Geometry of Four Dimensions. New York: Dover, Neville, E. Cambridge, England: Cambridge University Press, Rucker, R. Boston, MA: Houghton Mifflin, Sloane, N.

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## Math Awareness Month Math Expands (Dimension)

Or get it by Thu, Sep 26 with faster delivery. This can be written in terms of the four standard basis vectors e 1 , e 2 , e 3 , e 4 , given by. The dot product of Euclidean three-dimensional space generalizes to four dimensions as. It can be used to calculate the norm or length of a vector,. Minkowski spacetime is four-dimensional space with geometry defined by a non-degenerate pairing different from the dot product:.

This leads to many of the well-known apparent "paradoxes" of relativity. The cross product is not defined in four dimensions. Instead the exterior product is used for some applications, and is defined as follows:. This is bivector valued, with bivectors in four dimensions forming a six-dimensional linear space with basis e 12 , e 13 , e 14 , e 23 , e 24 , e They can be used to generate rotations in four dimensions.

In the familiar three-dimensional space in which we live, there are three coordinate axes —usually labeled x , y , and z —with each axis orthogonal i. The six cardinal directions in this space can be called up , down , east , west , north , and south.

## Geometry of Four Dimensions - Illustrated

Positions along these axes can be called altitude , longitude , and latitude. Lengths measured along these axes can be called height , width , and depth. Comparatively, four-dimensional space has an extra coordinate axis, orthogonal to the other three, which is usually labeled w.

To describe the two additional cardinal directions, Charles Howard Hinton coined the terms ana and kata , from the Greek words meaning "up toward" and "down from", respectively.

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A position along the w axis can be called spissitude , as coined by Henry More. As mentioned above, Herman Minkowski exploited the idea of four dimensions to discuss cosmology including the finite velocity of light. In appending a time dimension to three dimensional space, he specified an alternative perpendicularity, hyperbolic orthogonality. This notion provides his four-dimensional space with a modified simultaneity appropriate to electromagnetic relations in his cosmos.

Minkowski's world overcame problems associated with the traditional absolute space and time cosmology previously used in a universe of three space dimensions and one time dimension. The geometry of four-dimensional space is much more complex than that of three-dimensional space, due to the extra degree of freedom. Just as in three dimensions there are polyhedra made of two dimensional polygons , in four dimensions there are 4-polytopes made of polyhedra. In three dimensions, there are 5 regular polyhedra known as the Platonic solids.

In four dimensions, there are 6 convex regular 4-polytopes , the analogues of the Platonic solids. Relaxing the conditions for regularity generates a further 58 convex uniform 4-polytopes , analogous to the 13 semi-regular Archimedean solids in three dimensions. Relaxing the conditions for convexity generates a further 10 nonconvex regular 4-polytopes. In three dimensions, a circle may be extruded to form a cylinder. In four dimensions, there are several different cylinder-like objects. A sphere may be extruded to obtain a spherical cylinder a cylinder with spherical "caps", known as a spherinder , and a cylinder may be extruded to obtain a cylindrical prism a cubinder.

The Cartesian product of two circles may be taken to obtain a duocylinder.

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All three can "roll" in four-dimensional space, each with its own properties. In three dimensions, curves can form knots but surfaces cannot unless they are self-intersecting. In four dimensions, however, knots made using curves can be trivially untied by displacing them in the fourth direction—but 2D surfaces can form non-trivial, non-self-intersecting knots in 4D space.

The Klein bottle is an example of such a knotted surface. The set of points in Euclidean 4-space having the same distance R from a fixed point P 0 forms a hypersurface known as a 3-sphere. The hyper-volume of the enclosed space is:. Growing or shrinking R with time means expanding or collapsing universe, depending on the mass density inside. Research using virtual reality finds that humans, in spite of living in a three-dimensional world, can, without special practice, make spatial judgments about line segments, embedded in four-dimensional space, based on their length one dimensional and the angle two dimensional between them.

Each maze consisted of four path segments of random length and connected with orthogonal random bends, but without branches or loops i. The researchers found that some of the participants were able to mentally integrate their path after some practice in 4D the lower-dimensional cases were for comparison and for the participants to learn the method.

To understand the nature of four-dimensional space, a device called dimensional analogy is commonly employed. Dimensional analogy was used by Edwin Abbott Abbott in the book Flatland , which narrates a story about a square that lives in a two-dimensional world, like the surface of a piece of paper. From the perspective of this square, a three-dimensional being has seemingly god-like powers, such as ability to remove objects from a safe without breaking it open by moving them across the third dimension , to see everything that from the two-dimensional perspective is enclosed behind walls, and to remain completely invisible by standing a few inches away in the third dimension.

By applying dimensional analogy, one can infer that a four-dimensional being would be capable of similar feats from our three-dimensional perspective. Rudy Rucker illustrates this in his novel Spaceland , in which the protagonist encounters four-dimensional beings who demonstrate such powers. As a three-dimensional object passes through a two-dimensional plane, two-dimensional beings in this plane would only observe a cross-section of the three-dimensional object within this plane.

For example, if a spherical balloon passed through a sheet of paper, beings in the paper would see first a single point, then a circle gradually growing larger, until it reaches the diameter of the balloon, and then getting smaller again, until it shrank to a point and then disappeared. Similarly, if a four-dimensional object passed through a three dimensional hyper surface, one could observe a three-dimensional cross-section of the four-dimensional object—for example, a 4-sphere would appear first as a point, then as a growing sphere, with the sphere then shrinking to a single point and then disappearing.

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A useful application of dimensional analogy in visualizing higher dimensions is in projection. For instance, computer screens are two-dimensional, and all the photographs of three-dimensional people, places and things are represented in two dimensions by projecting the objects onto a flat surface. By doing this, the dimension orthogonal to the screen depth is removed and replaced with indirect information. The retina of the eye is also a two-dimensional array of receptors but the brain is able to perceive the nature of three-dimensional objects by inference from indirect information such as shading, foreshortening , binocular vision , etc.

Artists often use perspective to give an illusion of three-dimensional depth to two-dimensional pictures. The shadow , cast by a fictitious grid model of a rotating tesseract on a plane surface, as shown in the figures, is also the result of projections. Similarly, objects in the fourth dimension can be mathematically projected to the familiar three dimensions, where they can be more conveniently examined.

In this case, the 'retina' of the four-dimensional eye is a three-dimensional array of receptors. A hypothetical being with such an eye would perceive the nature of four-dimensional objects by inferring four-dimensional depth from indirect information in the three-dimensional images in its retina.