Chirality (mathematics)

In geometry, a figure is chiral (and said to have chirality) if it is not identical to its mirror image, or more particularly if it cannot be mapped to its mirror image by rotations and translations alone. A chiral object and its mirror image are said to be enantiomorphs. The word chirality is derived from the Greek χειρ (cheir), the hand, the most familiar chiral object; the word enantiomorph stems from the Greek εναντιος (enantios) 'opposite' and μορφη (morphe) 'form'. A non-chiral figure is called achiral.

The helix (and by extension spun string/twine, etc) and Möbius strip, as well as the S and Z-shaped tetrominoes of the popular video game Tetris, also exhibit chirality, although the last only in two-dimensional space.

Many other familiar objects exhibit the same chiral symmetry of the human body (or enantiomorphic) —gloves, glasses, shoes, legs on a pair of pants, etc. A similar notion of chirality is considered in knot theory, as explained below.

Chirality and symmetry group

A figure is achiral if and only if its symmetry group contains at least one orientation-reversing isometry. (In Euclidean geometry any isometry can be written as $v\mapsto Av+b$ with an orthogonal matrix $A$ and a vector $b$. The determinant of $A$ is either 1 or -1 then. If it is -1 the isometry is orientation-reversing, otherwise it is orientation-preserving.)

Chirality in three dimensions

In three dimensions, every figure which possesses a plane of symmetry or a center of symmetry is achiral. (A plane of symmetry of a figure $F$ is a plane $P$, such that $F$ is invariant under the mapping $(x,y,z)\mapsto (x,y,-z)$, when $P$ is chosen to be the $x$-$y$-plane of the coordinate system. A center of symmetry of a figure $F$ is a point $C$, such that $F$ is invariant under the mapping $(x,y,z)\mapsto (-x,-y,-z)$, when $C$ is chosen to be the origin of the coordinate system.) Note, however, that there are achiral figures lacking both plane and center of symmetry. An example is the figure

$F_{0}=\left\{(1,0,0),(0,1,0),(-1,0,0),(0,-1,0),(2,1,1),(-1,2,-1),(-2,-1,1),(1,-2,-1)\right\}$

which is invariant under the orientation reversing isometry $(x,y,z)\mapsto (-y,x,-z)$ and thus achiral, but it has neither plane nor center of symmetry. The figure

$F_{1}=\left\{(1,0,0),(-1,0,0),(0,2,0),(0,-2,0),(1,1,1),(-1,-1,-1)\right\}$

also is achiral as the origin is a center of symmetry, but it lacks a plane of symmetry.

Note also that achiral figures can have a center axis.

Chirality in two dimensions

In two dimensions, every figure which possesses an axis of symmetry is achiral, and it can be shown that every bounded achiral figure must have an axis of symmetry. (An axis of symmetry of a figure $F$ is a line $L$, such that $F$ is invariant under the mapping $(x,y)\mapsto (x,-y)$, when $L$ is chosen to be the $x$-axis of the coordinate system.) Consider the following pattern:

> > > > > > > > > >
 > > > > > > > > > >

This figure is chiral, as it is not identical to its mirror image:

...  > > > > > > > > > > ...
    > > > > > > > > > >

But if one prolongs the pattern in both directions to infinity, one receives an (unbounded) achiral figure which has no axis of symmetry. Its symmetry group is a frieze group generated by a single glide reflection.

Knot theory

A knot is called achiral if it can be continuously deformed into its mirror image, otherwise it is called chiral. For example the unknot and the figure-eight knot are achiral, whereas the trefoil knot is chiral.

See also

• Chirality (physics)
• Chirality (chemistry)
• Orientation (mathematics)
• Handedness