Four-velocity

In physics, in particular in special relativity and general relativity, a four-velocity is a four-vector, a vector in four-dimensional spacetime.[nb 1] It is the relativistic counterpart of velocity, which is a three-dimensional vector in space.

Events constitute the mathematical description of points in time and space, the set of all of them together forming a mathematical model of physical four-dimensional spacetime. The history of an object traces a curve in spacetime, called its world line. If the object is massive, so that its speed is less than the speed of light, the world line may be parametrized by the proper time of the object. The four-velocity is the rate of change of four-position with respect to the proper time along the curve. The velocity, in contrast, is the rate of change of the position in (three-dimensional) space of the object, as seen by an observer, with respect to the observer's time.

The magnitude of an object's four-velocity (the quantity obtained by applying the metric tensor to the four-velocity and itself) is always equal to the square of c, the speed of light. For an object at rest (with respect to the coordinate system) its four-velocity is parallel to the direction of the time coordinate. A four-velocity is thus the normalized future-directed timelike tangent vector to a world line, and is a contravariant vector. Though it is a vector, addition of two four-velocities does not yield a four-velocity: the space of four-velocities is not itself a vector space.[nb 2]

Velocity

The path of an object in three-dimensional space (in an inertial frame) may be expressed in terms of three spatial coordinate functions xi(t) of time t, where i is an index which takes values 1, 2, 3.

The three coordinates form the 3d position vector, written as a column vector

$\vec{x}(t) = \begin{bmatrix} x^1(t) \\ x^2(t) \\ x^3(t) \end{bmatrix} \,.$

The components of the velocity ${\vec{u}}$ (tangent to the curve) at any point on the world line are

${\vec{u}} = \begin{bmatrix}u^1 \\ u^2 \\ u^3\end{bmatrix} = {d \vec{x} \over dt}= \begin{bmatrix}\tfrac{dx^1}{dt} \\ \tfrac{dx^2}{dt} \\ \tfrac{dx^3}{dt}\end{bmatrix}.$

Each component is simply written

$u^i = {dx^i \over dt}$

Theory of relativity

In Einstein's theory of relativity, the path of an object moving relative to a particular frame of reference is defined by four coordinate functions xμ(τ), where μ is a spacetime index which takes the value 0 for the timelike component, and 1, 2, 3 for the spacelike coordinates. The zeroth component is defined as the time coordinate multiplied by c,

$x^{0} = ct\,,$

Each function depends on one parameter τ called its proper time. As a column vector,

$\mathbf{x} = \begin{bmatrix} x^0(\tau)\\ x^1(\tau) \\ x^2(\tau) \\ x^3(\tau) \\ \end{bmatrix}\,.$

Time dilation

From time dilation, the differentials in coordinate time t and proper time τ are related by

$dt = \gamma(u) d\tau$

where the Lorentz factor,

$\gamma(u) = \frac{1}{\sqrt{1-\frac{u^2}{c^2}}}\,,$

is a function of the Euclidean norm u of the 3d velocity vector $\vec{u}$:

$u = || \ \vec{u} \ || = \sqrt{ (u^1)^2 + (u^2)^2 + (u^3)^2} \,.$

Definition of the four-velocity

The four-velocity is the tangent four-vector of a world line. The four-velocity at any point of world line $\mathbf{x}(\tau)$ is defined as:

$\mathbf{U} = \frac{d\mathbf{x}}{d \tau}$

where $\mathbf{x}$ is the four-position and $\tau$ is the proper time.

The four-velocity defined here using the proper time of an object does not exist for world lines for objects such as photons travelling at the speed of light; nor is it defined for tachyonic world lines, where the tangent vector is spacelike.

Components of the four-velocity

The relationship between the time t and the coordinate time x0 is defined to be related to coordinate time by

$x^0 = ct .$

Taking the derivative of this with respect to the proper time τ, we find the Uμ velocity component for μ = 0:

$U^0 = \frac{dx^0}{d\tau} = \frac{d(ct)}{d\tau}= c\frac{dt}{d\tau} = c \gamma(u)$

and for the other 3 components to proper time we get the Uμ velocity component for μ = 1, 2, 3:

$U^i = \frac{dx^i}{d\tau} = \frac{dx^i}{dt} \frac{dt}{d\tau} = \frac{dx^i}{dt} \gamma(u) = \gamma(u) u^i$

where we have used the chain rule and the relationships

$u^i = {dx^i \over dt } \,,\quad \frac{dt}{d\tau} = \gamma (u)$

Thus, we find for the four-velocity $\mathbf{U}$:

$\mathbf{U} = \gamma \begin{bmatrix} c\\ \vec{u} \\ \end{bmatrix} .$

In terms of the synchronized clocks and rulers associated with a particular slice of flat spacetime, the three spacelike components of four-velocity define a traveling object's proper velocity $\gamma \vec{u} = d\vec{x}/d\tau$ i.e. the rate at which distance is covered in the reference map frame per unit proper time elapsed on clocks traveling with the object.