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A null geodesic is characterized by the vanishing of the tangent-vector norm, expressed as $\tilde{T}\cdot\tilde{T} \equiv g_{ij}T^iT^j = 0$. A non-null geodesic is distinguished by having a nonzero tangent-vector norm, $g_{ij}T^iT^j \neq 0$.

Conditions: $\tilde{T}$ is the tangent vector to the geodesic.; $g_{ij}$ are metric components.; The equality to zero defines the null case, and nonzero defines the non-null case.

A null geodesic is characterized by the vanishing of the tangent-vector norm, expressed as $\tilde{T}\cdot\tilde{T} \equiv g_{ij}T^iT^j = 0$. A non-null geodesic is distinguished by having a nonzero tangent-vector norm, $g_{ij}T^iT^j \neq 0$.

Conditions: $\tilde{T}$ is the tangent vector to the geodesic.; $g_{ij}$ are metric components.; The equality to zero defines the null case, and nonzero defines the non-null case.

An affine parameter is the special choice of the curve parameter $\lambda$ for which the scalar proportionality function $f(\lambda)$ in the tangent evolution equation vanishes identically. Specifically, the tangent vector $\tilde{T}$ satisfies $\frac{d\tilde{T}}{d\lambda} = f(\lambda)\tilde{T}$, and the parameter is affine if $f(\lambda)=0$ for all $\lambda$.

Conditions: $\tilde{T}$ is the tangent to the geodesic.; $f(\lambda)$ is a scalar function of the parameter.; The affine case is the subcase $f(\lambda)=0$ for all $\lambda$.

An affine parameter is the special choice of the curve parameter $\lambda$ for which the scalar proportionality function $f(\lambda)$ in the tangent evolution equation vanishes identically. Specifically, the tangent vector $\tilde{T}$ satisfies $\frac{d\tilde{T}}{d\lambda} = f(\lambda)\tilde{T}$, and the parameter is affine if $f(\lambda)=0$ for all $\lambda$.

Conditions: $\tilde{T}$ is the tangent to the geodesic.; $f(\lambda)$ is a scalar function of the parameter.; The affine case is the subcase $f(\lambda)=0$ for all $\lambda$.

A geodesic between two points is defined as the shortest length curve joining those two points. The video notes that while other equivalent definitions exist, this shortest-path formulation is the one used to frame the subsequent variational problem.

Conditions: Applies to a curve connecting two specified points in space-time.; The definition is presented as one usable formulation among equivalent alternatives.

A geodesic between two points is defined as the shortest length curve joining those two points. The video notes that while other equivalent definitions exist, this shortest-path formulation is the one used to frame the subsequent variational problem.

Conditions: Applies to a curve connecting two specified points in space-time.; The definition is presented as one usable formulation among equivalent alternatives.