JesseM said:
That would only be the angular speed of a circle that happened to coincide with the equator (which would actually be \theta = \pi/2 rather than \theta = 0 as I incorrectly stated earlier).
starthaus said:
"So what" is that since \theta = \pi/2 is the
only case of the equation below (from my post 76, and basically the same as your own equation) that actually corresponds to a circular orbit rather than a non-orbiting circular path:
d\tau /dt = \sqrt{1 - r_s/r}\sqrt{1 - (r sin(\theta) \omega/ c\sqrt{1-r_s/r})^2}
...then for a circular
orbit with d\theta = 0 it must be true that sin(\theta) = 1 and therefore the equation reduces to:
d\tau /dt = \sqrt{1 - r_s/r}\sqrt{1 - (r \omega/ c\sqrt{1-r_s/r})^2}
If you then make the substitution v = r \omega / \sqrt{1 - r_s/r}, which was exactly the substitution kev made in
post #8 (he defined u = r \left(\frac{d\phi}{dt}\right), equivalent to u = r\omega, and then he defined u = v \sqrt{1-\frac{r_s}{r}}, equivalent to v = r\omega / \sqrt{1 - r_s/r}), then this equation becomes:
d\tau /dt = \sqrt{1 - r_s/r}\sqrt{1 - v^2/c^2}
So the equation is correct for the special case of a circular orbit where d\theta = 0. Do you disagree? If not, then the symmetry argument I already mentioned shows why this would hold for
any circular orbit, even one where d\theta was not equal to 0 in our original coordinate system (since you could always rotate into a new coordinate system where d\theta
was equal to 0 on the orbit, and the metric would be exactly the same in this new coordinate system since the Schwarzschild metric is invariant under rotations)
Actually now that I've looked back at kev's post #8 more carefully I have no idea why in
post 28 you criticized him by saying:
You need to make
d\theta=dr=0
-v is equal to:
r\frac{d\phi}{dt} and not r\frac{d\theta}{dt}
...since it appears to me he
did make d\theta = 0, and he
did define the velocity in terms of \frac{d\phi}{dt} rather than \frac{d\theta}{dt}! I guess I shouldn't have taken your word for it that he did it differently there (even though you
could still get exactly the same final result by assuming a circular orbit where d\phi = 0 along a short segment, do you disagree? If you do disagree, I can demonstrate)
JesseM said:
Do you disagree that any
great circle on a sphere would correspond to a valid circular orbit, including a circle which could be divided into two halves of constant longitude (i.e. constant \phi), or plenty of circles where neither longitude nor latitude were constant?
starthaus said:
The point is that it doesn't.
The point is that
what doesn't? You didn't answer my question about whether you disagree that there are valid circular orbits in Schwarzschild spacetime which, in a given coordinate system, would have a description like the one above. If you do disagree then I think you need to do some thinking about how spherical coordinates work, in particular what the coordinate description would look like for an "upright" circle whose plane was at a right angle to the "horizontal" \theta = \pi/2 plane.
starthaus said:
The domain for \theta is [0,\pi]. Do you dispute that?
No, of course not, why do you imagine I would? In post #56 I gave the example of a complete circle where the coordinate description would be such that one half of the circle would have a constant r=R and \phi = \pi/2 while the other half would have constant r=R and \phi = -\pi/2, I thought it was fairly obvious that the points covered by each half would then be defined by varying \theta from 0 to \pi. Again, do you disagree that this would be a valid coordinate description for the set of points on a single continuous circle, one whose center is at r=0 and whose plane is at a right angle to the \theta = \pi/2 plane, and where d\phi = 0 along any infinitesimal segment of this circle? If not you should see why, despite the fact that kev actually made d\theta = 0 rather than d\phi = 0, it
would have been perfectly valid for him to do the reverse, either way there'd be a valid circular orbit meeting this condition, there'd be nothing non-rigorous or "hack"-y about such a starting assumption.