# Time dilation and Length contraction in Saddle Geometries

1. Jan 14, 2012

### kmarinas86

The way I've been reading it, elliptic geometries are due to a positive Gaussian curvature, while hyperbolic geometries are due to a negative Gaussian curvature.

Do local saddle curvatures mean local time dilation and length contraction, or do they mean local time acceleration and length expansion? Or do saddle geometries correspond to local time dilation and length expansion? Or do saddle geometries correspond to local time acceleration and length contraction?

Also, how would a localized "negative" energy density affect time and length relative to flat space?

2. Jan 15, 2012

### Naty1

Fantastic questions....I hope an 'expert(s)' will come across them and reply.

I think I have a few preliminary comments to help frame a reply, but I do not know enough
to provide a complete view.

(1) In both special and general relativity, Christoffel symbols used in writing the laws of physics allow transformations (more precisely, "diffeomorphism") and preserve the form of the laws. In special relativity (flat spacetime) Lorentz transformations will also preserve the form of the laws if Christoffel symbols are not used. So I'm pretty sure we need somebody who knows how Christoffel math works.

(2) An open universe has a negative Gaussian curvature, K = -1; A closed universe has positive Gaussian curvature, K = +1. Generalizations of curvature are the scalar curvature and Ricci curvature which are used in the Einstein field equations ....so another issue is how Gaussian curvature relates to these.

[http://en.wikipedia.org/wiki/Scalar_curvature#Surfaces

says in two dimensions, scalar curvature is exactly twice the Gaussian curvature but what goes on in higher diemnsions??]

(3) Regarding negative energy density: Anti de Sitter space is a solution of Einstein's field equation with a negative (attractive) cosmological constant Λ ,corresponding to a negative vacuum energy density and positive pressure. What you may be interested in is here:

http://en.wikipedia.org/wiki/Anti_d...nguished_from_spacetime_in_general_relativity

Synopsis: Seems like just typical curvature,

3. Jan 17, 2012

### Naty1

I'm sorry no one has responded so far: I cam across this insight in my notes, but am not positive about its full interpretation:

There seem to be some really good descriptions here: [talking about the Ricci scalar]
http://en.wikipedia.org/wiki/Scalar_curvature

4. Jan 17, 2012

### Staff: Mentor

It depends on the specific geometry, and on what you define as "saddle curvature" (see below). Do you have any particular examples of "saddle curvatures" in mind?

One example of what might be called "saddle curvature" is the curvature around a spherically symmetric massive body, i.e., the Schwarzschild solution. In the radial direction, tidal gravity separates geodesics, but in the tangential direction, tidal gravity brings them closer together. So the sign of the curvature is different in different directions.

5. Jan 17, 2012

### Staff: Mentor

Actually, Christoffel symbols are not tensors, so they themselves are not frame independent. They can appear in equations that are frame independent overall, but they themselves act like vector or tensor components that can change from frame to frame.

The FRW solutions are actually very special ideal cases, where there is a well-defined "sign" to the curvature. In most cases, even very simple ones, there isn't (see my previous post).

In general there isn't a unique "scalar curvature"; there are different scalars that can be derived from the curvature tensor by contracting it in different ways. The Ricci scalar is one; the Kretschmann scalar, for example, is another, distinct one:

http://en.wikipedia.org/wiki/Kretschmann_scalar

Two dimensions is a special case where, AFAIK, all of the possible scalars turn out to be the same.

AdS is equivalent to a K = -1 (negative curvature) FRW model where the only energy density is due to the cosmological constant. So yes, it is one of the very special cases where the sign of the curvature is simple and well-defined.

6. Jan 19, 2012

### Naty1

PeterDonis: thanks for helping out.

The Schwarzschild solution is a saddle curvature ?? I can't find an illustration....I thought a saddle shape was like the center one here:

http://en.wikipedia.org/wiki/Shape_of_the_universe

My earlier comment about Christoffel symbols was I think based in part on this reading:

http://en.wikipedia.org/wiki/Christoffel_symbols

"In mathematics and physics, the Christoffel symbols..... are numerical arrays of real numbers that describe, in coordinates, the effects of parallel transport in curved surfaces and, more generally, manifolds....

In general relativity, the Christoffel symbol plays the role of the gravitational force field with the corresponding gravitational potential being the metric tensor."