Astronomy and Astrophysics – Astrophysics – General Relativity and Quantum Cosmology
Scientific paper
1994-11-28
Phys.Lett. B349 (1995) 89-93
Astronomy and Astrophysics
Astrophysics
General Relativity and Quantum Cosmology
5 pages (10 preprint pages) ReVTeX
Scientific paper
10.1016/0370-2693(95)00195-Q
Local action principles on a manifold $\M$ are invariant (if at all) only under diffeomorphisms that preserve the boundary of $\M$. Suppose, however, that we wish to study only part of a system described by such a principle; namely, the part that lies in a bounded region $R$ of spacetime where $R$ is specified in some diffeomorphism invariant manner. In this case, a description of the physics within $R$ should be invariant under {\it all} diffeomorphisms regardless of whether they preserve the boundary of this region. The following letter shows that physics in such a region can be described by an action principle that $i$) is invariant under both diffeomorphisms which preserve the boundary of $R$ and those that do not, $ii$) leaves the dynamics of the part of the system {\it outside} the region $R$ completely undetermined, and $iii$) can be constructed without first solving the original equations of motion.
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