- #26

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... never mind. If the OP is happy then so am I.

Pete

Pete

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- #26

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... never mind. If the OP is happy then so am I.

Pete

Pete

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- #27

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Arg, Pete if I saw this earlier while I was still in the office I could have told you. It is in the chapter on derivative operators and curvature which should be chapter three.DavidWhitbeck - what page in wald?

I would have to study it carefully, but I think that you might be stipulating additional properties that Wald does not assume. If I could see axiomatically how you arrived at the covariant derivatives of vectors vanished then maybe I could understand your p.o.v.

In a previous post you said something to the effect of one can't define a covariant derivative operator without implicitly assuming something about the existence of an affine connection, I don't really get that remark and I'm probably not reading it correctly, I just never studied at that level, but it seems to me that you can only run into a problem with a definition if it's

(a) ill-defined, i.e. lacking in logical consistency

(b) well defined, but doesn't actually exist

I'm hoping that it's not (a). Are you saying that you can't prove the existence of such an operator without making additional assumptions?

- #28

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If you can take a look at it tommorow and note the page number I would be grateful. Thanks David!Arg, Pete if I saw this earlier while I was still in the office I could have told you. It is in the chapter on derivative operators and curvature which should be chapter three.

I recall reading the proof in more than one places but at the moment I can only recall the one inI would have to study it carefully, but I think that you might be stipulating additional properties that Wald does not assume. If I could see axiomatically how you arrived at the covariant derivatives of vectors vanished then maybe I could understand your p.o.v.

It was implied that one could take the vanishing of the covariant derivative of the metric tensor as an axiom. That was the comment that I was talking about. Regarding the metric tensor and the affine connection - Since Ohanian and Ruffini say this so beautifully I'll simply quote that text. From page 302In a previous post you said something to the effect of one can't define a covariant derivative operator without implicitly assuming something about the existence of an affine connection, I don't really get that remark and I'm probably not reading it correctly, ...

One can have an affine space with no metric defined on it and one can have a metric space with no affine connection defined on it. If one were to take the covariant derivative of the metric equals zero as an axiom then I don't see how we can keep those geometries seperate. I believe that it is the vanishing of the covariant derivative of the metric tensor that establishes the connection between the two geometries. I guess the problem I have is that stating the covariant derivative of the metric tensor = 0 as an axiom seems to me to border on a definition of the metric instead. Too high-guru math for me to see it as being obvious or not.Mathematically, a Riemannian space is a differentiable manifold endowed with a topological structure and a geometrical structure. In the discussion of the geometric structure of a curved space we can make a distinction between the affine geometry and the metric geometry. These two kinds of geometries correspond to two different ways in which we can detect the curvature of a space. One way is by examining the behavior of of parallel line segments, or parallel vectors. For example, on the surface of a sphere, we can readily detect the curvature by transporting a vector around a close path, always keeping the vector parallel to itself as possible. [fig not shown here] shows what happens if we parallel transport a vector around a triangular path on the sphere. The final vector differs in the direction from the initial vector, whereas on a flat surface the final vector would not differ. Such changes in a vector produced by parallel transport characterize the affine geometry (the word affine means connected, and refers to how parallels at different places are connected, or related). .... Another way in which we can detect curvature is by measurements of lengths and areas. For example, we can draw a circle on the sphere, and check the radius vs. circumference, or the radius vs. area. Both the circumference and the area of such a circle are smaller than for a circle on a flat surface [figure not shown here]. Such deviations from what we expect on a flat surface characterize the metric geometry.

See above. Does that address your question?I just never studied at that level, but it seems to me that you can only run into a problem with a definition if it's

(a) ill-defined, i.e. lacking in logical consistency

(b) well defined, but doesn't actually exist

I'm hoping that it's not (a). Are you saying that you can't prove the existence of such an operator without making additional assumptions?

Pete

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- #29

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Ah! I see I should have looked more closely at Ohanian! since the authors wroteI believe that it is the vanishing of the covariant derivative of the metric tensor that establishes the connection between the two geometries.

from which the authors show that the covariant derivative of the metric tensor is zero.Next, we establish the relation between the affine geometry and the metric geometry, that is, the relationship between the Christoffel symbols (...) and the metric tensor ...

Its nice to see that my instincts still work ... sometimes.

Pete

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- #30

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Wald defines derivative operators by five rules on page 31, and then on page 35 he proves that if you additionally require that it keeps the metric constant, then there exists a unique derivative operator that satisfies those six properties, and he calls it the covariant derivative.

So you go: parallel transport along geodesics => metric covariantly constant

Wald goes: metric covariantly constant=> parallel transport along geodesics

It looks like the person who is really right on this thread is Hurkyl for pointing out that you can do it either way.

So these are the five properties that a derivative operator must satisfy:

1. Linearity.

2. Leibnitz Rule.

3. Commutativity with contraction.

4. Tangent vectors satisfy [itex]t(f) = t^a\nabla_a f[/itex]

5. Torsion free-- [itex][\nabla_a,\nabla_b]f = 0[/itex]

- #31

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And the last one is optional.

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