Invariant Integrals and Tensor Densities
We would like to be
able to integrate a quantity over a particular range of coordinate values in
such a way that the integrand gives the same value in any other generalized
coordinate system. If the integrand is a pure scalar quantity, then this is
easily achieved because of the way that scalar quantities transform. Let be such a pure scalar quantity. The
transformation law that it obeys when a new primed coordinate system is
introduced is as follows:
(9.1) |
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Say that we are
considering the scalar quantity at two distinct points
.
We can sum the scalar function evaluated at the two distinct points in a new
primed coordinate system such that the following relation holds:
(9.2) |
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Here are the same points in the new coordinate
system. Unlike tensors of higher rank, a scalar field can be evaluated at two
different points and still be a scalar field.
There should be no problem making this behavior hold if we go to
infinitesimal sums.
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When we are integrating over coordinate ranges we want the following integral invariance to hold.
The word 'Quantity' is
meant to represent a tensor of some general type. However, with integrands of
this pattern we run into a problem in that what we are integrating is not
necessarily a pure tensor quantity. We know that the 4-D volume element transforms according to the following
Jacobian relation
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This relation implies
that the differential element transforms in a funny way. It's not
transforming like a scalar quantity and it's not transforming like a vector
quantity. This differential element transforms according to a rule that's
similar to tensors but is different in that a power of the transformation
Jacobian comes into the transformation. A new set of geometric quantities
called Tensor Densities can be defined in an
analogous manner to tensors but the transformations involve powers of the
transformation Jacobian. A general definition of the tensor density can be
written in the following way.
Tensor density: A tensor density, , of weight
transforms like a tensor except that the Wth
power of the Jacobian appears as a factor with the pattern shown below.
(9.6) |
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Since the differential
element transforms according to equation (9.5)
with the pattern
(9.7) |
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then must be a scalar density of weight
.
The integrand in (9.4)
will be a scalar only if the factor labelled 'Quantity' is a tensor density of
weight
.
To make integrals be independent of the coordinates, the integrand is
multiplied by the square root of the metric determinant as shown in the
following expression.
This works since the metric transforms according to
(9.9) |
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Since the right hand side of this equation is essentially the product of three matrices multiplied together, we can use the rule for the product of matrix determinants to give
(9.10) |
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The value g is negative for an indefinite metric so when we take the square root of this relation we insert a minus sign and the result is
(9.11) |
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Thus is a scalar density of weight
.
We then see that the integral
invariance given by (9.8) works since
we have made
(9.12) |
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and therefore (9.3) must be realized.
The covariant derivative of a tensor density has the following pattern
(9.13)
For example, the covariant derivative of a
vector density has the form
(9.14)
For the special case
when this leads to the important divergence
equation
(9.15) |
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In terms of a tensor
density formed from multiplying a tensor by
,
this divergence expression becomes
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It can be shown that the metric determinant, which acts as a scalar density of weight 2, satisfies the following relations.
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and
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Relations (9.16), (9.17), and (9.18) turn out to be of great use in the Lagrangian formulation of general relativity.
15/10/2002 3:07 PM