Trapped in a Menger Sponge
Saw something amusing today on a math blog. Under the heading of "weird things math/CS/physical science students build," this entry from the Cornell math club:
It's a Menger sponge, an object like a Sierpinski carpet, but in dimension 3 and with cubes, (he said like that instantly explained everything). You start with a big, solid cube and divide it into a 3-by-3-by-3 grid of smaller cubes. Then you knock out the centre piece of each face and the centre piece of the whole thing, leaving 20 pieces left, like a cube with a jack hollowed out in the middle. Repeat with each remaining piece, "ad infinitum," as they say. The Menger sponge is the limit of this process (more precisely, the intersection of the intermediary constructions). It has the really cool property that every curve (topological space of [Lebesgue covering] dimension one) embeds homeomorphically into the sponge, according to the Wiki article, making it a universal object for (these, topological) curves.
Needless to say, the first thing that flashed through my mind was half a script and the byline for Cube 3. One small problem: the thing is Lebesgue null. On the other hand, the reasonably high Hausdorff dimension (about 2.73) suggests something can be worked out, either by holding our noses and going or switching to the four-dimensional analogue.
I'm afraid the ol' UBC Math Club hasn't constructed anything quite so, well, intimidating. There's just this fun guy:
I can't recall seeing any curiously intriguing math-themed objects at Princeton, although I haven't been keeping my eyes open. It's more of a tigers-and-abstract-statuary sort of place. Next time I'm there I'll snap a shot of the bust of David Hilbert in the common room.
8 Comments:
So there is a challenge for you, come up with an appropriately math-like display.
Is the Menger sponge an example of fractals? Seems to be the case.
What would the limit of the surface area be for this contruct?
Second issue here (or third...), what is wiki, Lebesgue, Hausdorff?
Ignorance may be bliss but curiosity killed the cat.
This morning, over breakfast, I realised I ought to've given a link to explain _Cube_, which was in my mind the central joke of the entry: http://en.wikipedia.org/wiki/Cube_(movie) . "Seven people, trapped in a cube...." I think you mentioned once you saw "Cube 2: Hypercube" on the television.
A Menger sponge is indeed a fractal. It should have infinite surface area. I think each stage of the constuction multiplies the area by thirteen-ninths. But whether I can count or not, the Hausdorff dimension being strictly greater than 2 implies that the area is infinite.
"Wiki" is Wikipedia. Henri Lebesgue is (was) a mathematician. "Lebesgue covering dimension" is a concept of dimension for abstract topological spaces that conforms to our usual intuition if the space in question is an object sitting in some Euclidean space. "Lebesgue null" means that the sponge has zero volume (there's no space inside).
"Hausdorff dimension" is difficult to explain, especially without my real analysis textbook on hand. It's supposed to give, roughly, a measure of the fractional ("fractal") dimension. It's the unique (positive real) number for which the corresponding Hausdorff measure ("fractal volume") is neither zero nor infinite. I guess it gives in some sense a measure of the intricacy or coarseness of the fractal, but don't quote me on that.
I'll have to keep one eye open for a weird thing to build. It'll have to be pretty good to beat the Hilbert bust.
Now the memory comes back, so you are planning on submitting a script for the next movie in the series? Multi-talented!
So it will be Hausdorff dimensions discussed at lunch.
Check your email.
At the Physics Camp I've been working at this summer we had the children build a huge, 3D Sierpinski triangle. First they make little triangles out of paper and tape, and then they attach four of them together to make a bigger triangle. And so on.
It kind of looks like this: http://tinyurl.com/q783h
I think it's about 7 feet tall now.
Oh... it's beautiful. All those tetrahedra, like ten thousand grinning, Maypole-dancing children on the steps of an alien Khufu's mausoleum.
Or how about a large number of the Escher pictures linked and in multiple dimensions.
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