- #1
Agerhell
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The post-Newtonian expansion at the first post-Newtonian 1PN level under Schwarzschild conditions, a test-body in a spherically symmetric gravitational field, gives the following expression for the gravitational acceleration of the test-body: [tex]\frac{{\rm d}\bar{v}}{{\rm d}t}=-\frac{GM}{r^2}\left(1-\frac{4GM}{rc^2}+\frac{v^2}{c^2}\right)\hat{r}+\frac{4GM}{r^2}(\hat{r}{\cdot \hat{v}})\frac{v^2}{c^2}\hat{v}.[/tex]
For circular orbits this gives (setting the right hand terms equal to the centripetal acceleration ##v^2/r##) an orbital velocity of:
[tex]v=\sqrt{\frac{GM}{r}\frac{[1-4GM/(rc^2)]}{[1-GM/(rc^2)]}}[/tex]
Using the same procedure on the post-Newtonian expansion taken to the third post-Newtonian level, (see for instance http://arxiv.org/abs/gr-qc/0209089) one gets the orbital velocity of circular orbits:
[tex]v=\sqrt{\frac{GM}{r}\left[1-\frac{4GM}{rc^2}+9\left(\frac{GM}{rc^2}\right)^2-16\left(\frac{GM}{rc^2}\right)^3\right]}\left(1-\frac{GM}{rc^2}\right)^{-1/2}.[/tex]
Now in Schwarzschild coordinates the orbital velocity really should be ##v=\sqrt{GM/r}##, however the post-Newtonian expansion is derived using isotropic coordinates so maybe the orbital velocity for circular orbits should be different? Now my personal alternative to the post-Newtonian expansion expression for gravitational acceleration looks like:
[tex] \frac{{\rm d}\bar{v}}{{\rm d}t}=-\frac{GM}{r^2}(\hat{r}\cdot\hat{v})\left(1-3\frac{v^2}{c^2[1-2GM/(rc^2)]}+\frac{v^4}{c^4[1-2GM/(rc^2)]^2}\right)\hat{v}\nonumber \\+\frac{GM}{r^2}(\hat{r}\times\hat{v})\times \hat{v}.\hspace{4mm}[/tex]
(for details: http://vixra.org/abs/1303.0004)
which gives the classical orbital velocity for circular orbits, ##v=\sqrt{GM/r}##.
Now, which expression for gravitational acceleration results in the more correct expression for the orbital velocity of a body in circular orbit?
I tried to compare my expression to the post-Newtonian when I sent my paper in for peer-review. However, the reviewers report stated:
Now, are the reviewers arguments valid? Is the post-Newtonian expansion to be treated like some kind of golden standard that can not be put into question?
For circular orbits this gives (setting the right hand terms equal to the centripetal acceleration ##v^2/r##) an orbital velocity of:
[tex]v=\sqrt{\frac{GM}{r}\frac{[1-4GM/(rc^2)]}{[1-GM/(rc^2)]}}[/tex]
Using the same procedure on the post-Newtonian expansion taken to the third post-Newtonian level, (see for instance http://arxiv.org/abs/gr-qc/0209089) one gets the orbital velocity of circular orbits:
[tex]v=\sqrt{\frac{GM}{r}\left[1-\frac{4GM}{rc^2}+9\left(\frac{GM}{rc^2}\right)^2-16\left(\frac{GM}{rc^2}\right)^3\right]}\left(1-\frac{GM}{rc^2}\right)^{-1/2}.[/tex]
Now in Schwarzschild coordinates the orbital velocity really should be ##v=\sqrt{GM/r}##, however the post-Newtonian expansion is derived using isotropic coordinates so maybe the orbital velocity for circular orbits should be different? Now my personal alternative to the post-Newtonian expansion expression for gravitational acceleration looks like:
[tex] \frac{{\rm d}\bar{v}}{{\rm d}t}=-\frac{GM}{r^2}(\hat{r}\cdot\hat{v})\left(1-3\frac{v^2}{c^2[1-2GM/(rc^2)]}+\frac{v^4}{c^4[1-2GM/(rc^2)]^2}\right)\hat{v}\nonumber \\+\frac{GM}{r^2}(\hat{r}\times\hat{v})\times \hat{v}.\hspace{4mm}[/tex]
(for details: http://vixra.org/abs/1303.0004)
which gives the classical orbital velocity for circular orbits, ##v=\sqrt{GM/r}##.
Now, which expression for gravitational acceleration results in the more correct expression for the orbital velocity of a body in circular orbit?
I tried to compare my expression to the post-Newtonian when I sent my paper in for peer-review. However, the reviewers report stated:
"The main argument put forward for the advantage of the author's model over post-Newtonian expansions is that the velocity of a circular orbit as a function of the radial coordinate r is correctly recovered. The coordinate r is not an observable, it is a gauge dependent object, and so basing a conclusion on the behaviour of functions of that variable is invalid. The post-Newtonian expansion is well known to correctly describe all observable quantities for orbits in the Solar System and in the binary pulsar systems, for instance. The author should be demonstrating that his model reproduces post-Newtonian observables, not that it gives different results."
Now, are the reviewers arguments valid? Is the post-Newtonian expansion to be treated like some kind of golden standard that can not be put into question?