# Perfect Shapes in Higher Dimensions - Numberphile

TLDRThis script explores the fascinating world of regular polytopes, starting with the well-known Platonic solids in three dimensions and extending the concept to higher dimensions. It explains why there are exactly five Platonic solids and delves into the properties of regular polygons that lead to this number. The professor illustrates the construction of four-dimensional polytopes, such as the hypercube, 5-cell, 24-cell, and 600-cell, using creative visualization techniques like projections and wireframes. The discussion highlights the unique characteristics of dimensions beyond three, revealing a surprising scarcity of regular polytopes in higher dimensions, and concludes with a reflection on the peculiar beauty and complexity of living in a multidimensional universe.

###### Takeaways

- π The script discusses the concept of regular polytopes, which are higher-dimensional analogues to 2D polygons and 3D polyhedra.
- πΆ There are exactly five Platonic solids in three dimensions: the tetrahedron, cube, octahedron, dodecahedron, and icosahedron.
- πΊ The number of regular polytopes in dimensions higher than three decreases, with only 6 in four dimensions, and then 3 in each dimension from five onwards.
- π The sequence 5, 6, 3, 3, 3... represents the number of regular polytopes in the respective dimensions starting from three dimensions up.
- π€ The reason for exactly five Platonic solids is due to geometric constraints involving the arrangement of regular polygons around vertices and edges.
- π The script explains that regular polygons like triangles, squares, and pentagons can tessellate a plane, but hexagons and higher cannot fully enclose a three-dimensional shape without being flat or warped.
- π¨ M.C. Escher's art is mentioned, featuring an icosahedron, showing how mathematical concepts can be intuitively represented in art.
- π The script humorously mentions the Utah teapot as a 'sixth Platonic solid' in the context of computer graphics, highlighting its significance in rendering and modeling software.
- π The '5 Cell' or 'simplex' and the '24 Cell' are mentioned as examples of four-dimensional regular polytopes, formed by tessellating spaces with three-dimensional Platonic solids.
- π The concept of duality in polytopes is introduced, where for every measure polytope (like a cube), there is a corresponding cross polytope (like an octahedron), swapping vertices and faces.
- π§© The script concludes by emphasizing the uniqueness of two and three dimensions for having a variety of regular polytopes, suggesting a sense of 'luck' in living in these dimensions.

###### Q & A

### What is a regular polytope?

-A regular polytope is a higher-dimensional generalization of a polygon (2D), polyhedron (3D), and so on. It is a figure that exists in any number of dimensions, where each face is a regular polygon, and the figure is symmetrical across all its elements.

### What are the five platonic solids?

-The five platonic solids are the tetrahedron, cube (or hexahedron), octahedron, dodecahedron, and icosahedron. Each of these solids is composed of identical regular polygons and has the same number of faces meeting at each vertex.

### Why does the sequence 5, 6, 3, 3, 3, 3, 3, 3, 3 suggest the number of regular polytopes in different dimensions?

-The sequence represents the number of regular polytopes that exist in each dimension starting from three dimensions up to nine and higher. The numbers correspond to the platonic solids (5), the 6 regular polytopes in four dimensions, and then 3 for each dimension from five to nine and beyond, indicating a pattern where only three types of regular polytopes can exist in these higher dimensions.

### What is the significance of the dihedral angle in determining the existence of regular polytopes?

-The dihedral angle is the angle between two faces that meet at an edge in a polytope. For a polytope to be regular in higher dimensions, the dihedral angles must allow for at least three of the same polytopes to fit around an edge without overlapping, which means the angle must be less than 120 degrees.

### Why can't a sphere be considered a platonic solid?

-A sphere cannot be considered a platonic solid because it doesn't have flat faces, edges, or vertices. All platonic solids are composed of congruent regular polygons meeting at each vertex, whereas a sphere is a continuous surface without any flat faces.

### What is the Utah teapot, and why is it considered a 'sixth platonic solid' in the context of computer graphics?

-The Utah teapot is a 3D model that has become a standard object in the computer graphics community for testing rendering and modeling software. It is jokingly referred to as the 'sixth platonic solid' due to its widespread use and recognition, although it does not have the mathematical properties of a platonic solid.

### How does the concept of extrusion help in visualizing higher-dimensional hypercubes?

-Extrusion is a method where a lower-dimensional shape is extended into a higher dimension by moving it perpendicularly to its own plane. For example, a square (2D) can be extruded to form a cube (3D). This concept helps in visualizing higher-dimensional hypercubes by imagining the process of extruding a cube into a four-dimensional hypercube and so on.

### What is the significance of the number 1152 in the context of the 24-cell polytope?

-The number 1152 represents the total number of symmetries that the 24-cell polytope has in four-dimensional space. It is calculated by multiplying the symmetries of an octahedron (48) by the number of octahedra in the 24-cell (24), resulting in 1152 possible orientations.

### How can the concept of duals be applied to regular polytopes?

-The concept of duals in regular polytopes refers to a pair of polytopes where the faces and vertices of one correspond to the vertices and faces of the other, respectively. For example, the cube and the octahedron are duals in three dimensions. In higher dimensions, this concept is used to create cross polytopes, which are the duals of measure polytopes like the hypercube.

### What are the three series of regular polytopes that exist in higher dimensions?

-The three series of regular polytopes that exist in higher dimensions are the simplex series (which includes the 5-cell), the measure polytope series (which includes the hypercube), and the dual series, which includes the cross polytopes that are the duals of the measure polytopes.

###### Outlines

##### π’ Introduction to Regular Polytopes

The video begins with a numerical sequence puzzle that leads into the topic of regular polytopes. The professor explains that a polytope is a higher-dimensional generalization of polygons and polyhedra. The focus then shifts to the concept of platonic solids, which are the three-dimensional regular polytopes. Five platonic solids are introduced: the tetrahedron, cube, octahedron, dodecahedron, and icosahedron. The discussion includes the number of faces for each solid and their representation in art, such as M.C. Escher's icosahedron pattern. The video aims to explain why there are exactly five platonic solids and no more, setting the stage for a deeper exploration of regular polytopes in various dimensions.

##### π Understanding Platonic Solids Through Geometry

This paragraph delves into the geometric principles behind the existence of exactly five platonic solids. The professor discusses the importance of regular polygons in forming these solids and how different polygons can be combined to create three-dimensional shapes. It is explained that only certain numbers of regular polygons can come together to form a solid without leaving gaps or overlapping. The tetrahedron, made from three equilateral triangles, and the octahedron, formed by combining two square pyramids, are used as examples. The discussion also touches on why certain polygon shapes, like hexagons and heptagons, cannot form platonic solids. The paragraph concludes with the realization that only triangles and squares can be used to create the known platonic solids.

##### π€ The Limitations of Higher-Dimensional Platonic Solids

The conversation continues with an exploration of the limitations of creating higher-dimensional platonic solids. The professor explains that while there are infinitely many regular polygons in two dimensions, the number of possible three-dimensional shapes is limited to five. The discussion moves on to four dimensions, where it is shown that only six regular polytopes exist. The limitations are due to the angles and the way they can fit together in space. The professor uses the example of the Utah teapot, a humorous reference to a common object in computer graphics, to illustrate the idea of a 'sixth platonic solid.' The paragraph ends with an introduction to the concept of creating four-dimensional regular polytopes from three-dimensional platonic solids.

##### π Exploring Four-Dimensional Polytopes

This section of the video focuses on the exploration of four-dimensional polytopes, specifically the 'hypercube' and its construction. The professor describes the process of creating a hypercube by using cubes as the building blocks, similar to how a cube is made from squares. The concept of 'popping out' into the fourth dimension is introduced to explain how the hypercube can be formed without overlapping. The video also discusses the challenges of visualizing four-dimensional objects and the use of projections and wireframes to help understand their structure. The discussion includes the trade-offs between using shadows versus wireframes for better understanding of the object's shape.

##### π The Dihedral Angle and Higher Dimensional Polytopes

The video script explains how the dihedral angles of platonic solids determine the possibility of creating higher-dimensional regular polytopes. The professor discusses the dihedral angles of various three-dimensional shapes and their implications for forming four-dimensional polytopes. The '24 Cell,' made from octahedra, and the '120 Cell,' composed of dodecahedra, are introduced as examples of four-dimensional polytopes. The limitations of the icosahedron due to its large dihedral angle are also highlighted. The paragraph concludes with a general method for determining the feasibility of creating higher-dimensional polytopes based on the dihedral angles of existing ones.

##### π Visualizing Higher-Dimensional Simplexes and Hypercubes

This paragraph discusses the visualization of higher-dimensional simplexes and hypercubes. The professor explains the concept of a simplex as a complete graph where every vertex is connected to every other vertex. The challenge of creating a model that represents this connectivity in three dimensions is addressed, with the suggestion to start with an octahedron and distort it to avoid intersections. The 'measure polytope' or 'hypercube sequence' is introduced as a way to visualize higher dimensions by extruding shapes stepwise. The paragraph also explores different projections and models to represent these higher-dimensional shapes, such as the 'Rhombic Triacontahedron,' and discusses the practical applications of these models.

##### π The Diversity of Regular Polytopes Across Dimensions

The final paragraph of the video script wraps up the discussion on the diversity of regular polytopes across different dimensions. The professor reflects on the number of regular polytopes in two and three dimensions and contrasts it with the limited number in higher dimensions. The conclusion is that there are only three types of regular polytopes in dimensions four and above. The professor and Brady humorously discuss the 'interestingness' of living in three dimensions, with a nod to the four-dimensional space-time continuum. The video ends with a summary of the key points made throughout the script, emphasizing the unique properties of regular polytopes in various dimensions.

###### Mindmap

###### Keywords

##### π‘Regular Polytopes

##### π‘Platonic Solids

##### π‘Dihedral Angles

##### π‘Tetrahedron

##### π‘Hypercube

##### π‘Cross-Polytope

##### π‘Duality

##### π‘Measure Polytope

##### π‘Four-Dimensional Space

##### π‘Symmetry

##### π‘Projection

###### Highlights

Introduction to the concept of regular polytopes in various dimensions, starting with a sequence puzzle.

Explanation of a polytope as a general term for polygons, polyhedra, and higher-dimensional figures.

Discussion on the five Platonic solids: tetrahedron, cube, octahedron, dodecahedron, and icosahedron.

M.C. Escher's icosahedron pattern as an artistic representation of a mathematical concept.

The uniqueness of the five Platonic solids and the impossibility of a sixth in three dimensions.

Exploration of regular polygons in two dimensions and their relation to Platonic solids.

Demonstration of how three-dimensional Platonic solids are constructed from regular polygons.

The humorous mention of the Utah teapot as a 'sixth Platonic solid' in computer graphics.

Introduction to four-dimensional regular polytopes and their construction from three-dimensional solids.

Description of the hypercube and its construction from cubes in four-dimensional space.

Challenges of visualizing four-dimensional objects through shadows and wireframe projections.

Explanation of the '5 Cell' or simplex, a four-dimensional polytope constructed from tetrahedra.

The '24 Cell', made from octahedra, described as the most beautiful four-dimensional regular polytope.

The '120 Cell', a four-dimensional polytope composed of dodecahedron crust.

Inability to create a four-dimensional polytope from an icosahedron due to its dihedral angle.

General method for determining the existence of higher-dimensional regular polytopes based on dihedral angles.

The simplex series and hypercube series as the two sequences of regular polytopes in higher dimensions.

Discussion on the dual nature of polytopes, such as the cube and octahedron in three dimensions.

The concept of measure polytopes and their role in measuring the volume of higher-dimensional spaces.

The conclusion that there are only three types of regular polytopes in dimensions higher than four.

###### Transcripts

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