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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.