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Line element

In differential geometry, the line element, commonly denoted as ds^2, is a quadratic differential form that defines the infinitesimal squared arc length or interval between two infinitesimally close points on a manifold, given by ds^2 = g_{ij} \, dx^i \, dx^j, where g_{ij} are the components of the metric tensor and dx^i represent infinitesimal coordinate differentials.^[1] This expression encapsulates the intrinsic geometry of the space, allowing for the measurement of distances, angles, and curvatures independent of any embedding in a higher-dimensional Euclidean space.^[2] In Riemannian geometry, the metric tensor g_{ij} is a smooth, symmetric, and positive definite (0,2)-tensor field on the manifold, providing the coefficients for the line element and inducing an inner product on the tangent spaces at each point, enabling the computation of lengths of tangent vectors as \sqrt{g_{ij} v^i v^j} for a vector v.^[1] In Riemannian geometry, the arc length along a curve is obtained by integrating the line element: L = \int_a^b \sqrt{g_{ij} \frac{dx^i}{dt} \frac{dx^j}{dt}} \, dt, generalizing the Euclidean distance to curved spaces.^[1] For surfaces embedded in \mathbb{R}^3, the line element corresponds to the first fundamental form, expressed as ds^2 = E \, du^2 + 2F \, du \, dv + G \, dv^2, where E = \mathbf{r}_u \cdot \mathbf{r}_u, F = \mathbf{r}_u \cdot \mathbf{r}_v, and G = \mathbf{r}_v \cdot \mathbf{r}_v for a parametrization \mathbf{r}(u,v), capturing the surface's intrinsic metric properties.^[2] In pseudo-Riemannian geometry, such as spacetime, the metric tensor has an indefinite signature, and the line element defines spacetime intervals, with proper time for timelike paths given by \tau = \int \sqrt{ - ds^2 }. Beyond pure mathematics, the line element plays a central role in physics, particularly in general relativity, where it describes the geometry of spacetime via metrics like the Minkowski line element ds^2 = -dt^2 + dx^2 + dy^2 + dz^2 (in units where c = 1) in flat spacetime or the Schwarzschild metric ds^2 = -(1 - 2m/r) dt^2 + (1 - 2m/r)^{-1} dr^2 + r^2 d\theta^2 + r^2 \sin^2\theta d\phi^2 (in units where G = c = 1) for black holes, determining geodesics as paths of extremal proper time or distance.^[2] In Riemannian geometry, it facilitates the study of geodesics, curvature, and isometries, with examples in curvilinear coordinates such as spherical systems yielding ds^2 = dr^2 + r^2 d\theta^2 + r^2 \sin^2\theta d\phi^2, highlighting how the metric adapts to the coordinate choice while preserving geometric invariants.^[2]

Fundamentals

Definition and arc length

The line element emerged from the calculus of variations in the 18th century, where it served as a foundational tool for determining geodesics—the curves of extremal length on surfaces. Leonhard Euler laid the groundwork in his 1744 treatise Methodus inveniendi lineas curvas maximi minimive proprietate gaudentes, applying variational principles to minimize path lengths and deriving equations now known as the Euler-Lagrange equations for such problems.^[3] Joseph-Louis Lagrange advanced these ideas in 1760, refining the analytical framework to eliminate purely geometric constructions and extending applications to minimal surfaces and geodesics.^[3] Carl Friedrich Gauss further developed the concept in 1827, introducing the line element as a measure intrinsic to curved surfaces, independent of embedding in Euclidean space, which enabled the study of geodesic properties without reference to external coordinates.^[4] In modern differential geometry, the line element ds quantifies the infinitesimal arc length along a smooth curve \gamma in a metric space, parameterized by local coordinates x^i(t), where t varies over an interval [t_1, t_2]. It is defined as ds = \sqrt{ds^2}, with ds^2 denoting the squared line element, a positive definite quadratic form that captures the local geometry; the metric tensor encodes this form, ensuring coordinate independence.^[5] Infinitesimally, ds arises from the norm of the tangent vector \dot{\gamma}(t) = \frac{dx^i}{dt} \frac{\partial}{\partial x^i}, representing the instantaneous direction and magnitude of motion along the curve at each point. This interpretation aligns with first principles: as the parameter increment dt approaches zero, ds approximates the Euclidean distance between nearby points on the curve, scaled by the local metric structure.^[6] The finite arc length s of the curve from t_1 to t_2 is obtained by integrating the line element:

s = \int_{t_1}^{t_2} ds = \int_{t_1}^{t_2} \sqrt{ds^2} \, dt,

where the integrand is the speed \sqrt{ds^2 / dt^2}, ensuring reparameterization invariance when the curve is regular (nonzero speed).^[5] For proper setup with parametric curves, one assumes the parameterization is smooth and the tangent vector nowhere vanishes, allowing the integral to accumulate the total length along the path.^[7] As a representative example, consider a simple parametric curve in the plane, such as the graph of a parabola traced by \gamma(t) = (t, t^2 / 2) for t \in [0, 1]. The arc length is computed as s = \int_0^1 \sqrt{1 + t^2} \, dt = \frac{1}{2} \left[ t \sqrt{1 + t^2} + \sinh^{-1} t \right]_0^1 \approx 1.148, illustrating how the integral aggregates infinitesimal segments to yield the total curved distance, exceeding the straight-line separation of endpoints.^[7]

Relation to the metric tensor

The metric tensor g_{ij} is defined as a smooth, symmetric, non-degenerate bilinear form on the tangent spaces of a manifold, assigning an inner product to each tangent space T_p M at every point p \in M.^[8] In local coordinates, its components g_{ij} form a symmetric matrix that varies smoothly across the manifold.^[8] The squared line element ds^2 is expressed using the metric tensor via the equation

ds^2 = g_{ij} \, dx^i \, dx^j,

where the Einstein summation convention is employed, implying summation over repeated indices i and j from 1 to the dimension of the manifold.^[8] This quadratic form captures the infinitesimal squared distance between nearby points.^[8] Under a coordinate transformation x^i = x^i(\tilde{x}^k), the components of the metric tensor transform as \tilde{g}_{kl} = \frac{\partial x^i}{\partial \tilde{x}^k} \frac{\partial x^j}{\partial \tilde{x}^l} g_{ij}, ensuring that ds^2 remains invariant, thereby defining distances independently of the choice of coordinates.^[8] For Riemannian manifolds, the metric is positive-definite, meaning g_{ij} v^i v^j > 0 for all non-zero vectors v, with signature (n, 0) in n-dimensions.^[8] In pseudo-Riemannian cases, the metric has an indefinite signature (p, q) with p + q = n and both p, q > 0, allowing for both positive and negative eigenvalues, as in Lorentzian metrics with signature (1, n-1). In cases where g_{ij} \frac{dx^i}{dt} \frac{dx^j}{dt} > 0 (e.g., in Riemannian manifolds or spacelike curves in pseudo-Riemannian ones), the arc length s of a curve \gamma(t) parameterized by t \in [a, b] is

s = \int_a^b \sqrt{g_{ij} \frac{dx^i}{dt} \frac{dx^j}{dt}} \, dt,

which is independent of the parameterization.^[8] For unit speed curves, the parameterization satisfies g_{ij} \frac{dx^i}{dt} \frac{dx^j}{dt} = 1, normalizing the speed to unity along the curve. In pseudo-Riemannian manifolds, for timelike curves where the form is negative, proper time is defined using \int_a^b \sqrt{ - g_{ij} \frac{dx^i}{dt} \frac{dx^j}{dt} } \, dt (assuming signature with negative time component); further details are covered in subsequent sections on spacetime applications.^[8]

Euclidean spaces

Cartesian coordinates

In flat Euclidean space, the line element in Cartesian coordinates provides the simplest expression for measuring infinitesimal distances. For an n-dimensional space with coordinates x^1, x^2, \dots, x^n, the line element is given by

ds^2 = \delta_{ij} \, dx^i \, dx^j = \sum_{i=1}^n (dx^i)^2,

where \delta_{ij} is the Kronecker delta, which equals 1 if i = j and 0 otherwise, representing the flat metric tensor in this coordinate system.^[9] This form arises as a special case of the general line element ds^2 = g_{ij} \, dx^i \, dx^j, where the metric components g_{ij} are constant and diagonal. The arc length s along a curve parameterized by t, with position vector \mathbf{r}(t) = (x^1(t), \dots, x^n(t)), is computed by integrating the line element:

s = \int_a^b \sqrt{\sum_{i=1}^n \left( \frac{dx^i}{dt} \right)^2} \, dt = \int_a^b \|\mathbf{r}'(t)\| \, dt.

For a straight line between two points, this integral simplifies directly to the Euclidean distance \sqrt{\sum_{i=1}^n (\Delta x^i)^2}, generalizing the Pythagorean theorem from two dimensions—where ds^2 = dx^2 + dy^2 yields the hypotenuse length \sqrt{(\Delta x)^2 + (\Delta y)^2}—to higher dimensions.^[10]^[11] Under orthogonal transformations, such as rotations, the Cartesian coordinates can be changed via a matrix R with R^T R = I, preserving the flat metric: the line element remains ds^2 = \sum_{i=1}^n (dx'^i)^2 in the new coordinates x'^i = R^i_j x^j, since the metric tensor transforms as \delta_{ij} under such bases.^[12] This invariance ensures that distances and angles are unchanged, reflecting the isotropy of Euclidean space. In applications, the line element primarily defines distances between points along paths, essential for optimization problems like shortest paths (geodesics, which are straight lines here) and in vector calculus for line integrals. While it extends to higher-dimensional forms like the volume element dV = dx^1 \wedge \cdots \wedge dx^n for integration over regions, the focus remains on one-dimensional arc lengths.^[13] Historically, the conceptual foundation traces to Euclidean geometry in Euclid's Elements (circa 300 BCE), where the Pythagorean theorem underpins finite distances, evolving into the infinitesimal line element through 17th- and 18th-century calculus developments in arc length by Leibniz and Euler, and formalized in n-dimensional vector spaces by the 19th century.^[4]

Curvilinear coordinates

In Euclidean space, orthogonal curvilinear coordinates provide a system where the coordinate curves intersect at right angles, allowing for a simplified representation of distances and geometry that aligns with the inherent symmetries of certain problems.^[14] Common examples include two-dimensional polar coordinates (r, \theta), where r is the radial distance and \theta is the azimuthal angle, and three-dimensional cylindrical coordinates (r, \theta, z) or spherical coordinates (r, \theta, \phi), with \theta as the polar angle and \phi as the azimuthal angle.^[15] These systems are particularly advantageous for problems exhibiting rotational or spherical symmetry, such as calculating fields around circular or spherical objects, where the coordinate choice reduces the complexity of equations compared to Cartesian coordinates—for instance, describing a circle requires a constant r in polar coordinates rather than a varying relation like x^2 + y^2 = r^2.^[16] The line element in an n-dimensional orthogonal curvilinear coordinate system (u^1, u^2, \dots, u^n) takes the diagonal form

ds^2 = h_1^2 (du^1)^2 + h_2^2 (du^2)^2 + \dots + h_n^2 (du^n)^2,

where the h_i are the scale factors, also known as Lamé coefficients, which account for the stretching or compression along each coordinate direction.^[14] These scale factors are derived from the position vector \mathbf{r}(u^1, u^2, \dots, u^n) in Euclidean space, with h_i = \left| \frac{\partial \mathbf{r}}{\partial u^i} \right|, representing the magnitude of the infinitesimal displacement vector along the u^i-direction.^[15] In two-dimensional polar coordinates, the scale factors are h_r = 1 and h_\theta = r, yielding the line element

ds^2 = dr^2 + r^2 d\theta^2.

The arc length along a path is found by integrating ds = \sqrt{dr^2 + r^2 d\theta^2}; for example, the circumference of a circle at fixed r is \int_0^{2\pi} r \, d\theta = 2\pi r.^[16] For three-dimensional spherical coordinates, the scale factors are h_r = 1, h_\theta = r, and h_\phi = r \sin \theta, giving

ds^2 = dr^2 + r^2 d\theta^2 + r^2 \sin^2 \theta \, d\phi^2.

Here, the arc length integral along, say, a great circle (fixed r, \theta = \pi/2, varying \phi) simplifies to \int_0^{2\pi} r \, d\phi = 2\pi r, highlighting the coordinate system's efficiency in symmetric geometries.^[15] This form emerges as a special case of the Cartesian line element when scale factors are unity, but curvilinear systems introduce position-dependent factors to better capture curved symmetries.^[14]

General manifolds

Riemannian manifolds

In a Riemannian manifold, the line element generalizes the notion of infinitesimal distance to curved spaces, providing a way to measure lengths and angles intrinsically without reference to an embedding. A Riemannian manifold is a smooth manifold equipped with a positive-definite metric tensor g_{ij}(x) at each point, which varies smoothly with position x. The line element is expressed in local coordinates as

ds^2 = g_{ij}(x) \, dx^i \, dx^j,

where summation over repeated indices i, j = 1, \dots, n is implied (Einstein notation), and the components g_{ij} form a symmetric positive-definite matrix that may include off-diagonal terms, reflecting the possible non-orthogonality of the coordinate basis. This form allows the metric to capture the local geometry, including curvature, by how g_{ij} changes across the manifold.^[17]^[12] Under a change of coordinates from x^i to x'^k, the line element remains invariant, ensuring that distances are independent of the coordinate choice. The metric transforms as a tensor via the Jacobian matrix of the transformation:

g'_{kl}(x') = \frac{\partial x^m}{\partial x'^k} \frac{\partial x^n}{\partial x'^l} g_{mn}(x),

where the partial derivatives account for how infinitesimal displacements dx^i map to dx'^k. This transformation law preserves the positive-definiteness and the geometric structure, confirming that the metric defines an intrinsic geometry on the manifold.^[18]^[19] Geodesics on a Riemannian manifold are the curves that locally minimize the arc length, analogous to straight lines in Euclidean space. The arc length L of a curve \gamma(t) parameterized by t \in [a, b] is given by

L = \int_a^b \sqrt{g_{ij}(\gamma(t)) \frac{dx^i}{dt} \frac{dx^j}{dt}} \, dt.

To find the extremal paths, one applies the calculus of variations: the Euler-Lagrange equations for the Lagrangian \mathcal{L} = \sqrt{g_{ij} \dot{x}^i \dot{x}^j} (where \dot{x} = dx/dt) yield the geodesic equation

\frac{d^2 x^k}{dt^2} + \Gamma^k_{ij} \frac{dx^i}{dt} \frac{dx^j}{dt} = 0,

with the Christoffel symbols of the second kind defined as

\Gamma^k_{ij} = \frac{1}{2} g^{kl} \left( \partial_i g_{jl} + \partial_j g_{il} - \partial_l g_{ij} \right),

where g^{kl} is the inverse metric (g^{kl} g_{lm} = \delta^k_m) and \partial_i = \partial / \partial x^i. These symbols encode the curvature effects through the derivatives of the metric.^[20]^[21] A concrete example is the metric on a two-dimensional surface of revolution, obtained by rotating a curve in the rz-plane around the z-axis in \mathbb{R}^3. If the generating curve is parameterized as r = r(z) with z as the meridional coordinate and \theta the azimuthal angle, the induced Riemannian metric from the Euclidean embedding is

ds^2 = \left(1 + \left(\frac{dr}{dz}\right)^2 \right) dz^2 + r(z)^2 \, d\theta^2,

which is diagonal but position-dependent, illustrating how embedding in higher dimensions determines the intrinsic geometry. For instance, a sphere of radius R arises when r(z) = \sqrt{R^2 - z^2} for |z| < R, yielding the metric ds^2 = \frac{R^2}{R^2 - z^2} dz^2 + (R^2 - z^2) d\theta^2, though geodesics (great circles) can be computed intrinsically without the embedding.^[22]^[23]^[24] The invariant nature of ds^2 under diffeomorphisms underscores its role in defining the manifold's geometry solely through the metric, independent of any external coordinates or embeddings. This tensorial object encapsulates all information about lengths, angles, and volumes, forming the foundation for studying curvature via the Riemann tensor derived from second derivatives of g_{ij}. In contrast to curvilinear coordinates in Euclidean space, where the metric is derived from a flat background and often diagonalizes to orthogonal form, the general Riemannian case admits arbitrary variation and mixing of components, enabling the description of truly curved geometries.^[25]^[12]

Pseudo-Riemannian manifolds

In pseudo-Riemannian manifolds, the line element generalizes the Riemannian case by incorporating a metric tensor with an indefinite signature, allowing for both positive and negative eigenvalues in its quadratic form. The signature is denoted as (p, q), where p is the number of positive eigenvalues and q the number of negative ones, with the total dimension n = p + q. This contrasts with the positive-definite (n, 0) signature of Riemannian metrics, enabling structures that distinguish between different types of directions in the tangent space. A prominent example is the Lorentzian signature (3, 1) or (1, 3), commonly used in spacetime contexts, where the metric has three spatial positive directions and one temporal negative direction (or vice versa, depending on convention).^[26] The line element on a pseudo-Riemannian manifold is expressed in local coordinates as

ds^2 = g_{ij} \, dx^i \, dx^j,

where g_{ij} is the indefinite metric tensor, symmetric and non-degenerate but not positive definite. Unlike in the Riemannian setting, the sign of ds^2 classifies infinitesimal displacements: spacelike if ds^2 > 0, timelike if ds^2 < 0, and null if ds^2 = 0. This classification arises directly from the signature, partitioning the tangent space into orthogonal subspaces corresponding to positive, negative, and degenerate directions. Under smooth coordinate transformations, the line element transforms as a scalar, preserving its form and the metric's signature locally, just as in the Riemannian case.^[26] The indefinite nature introduces a causal structure, geometrically manifested through light cones at each point. These cones are the sets of null directions where ds^2 = 0, forming double cones that separate timelike interiors (where causal influences can propagate) from spacelike exteriors (where they cannot). In a manifold of Lorentzian signature, the light cone divides the tangent space into future and past timelike regions, with spacelike vectors lying outside. This structure is intrinsic to the metric and holds generally, without reference to flat models.^[27] For an illustrative example, consider a two-dimensional pseudo-Riemannian manifold with (1, 1) signature, such as a curved surface equipped with a metric ds^2 = -f(u,v)^2 \, du^2 + g(u,v)^2 \, dv^2, where f and g are positive smooth functions ensuring non-degeneracy. Here, curves with ds^2 < 0 are timelike, tracing paths within the light cones, while the metric's curvature affects geodesic behavior without altering the fundamental interval classification. Such manifolds demonstrate how the line element adapts to indefinite metrics while retaining tensorial properties under diffeomorphisms.^[26]

Applications in spacetime

Minkowski spacetime

In Minkowski spacetime, the line element describes the invariant spacetime interval in flat four-dimensional spacetime, as formulated in special relativity. The metric takes the form

ds^2 = -c^2 dt^2 + dx^2 + dy^2 + dz^2,

where c is the speed of light, t is the coordinate time, and x, y, z are spatial coordinates, employing the metric signature (-, +, +, +). Equivalently, in abstract index notation, it is expressed as ds^2 = \eta_{\mu\nu} dx^\mu dx^\nu, with \eta_{\mu\nu} = \operatorname{diag}(-1, 1, 1, 1) (in units where c=1). This form was introduced by Hermann Minkowski to geometrize Einstein's special relativity, unifying space and time into a single manifold where the line element remains invariant under coordinate transformations.^[28] For timelike paths, where ds^2 < 0, the line element defines the proper time \tau experienced by an observer along their worldline. The infinitesimal proper time is given by d\tau = \frac{|ds|}{c} = \sqrt{-ds^2}/c, and the total proper time for a path is the integral \tau = \frac{1}{c} \int \sqrt{-ds^2}. This quantity is Lorentz invariant, representing the time measured by a clock moving with the observer, distinct from coordinate time in any inertial frame. In special relativity, this leads to time dilation effects, where moving clocks tick slower relative to stationary ones.^[29] The line element is preserved under Lorentz transformations, which include spatial rotations and boosts between inertial frames. Boosts along the x-direction, for instance, mix time and space coordinates while leaving ds^2 unchanged, embodying the principle of relativity that physical laws are the same in all inertial frames. These transformations form the Poincaré group, ensuring the invariance of the spacetime interval as a fundamental postulate.^[29]^[28] Null geodesics correspond to paths where ds^2 = 0, defining the worldlines of light rays propagating at speed c. These form light cones at each event, separating timelike (inside the cone) and spacelike (outside) intervals, with the cone's generators tracing null directions. In inertial coordinates, such paths satisfy dx = \pm c dt (with dy = dz = 0 for radial propagation), illustrating the causal structure of spacetime.^[29] Minkowski spacetime uses Cartesian-like coordinates tied to inertial frames, where observers are at rest relative to the axes. Boosts can be parameterized using rapidity \phi, defined by v = c \tanh \phi, which adds hyperbolically under velocity composition: \phi_{\text{total}} = \phi_1 + \phi_2. This parameterization simplifies Lorentz transformations, expressing them via hyperbolic functions and highlighting the geometric analogy to rotations in Euclidean space.^[28]

Curved spacetimes

In general relativity, the line element describing curved spacetimes takes the form

ds^2 = g_{\mu\nu}(x) \, dx^\mu \, dx^\nu,

where the metric tensor g_{\mu\nu} varies with position and is determined by the Einstein field equations G_{\mu\nu} = \frac{8\pi G}{c^4} T_{\mu\nu}, which relate spacetime curvature to the distribution of matter and energy.^[30] These equations, formulated in 1915, provide the foundation for modeling gravitational effects as geometric distortions of spacetime.^[30] A seminal exact solution is the Schwarzschild metric, derived by Karl Schwarzschild in 1916 shortly after the field equations, which applies to the vacuum exterior of a static, spherically symmetric mass M and is given by

ds^2 = -\left(1 - \frac{2GM}{c^2 r}\right) c^2 dt^2 + \left(1 - \frac{2GM}{c^2 r}\right)^{-1} dr^2 + r^2 d\Omega^2,

where d\Omega^2 = d\theta^2 + \sin^2\theta \, d\phi^2.^[31] This metric is interpreted in modern general relativity as describing the spacetime around a non-rotating black hole, featuring an event horizon at r_s = 2GM/c^2, the boundary beyond which radial null geodesics—paths of light rays—cannot escape.^[32] For timelike observers, proper time \tau along geodesics is measured by c^2 d\tau^2 = -ds^2, which slows relative to coordinate time t near the horizon due to gravitational redshift.^[33] Radial null geodesics in this geometry, satisfying ds^2 = 0 with d\theta = d\phi = 0, approach the horizon in finite proper distance but infinite coordinate time for distant observers, highlighting the causal structure of the spacetime.^[33] In cosmology, the Friedmann–Lemaître–Robertson–Walker (FLRW) metric models homogeneous and isotropic expanding universes as

ds^2 = -c^2 dt^2 + a(t)^2 \left[ \frac{dr^2}{1 - k r^2} + r^2 d\Omega^2 \right],

where a(t) is the time-dependent scale factor governing expansion and k parameterizes spatial curvature (k = 0, +1, -1 for flat, closed, or open geometries, respectively). This form, first proposed in 1922, arises as a solution to the Einstein equations with a perfect fluid source and underpins the standard Big Bang model. For rotating masses, the Kerr metric extends the Schwarzschild solution to include angular momentum, describing the spacetime around rotating black holes while preserving asymptotic flatness.^[34] Geodesic motion in these curved spacetimes follows the equation

\frac{d^2 x^\mu}{d\tau^2} + \Gamma^\mu_{\nu\sigma} \frac{dx^\nu}{d\tau} \frac{dx^\sigma}{d\tau} = 0,

where the Christoffel symbols \Gamma^\mu_{\nu\sigma} = \frac{1}{2} g^{\mu\lambda} (\partial_\nu g_{\sigma\lambda} + \partial_\sigma g_{\nu\lambda} - \partial_\lambda g_{\nu\sigma}) encode the connection and cause paths to deviate from straight lines in flat space, reflecting gravitational deflection.^[35] This formulation, central to general relativity, predicts phenomena like orbital precession and light bending observed in astrophysical tests.^[35]

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Revisiting timelike and null geodesics in the Schwarzschild spacetime
Oct 18, 2022 · The main result presented in this paper is a concise formula describing all types of timelike and null trajectories in the Schwarzschild metric ...
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Gravitational Field of a Spinning Mass as an Example of ...
Feb 21, 2014 · A novel solution to Einstein's gravitational equations, discovered in 1963, turned out to describe the curvature of space around every astrophysical black hole.
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[PDF] General Relativity - DAMTP - University of Cambridge
If we wished, we could substitute the Christoffel symbols into the geodesic equation (1.7) to determine the equations of motion. However, it's typically ...