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alc95
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Homework Statement
Hartle, Gravity, P9.8
A spaceship is moving without power in a circular orbit about a black hole of mass M, with Schwarzschild radius 7M.
(a) What is the period as measured by an observer at infinity?
(b) What is the period as measured by a clock on the spaceship?
Homework Equations
Eqn 9.46 from Hartle:
[tex]
\omega =\sqrt{(M/r^3)}
[/tex]
Proper time:
[tex]
d\tau^2=-ds^2
[/tex]
and the Schwarzschild metric.
The Attempt at a Solution
(a) This part is fine. Using T=2*pi/omega and substituting r=7M, T=14*pi*sqrt(7)*M.
However, I'm not sure how Eqn 9.46 is derived? Can it be derived using the Schwarzschild metric and noting that the coordinate r is constant?
(b) The clock in the spaceship measures proper time:
[tex]
d\tau=\sqrt{(1-2M/r)dt^2+r^2d\phi^2}=\sqrt{(1-2M/r)+M/r}dt
[/tex]
Here, [tex]d\phi^2=M/r^3dt[/tex]
[tex]
T'=\sqrt{(1-2M/r)+M/r}T
[/tex]
Is my reasoning correct? Assuming this is correct, is all that remains is to substitute r=7M and the period T from part (a)?