# Geodesics

##### Straight Lines

When we think of a straight line, we usually think of a line in the Euclidean sense; that is, $c(t)=p+tX$, where $p$ is a point contained in the line, $t$ is a real number, and $X$ is a vector that points parallel to the line. If we consider Euclidean space as a manifold, we would say that $X$ is in the tangent space $T_{c(t)}(\mathbb E^n)$, because $c'(t)=X$. One important observation to make is that all along $c(t)$, $X$ never changes; i.e., we never accelerate. That is, if we move along the curve, we never speed up or slow down, and we never turn.

In the language of my post on covariant derivatives, this is easy to express:

$\nabla_{c'} c' \equiv 0$

The geometric interpretation is simple here: in the direction of the velocity vector, the velocity vector doesn’t change. You can probably see the punchline coming by now. If we generalize to a curve $c(t)$ on a manifold $M$, $c(t)$ is a geodesic if $\nabla_{c'} c' \equiv 0$.

Now, you may notice that we can trace out the same curve if we tweak the parameter $t$ so that we could accelerate on the curve (we wouldn’t turn, but we could speed up or slow down). That is, we could have an alternate parametrization. But in order to have a geodesic, we need $\nabla_{c'} c' \equiv 0$, so $\nabla_{c'}\left = 2\left<\nabla_{c'} c',c' \right> = 0$, and therefore $\|c'\|$ is a constant along the curve. This gives us a unique parametrization of the curve, up to a constant scaling factor on the parameter. In fact, if we consider such a scaling factor, we get that $\nabla_{c'(st)} c'(st) = \nabla_{s c'(t)} s c'(t) = s^2\nabla_{c'(t)} c'(t) = 0$, so a geodesic with a constant scaling factor on its parameter is still a geodesic (and obviously has the same image). This motivates the following definition: if $\|c'\| = 1$ then the geodesic is called a normal geodesic.

##### The Exponential Map

Say that some curve $\gamma(t)$ is a geodesic. Then $\nabla_{\gamma'}\gamma' = 0$ is a second-order differential equation in $t$. If we assume that $\gamma(0) = p$, and $\gamma'(0) = v$, then we have the required conditions for existence and uniqueness of a solution to the differential equation. That is, given a point $p\in M$ and tangent vector $v\in T_p(M)$, there is a unique geodesic $\gamma_v$ that passes through $p$ with velocity $v$.

The exponential map $\text{exp}_p:T_p(M)\to M$ is defined as $\text{exp}_p(v) = \gamma_v(1)$, assuming that 1 is in the domain of $\gamma_v$. The exponential map is fairly important when talking about Riemannian manifolds, and it turns out that it is smooth and a local diffeomorphism. The latter means that there is a neighborhood around $p$ where its unique inverse exists. This inverse is the logarithmic map, or $\text{log}_p:M\to T_p(M)$.

The exponential map is so important, in fact, that it appears in many of the important theorems in Riemannian geometry, like the Hopf-Rinow Theorem and the Cartan-Hadamard Theorem. It’s also essential to understanding the effects of curvature on a Riemannian manifold.

##### Arc Length

At this point we can ask about the relationship between arc length and geodesics. Assume that we have some smooth function $\alpha : [a,b]\times(-\epsilon,\epsilon)\to M$. We can compute the change in arc length $L[c_s]$ over the family of curves $c_s = \alpha | [a,b]\times\{s\}$:

$\frac d{ds}L[c_s] = \frac d{ds}\int_a^b\left^{1/2}dt = \int_a^b\nabla_S\left^{1/2}dt$
$= \frac 1 2\int_a^b\left^{-1/2}\nabla_S\leftdt = \int_a^b\left^{-1/2}\left<\nabla_S T,T\right>dt$

The variables $S,T$ that we substitute here are fields of tangent vectors corresponding to the differential of $\alpha$ with respect to the variables $s,t$. The rest is just calculus. Since $s,t$ are independent of each other, we know that their derivatives commute and so we can say that $[T,V] = 0$. This means that we can make the switch $\nabla_S T = \nabla_T S$:

$\frac d{ds}L[c_s] = \int_a^b\left^{-1/2}\left<\nabla_T S,T\right>dt$
$= \int_a^b\left^{-1/2}\left(T\left-\left\right)dt$

If we consider the curve $c_0$, and consider that we can always reparametrize a curve without loss of generality so that $l = \left^{1/2}$ is a constant,

$\frac d{ds}L[c_s]\mid_{s = 0} = l^{-1} \left(\left\mid_a^b-\int_a^b\leftdt\right)$

This is called the first variation formula. The function $\alpha$ is called a variation. If we assume that all the $c_s$ are curves that join two points in $M$, then we know that $S$ vanishes at the endpoints. If we further assume that $c_0$ is a geodesic, then the integral vanishes (because $\nabla_T T = 0$). What this means is that geodesics are critical points of the arc length function $L$ for curves that join two points.

We can’t claim that a geodesic segment minimizes the distance between two points (though there is a unique minimizing geodesic segment; for that we need the second variation formula, which I won’t get into in this post). To see this, consider the case when $M$ is a sphere, with the usual angular metric. If we consider any two distinct points, there is a great circle path that joins them that is of length the angular distance between them, $\delta$. However, there is also a path of length $2\pi - \delta$ that goes around “the long way” that joins the points as well. This path happens to be the longest one that you can take, and it’s also a geodesic segment. Obviously this would be a maximum of the first variation formula.

It’s easy to see that the first variation formula gives us a lot of power in talking about the geometry of a Riemannian manifold. The source that I use actually motivates the definition of a geodesic from an effort to minimize the first variation formula. I prefer to motivate it from the “straight line” perspective.

##### Sources

Much of this material comes from Comparison Theorems in Riemannian Geometry by Jeff Cheeger and David G. Ebin.