Solution
ID: past-exam-of-the-relativity-course-of-the-university-of-cambridge/2012/paper-1/3/solution
Past exam of the relativity course of the University of Cambridge 2012 Paper 1 3 Solution by
Codex 0 2026-10-20
The Einstein tensor and its contractions areTo match the supplied Ricci tensor components, use the Riemann curvature tensor conventionThis convention also fixes the sign used for geodesic deviation above. In vacuum, tracing the Einstein field equations gives . Substitution therefore yields .
Differentiate the angular field equation to obtainConsequently and . The other angular component follows from . Thus the single angular equation implies all the displayed vacuum equations wherever this coordinate chart is regular.
Integrating gives . Match with the Newtonian gravitational potential in the weak-field overlap region to identify . Hence the Schwarzschild-de Sitter spacetime hasNegative instead gives the Schwarzschild--anti-de Sitter black hole case. This matching fixes the normalization locally; a spacetime with nonzero is not asymptotically flat.
For timelike geodesics in the orbital plane, take proper time as parameter and use . The cyclic coordinates give and ; the angular canonical momentum is in this signature. The four-velocity normalization then givesHere is conserved Killing energy per unit rest mass in the chosen time normalization, and is conserved angular momentum per unit rest mass. There is no general interpretation of as energy measured at an asymptotically flat infinity when .
For radial motion , differentiation gives . The radial Euler-Lagrange equation extends the resulting expression through a turning point where division by would fail. ThusThis is the proper-time second derivative of the areal radius, not a nonzero four-acceleration of the freely falling particle. Positive contributes an outward term; negative reinforces inward attraction. Restoring units in the slow-motion, weak-field Newtonian limit givesThe extra term describes the gravitational effect of a cosmological constant, not drag or a contact force. The approximation requires , and speed much less than .
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