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Exact relativistic helix in a uniform magnetic field
Exact global coordinate-time solutions of the adopted flat external-field charged test-particle model for a uniform constant magnetic field. The force law is conditional on the local RPM action postulate and its draft equivalence and mass-shell suppliers. The result describes model orbits only, with no self-field, radiation reaction, measurement, or empirical claim.
Statement
Use the flat SI charged-particle model and local external-action adoption of Charged and spinning test-body quantities and External-field charged-particle action, with the conditional equation and coordinate Lorentz force of Stationary charged action and Lorentz force and Charged test-body mass shell and laboratory work, adopt the flat massive-particle momentum model Adopted massive-particle momentum, observer measurement and isolated-collision conservation, and assume of The Axiom of Countable Choice () for the canonical tangent-bundle smooth structure required by the current local IVP and action-equivalence carriers. Let be in kilograms, in coulombs, in metres per second, and in tesla; , . The smooth global potential , has . Spatial coordinates are in metres, coordinate time in seconds, and initial momentum components in kilograms metres per second. Given initial position and momentum at , define
Here has momentum units, is dimensionless, and the signed angular frequency has units of inverse seconds; all displayed positions are in metres and is in joules.
For , the exact momentum and energy are
and the exact position is
The transverse projection is a circle of radius when ; it is a point when . In that case the full spatial orbit is a straight line parallel to if , and a stationary point if . If and , the circular orbit is planar. The field does no work. If (including or ), the solution is free motion , , . These formulas give a global future-timelike orbit and are unique on every common connected time interval; they are conditional model deductions, not empirical claims.
Facts & Assumptions
Given: A named inertial SI chart, , constant , , the uniform field and explicit potential above, the adopted local action and flat massive-particle models, and for the canonical tangent-bundle smooth structure.
Charged and spinning test-body quantities: the charged test-body field is an alternating two-form, , , and in the flat SI chart , ; local potentials represent .
External-field charged-particle action: model trajectories are stationary for the fixed-endpoint external-charge action in a local potential domain.
Stationary charged action and Lorentz force: under the local hypotheses and , stationarity is equivalent to .
Charged test-body mass shell and laboratory work: in the flat inertial specialization, and .
Adopted massive-particle momentum, observer measurement and isolated-collision conservation: a flat-model massive particle has constant and kinetic four-momentum .
The Axiom of Countable Choice (): assume only countable choice for the canonical tangent-bundle smooth structure, as stated by the current RPM suppliers.
Energy, momentum, invariant mass and four-force of relativistic particles, and necessary collision thresholds: in an inertial frame, , , and .
Norms of four-velocity and four-acceleration, and the observer rest-space decomposition: future timelike chart motion has .
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: a differentiable scalar function on an interval with zero derivative is constant.
The cross product in : the right-handed coordinate convention gives and .
Sums, scalar multiples, products and quotients: , , , and when : differentiable real components obey the sum and product rules, so .
Proof
The potential components satisfy and all electric components vanish, by the tensor convention [F1]. The potential is smooth on all of Minkowski space, so the local-potential assumption [A1] applies, and under [A3], [F2] supplies the covariant equation. The coordinate force and energy equations are therefore those of [F3].
With , the actual current carrier [F3] under [A3] gives . The adopted massive-particle model [A2] and inertial relations [F4] give and . Thus and . Using [F3], [F8], and , one has , so , , because . Differentiation using [F6] verifies the stated sinusoidal momenta and their initial values. The sum and product rules [F9] show that the rotating components also satisfy , so and the displayed constant obeys the shell and ; when , the force is zero and momentum is constant.
For , differentiating the proposed and with [F6] gives and ; differentiating gives . These are exactly the velocity components from step 2.1, and all increments vanish at . For , the stated free position has constant derivative .
For , set and . The transverse coordinates relative to the fixed centre are ; their squared distance from that centre is . Since and , the radius is . If , and , so the transverse projection is a point. Then makes the full spatial orbit an axial straight line when and a stationary point when . If and , is constant and the circle is planar. The energy constancy in step 2.1 proves zero magnetic work.
The displayed curves satisfy the force equation and initial data by steps 2.1 and 3.1. Any other solution with the same data has constant energy by [F3], so step 2.1 puts its momentum in the same constant-coefficient system , , . For the difference of two such momenta, the product rule [F9] gives , and . With zero initial difference, [F7] makes and on every common connected interval, hence both momenta agree, including . Their velocities then agree by [F4], so each position difference has zero derivative and zero initial value; [F7] gives equality. The formulas are smooth for all real ; , and in the rotating case the denominator is nonzero. The resulting worldline is timelike by [F5].
Depends on
Charged and spinning test-body quantities
External-field charged-particle action
Stationary charged action and Lorentz force
Charged test-body mass shell and laboratory work
The Axiom of Countable Choice ()
The electromagnetic two-form, its observer decomposition and the four-current
Adopted massive-particle momentum, observer measurement and isolated-collision conservation
Energy, momentum, invariant mass and four-force of relativistic particles, and necessary collision thresholds
Norms of four-velocity and four-acceleration, and the observer rest-space decomposition
The cross product in
Sine and cosine defined by their real power series
The derivatives of sine and cosine are cosine and minus sine
Continuity and derivatives of positive-base real powers
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
Sums, scalar multiples, products and quotients: , , , and when