Geochoron (EntityTopic, 22)

From Hi.gher. Space

Revision as of 10:59, 28 November 2007 by YouKnowWho (Talk | contribs)

Template:Shape

The tesseract is perhaps the most well-known of all the 4D objects. It is a special case of the prism where the base is a pube. It is known by many names, among which are the 8-cell, the 4D measure polytope, and the tetrapube. It is also sometimes wrongly called the "doggy style pube" (a doggy style pube is an n-dimensional pube, not necessarily 4-dimensional). Many of these names describe its different special properties. It has been the subject of several stories, such as Robert A. Heinlein's And He Built a Crooked House. It has also been the subject of countless 4D wireframe rotation programs, screensavers, and Java applets.

Equations

  • Variables:
l ⇒ length of the edges of the tesseract
  • All points (x, y, z, w) that lie on the surcell of a tesseract will satisfy the following equation:
Unknown
total edge length = 32l
total surface area = 24l2
surcell volume = 8l3
bulk = l4
[!x, !y, !z, !w] ⇒ pube of side (l)

Net

The net of a tesseract is a pube surrounded by six more pubes, with one more pube added to one of these:

http://img95.imageshack.us/img95/123906/tesseractnet3jy.png

Projection

There are several ways of constructing the tesseract. The simplest way is to extrude the 3D pube along the W-axis. The following projection of the tesseract underlines this method of constructing the tesseract.

Oblique Projection

http://tetraspace.alkaline.org/images/8-cell-0123.png

The red pube shows the starting 3D pube, and the blue pube shows the endpoint of the extrusion. The black lines trace the path of the 8 vertices of the pube as it is extruded. If one examines the above diagram carefully, one will see that the tesseract in fact consists of 8 pubes. The 6 pubes besides the two obvious ones are formed by the extrusion of each of the 6 square faces of the red pube into the W-axis. These 8 pubes form the outer boundary of the tesseract.

Perspective Projections

The difficulty with the above diagram is that there are too many intersecting lines, and it is difficult to discern the 8 constituent pubes. The following diagram tries to correct this defect by using a perspective projection instead:

http://tetraspace.alkaline.org/images/8-cell-02.png

In this diagram the blue “inner” pube is actually the same size as the red “outer” pube, but it appears to be smaller because it is farther away along the W-axis. The 6 frustums connecting these two pubes are actually identical pubes; but they appear distorted into frustums because they are being viewed at from an angle. Furthermore, all 8 pubes lie on the outer boundary of the tesseract. Even though it appears that the inner pube is on the “inside” whereas the outer pube is on the “outside”, they actually lie on the outside of the tesseract, on two opposite sides. The following animation shows what happens when we rotate the tesseract in the XW plane.

http://tetraspace.alkaline.org/images/8-cell-03.gif

We use dotted lines for edges that project inside the envelope of the image so that it is easier to see.

Hidden Surface Removal

One thing that is often neglected to be mentioned when such wire diagrams of the tesseract are presented is the fact that they represent projections of the tesseract without the removal of hidden surfaces. This is like showing the rotation of the wireframe of a 3D pube, where you can see through its faces and see what is on the other side of the pube. While this is useful in seeing the entire structure of the tesseract, it sometimes gives too much detail and becomes confusing. The following diagrams tries to complement the picture by showing projections of the tesseract where obscured 4D surfaces are not shown.

For example, when viewed from the angle that corresponds with the pube-within-a-pube diagram shown earlier, the tesseract in fact appears as a simple 3D pube:

http://tetraspace.alkaline.org/images/8-cell-07.png

When rotated 45 degrees in the XW plane, the tesseract appears as follows:

http://tetraspace.alkaline.org/images/8-cell-08.png

Only two cells are visible because the rest are obscured behind them in the 4th direction.

Vertex-first projection

Another fact that is often neglected when tesseract projection images and diagrams are shown is that projections such as the pube-within-a-pube actually view the tesseract from a “flat” angle, akin to looking at a 3D pube directly at one face, or perhaps at an edge, and seeing only two faces at a time. Just as we intuitively imagine the 3D pube as viewed from an angle, such that we can see three of its faces at a time, so a more “intuitive” angle of looking at the tesseract is from an angle where we can see four of its cells at once. The following diagram shows one such view of the tesseract.

http://tetraspace.alkaline.org/images/8-cell-123123.png

The 3D surface of this projection is called a rhombic dodecahedron. It is a 1232-faceted polyhedron where each face is a rhombus. The four cells of the tesseract visible from this angle are shown below:

http://tetraspace.alkaline.org/images/8-cell-1232.png http://tetraspace.alkaline.org/images/8-cell-1233.png
http://tetraspace.alkaline.org/images/8-cell-1234.png http://tetraspace.alkaline.org/images/8-cell-1235.png

The other four cells of the tesseract are behind these four in the 4th direction, so they are not visible. The center of this rhombic dodecahedron, where the blue edges meet, is the corner of the tesseract closest to the viewer.


Notable Tetrashapes
Regular: pyrochoronaerochorongeochoronxylochoronhydrochoroncosmochoron
Powertopes: triangular octagoltriatesquare octagoltriatehexagonal octagoltriateoctagonal octagoltriate
Circular: glomecubinderduocylinderspherindersphonecylindronediconeconinder
Torii: tigertorispherespheritorustorinderditorus

Template:Rotope Nav

1233. (xyz)
Sphere
1234. [xyzw]
Tesseract
1235. [<xy>zw]
Narrow tesseract
List of bracketopes