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patrick_gold
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I'm currently doing a course on general relativity, and I'm struggling with the following exercise - I would greatly appreciate the help anyone might offer. Whilst not technically homework (not assessed or in a tutorial set) it is from a collection of difficult questions from a recommended resource. It is as follows:
Weyl's solution to the field equations in a vacuum has that
$$ ds^{2} = e^{2U}dt^{2} -e^{-2U}(r^{2}d\phi ^{2} + e^{2V}(dr^{2} +dz^2)) $$ $$ where \; \; U=U(r,z),\; V=V(r,z); $$
and the field equations reduce to Laplace's equation in cylindrical coordinates, that being
$$ \nabla^{2}U = U_{,rr} + r^{-1}U_{,r} + U_{,zz}=0, $$
as given U we can find V by integration:
$$ V_{,r}=r((U_{,r})^{2}-(U_{,z})^2)\;, \; \; \; V_{,z}=2rU_{,r}U_{,z} \;. $$
This suggests a way of generating solutions to the Einstein Fields equations: Laplace's
equation is linear, so just add arbitrary (axially symmetric) solutions of it to get U
and then find V .
(a) Why is the Weyl metric static?
(b) Explain why adding a constant to either (or both) U and V is geometrically
(and hence physically) irrelevant.
(c) If $$U\equiv 0 $$ we clearly have flat space. Show that if $$U = V$$ we also have flat space. [Hint: perhaps we might change coordinates (something about hyperbolic trig functions and rapidity?)] This suggests generating GR solutions from Newtonian ones will not be as easy as it seems.
(d)The simplest non-trivial solution to Laplace's equation is the (spherically symmetric) potential of a point particle: $$U=-\frac{m}{\sqrt{r^{2}+z^{2}}}$$. Show this is a solution to $$\nabla^{2}U=0$$, find the corresponding V and explain why the metric is asymptotically flat.
(e) The solution is part (d) is known as the Curzon-Chazy solution. Prove it is not Schwarzschild and so not spherically symmetric. (Thus once again demonstrating that the Newtonian solution and the Einsteinian solutions are not simply related.)
*Nota bene c=1 here.
See question statement
I've struggled a bit, there's some obvious substitution work in here verifying things are solutions but I'm not very confident with the rest of the material.
Homework Statement
Weyl's solution to the field equations in a vacuum has that
$$ ds^{2} = e^{2U}dt^{2} -e^{-2U}(r^{2}d\phi ^{2} + e^{2V}(dr^{2} +dz^2)) $$ $$ where \; \; U=U(r,z),\; V=V(r,z); $$
and the field equations reduce to Laplace's equation in cylindrical coordinates, that being
$$ \nabla^{2}U = U_{,rr} + r^{-1}U_{,r} + U_{,zz}=0, $$
as given U we can find V by integration:
$$ V_{,r}=r((U_{,r})^{2}-(U_{,z})^2)\;, \; \; \; V_{,z}=2rU_{,r}U_{,z} \;. $$
This suggests a way of generating solutions to the Einstein Fields equations: Laplace's
equation is linear, so just add arbitrary (axially symmetric) solutions of it to get U
and then find V .
(a) Why is the Weyl metric static?
(b) Explain why adding a constant to either (or both) U and V is geometrically
(and hence physically) irrelevant.
(c) If $$U\equiv 0 $$ we clearly have flat space. Show that if $$U = V$$ we also have flat space. [Hint: perhaps we might change coordinates (something about hyperbolic trig functions and rapidity?)] This suggests generating GR solutions from Newtonian ones will not be as easy as it seems.
(d)The simplest non-trivial solution to Laplace's equation is the (spherically symmetric) potential of a point particle: $$U=-\frac{m}{\sqrt{r^{2}+z^{2}}}$$. Show this is a solution to $$\nabla^{2}U=0$$, find the corresponding V and explain why the metric is asymptotically flat.
(e) The solution is part (d) is known as the Curzon-Chazy solution. Prove it is not Schwarzschild and so not spherically symmetric. (Thus once again demonstrating that the Newtonian solution and the Einsteinian solutions are not simply related.)
*Nota bene c=1 here.
Homework Equations
See question statement
The Attempt at a Solution
I've struggled a bit, there's some obvious substitution work in here verifying things are solutions but I'm not very confident with the rest of the material.