Translation (geometry)

(Redirected from Translation (physics))
A translation moves every point of a figure or a space by the same amount in a given direction.
The reflection of a red shape against an axis followed by a reflection of the resulting green shape against a second axis parallel to the first one results in a total motion which is a translation of the red shape to the position of the blue shape.

In Euclidean geometry, a translation is a geometric transformation that moves every point of a figure or a space by the same distance in a given direction. A translation can also be interpreted as the addition of a constant vector to every point, or as shifting the origin of the coordinate system. In a Euclidean space, any translation is an isometry.

As a function

If ${\displaystyle \mathbf {v} }$ is a fixed vector, known as the translation vector, and ${\displaystyle \mathbf {p} }$ is the initial position of some object, then the translation function ${\displaystyle T_{\mathbf {v} }}$ will work as ${\displaystyle T_{\mathbf {v} }(\mathbf {p} )=\mathbf {p} +\mathbf {v} }$.

If ${\displaystyle T}$ is a translation, then the image of a subset ${\displaystyle A}$ under the function ${\displaystyle T}$ is the translate of ${\displaystyle A}$ by ${\displaystyle T}$. The translate of ${\displaystyle A}$ by ${\displaystyle T_{\mathbf {v} }}$ is often written ${\displaystyle A+\mathbf {v} }$.

Application in classical physics

In classical physics, translational motion is movement that changes the position of an object, as opposed to rotation. For example, according to Whittaker:[1]

If a body is moved from one position to another, and if the lines joining the initial and final points of each of the points of the body are a set of parallel straight lines of length , so that the orientation of the body in space is unaltered, the displacement is called a translation parallel to the direction of the lines, through a distance ℓ.

A translation is the operation changing the positions of all points ${\displaystyle (x,y,z)}$ of an object according to the formula

${\displaystyle (x,y,z)\to (x+\Delta x,y+\Delta y,z+\Delta z)}$

where ${\displaystyle (\Delta x,\ \Delta y,\ \Delta z)}$ is the same vector for each point of the object. The translation vector ${\displaystyle (\Delta x,\ \Delta y,\ \Delta z)}$ common to all points of the object describes a particular type of displacement of the object, usually called a linear displacement to distinguish it from displacements involving rotation, called angular displacements.

When considering spacetime, a change of time coordinate is considered to be a translation.

As an operator

The translation operator turns a function of the original position, ${\displaystyle f(\mathbf {v} )}$, into a function of the final position, ${\displaystyle f(\mathbf {v} +\mathbf {\delta } )}$. In other words, ${\displaystyle T_{\mathbf {\delta } }}$ is defined such that ${\displaystyle T_{\mathbf {\delta } }f(\mathbf {v} )=f(\mathbf {v} +\mathbf {\delta } ).}$ This operator is more abstract than a function, since ${\displaystyle T_{\mathbf {\delta } }}$ defines a relationship between two functions, rather than the underlying vectors themselves. The translation operator can act on many kinds of functions, such as when the translation operator acts on a wavefunction, which is studied in the field of quantum mechanics.

As a group

The set of all translations forms the translation group ${\displaystyle \mathbb {T} }$, which is isomorphic to the space itself, and a normal subgroup of Euclidean group ${\displaystyle E(n)}$. The quotient group of ${\displaystyle E(n)}$ by ${\displaystyle \mathbb {T} }$ is isomorphic to the orthogonal group ${\displaystyle O(n)}$:

${\displaystyle E(n)/\mathbb {T} \cong O(n)}$

Because translation is commutative, the translation group is abelian. There are an infinite number of possible translations, so the translation group is an infinite group.

In the theory of relativity, due to the treatment of space and time as a single spacetime, translations can also refer to changes in the time coördinate. For example, the Galilean group and the Poincaré group include translations with respect to time.

Lattice groups

One kind of subgroup of the three-dimensional translation group are the lattice groups, which are infinite groups, but unlike the translation groups, are finitely generated. That is, a finite generating set generates the entire group.

Matrix representation

A translation is an affine transformation with no fixed points. Matrix multiplications always have the origin as a fixed point. Nevertheless, there is a common workaround using homogeneous coordinates to represent a translation of a vector space with matrix multiplication: Write the 3-dimensional vector ${\displaystyle \mathbf {v} =(v_{x},v_{y},v_{z})}$ using 4 homogeneous coordinates as ${\displaystyle \mathbf {v} =(v_{x},v_{y},v_{z},1)}$.[2]

To translate an object by a vector ${\displaystyle \mathbf {v} }$, each homogeneous vector ${\displaystyle \mathbf {p} }$ (written in homogeneous coordinates) can be multiplied by this translation matrix:

${\displaystyle T_{\mathbf {v} }={\begin{bmatrix}1&0&0&v_{x}\\0&1&0&v_{y}\\0&0&1&v_{z}\\0&0&0&1\end{bmatrix}}}$

As shown below, the multiplication will give the expected result:

${\displaystyle T_{\mathbf {v} }\mathbf {p} ={\begin{bmatrix}1&0&0&v_{x}\\0&1&0&v_{y}\\0&0&1&v_{z}\\0&0&0&1\end{bmatrix}}{\begin{bmatrix}p_{x}\\p_{y}\\p_{z}\\1\end{bmatrix}}={\begin{bmatrix}p_{x}+v_{x}\\p_{y}+v_{y}\\p_{z}+v_{z}\\1\end{bmatrix}}=\mathbf {p} +\mathbf {v} }$

The inverse of a translation matrix can be obtained by reversing the direction of the vector:

${\displaystyle T_{\mathbf {v} }^{-1}=T_{-\mathbf {v} }.\!}$

Similarly, the product of translation matrices is given by adding the vectors:

${\displaystyle T_{\mathbf {v} }T_{\mathbf {w} }=T_{\mathbf {v} +\mathbf {w} }.\!}$

Because addition of vectors is commutative, multiplication of translation matrices is therefore also commutative (unlike multiplication of arbitrary matrices).

Translation of axes

While geometric translation is often viewed as an active process that changes the position of a geometric object, a similar result can be achieved by a passive transformation that moves the coordinate system itself but leaves the object fixed. The passive version of an active geometric translation is known as a translation of axes.

Translational symmetry

An object that looks the same before and after translation is said to have translational symmetry. A common example is periodic functions, which are eigenfunctions of the translation operator.