A four-dimensional space (4D) is a mathematical extension of the concept of three-dimensional or 3D space. Three-dimensional space is the simplest possible abstraction of the observation that one only needs three numbers, called '' dimensions'', to describe the sizes or locations of objects in the everyday world. For example, the volume of a rectangular box is found by measuring and multiplying its length, width, and height (often labeled ''x'', ''y'', and ''z''). The idea of adding a fourth dimension began with Jean le Rond d'Alembert's "Dimensions" being published in 1754, was followed by

Joseph-Louis Lagrange Joseph-Louis Lagrange (born Giuseppe Luigi LagrangiaBernhard Riemann Georg Friedrich Bernhard Riemann (; 17 September 1826 – 20 July 1866) was a German mathematician who made contributions to analysis, number theory, and differential geometry. In the field of real analysis, he is mostly known for the first rig ...

. In 1880,

Charles Howard Hinton Charles Howard Hinton (1853 – 30 April 1907) was a British mathematician and writer of science fiction Science fiction (sometimes shortened to Sci-Fi or SF) is a genre of speculative fiction which typically deals with imaginative and ...

popularized these insights in an essay titled " What is the Fourth Dimension?", which explained the concept of a " four-dimensional cube" with a step-by-step generalization of the properties of lines, squares, and cubes. The simplest form of Hinton's method is to draw two ordinary 3D cubes in 2D space, one encompassing the other, separated by an "unseen" distance, and then draw lines between their equivalent vertices. This can be seen in the accompanying animation whenever it shows a smaller inner cube inside a larger outer cube. The eight lines connecting the vertices of the two cubes in this case represent a ''single direction'' in the "unseen" fourth dimension. Higher-dimensional spaces (i.e., greater than three) have since become one of the foundations for formally expressing modern mathematics and physics. Large parts of these topics could not exist in their current forms without the use of such spaces. Einstein's concept of spacetime uses such a 4D space, though it has a Minkowski structure that is slightly more complicated than Euclidean 4D space. Single locations in 4D space can be given as vectors or '' n-tuples'', i.e., as ordered lists of numbers such as . It is only when such locations are linked together into more complicated shapes that the full richness and geometric complexity of higher-dimensional spaces emerge. A hint to that complexity can be seen in the accompanying 2D animation of one of the simplest possible 4D objects, the tesseract (equivalent to the 3D

cube In geometry, a cube is a three-dimensional solid object bounded by six square faces, facets or sides, with three meeting at each vertex. Viewed from a corner it is a hexagon and its net is usually depicted as a cross. The cube is the only r ...

; see also

hypercube In geometry, a hypercube is an ''n''-dimensional analogue of a square () and a cube (). It is a closed, compact, convex figure whose 1- skeleton consists of groups of opposite parallel line segments aligned in each of the space's dimensions, ...

History

Lagrange Joseph-Louis Lagrange (born Giuseppe Luigi Lagrangiamechanics can be viewed as operating in a four-dimensional space — three dimensions of space, and one of time. In 1827, Möbius realized that a fourth dimension would allow a three-dimensional form to be rotated onto its mirror-image; by 1853, Ludwig Schläfli had discovered all the regular polytopes that exist in higher dimensions, including the four-dimensional analogues of the Platonic solids, but his work was not published until after his death. Higher dimensions were soon put on firm footing by

Bernhard Riemann Georg Friedrich Bernhard Riemann (; 17 September 1826 – 20 July 1866) was a German mathematician who made contributions to analysis, number theory, and differential geometry. In the field of real analysis, he is mostly known for the first rig ...

's 1854 thesis, ''Über die Hypothesen welche der Geometrie zu Grunde liegen'', in which he considered a "point" to be any sequence of coordinates (''x''₁, ..., ''x''_''n''). The possibility of geometry in higher dimensions, including four dimensions in particular, was thus established. An arithmetic of four dimensions, called

quaternion In mathematics, the quaternion number system extends the complex numbers. Quaternions were first described by the Irish mathematician William Rowan Hamilton in 1843 and applied to mechanics in three-dimensional space. Hamilton defined a quatern ...

s, was defined by William Rowan Hamilton in 1843. This

associative algebra In mathematics, an associative algebra ''A'' is an algebraic structure with compatible operations of addition, multiplication (assumed to be associative), and a scalar multiplication by elements in some field ''K''. The addition and multiplic ...

was the source of the science of vector analysis in three dimensions as recounted in ''

A History of Vector Analysis ''A History of Vector Analysis'' (1967) is a book on the history of vector analysis by Michael J. Crowe, originally published by the University of Notre Dame Press. As a scholarly treatment of a reformation in technical communication, the text i ...

''. Soon after, tessarines and coquaternions were introduced as other four-dimensional algebras over R. One of the first major expositors of the fourth dimension was

, starting in 1880 with his essay ''What is the Fourth Dimension?'', published in the Dublin University magazine. He coined the terms '' tesseract'', ''ana'' and ''kata'' in his book ''

A New Era of Thought ''A New Era of Thought'' is a non-fiction work written by Charles Howard Hinton, published in 1888 and reprinted in 1900 by Swan Sonnenschein & Co. Ltd., London. ''A New Era of Thought'' is about the fourth dimension and its implications on human ...

'' and introduced a method for visualising the fourth dimension using cubes in the book ''Fourth Dimension''. Hinton's ideas inspired a fantasy about a "Church of the Fourth Dimension" featured by Martin Gardner in his January 1962 " Mathematical Games column" in '' Scientific American''. In 1886, Victor Schlegel described his method of visualizing four-dimensional objects with Schlegel diagrams. In 1908,

Hermann Minkowski Hermann Minkowski (; ; 22 June 1864 – 12 January 1909) was a German mathematician and professor at Königsberg, Zürich and Göttingen. He created and developed the geometry of numbers and used geometrical methods to solve problems in number t ...

presented a paper consolidating the role of time as the fourth dimension of spacetime, the basis for Einstein's theories of special and general relativity. But the geometry of spacetime, being

non-Euclidean In mathematics, non-Euclidean geometry consists of two geometries based on axioms closely related to those that specify Euclidean geometry. As Euclidean geometry lies at the intersection of metric geometry and affine geometry, non-Euclidean geo ...

, is profoundly different from that explored by Schläfli and popularised by Hinton. The study of Minkowski space required new mathematics quite different from that of four-dimensional Euclidean space, and so developed along quite different lines. This separation was less clear in the popular imagination, with works of fiction and philosophy blurring the distinction, so in 1973

H. S. M. Coxeter Harold Scott MacDonald "Donald" Coxeter, (9 February 1907 – 31 March 2003) was a British and later also Canadian geometer. He is regarded as one of the greatest geometers of the 20th century. Biography Coxeter was born in Kensington t ...

felt compelled to write:

Vectors

Mathematically, four-dimensional space is a space with four spatial dimensions, that is a space that needs four parameters to specify a point in it. For example, a general point might have position vector a, equal to :

\mathbf = \begin a_1 \\ a_2 \\ a_3 \\ a_4 \end.

This can be written in terms of the four

standard basis In mathematics, the standard basis (also called natural basis or canonical basis) of a coordinate vector space (such as \mathbb^n or \mathbb^n) is the set of vectors whose components are all zero, except one that equals 1. For example, in the c ...

vectors (e₁, e₂, e₃, e₄), given by :

\mathbf_1 = \begin 1 \\ 0 \\ 0 \\ 0 \end; \mathbf_2 = \begin 0 \\ 1 \\ 0 \\ 0 \end; \mathbf_3 = \begin 0 \\ 0 \\ 1 \\ 0 \end; \mathbf_4 = \begin 0 \\ 0 \\ 0 \\ 1 \end,

so the general vector a is :

\mathbf = a_1\mathbf_1 + a_2\mathbf_2 + a_3\mathbf_3 + a_4\mathbf_4.

Vectors add, subtract and scale as in three dimensions. The dot product of Euclidean three-dimensional space generalizes to four dimensions as :

\mathbf \cdot \mathbf = a_1 b_1 + a_2 b_2 + a_3 b_3 + a_4 b_4.

It can be used to calculate the norm or

length Length is a measure of distance. In the International System of Quantities, length is a quantity with dimension distance. In most systems of measurement a base unit for length is chosen, from which all other units are derived. In the Interna ...

of a vector, :

\left,  \mathbf \ = \sqrt = \sqrt,

and calculate or define the angle between two non-zero vectors as :

\theta = \arccos.

Minkowski spacetime is four-dimensional space with geometry defined by a non-degenerate pairing different from the dot product: :

\mathbf \cdot \mathbf = a_1 b_1 + a_2 b_2 + a_3 b_3 - a_4 b_4.

As an example, the distance squared between the points (0,0,0,0) and (1,1,1,0) is 3 in both the Euclidean and Minkowskian 4-spaces, while the distance squared between (0,0,0,0) and (1,1,1,1) is 4 in Euclidean space and 2 in Minkowski space; increasing

b_4

actually decreases the metric distance. This leads to many of the well-known apparent "paradoxes" of relativity. The

cross product In mathematics, the cross product or vector product (occasionally directed area product, to emphasize its geometric significance) is a binary operation on two vectors in a three-dimensional oriented Euclidean vector space (named here E), and is ...

is not defined in four dimensions. Instead the exterior product is used for some applications, and is defined as follows: :

\begin
\mathbf \wedge \mathbf = (a_1b_2 - a_2b_1)\mathbf_ + (a_1b_3 - a_3b_1)\mathbf_ + (a_1b_4 - a_4b_1)\mathbf_ + (a_2b_3 - a_3b_2)\mathbf_ \\
+ (a_2b_4 - a_4b_2)\mathbf_ + (a_3b_4 - a_4b_3)\mathbf_. \end

This is bivector valued, with bivectors in four dimensions forming a six-dimensional linear space with basis (e₁₂, e₁₃, e₁₄, e₂₃, e₂₄, e₃₄). They can be used to generate rotations in four dimensions.

Orthogonality and vocabulary

In the familiar three-dimensional space of daily life, there are three coordinate axes—usually labeled ''x'', ''y'', and ''z''—with each axis

orthogonal In mathematics, orthogonality is the generalization of the geometric notion of ''perpendicularity''. By extension, orthogonality is also used to refer to the separation of specific features of a system. The term also has specialized meanings in ...

(i.e. perpendicular) to the other two. The six cardinal directions in this space can be called ''up'', ''down'', ''east'', ''west'', ''north'', and ''south''. 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,

coined the terms ''ana'' and ''kata'', from the Greek words meaning "up toward" and "down from", respectively. As mentioned above, Hermann 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.

Geometry

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-polytope In geometry, a 4-polytope (sometimes also called a polychoron, polycell, or polyhedroid) is a four-dimensional polytope. It is a connected and closed figure, composed of lower-dimensional polytopal elements: vertices, edges, faces (polygons), an ...

s 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 solid In geometry, an Archimedean solid is one of the 13 solids first enumerated by Archimedes. They are the convex uniform polyhedra composed of regular polygons meeting in identical vertices, excluding the five Platonic solids (which are composed ...

s in three dimensions. Relaxing the conditions for convexity generates a further 10 nonconvex regular 4-polytopes. In three dimensions, a circle may be

extrude Extrusion is a process used to create objects of a fixed cross-sectional profile by pushing material through a die of the desired cross-section. Its two main advantages over other manufacturing processes are its ability to create very complex ...

d 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 In mathematics, specifically set theory, the Cartesian product of two sets ''A'' and ''B'', denoted ''A''×''B'', is the set of all ordered pairs where ''a'' is in ''A'' and ''b'' is in ''B''. In terms of set-builder notation, that is : A\ti ...

of two circles may be taken to obtain a duocylinder. 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. Because these surfaces are two-dimensional, they can form much more complex knots than strings in 3D space can. The

Klein bottle In topology, a branch of mathematics, the Klein bottle () is an example of a non-orientable surface; it is a two-dimensional manifold against which a system for determining a normal vector cannot be consistently defined. Informally, it is a o ...

is an example of such a knotted surface. Another such surface is the real projective plane.

Hypersphere

The set of points in

Euclidean 4-space A four-dimensional space (4D) is a mathematical extension of the concept of three-dimensional or 3D space. Three-dimensional space is the simplest possible abstraction of the observation that one only needs three numbers, called ''dimensions'', ...

having the same distance R from a fixed point P₀ forms a hypersurface known as a

3-sphere In mathematics, a 3-sphere is a higher-dimensional analogue of a sphere. It may be embedded in 4-dimensional Euclidean space as the set of points equidistant from a fixed central point. Analogous to how the boundary of a ball in three dimensi ...

. The hyper-volume of the enclosed space is: :

\mathbf V = \begin \frac \end \pi^2 R^4

This is part of the Friedmann–Lemaître–Robertson–Walker metric in General relativity where ''R'' is substituted by function ''R''(''t'') with ''t'' meaning the cosmological age of the universe. Growing or shrinking ''R'' with time means expanding or collapsing universe, depending on the mass density inside.

Cognition

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. The researchers noted that "the participants in our study had minimal practice in these tasks, and it remains an open question whether it is possible to obtain more sustainable, definitive, and richer 4D representations with increased perceptual experience in 4D virtual environments". In another study, the ability of humans to orient themselves in 2D, 3D, and 4D mazes has been tested. Each maze consisted of four path segments of random length and connected with orthogonal random bends, but without branches or loops (i.e. actually labyrinths). The graphical interface was based on John McIntosh's free 4D Maze game. The participating persons had to navigate through the path and finally estimate the linear direction back to the starting point. 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).

Dimensional analogy

To understand the nature of four-dimensional space, a device called ''dimensional analogy'' is commonly employed. Dimensional analogy is the study of how (''n'' − 1) dimensions relate to ''n'' dimensions, and then inferring how ''n'' dimensions would relate to (''n'' + 1) dimensions. 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 the three-dimensional perspective. Rudy Rucker illustrates this in his novel '' Spaceland'', in which the protagonist encounters four-dimensional beings who demonstrate such powers.

Cross-sections

As a three-dimensional object passes through a two-dimensional plane, two-dimensional beings in this plane would only observe a

cross-section Cross section may refer to: * Cross section (geometry) ** Cross-sectional views in architecture & engineering 3D *Cross section (geology) * Cross section (electronics) * Radar cross section, measure of detectability * Cross section (physics) **Ab ...

of the three-dimensional object within this plane. For example, if a sphere 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 sphere, and then getting smaller again, until it shrank to a point and then disappearing. The 2D beings would not see a circle in the same way as three-dimensional beings do; rather, they only see a one-dimensional projection of the circle on their 1D "retina". 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 hypersphere would appear first as a point, then as a growing sphere (until it reaches the "hyperdiameter" of the hypersphere), with the sphere then shrinking to a single point and then disappearing. This means of visualizing aspects of the fourth dimension was used in the novel ''Flatland'' and also in several works of

. And, in the same way three-dimensional beings (such as humans with a 2D retina) can see all the sides and the insides of a 2D shape simultaneously, a 4D being could see all faces and the inside of a 3D shape at once with their 3D retina.

Projections

A useful application of dimensional analogy in visualizing higher dimensions is in

projection Projection, projections or projective may refer to: Physics * Projection (physics), the action/process of light, heat, or sound reflecting from a surface to another in a different direction * The display of images by a projector Optics, graphic ...

. A projection is a way for representing an ''n''-dimensional object in dimensions. 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 Eyes are organs of the visual system. They provide living organisms with vision, the ability to receive and process visual detail, as well as enabling several photo response functions that are independent of vision. Eyes detect light and conv ...

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.).

Artist An artist is a person engaged in an activity related to creating art, practicing the arts, or demonstrating an art. The common usage in both everyday speech and academic discourse refers to a practitioner in the visual arts only. However, th ...

s 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. The perspective projection of three-dimensional objects into the retina of the eye introduces artifacts such as foreshortening, which the brain interprets as depth in the third dimension. In the same way, perspective projection from four dimensions produces similar foreshortening effects. By applying dimensional analogy, one may infer four-dimensional "depth" from these effects. As an illustration of this principle, the following sequence of images compares various views of the three-dimensional

with analogous projections of the four-dimensional tesseract into three-dimensional space.

Shadows

A concept closely related to projection is the casting of shadows. Schlegel wireframe 8-cell

If a light is shone on a three-dimensional object, a two-dimensional shadow is cast. By dimensional analogy, light shone on a two-dimensional object in a two-dimensional world would cast a one-dimensional shadow, and light on a one-dimensional object in a one-dimensional world would cast a zero-dimensional shadow, that is, a point of non-light. Going the other way, one may infer that light shone on a four-dimensional object in a four-dimensional world would cast a three-dimensional shadow. If the wireframe of a cube is lit from above, the resulting shadow on a flat two-dimensional surface is a square within a square with the corresponding corners connected. Similarly, if the wireframe of a tesseract were lit from "above" (in the fourth dimension), its shadow would be that of a three-dimensional cube within another three-dimensional cube suspended in midair (a "flat" surface from a four-dimensional perspective). (Note that, technically, the visual representation shown here is actually a two-dimensional image of the three-dimensional shadow of the four-dimensional wireframe figure.)

Bounding volumes

Dimensional analogy also helps in inferring basic properties of objects in higher dimensions. For example, two-dimensional objects are bounded by one-dimensional boundaries: a square is bounded by four edges. Three-dimensional objects are bounded by two-dimensional surfaces: a cube is bounded by 6 square faces. By applying dimensional analogy, one may infer that a four-dimensional cube, known as a tesseract, is bounded by three-dimensional volumes. And indeed, this is the case: mathematics shows that the tesseract is bounded by 8 cubes. Knowing this is key to understanding how to interpret a three-dimensional projection of the tesseract. The boundaries of the tesseract project to ''volumes'' in the image, not merely two-dimensional surfaces.

Visual scope

People have a spatial self-perception as beings in a three-dimensional space, but are visually restricted to one dimension less: the eye sees the world as a projection to two dimensions, on the surface of the retina. Assuming a four-dimensional being were able to see the world in projections to a hypersurface, also just one dimension less, i.e., to three dimensions, it would be able to see, e.g., all six faces of an opaque box simultaneously, and in fact, what is inside the box at the same time, just as people can see all four sides and simultaneously the interior of a rectangle on a piece of paper. The being would be able to discern all points in a 3-dimensional subspace simultaneously, including the inner structure of solid 3-dimensional objects, things obscured from human viewpoints in three dimensions on two-dimensional projections. Brains receive images in two dimensions and use reasoning to help picture three-dimensional objects.

Limitations

Reasoning by analogy from familiar lower dimensions can be an excellent intuitive guide, but care must be exercised not to accept results that are not more rigorously tested. For example, consider the formulas for the area of a circle (

A = \pi r^2

) and the volume of a sphere (

V = \frac\pi r^3

). One might guess that the volume of the 3-sphere in four-dimensional space is

V=6\pi r^3

, or perhaps

V=8\pi r^3

, but neither of these is right. The actual formula is

V=2\pi^2 r^3

References

External links

"Dimensions" videos, showing several different ways to visualize four dimensional objects''Science News'' article summarizing the "Dimensions" videos, with clips
* ''Flatland: a Romance of Many Dimensions'' (second edition)
Frame-by-frame animations of 4D - 3D analogies
{{DEFAULTSORT:Fourth Dimension Dimension Multi-dimensional geometry Special relativity 4 (number)