Canonical Measure and Volume Form
In analogy with the case of surfaces, a metric tensor on an n-dimensional paracompact manifold M gives rise to a natural way to measure the n-dimensional volume of subsets of the manifold. The resulting natural positive Borel measure allows one to develop a theory of integrating functions on the manifold by means of the associated Lebesgue integral.
A measure can be defined, by the Riesz representation theorem, by giving a positive linear functional Λ on the space C0(M) of compactly supported continuous functions on M. More precisely, if M is a manifold with a (pseudo-)Riemannian metric tensor g, then there is a unique positive Borel measure μg such that for any coordinate chart (U,φ),
for all ƒ supported in U. Here det g is the determinant of the matrix formed by the components of the metric tensor in the coordinate chart. That Λ is well-defined on functions supported in coordinate neighborhoods is justified by Jacobian change of variables. It extends to a unique positive linear functional on C0(M) by means of a partition of unity.
If M is in addition oriented, then it is possible to define a natural volume form from the metric tensor. In a positively oriented coordinate system (x1,...,xn) the volume form is represented as
where the dxi are the coordinate differentials and the wedge ∧ denotes the exterior product in the algebra of differential forms. The volume form also gives a way to integrate functions on the manifold, and this geometric integral agrees with the integral obtained by the canonical Borel measure.
Read more about this topic: Metric Tensor
Famous quotes containing the words canonical, measure, volume and/or form:
“If God bestowed immortality on every man then when he made him, and he made many to whom he never purposed to give his saving grace, what did his Lordship think that God gave any man immortality with purpose only to make him capable of immortal torments? It is a hard saying, and I think cannot piously be believed. I am sure it can never be proved by the canonical Scripture.”
—Thomas Hobbes (1579–1688)
“From whatever you wish to know and measure you must take your leave, at least for a time. Only when you have left the town can you see how high its towers rise above the houses.”
—Friedrich Nietzsche (1844–1900)
“Measured by any standard known to science—by horse-power, calories, volts, mass in any shape,—the tension and vibration and volume and so-called progression of society were full a thousand times greater in 1900 than in 1800;Mthe force had doubled ten times over, and the speed, when measured by electrical standards as in telegraphy, approached infinity, and had annihilated both space and time. No law of material movement applied to it.”
—Henry Brooks Adams (1838–1918)
“The form of act or thought mattered nothing. The hymns of David, the plays of Shakespeare, the metaphysics of Descartes, the crimes of Borgia, the virtues of Antonine, the atheism of yesterday and the materialism of to-day, were all emanation of divine thought, doing their appointed work. It was the duty of the church to deal with them all, not as though they existed through a power hostile to the deity, but as instruments of the deity to work out his unrevealed ends.”
—Henry Brooks Adams (1838–1918)