- #1
homer
- 46
- 0
Homework Statement
Consider a particle moving through Minkowski space with worldline [itex]x^\mu(\lambda)[/itex]. Here [itex]\lambda[/itex] is a continuous parameter which labels different points on the worldline and [itex]x^\mu = (t,x,y,z)[/itex] denotes the usual Cartesian coordinates. We will denote [itex]\partial/\partial \lambda[/itex] by a dot. In this problem we will assume that the trajectory of the particle obeys the equation of motion [itex]\ddot{x}^\mu = 0[/itex].
(a) Show that this trajectory describes a particle moving at constant velocity.
(b) Show that this trajectory is a local minimum of the action
[tex]
S = \int ds = \int d\lambda\,\sqrt{\eta_{\mu\nu} \dot{x}^\mu \dot{x}^\nu}
[/tex]
(c) Consider a new coordinate system [itex]x^{\mu'}[/itex] which differs from the original Cartesian coordinate system; as before, the Cartesian coordinates [itex]x^\mu[/itex] can be written as a function of these new coordinates [itex]x^\mu = x^\mu(x^{\mu'})[/itex]. Show that the equation of motion can be written in these new [itex]x^{\mu'}[/itex] coordinates as
[tex]
\ddot{x}^{\mu'} + \Gamma_{\nu'\lambda'}^{\mu'}\dot{x}^{\nu'}\dot{x}^{\lambda'} = 0
[/tex]
for some [itex]\Gamma^{\mu'}_{\nu'\lambda'}[/itex] which you must compute; [itex]\Gamma^{\mu'}_{\nu'\lambda'}[/itex] is known as the Christoffel symbol. These extra Christoffel terms in the equation of motion can be thought of as "fictitious" forces that arise in an accelerated reference frame.
(* I only need help with part c *)
Homework Equations
Jacobian matrix:
[tex]
J_{\beta}^{\alpha'} = \frac{\partial x^{\alpha'}}{\partial x^{\beta}}
[/tex]
Derivaitves:
[itex]\dot{x}^{\mu'} = J_{\mu}^{\mu'} \dot{x}^\mu[/itex]
[itex]\dot{x}^{\mu} = J_{\mu'}^{\mu} \dot{x}^{\mu'}[/itex]Notation:
[itex]\partial_\mu = \partial/\partial x^\mu[/itex]
The Attempt at a Solution
I feel like I'm just spinning my wheels on this problem, and don't know where to go with it. This is from PHYS 514: General Relativity at McGill. Since I'm not actually taking this class I have no graders nor TA's ask when I get stuck as I learn how to do summation convention calculations. We haven't introduced Christoffel symbols yet in the class videos for the week of this assignment, so I assume we should only find them by deriving the equation of motion in the primed coordinate system. This is what I have come up with so far, but I have no idea if I made an error because this is my first time doing these kind of calculations (in this notation I mean).
Recall we earlier showed that
[tex]
\dot{x}^{\mu'} = J_{\mu}^{\mu'}\dot{x}^\mu\:,
\qquad
\dot{x}^\nu = J_{\nu'}^{\nu} \dot{x}^{\nu'}.
[/tex]
Differentiating the left equation of with respect to [itex]\lambda[/itex] then gives
[tex]
\ddot{x}^{\mu'} = \frac{d}{d\lambda}\big(J_{\mu}^{\mu'}\dot{x}^\mu\big)
= \frac{d}{d\lambda}\big(J_{\mu}^{\mu'}\big)\,\dot{x}^\mu + J_{\mu}^{\mu'}\ddot{x}^\mu.
[/tex]
But since [itex]\ddot{x}^\mu = 0[/itex], this simplifies to
[tex]
\ddot{x}^{\mu'} = \frac{d}{d\lambda}\big(J_{\mu}^{\mu'}\big)\,\dot{x}^\mu.
[/tex]
We can compute the derivative of the Jacobian by swapping the order of derivatives as
[tex]
\frac{d}{d\lambda}\big(J_{\mu}^{\mu'}\big)
=
\frac{d}{d\lambda}\Big(\partial_\mu x^{\mu'}\Big)
= \partial_{\mu}\dot{x}^{\mu'}.
[/tex]
Thus we have
[tex]
\ddot{x}^{\mu'} = \big(\partial_\mu \dot{x}^{\mu'}\big)\dot{x}^\mu.
[/tex]
Since we can write [itex]\dot{x}^{\mu'} = J_{\nu}^{\mu'}\dot{x}^\nu[/itex] and [itex]\dot{x}^\mu = J_{\nu'}^{\mu}\dot{x}^{\nu'}[/itex], we can write the equation above as
[tex]
\ddot{x}^{\mu'} = \partial_\mu\big(J^{\mu'}_{\nu}\dot{x}^\nu\big)J_{\nu'}^{\mu}\dot{x}^{\nu'}.
[/tex]
Writing [itex]\dot{x}^\nu = J^{\nu}_{\lambda'}\dot{x}^{\lambda'}[/itex], this equation becomes
[tex]
\ddot{x}^{\mu'} = \partial_\mu\big(J^{\mu'}_{\nu}J^{\nu}_{\lambda'}\dot{x}^{\lambda'}\big)J_{\nu'}^{\mu}\dot{x}^{\nu'}.
[/tex]
Applying the product rule for differentiation, we thus find
\begin{align*}
\ddot{x}^{\mu'}
& = \Big(
\partial_\mu\big(J^{\mu'}_{\nu}J^{\nu}_{\lambda'}\big)\dot{x}^{\lambda'} +
J_{\nu}^{\mu'}J^{\nu}_{\lambda'}\partial_\mu \dot{x}^{\lambda'}
\Big)J_{\nu'}^{\mu}\dot{x}^{\nu'} \\
& =
J_{\nu'}^{\mu}\partial_\mu\big(
J^{\mu'}_{\nu}J^{\nu}_{\lambda'}
\big)\dot{x}^{\lambda'}\dot{x}^{\nu'} +
J_{\nu'}^{\mu}J_{\nu}^{\mu'}J^{\nu}_{\lambda'}
\big(\partial_\mu \dot{x}^{\lambda'}\big)\dot{x}^{\nu'}.
\end{align*}
AND HERE IS WHERE I'M STUCK
Any help would be greatly appreciated, as this is a somewhat daunting subject to go it alone.
Last edited: