# Generality of theta=pi/2 in Schwarzchild

1. May 14, 2010

### ghetom

I'm doing some work on schwarzchild orbits, and everything assumes that $$\theta=\pi/2$$ and claims that this doesn't compromise generality. It seems pretty obvious (since all the geodesics are planar or radial and the metric is spherically symmetric), but can anyone prove that $$\theta=\pi/2$$ is fully general?

here's the metric if you need it:
$$ds^2=fdt^2-(1/f)dr^2+r^2d\Omega^2$$
where
$$f=1-(2m/r)$$

2. May 14, 2010

### George Jones

Staff Emeritus
3. May 14, 2010

### ghetom

possibly, but I don't have a copy, and the relevant pages aren't available online.

4. May 14, 2010

### George Jones

Staff Emeritus
I not sure how omitted pages are chosen. When I looked at the book using Google, pages 177 and 178 were viewable.

5. May 14, 2010

### ghetom

How strange - I definitely can't read them.

6. May 14, 2010

### DrGreg

I suspect Google Books has some sort of algorithm that either chooses randomly which pages you can see, or else counts the number of pages that you choose to view and stops after a certain threshold. I'm guessing that from my experience.

P.S. In the case of the link given earlier in this thread, when I tried, pages 168 to 210 were not available.

Last edited: May 14, 2010
7. May 14, 2010

### yuiop

In my case I could see just about every page in the book except pages 177 and 178. Maybe the algorithm blocks the most popular pages and everyone rushing to see the page referenced by George makes it unviewable?

8. May 14, 2010

### Russell E

I don't think your question is entirely clear. Are you asking whether the physics on some other plane through the origin might be different than the physics on the theta=pi/2 plane? That would contradict spherical symmetry. In essense, the stipulation of spherical symmetry is tantamount to the stipulation that all planes (and rays) through the origin have equivalent physics. So your question is "can anyone prove that a spherically symmetrical field is spherically symmetrical?".

One could question whether the gravitational field of a spherical mass (or a hypothetical mass point) is really spherically symmetrical, but by the principle of sufficient reason it would be hard to justify any such hypothesis without also positing some lack of homogeneity and isotropy in the universe. But this probably isn't what you are asking, because you asked for a "proof", whereas this would be an empirical question.

By the way, I was able to view 177 and 178, but I don't think that discussion addresses the question, because it assumes that a spherically symmetrical field is spherically symmetrical. It doesn't present any "justification" of this, it treats it as tautological, which it is.

I suppose one other possible interpretation of the question is: How do we know that a test particle will move entirely in a single plane. Needless to say, this isn't related to the theta=pi/2 plane, and obviously a test particle need not remain in that particular plane, but one could ask whether a test particle's motion might not be confined to a single plane. This too is rather tautological, since the trajectory at any point is in some plane, and there is no momentum outside that plane, so what would cause it to diverge from the plane? And in which direction would it diverge, since by symmetry it would have no more reason to go one way as the other. If this is what you're asking, then the "proof" you're looking for is simply "conservation of angular momentum".

Last edited: May 14, 2010
9. May 15, 2010

### ghetom

Thanks. I was looking for a mathematical 'proof,' but when you state it like that, it becomes blatantly obvious!