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Killing particle charge and force balance
Conditional scalar balance for a test particle and a specified Killing field; energy interpretation only for a future unit time-translation field, with no global symmetry assumption.
Statement
Let be an open region of the prescribed background and let be a smooth Killing field on . For a massive force-postulate trajectory , parameterized by proper time on a nonempty open interval with , constant , , and , the scalar obeys
Thus it is constant when the body is free (). For a free null geodesic on a nonempty open interval , with , affine parameter , and tangent momentum , is constant along as well. If is a future unit timelike observer field along the applicable curve, then has energy units and is the energy defined by that observer; for a general Killing field it is only a conserved scalar, not automatically an energy.
Facts & Assumptions
Given: The smooth metric-compatible torsion-free connection, Killing field, force law, and null branch in the Statement.
A Killing field satisfies (Killing fields of a Lorentzian metric).
On the massive branch and with are the adopted constant-mass force assumptions (Mechanical momentum and observer energy, Prescribed constant-mass force law).
The null sector adopts independent einbein variation (Massless test-particle action and momentum scale); its nonzero future-null branch satisfies and (Positive-einbein variation and massive/null sector equations).
The Levi-Civita connection is metric-compatible and torsion-free; in a chart these properties give and (Existence and uniqueness of the metric-selected connection of a nondegenerate metric, and its extension to tensor fields, Torsion freedom and metric compatibility in chart components).
A differentiable scalar function with zero derivative on an interval is constant (A function continuous on an interval whose derivative vanishes at every interior point of is constant on ; consequently two such functions with the same derivative differ by a constant).
Proof
In any chart, differentiating the flow pullback that defines gives : in the coordinate expression , use and . On the other hand, gives . Expanding and substituting [F5], the terms with cancel by torsion-freeness, and the remaining metric-derivative terms become by metric compatibility. Thus on this region, without an orientability hypothesis. Since is Killing by [F1], this symmetrized covariant derivative vanishes. Now metric compatibility [F2] gives . With , the first term is . Substituting [F3] proves the forced balance .
If , step 1.1 gives zero derivative, so [F6] makes the scalar constant on the connected parameter interval. On a free null curve, [F2] and [F4] give ; the same locally derived identity from step 1.1 with makes this zero, hence the null scalar is constant.
If is a future unit observer, [F3] gives by symmetry of . With in kg m s and in m s, this has units joules. For other Killing fields that observer definition does not apply, so the conserved quantity remains only a scalar.
Depends on
Killing fields of a Lorentzian metric
Existence and uniqueness of the metric-selected connection of a nondegenerate metric, and its extension to tensor fields
Torsion freedom and metric compatibility in chart components
Covariant differentiation along curves: metric pairing, parallel transport and geodesic speed
A function continuous on an interval whose derivative vanishes at every interior point of is constant on ; consequently two such functions with the same derivative differ by a constant
Mechanical momentum and observer energy
Prescribed constant-mass force law
Massless test-particle action and momentum scale
Positive-einbein variation and massive/null sector equations