathematics, CS, and physics”
> 19. Given a solid ball in 3‑dimensional space, there exists a decomposition of the ball into a finite number of disjoint subsets, which can then be put back together in a different way to yield two identical copies of the original ball. While you're at it, you can completely turn that sphere inside out without creating any holes or creases[1]. [1] [this video]
Cites ykonstant · 2022-04 · on “Life’s preference for symmetry is like ‘a new law of nature’”
Here are two classic youtube videos on the topic of sphere eversion; they mostly overlap, but I am posting both for double the hilarity in the comment sections :) (the full 21 minute one) [this video] (the first 10 minutes) [LINK] If I remember correctly, I think the 10 minute one has the more hilarious comment section.
Cites gosub100 · 2025-01 · on “Mathematicians discover new way for spheres to 'kiss'”
Sort of a tangent, but here's a 20m video explaining how to invert a sphere without tearing it: [this video]