# Confusion regarding conformal transformations

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• nigelscott
The conformal invariance of a metric is a property of a metric that states that the metric is invariant with respect to a conformal transformation. This means that if we change the coordinate system in which the metric is measured, the metric will still be correct. The conformal invariance of a metric is often expressed using the following equation:gμν(x)dxμdxν = g'μν(x)dx'μdx'νwhere the superscripts on the left side denote the original coordinate system and the superscripts on the right side denote the new coordinate system. This equation states that the distances between points in the two systems are thef

#### nigelscott

I am confused about conformal transformations on Riemannian manifolds. Here's what I have so far.

1. Under a conformal transformation the metric changes by:

g' -> Ω2g

2. Under a Weyl transformation the metric changes by:

g' -> exp(-2f)g

3. Any 2D Riemann manifold is locally conformally flat and the metric can be defined in terms of isothermal coordinates.

g = exp(f)(du2 + dv2 )

Where u and v are Euclidean.

How are these all tied together? There appears to be a commonality in form but the multiplying functions are different. Where does the exponential come from?

OK, I have modified my original questions. I think (hope) I now have a better understanding. Perhaps somebody could critique this. However, I am still a little confused about the relationship between λ and Ω and the difference in exponents. Thanks.

Conformal Manifolds:

A manifold, M, is said to be conformal if two Riemannian metrics, g and g', on the smooth manifold M are related by the equation:

g'μν = Ω2gμν

Where Ω is a real-valued smooth function defined on the manifold. Metrics obeying this equation are said to be conformally equivalent.

This equation represents a general coordinate transformation, x -> x', such that x' = f(x) has the following effect on the metric:

g'μν = (∂xρ/∂x'μ)(∂xσ/∂x'ν)gρσ

Under these conditions both the metric and the Weyl tensor are invariant meaning:

gμν(x)dxμdxν = g'μν(x)dx'μdx'ν or ds2 = ds'2.

and

Cabcd = C'abcd

Conformal Transformations:

A conformal transformation can be defined as a subset or special case of the above general coordinate transformation which acts as a rescaling of the metric x -> x by a scale factor, λ . This rescaling of the metric is often referred to as WEYL transformation and the 2 terms are often used interchangeably, although they are in fact different things. Therefore:

gμν = λgμν

From above:

gμν = (∂x'ρ/∂xμ)(∂'xσ/∂xν)g'ρσ

So we can write:

(∂x'ρ/∂xμ)(∂'xσ/∂xν)g'ρσ = λgμν

or

gμν = λ-1{(∂x'ρ/∂xμ)(∂'xσ/∂xν)g'ρσ}

In other words any coordinate transformation such that g'μν = λgμν can be 'undone' by a Weyl transformation. The Weyl transformation takes us to a coordinate system where the metric has the same form as the one we started with, but the points have all been moved around and pushed closer together or farther apart depending on the scale factor. Therefore, under a Weyl transformation:

ds2 -> λds2

The Weyl transformation changes the proper distances at each point by a scalar factor λ; and, therefore, has no directional meaning. It is a local change of scale which preserves the angles between all lines. It is not a coordinate transformation on the space or spacetime.

Conformal Flatness:

A necessary condition for a Riemannian manifold to be conformally flat is that the WEYL TENSOR vanish. The interpretation of this is that if a manifold (M,g) has a neighborhood (U,h) at each point such that g'μν = η'μν = Ω2ημν it is locally conformally flat. Under these circumstances the Riemannian metric is conformal to the Euclidean metric and can be described by a local coordinate system in which the metric ds2 satisfies:

ds2 = Ω2(dx12 + dx22 + ... dxn2)

These are referred to as ISOTHERMAL COORDINATES.

The multiplying functions can be defined as you want and are conventional. In 2d, a metric has 3 components, so gaugefixing your 2 gct's leaves you with one function for the metric. A conformal metric can the be used to write the metric in the form you give.

Hope this helps.

Thanks. I think I am almost there. By convention Ω = exp(2f). Both the CT and the WT are applicable to d ≥ 2 but for d = 2, f = f(z) and is holomorphic. The scale factor becomes |df/dz|2. Correct?

I am confused about conformal transformations on Riemannian manifolds. Here's what I have so far.

1. Under a conformal transformation the metric changes by:

g' -> Ω2g

2. Under a Weyl transformation the metric changes by:

g' -> exp(-2f)g

3. Any 2D Riemann manifold is locally conformally flat and the metric can be defined in terms of isothermal coordinates.

g = exp(f)(du2 + dv2 )

Where u and v are Euclidean.

How are these all tied together? There appears to be a commonality in form but the multiplying functions are different. Where does the exponential come from?
You may need to look at this post
Conformal Invariance Klein Gordon Action