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Tetrad invariance of the future massive mass-shell measure

Statement

Let x be an event of a time-oriented smooth Lorentzian four-manifold, let M∗>0, and let e and e′ be any two smooth future orthonormal tetrads near x. The measures dμM∗,x,e and dμM∗,x,e′ defined in Canonical future massive mass-shell measure agree on every Borel subset of the future massive shell at x. Hence the fiber measure dμM∗,x is independent of the chosen future orthonormal tetrad.

Facts & Assumptions

Given: A future massive shell in one tangent space, mass energy M∗>0, two future orthonormal tetrads, and the countable-choice assumption carried by the cited change-of-variables supplier.

[F1]

In tetrad coordinates the shell is P(q)=(E(q),q), E(q)=M∗2+∣q∣2, and its measure is d3q/E(q) (Canonical future massive mass-shell measure).

[F2]

If eA′=BCAeC, the basis-change theorem gives BTηB=η and B00>0. Its inverse Λ=B−1, which sends the first tetrad's component list to the second's, is also Lorentz and maps the chosen future timelike component and its massive shell onto themselves: B maps the component into itself, and as an invertible linear map it maps that connected component onto the component containing Be0, which is the future one; the inverse has the same property. Taking determinants in ΛTηΛ=η gives ∣det⁡Λ∣=1 (The change between two future-directed tetrads is a Lorentz matrix, Time orientation, future and past timelike/null/causal vectors, and the closed cones, The timelike set has exactly two convex components, described in orthonormal coordinates).

[F3]

If Q:R3→R3 is a C1 diffeomorphism, then for every nonnegative Borel h, ∫h(q′)d3q′=∫h(Q(q))∣det⁡DQ(q)∣d3q, assuming ACω (Borel change of variables from the compact-support formula and Radon uniqueness, The Axiom of Countable Choice (ACω)).

Proof

1.1givenF1F2algebra

Write P(q)=(E(q),q) in the first tetrad and let q′=Q(q) be the spatial coordinates of the same future-shell vector in the second tetrad. The tetrad change Λ maps the entire future shell bijectively to itself; the spatial coordinate and its inverse are smooth, so Q is a global C1 diffeomorphism of R3. Form the 4×4 matrix C(q)=[P(q),∂1P(q),∂2P(q),∂3P(q)]. Block elimination gives det⁡C(q)=E(q)−∣q∣2/E(q)=M∗2/E(q).

2.1step 1.1F2algebra

The chain rule on the shell gives ΛC(q)=C(q′)diag⁡(1,DQ(q)). Taking absolute determinants and using [F2] and step 1.1 yields M∗2E(q)=M∗2E(q′)∣det⁡DQ(q)∣,∣det⁡DQ(q)∣=E(q′)E(q).

3.1step 2.1F3algebra∎

For any Borel A on the shell, apply [F3] to the nonnegative Borel function h(q′)=1A(Pe′(q′))/E(q′). Then μM∗,x,e′(A)=∫R31A(Pe(q))E(q′)∣det⁡DQ(q)∣ d3q=∫R31A(Pe(q))E(q) d3q=μM∗,x,e(A), where the first equality uses the common geometric vector and the second uses step 2.1. This is equality of positive Borel measures, completing the proof.

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