First shape found that can't pass through itself

383 pointsposted 20 hours ago
by fleahunter

104 Comments

teekert

12 minutes ago

I feel like the next Voyager type vessel should include this shape made from gold or titanium or something. Also add an Einstein-tile. What more should we include?

Perhaps that knot that has a none additive “unknot” from a recent Stand-up Maths episode as well…

All Easter-eggs from our universe we found so far.

teo_zero

12 hours ago

Misleading title. Other shapes have been well known for years, like a sphere. The novelty here is the first polyhedron that can't pass through itself.

jibal

12 hours ago

convex polyhedron

(but your point about the title is valid)

cluckindan

11 hours ago

A sphere can be approximated by a polyhedron. Somewhat obviously, all such polyhedra would seem to have the Rupert property. This new Nopert seems to differ in one key detail: some of the vertices near the flat top/bottom are at a shallower angle to the vertical axis than the vertices below/above them.

Can you pass the T-shaped tetromino through itself?

mkl

11 hours ago

The T-shaped tetromino is not convex, so not part of the conjecture. There are many nonconvex shapes that don't have the Rupert property.

boothby

3 hours ago

Nevertheless, the t-shaped tetronimo (assuming four glued cubes) has a shadow shaped like a bar of length two. I believe that such a shadow will pass through a bar of length three, with a tilt similar to the cube's.

brabel

2 hours ago

If the remaining edge has exactly zero thickness , it means it doesn’t fit. I think that would be the case in that example?

debtta

an hour ago

I don't think that's the case, how can you turn a bar of length two but still have it fit within the width of the bar of length three?

AmbroseBierce

11 hours ago

For laymen's sake I think the title should say "First shape (without curves) found that [...]"

KernalSanders

an hour ago

The article isn't really for the layperson. It's confusing why several people are nitpicking at the title.

integralid

32 minutes ago

Because not-laypoeople ale precisely the kind of people who would nitpick the technically incorrect title.

bonoboTP

2 hours ago

And not "pass through itself" but "pass through its copy"

ekianjo

10 hours ago

Why wouldn't a sphere pass through itself? The projected shadow has the same size as its diameter

nyrikki

9 hours ago

A polyhedron has the Rupert property if a polyhedron of the same or larger size and the same shape as can pass through a hole in the original polyhedron.

A sphere is a surface of constant width, which the polyhedron approximation is not.

> The projected shadow has the same size as its diameter

Thus this is exactly why the sphere doesn't have the Rupert property.

NL807

4 hours ago

Ok, so by that definition a geodesic sphere has the Rupert property, as the sphere is an approximation made up of equilateral triangles. What if we perform isotropic subdivision on the equilateral triangles, such that each inserted point lies on the sphere, centred on each base triangle. We then subdivide each base triangle by constructing 3 new triangles around the inserted point. Thus at each iteration, geodesic sphere of N triangles is subdivided into 3*N triangles. If we continue with the subdivision, each iteration is a refinement of the geodesic sphere, and the geometric approximation gets closer to the shape of a true sphere. As N approaches infinity, the Rupert property holds true (according to the definition). What happens at infinity?

mechanicalpulse

2 hours ago

At infinity, the shape becomes a sphere and all orientations of it are identical. It is no longer a convex polyhedron and, thus, not subject to consideration.

tempestn

3 hours ago

I would guess the margin goes toward 0.

ted_dunning

3 hours ago

Why do you say that the Rupert property applies for all finite N?

Reubend

10 hours ago

Wouldn't you need a little material "left over" to claim that it can pass through itself? Two spheres of equal size wouldn't work because they would occupy exactly the same space.

dguest

2 hours ago

Yes!

The "pass through itself" criteria is the same as "has one shadow that fits entirely inside another shadow". If you allow "one shadow equals another shadow" then it's trivially true for every shape because a shadow equals itself.

Note that this "shadow" language assumes a point light source at infinity, i.e. all the rays are parallel.

smallerize

9 hours ago

That's trivially true for every shape, so it's probably not interesting in the context of this puzzle.

the_arun

9 hours ago

I think Sphere is a outlier for this context.

zelphirkalt

2 hours ago

That would depend on the light source and its size and distance.

king_geedorah

14 hours ago

Rather interesting solution to the problem. You can't test every possibility, so you pick one and get to rule out a bunch of other ones in the same region provided you can determine some other quality of that (non) solution.

I watched a pretty neat video[0] on the topic of ruperts / noperts a few weeks ago, which is a rather fun coincidence ahead of this advancement.

[0] https://www.youtube.com/watch?v=QH4MviUE0_s

anyfoo

13 hours ago

Not that coincidental. tom7 is mentioned in the article itself, and in his video's heartbreaking conclusion, he mentions the work presented in the article at the end. tom7 was working on proving the same thing!

cubefox

2 hours ago

And he tried to disprove the general conjecture, that every convex polyhedron has the Rupert property, by proving that the snub cube [1] doesn't have it. Which is an Archimedean solid and a much more "natural" shape than the Noperthedron, which was specifically constructed for the proof. (It might even be the "simplest" complex polyhedron without the property?)

So if he proves that the snub cube doesn't have the Rupert property, he could still be the first to prove that not all Archimedean solids have it.

1: https://en.wikipedia.org/wiki/Snub_cube

brabel

2 hours ago

Wouldn’t this problem be related to the problem of finding whether two shapes collide in 3d space? That would probably be one of the most studied problems in geometry as simulations and games must compute that as fast as possible for many shapes.

xz18r

an hour ago

Truly a special gem of a channel.

tempestn

12 hours ago

I really like the level of detail in this article. It was enough that I felt like I could get an actual understanding of the work done, but not into such mathematical detail that it was difficult to follow.

PunchyHamster

14 minutes ago

Showing animation for every other shape but one that found, why

zem

10 hours ago

I'd only heard of Prince Rupert because of his eponymous "prince Rupert's drops", but apparently he had not just one but several dazzling careers https://en.wikipedia.org/wiki/Prince_Rupert_of_the_Rhine

KernalSanders

an hour ago

That military career is quite a rollercoaster. Quick-thinking but also youthfully impatient, clearly disciplined enough to rise in the ranks but kicked all around based on how history went. It's pretty amazing that his achievements spanned quite different areas beyond just the military.

juris

3 hours ago

Bah, but with those two flat sides I cannot use it for D&D! I’m really rooting for you, rhombicosidodecahedron!

sans_souse

3 hours ago

Actually, perhaps you're onto something there - didn't Rupert's original conjecture specify polyhedron dice? Perhaps symmetry is one of the requirements for the law..

n1b0m

27 minutes ago

Are there other mathematical discoveries that came from the result of a wager?

n4r9

4 hours ago

Given how hard it was to find one example, the next result is bound to be something like "almost all convex polyhedra cannot pass through themselves".

jmkd

13 hours ago

Layperson question: aren't the nopert candidates just increasingly close to being spheres, which cannot have Rupert tunnels?

tmiku

13 hours ago

Yes, they get visually more sphere-like as more faces are added. But spheres are obviously/trivially non-Rupert, while the question of whether a convex polyhedron can be non-Rupert is more interesting.

gitaarik

12 hours ago

Would be interesting to see how much sides you can keep adding before the shape can't pass through itself. Or maybe you can indefinely keep passing them through, occasionally encountering noperts. Or maybe the noperts gradually increase, eventually making the no-nopperts harder to find. Who knows, let's find out.

KernalSanders

an hour ago

You'd probably end up with tighter and tighter tolerances such as they mention with the triakis tetrahedron.

The challenge is that it gets computationally intensive the more sides that you add if you don't have shortcuts like ruling out entire blocks of orientations in their parameter space (they figured out that if one shadow, projection, protrudes significantly, then you'd need a large rotation to get that protrusion into the other shadow, thus removing all of those rotational angles and reducing the number of orientations needed to check). More sides and more symmetry make it much harder to test a candidate, but you have an interesting idea.

maplant

12 hours ago

But importantly, they’re NOT!

somat

10 hours ago

Does it have to be straight through? I can imagine a scenario where the moving shape has to be rotated as it passes through. sort of analogous to some of those block puzzles or getting a sofa around a corner.

The article does say straight through and most analyses has been done with variation of the shadow technique, which has to be straight through. But the original bet. The thing that started this whole line of thought just said you had to get one through its copy, I think rotating is is an acceptable technique in this problem.

jstanley

7 hours ago

This is specifically about convex polyhedra, I don't see how rotating could help.

somat

5 hours ago

I don't really know, I am currently farting around with blender trying to see, but that is far from rigorous. and going poorly. but let me explain my thought process.

Note the egg shape in the article. specifically the widest band around the equator. now imagine one passing straight down through the other. one edge ring would pass through the shadow if it has a slight rotation offset but it is blocked by the next edge ring up, which could also fit but requires a different offset, so if you could change that rotational offset while it is passing through would it fit?

boothby

3 hours ago

I have the same question -- the problem of moving a couch around a corner is a nonconvex problem, but I suspect that pivoting, or perhaps a helical "rifling" motion, may avoid a vertex:face contact.

halapro

4 hours ago

Sorry for the silly question, but why spend time on this? Is it just for fun or is all mathematical exploration eventually useful? This feels closer to art than engineering.

mikepurvis

3 hours ago

Mathematicians spent decades agonizing about matrix transformations and surface normals, all entirely in the abstract, and then in the 80s that math turned out to be suddenly extremely practical and relevant to the field of computer graphics.

kvdveer

3 hours ago

The problem itself might not be very applicable, but the techniques used to solve it might be.

That said, researching something solely for the sake of curiosity can be a valid endeavour. Many profound scientific discoveries have been made by researching topics with no obvious application.

staplers

3 hours ago

Things like this sometimes lead to practical inventions like velcro or self-locking mechanisms that could be useful. All it takes is someone to connect the dots or find a use case for it and change the world in a small way.

cyode

11 hours ago

I'd love to have an in-print magazine with articles of this subject matter and level of detail. Especially for older kids...accessible and interesting content without all the internet's distractions.

Googling says Quanta is online only. Anyone know of similar publications that print?

kiicia

9 hours ago

Scientific American

fuglede_

4 hours ago

The triakis tetrahedron fit really is crazy close: https://youtu.be/jDTPBdxmxKw

dheera

4 hours ago

I don't understand why this is "hard". Doesn't a donut have this coveted property? I can't think of a way to drill a hole in a donut that would allow a donut through.

boothby

4 hours ago

A donut is nonconvex. The title leaves that very crucial word out.

psychoslave

3 hours ago

What, I can't believe no one came with a term like "anisotransient" for such a property.

dotancohen

2 hours ago

What about the sphere? Surely a hole bored through a sphere, no matter its size, could not pass a sphere of equal size?

dnw

13 hours ago

> Noperthedron (after “Nopert,” a coinage by Murphy that combines “Rupert” and “nope”).

A good sense of humor to go with the math.

867-5309

13 hours ago

this logical falsehood annoyed me since nopert is no+Rupert, whereas nope+Rupert would in fact be nopepert

strbean

13 hours ago

That's not how portmanteaus work.

foobarbecue

8 hours ago

Very true. Portmanteaus work by holding your luggage for you.

stephenlf

12 hours ago

Tom7 also has a couple of videos about portmanteaus

thaumasiotes

6 hours ago

This is actually a really interesting point. English portmanteaus usually work by combining all of one word with "half" (broadly construed) of the second word. Nopert fits the pattern precisely, including all of nope and half of Rupert.

The reason I find this so interesting is that Mandarin Chinese portmanteaus take a different standard form: instead of combining all of one word with half of the other word, they combine half of one word with half of the other word.

Think about how much you'd need to know about the structure of an arbitrary language before you'd feel confident predicting how it creates portmanteaus.

jibal

12 hours ago

Perhaps you should review what "logical falsehood" means, because that's not one.

burkaman

12 hours ago

The coiner gets to pick the combination that sounds the best, there is no correct choice. We could have gotten breakfunch and mototel, but some person or collection of people decided that brunch and motel work better.

biot

13 hours ago

Presumably a simple sphere would trivially qualify as being unable to pass through itself.

smokel

13 hours ago

The puzzle applies only to convex polyhedra.

biot

an hour ago

The article says:

> The full menagerie of shapes is too diverse to get a handle on, so mathematicians tend to focus on convex polyhedra

The phrase "tend to focus on" suggests it's not an exclusive thing. However, you're right -- it appears that the Rupert property only applies to convex polyhedra, so the article title and text is at the very least incomplete given that a sphere is a shape.

LostMyLogin

13 hours ago

A sphere is not a convex polyhedron

guelo

12 hours ago

At the limit of faces they are.

teraflop

12 hours ago

Sure, and pi is the limit of a sequence of rational numbers, but lots of properties that hold for rational numbers don't hold for pi.

guelo

11 hours ago

As you approach sphere you lose Rupertness.

burnt-resistor

an hour ago

Convex polyhedra are required to be finite polytopes.

jibal

12 hours ago

A sphere has no faces so it's not a convex poloyhedron.

burnt-resistor

an hour ago

Correction: a sphere has infinite faces so it's not an "convex poloyhedron [sic]." A convex polyhedron must have finite faces, so apeirotopes aren't allowed.

akoboldfrying

8 hours ago

Limiting behaviour can be counterintuitive. As you add vertices to a polyhedron, some properties approach those of a sphere (volume, surface area), but others just get further and further away (number of surface discontinuities). It's not at all obvious which way "Rupertness" will go, or even whether it's monotone with respect to vertex addition.

Havoc

8 hours ago

> a researcher at A&R Tech, an Austrian transportation systems company

Austrian transport companies research this stuff?!?

I’m both impressed and confused

cool_dude85

8 hours ago

It seems like both the authors on this paper were hobbyists (though, to be fair, trained mathematicians/statisticians, as one has a masters and the other a PhD).

megablast

8 hours ago

You should see what their patent office researchers get up to.

TheOtherHobbes

11 hours ago

Prince Rupert was an incredibly interesting character. This problem was a minor footnote in an impressively rich life.

mrguyorama

14 hours ago

Fans of "Tom7" should be very recently familiar with this!

He released a video about the Ruperts problems and his attempt to find a Nopert on just Sept 16th!

https://www.youtube.com/watch?v=QH4MviUE0_s

With this and the Knotting conjecture being disproven, there are have some really interesting math developments just recently!

Tom regularly releases wonderful videos to go with SIGBOVIK papers about fun and interesting topics, or even just interesting narratives of personal projects. He has that weird kind of computer comedy that you also get from like Foone, the kind where making computers do weird things that don't make sense is fun, the kind where a waterproof RJ45 to HDMI adapter (passive) tickles that odd part of your brain.

chaps

13 hours ago

His videos are some of the best out there. Super funny, depth that's rarely seen elsewhere, and a refreshingly scrappy academic approach. His video on kerning being an incomputable problem is filled with rigor and worth a watch.

Highly recommend all of his videos!

willmadden

5 hours ago

The sphere and anything cylindrical...

MichaelDickens

5 hours ago

The title says "first shape found" but the article clarifies that it's really the first convex polyhedron. A sphere isn't a convex polyhedron, so it doesn't quality for the (now-disproven) conjecture.

dyauspitr

8 hours ago

What does this mean? Does it mean that an object can pass through the largest 2D projection of itself?

ratelimitsteve

14 hours ago

it intuitively feels impossible because it sounds like the definition of "can pass through itself" is really "has at least one orientation where all of the sides of one instance are at most as long as all of the sides of the other instance" and then however you define an orientation an instance of a shape in orientation X should be able to pass through an instance of the same shape and size in the same orientation

strbean

13 hours ago

The criteria is "pass through itself without cutting in half". Presumably that extends to "without deleting the object entirely", which is what would happen to pass through in the same orientation.

jibal

12 hours ago

Notably, a sphere is non-Rupert (but a soccer ball is not ... it can pass through a tiny fringe).

thaumasiotes

6 hours ago

> Notably, a sphere is non-Rupert (but a soccer ball is not ...

A soccer ball is a sphere. It has decorative polygons projected onto its spherical surface, but having a color scheme doesn't stop it from being a sphere.

jibal

12 hours ago

My intuition is very different (and happens to fit reality). Note that convex polyhedra can have asymmetries.

hyperhello

14 hours ago

Yes, and when you think of it that way, it sounds like a partial ordering with a base case. If angle A can pass through angle B, and angle B can pass through angle C…

moralestapia

14 hours ago

>Prince Rupert of the Rhine, a 17th-century army officer, naval commander, colonial governor and gentleman scientist, won a bet about whether it’s possible to pass a cube through another.

Based.

greenchair

13 hours ago

I aspire to be a gentleman scientist!

dinkblam

13 hours ago

I conspire to be a colonial governor!

AaronAPU

12 hours ago

I’d be happy just winning a bet!

jstanley

6 hours ago

Good news: you can start today.