Acceleration (differential geometry)

In mathematics and physics, acceleration is the rate of change of velocity of a curve with respect to a given linear connection. This operation provides us with a measure of the rate and direction of the "bend".

Formal definition

Let be given a differentiable manifold $M$, considered as spacetime (not only space), with a connection $\Gamma$. Let $\gamma :ℝ\to M$ be a curve in $M$ with tangent vector, i.e. (spacetime) velocity, $\dot{\gamma }(\tau )$, with parameter $\tau$.

The (spacetime) acceleration vector of $\gamma$ is defined by ${\mathrm{\nabla }}_{\dot{\gamma }}\dot{\gamma }$, where $\mathrm{\nabla }$ denotes the covariant derivative associated to $\Gamma$.

It is a covariant derivative along $\gamma$, and it is often denoted by

${\mathrm{\nabla }}_{\dot{\gamma }}\dot{\gamma }=\frac{\mathrm{\nabla }\dot{\gamma }}{d\tau }.$

With respect to an arbitrary coordinate system $(x^{\mu })$, and with $({\Gamma }^{\lambda }{}_{\mu \nu })$ being the components of the connection (i.e., covariant derivative ${\mathrm{\nabla }}_{\mu }:={\mathrm{\nabla }}_{\partial /\partial x^{\mu }}$) relative to this coordinate system, defined by

${\mathrm{\nabla }}_{\partial /\partial x^{\mu }}\frac{\partial }{\partial x^{\nu }}={\Gamma }^{\lambda }{}_{\mu \nu }\frac{\partial }{\partial x^{\lambda }},$

for the acceleration vector field $a^{\mu }:=({\mathrm{\nabla }}_{\dot{\gamma }}\dot{\gamma }{)}^{\mu }$ one gets:

$a^{\mu }=v^{\rho }{\mathrm{\nabla }}_{\rho }{v}^{\mu }=\frac{d{v}^{\mu }}{d\tau }+{\Gamma }^{\mu }{}_{\nu \lambda }{v}^{\nu }{v}^{\lambda }=\frac{d^2{x}^{\mu }}{d{\tau }^2}+{\Gamma }^{\mu }{}_{\nu \lambda }\frac{d{x}^{\nu }}{d\tau }\frac{d{x}^{\lambda }}{d\tau },$

where $x^{\mu }(\tau ):={\gamma }^{\mu }(\tau )$ is the local expression for the path $\gamma$, and $v^{\rho }:=(\dot{\gamma }{)}^{\rho }$.

The concept of acceleration is a covariant derivative concept. In other words, in order to define acceleration an additional structure on $M$ must be given.

Using abstract index notation, the acceleration of a given curve with unit tangent vector ${\xi }^a$ is given by ${\xi }^b{\mathrm{\nabla }}_b{\xi }^a$.

See also

• Acceleration
• Covariant derivative