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4D Polytopes and 3D Models of Them. George W. Hart Stony Brook University. Goals of This Talk. Expand your thinking. Visualization of four- and higher-dimensional objects. Show Rapid Prototyping of complex structures. Note: Some Material and images adapted from Carlo Sequin.

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4d polytopes and 3d models of them

4D Polytopesand 3D Models of Them

George W. Hart

Stony Brook University


Goals of this talk
Goals of This Talk

  • Expand your thinking.

  • Visualization of four- and higher-dimensional objects.

  • Show Rapid Prototyping of complex structures.

Note: Some Material and images adapted from Carlo Sequin


What is the 4th dimension
What is the 4th Dimension ?

Some people think:“it does not really exist” “it’s just a philosophical notion”“it is ‘TIME’ ” . . .

But, a geometric fourth dimension is as useful and as real as 2D or 3D.


Higher dimensional spaces
Higher-dimensional Spaces

Coordinate Approach:

  • A point (x, y, z) has 3 dimensions.

  • n-dimensional point: (d1, d2, d3, d4, ..., dn).

  • Axiomatic Approach:

  • Definition, theorem, proof...

  • Descriptive Geometry Approach:

  • Compass, straightedge, two sheets of paper.


What is a regular polytope
What Is a Regular Polytope?

  • “Polytope” is the generalization of the terms “Polygon” (2D), “Polyhedron” (3D), … to arbitrary dimensions.

  • “Regular” means: All the vertices, edges, faces… are equivalent.

  • Assume convexity for now.

  • Examples in 2D: Regular n-gons:


Regular convex polytopes in 3d
Regular Convex Polytopes in 3D

The Platonic Solids:

There are only 5. Why ? …


Why only 5 platonic solids
Why Only 5 Platonic Solids ?

Try to build all possible ones:

  • from triangles: 3, 4, or 5 around a corner;

  • from squares: only 3 around a corner;

  • from pentagons: only 3 around a corner;

  • from hexagons:  floor tiling, does not close.

  • higher n-gons:  do not fit around vertex without undulations (not convex)


Constructing a d 1 d polytope
Constructing a (d+1)-D Polytope

Angle-deficit = 90°

2D

3D

Forcing closure:

?

3D

4D

creates a 3D corner

creates a 4D corner


Seeing a polytope
“Seeing a Polytope”

  • Real “planes”, “lines”, “points”, “spheres”, …, do not exist physically.

  • We understand their properties and relationships as ideal mental models.

  • Good projections are very useful. Our visual input is only 2D, but we understand as 3D via mental construction in the brain.

  • You are able to “see” things that don't really exist in physical 3-space, because you “understand” projections into 2-space, and you form a mental model.

  • We will use this to visualize 4D Polytopes.


Projections
Projections

  • Set the coordinate values of all unwanted dimensions to zero, e.g., drop z, retain x,y, and you get a orthogonal projection along the z-axis. i.e., a 2D shadow.

  • Linear algebra allows arbitrary direction.

  • Alternatively, use a perspective projection: rays of light form cone to eye.

  • Can add other depth queues: width of lines, color, fuzziness, contrast (fog) ...


Wire frame projections
Wire Frame Projections

  • Shadow of a solid object is mostly a blob.

  • Better to use wire frame, so we can see components.


Oblique projections
Oblique Projections

  • Cavalier Projection

3D Cube  2D

4D Cube  3D ( 2D )


Projections vertex edge face cell centered
Projections:VERTEX/ EDGE /FACE/CELL – centered

  • 3D Cube:

    Paralell proj.

    Persp. proj.

  • 4D Cube:

    Parallel proj.

    Persp. proj.


3d objects need physical edges
3D Objects Need Physical Edges

Options:

  • Round dowels (balls and stick)

  • Profiled edges – edge flanges convey a sense of the attached face

  • Flat tiles for faces– with holes to make structure see-through.


Edge treatments
Edge Treatments

(Leonardo Da Vinci)


How do we find all 4d polytopes
How Do We Find All 4D Polytopes?

  • Sum of dihedral angles around each edge must be less than 360 degrees.

  • Use the Platonic solids as “cells”

    Tetrahedron: 70.5°

    Octahedron: 109.5°

    Cube: 90°

    Dodecahedron: 116.5°

    Icosahedron: 138.2°.


All regular convex 4d polytopes
All Regular Convex 4D Polytopes

Using Tetrahedra (70.5°):

3 around an edge (211.5°)  (5 cells) Simplex

4 around an edge (282.0°)  (16 cells) Cross polytope

5 around an edge (352.5°)  (600 cells)

Using Cubes (90°):

3 around an edge (270.0°)  (8 cells) Hypercube

Using Octahedra (109.5°):

3 around an edge (328.5°)  (24 cells)

Using Dodecahedra (116.5°):

3 around an edge (349.5°)  (120 cells)

Using Icosahedra (138.2°):

 none: angle too large.


5 cell or 4d simplex
5-Cell or 4D Simplex

  • 5 cells, 10 faces, 10 edges, 5 vertices.

Carlo Sequin

Can make with Zometool also


16 cell or 4d cross polytope
16-Cell or “4D Cross Polytope”

  • 16 cells, 32 faces, 24 edges, 8 vertices.


4d hypercube or tessaract
4D Hypercube or “Tessaract”

  • 8 cells, 24 faces, 32 edges, 16 vertices.




Hypercube unfolded net
Hypercube Unfolded -- “Net”

One of the 261 different unfoldings


Corpus hypercubus
Corpus Hypercubus

Salvador Dali

“Unfolded”Hypercube


24 cell
24-Cell

  • 24 cells, 96 faces, 96 edges, 24 vertices.

  • (self-dual).


24 cell net in 3d
24-Cell “Net” in 3D

Andrew Weimholt


120 cell
120-Cell

  • 120 cells, 720 faces, 1200 edges, 600 vertices.

  • Cell-first parallel projection,(shows less than half of the edges.)


120 cell model
120-cell Model

Marc Pelletier


120 cell1
120-Cell

Carlo Séquin

Thin face frames, Perspective projection.



Smallest 120 cell
(smallest ?) 120-Cell

Wax model, made on Sanders machine


120 cell perspective projection1
120-Cell – perspective projection

Selective laser sintering


3d printing zcorp
3D Printing — Zcorp



120 cell soap bubble
120-Cell Soap Bubble

John Sullivan

Stereographic projection preserves 120 degree angles


120 cell net
120-Cell “Net”

with stack of 10 dodecahedra

George Olshevski


600 cell 2d projection
600-Cell -- 2D projection

  • Total: 600 tetra-cells, 1200 faces, 720 edges, 120 vertices.

  • At each Vertex: 20 tetra-cells, 30 faces, 12 edges.

  • Oss, 1901

Frontispiece of Coxeter’s book “Regular Polytopes,”


600 cell
600-Cell

Cross-eye Stereo Picture by Tony Smith


600 cell1
600-Cell

  • Dual of 120 cell.

  • 600 cells, 1200 faces, 720 edges, 120 vertices.

  • Cell-first parallel projection,shows less than half of the edges.

  • Can make with Zometool


600 cell2
600-Cell

Straw model by David Richter


Slices through the 600 cell
Slices through the 600-Cell

At each Vertex: 20 tetra-cells, 30 faces, 12 edges.

Gordon Kindlmann


History 3d models of 4d polytopes
History3D Models of 4D Polytopes

  • Ludwig Schlafli discovered them in 1852. Worked algebraically, no pictures in his paper. Partly published in 1858 and 1862 (translation by Cayley) but not appreciated.

  • Many independent rediscoveries and models.


Stringham 1880
Stringham (1880)

  • First to rediscover all six

  • His paper shows cardboard models of layers

3 layers of 120-cell

(45 dodecahedra)


Victor schlegel 1880 s
Victor Schlegel (1880’s)

Invented “Schlegel Diagram”

3D  2D perspective transf.

Used analogous 4D  3D

projection in educational

models.

Built wire and thread models.

Advertised and sold models via commercial catalogs: Dyck (1892) and Schilling (1911).

Some stored at Smithsonian.

Five regular polytopes


Sommerville s description of models
Sommerville’s Description of Models

“In each case, the external boundary of the projection represents one of the solid boundaries of the figure. Thus the 600-cell, which is the figure bounded by 600 congruent regular tetrahedra, is represented by a tetrahedron divided into 599 other tetrahedra … At the center of the model there is a tetrahedron, and surrounding this are successive zones of tetrahedra. The boundaries of these zones are more or less complicated polyhedral forms, cardboard models of which, constructed after Schlegel's drawings, are also to be obtained by the same firm.”


Cardboard models of 120 cell
Cardboard Models of 120-Cell

From Walther Dyck’s 1892 Math and Physics Catalog


Paul s donchian s wire models
Paul S. Donchian’s Wire Models

  • 1930’s

  • Rug Salesman with

  • “visions”

  • Wires doubled to show how front overlays back

  • Widely displayed

  • Currently on view at the Franklin Institute


Zometool
Zometool

  • 1970 Steve Baer designed and produced "Zometool" for architectural modeling

  • Marc Pelletier discovered (when 17 years old) that the lengths and directions allowed by this kit permit the construction of accurate models of the 120-cell and related polytopes.

  • The kit went out of production however, until redesigned in plastic in 1992.


120 cell2
120 Cell

  • Zome Model

  • Orthogonal projection


Uniform 4d polytopes
Uniform 4D Polytopes

  • Analogous to the 13 Archimedean Solids

  • Allow more than one type of cell

  • All vertices equivalent

  • Alicia Boole Stott listed many in 1910

  • Now over 8000 known

  • Cataloged by George Olshevski and Jonathan Bowers





Ambo 600-Cell

Bridges Conference, 2001


Ambo 120 cell
Ambo 120-Cell

Orthogonal projection

Stereolithography

Can do with Zome


Expanded 120 cell
Expanded 120-Cell

Mira Bernstein,

Vin de Silva, et al.



Big polytope net
Big Polytope “Net”

George Olshevski



48 truncated cubes
48 Truncated Cubes

Poorly designed FDM model



Duo prisms nets
Duo-Prisms - “Nets”

Andrew Weimholt

George Olshevski

Andrew Weimholt

Robert Webb


Grand antiprism net
Grand Antiprism “Net”

with stack of 10 pentagonal antiprisms

George Olshevski


Non-Convex Polytopes

  • Components may pass

  • through each other

  • Slices may be useful

  • for visualization

  • Slices may be

  • disconnected

Jonathan Bowers


Beyond 4 dimensions
Beyond 4 Dimensions …

  • What happens in higher dimensions ?

  • How many regular polytopes are therein 5, 6, 7, … dimensions ?

  • Only three regular types:

    • Hypercubes — e.g., cube

    • Simplexes — e.g., tetrahedron

    • Cross polytope — e.g., octahedron


Hypercubes
Hypercubes

  • A.k.a. “Measure Polytope”

  • Perpendicular extrusion in nth direction:

1D 2D 3D 4D


Orthographic projections
Orthographic Projections

Parallel lines remain parallel


Simplex series
Simplex Series

  • Connect all the dots among n+1 equally spaced vertices:(Put next one “above” center of gravity). 1D 2D 3D

This series also goes on indefinitely.


7d simplex
7D Simplex

A warped cube avoids intersecting diagonals.

Up to 6D can be constructed with Zometool.

Open problem: 7D constructible with Zometool?


Cross polytope series
Cross Polytope Series

  • Place vertex in + and – direction on each axis,a unit-distance away from origin.

  • Connect all vertex pairs that lie on different axes. 1D 2D 3D 4D

A square frame for every pair of axes

6 square frames= 24 edges


6d cross polytope
6D Cross Polytope

12 vertices suggestsusing icosahedron

Can do with Zometool.


6d cross polytope1
6D Cross Polytope

Chris Kling


Some references
Some References

  • Ludwig Schläfli: “Theorie der vielfachen Kontinuität,” 1858, (published in 1901).

  • H. S. M. Coxeter: “Regular Polytopes,” 1963, (Dover reprint).

  • Tom Banchoff, Beyond the Third Dimension, 1990.

  • G.W. Hart, “4D Polytope Projection Models by 3D Printing” to appear in Hyperspace.

  • Carlo Sequin, “3D Visualization Models of the Regular Polytopes…”, Bridges 2002.


Puzzle
Puzzle

  • Which of these shapes can / cannot be folded into a 4D hypercube?

  • Hint:Hold the red cube still and fold the others around it.

Scott Kim


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