- #1
Tomishiyo
- 18
- 1
Homework Statement
The metric for a given particle traveling in the presence of a gravitational field is [itex]g_{\mu\nu}=\eta_{\mu\nu}+h_{\mu\nu}[/itex], where [itex]\eta_{\mu\nu}[/itex] is the Minkowski metric, [itex]h_{00}=-2\phi[/itex] ([itex]\phi[/itex] the Newtonian gravitational potential); [itex]h_{i0}=0[/itex]; and [itex]h_{ij}=-2\phi\delta_{ij}[/itex]. Units are given in a system such that [itex]c=\hbar=k_B=1[/itex]. Find the equations of motion for a massive particle traveling in this field.
(a) Show that [itex]\Gamma^{0}_{\ \ 00}=\partial\phi/\partial t[/itex] and [itex]\Gamma^{i}_{\ \ 00}=\delta^{ij}\partial\phi/\partial x^j[/itex].
(b) Show that the time component of the geodesic equation implies that the energy [itex]p^0+m\phi[/itex] is conserved.
(c) Show that the space part of the geodesic equation lead to [itex]d^2x^i/dt^2=-m\delta^{ij}\partial\phi/\partial x^j[/itex] in agreement with Newtonian theory. Use the fact that the particle is nonrelativistic so [itex]p^0>>p^i[/itex].
Homework Equations
Affine Connections (or Christoffer Symbol) Equation:
[tex]\Gamma^{\mu}_{\ \ \alpha\beta}=\frac{g^{\mu\nu}}{2}\left(\frac{\partial g_{\alpha\nu}}{\partial x^{\beta}}+\frac{\partial g_{\beta\nu}}{\partial x^{\alpha}}-\frac{\partial g_{\alpha\beta}}{\partial x^{\nu}}\right)[/tex]
Geodesic Equation:
[tex]\frac{d^2x^{\mu}}{d\lambda^2}=-\Gamma^{\mu}_{\ \ \alpha\beta}\frac{dx^{\alpha}}{d\lambda}\frac{dx^{\beta}}{d\lambda}.[/tex]
The Attempt at a Solution
(a) I've managed this part, as long as I make a non-stated (the exercise and the textbook didn't say anything about it, I'm not omitting information in the exercise statement) weak field approximation: [itex]\phi<<1[/itex]. It seems to be a reasonable assumption, since if it is not the case I can see no other way of the connections yielding such a result, and because I've already seen such approximation in other textbooks. My trouble starts in the next item.
(b) Here is where trouble starts. First, I write the time component of the geodesic equation as:
[tex]\frac{d^2x^{0}}{d\lambda^2}=-\Gamma^{0}_{\ \ \alpha\beta}\frac{dx^{\alpha}}{d\lambda}\frac{dx^{\beta}}{d\lambda}=-\Gamma^{0}_{\ \ 00}\left(\frac{dx^{0}}{d\lambda}\right)^2-2\Gamma^{0}_{\ \ 0i}\frac{dx^{0}}{d\lambda}\frac{dx^{i}}{d\lambda}-\Gamma^{0}_{\ \ ij}\frac{dx^{i}}{d\lambda}\frac{dx^{j}}{d\lambda},[/tex]
since Christoffer symbols are symmetric with respect to the bottom indices. Here I am assuming the usual conventions that Greek indices run from 0 to 3 (0 time coordinate; 1,2,3 spatial coordinates), Latin indices run from 1 to 3, and [itex]\lambda[/itex] is a monotonically increasing motion parameter. The first needed Christoffer symbol is given by letter (a). The remaining two are:
[tex]\Gamma^{0}_{\ \ 0i}=\frac{\partial\phi}{\partial x^i}[/tex]
and
[tex]\Gamma^{0}_{\ \ ij}=-\delta_{ij}\frac{\partial\phi}{\partial t}.[/tex]
Please, if it is in any way unclear how did I obtained this connections or if you think that I've done it wrong (or my difficulties are related to that), do ask me to show the calculations. I've omitted them not for laziness, but simply because I don't think that is the problem and I think the problem will be better to visualize this way.
So, plugging in the connections the geodesic equation becomes:
[tex]\frac{d^2x^{0}}{d\lambda^2}=-\frac{\partial \phi}{\partial t}\left(\frac{dx^{0}}{d\lambda}\right)^2-2\frac{\partial \phi}{\partial x^i}\frac{dx^{0}}{d\lambda}\frac{dx^{i}}{d\lambda}-\delta_{ij}\frac{\partial \phi}{\partial t}\frac{dx^{i}}{d\lambda}\frac{dx^{j}}{d\lambda}.[/tex]
We implicitly define the [itex]\lambda[/itex] parameter by:
[tex]P^{\mu}\equiv(E;p^i)\equiv\frac{dx^{\mu}}{d\lambda},[/tex]
where [itex]E[/itex] is the particle's energy and [itex]p^i[/itex] is the momentum vector. In this way,
[tex]p^0=E; \ \ \frac{d}{d\lambda}=\frac{dx^0}{d\lambda}\frac{d}{dx^0}=E\frac{\partial}{\partial t}; \ \ p^i=\frac{dx^i}{d\lambda},[/tex]
and thus the geodesic equation yields:
[tex]E\frac{dE}{dt}=-\frac{\partial \phi}{\partial t}E^2-2E\frac{\partial \phi}{\partial x^i}p^i+\delta_{ij}\frac{\partial \phi}{\partial t}p^i p^j.[/tex]
And I'm stuck in the above equation. I can't express the second term in terms of [itex]E[/itex], [itex]m[/itex] and [itex]\phi[/itex]. I can't think of any possible relation to that. The third term is another problem. At first, I thought to use the energy-momentum relation [itex]E^2=m^2+p^2[/itex], but that relation holds only for free-particles, and so I can't state that the 4-momentum magnitude is [itex]g_{\mu\nu}P^{\mu}P^{\nu}=-m^2[/itex]. My textbooks does it in a similar exercise: in the solved exercise, it was showing that the time-component of the geodesic equation entails that the energy of a massless particle decays as the scale-factor in a Friedman-Robertson-Walker expanding universe. In this exercise, it used the fact that the four-momentum magnitude for a massless particle was zero (I got a little bit confused with the argument, because I thought you needed to correct this result with the different metric, but in the textbook it seems the author just assumed the magnitude to be [itex]-m^2[/itex] and then stated it was zero since the mass is zero). So, I don't know what [itex]g_{\mu\nu}P^{\mu}P^{\nu}[/itex] is supposed to be in this modified metric and thus I can't express the last term in terms of the quantities of interest. A possible solution that has occurred me if to redefine [itex]\lambda[/itex] in a more convenient way, but I couldn't think in any good definition to this case.
If you read me up to now, I deeply appreciate your patience, and I'll be very grateful if you can help me.
Thank you!