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Exact relativistic helix in a uniform magnetic field

Physical scope

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 ACω of The Axiom of Countable Choice (ACω) for the canonical tangent-bundle smooth structure required by the current local IVP and action-equivalence carriers. Let m>0 be in kilograms, q∈R in coulombs, c>0 in metres per second, and B0∈R in tesla; E=0, B=B0e3. The smooth global potential ϕ=0, A=(−B0x2/2,B0x1/2,0) has ∇×A=B0e3. Spatial coordinates are in metres, coordinate time t in seconds, and initial momentum components in kilograms metres per second. Given initial position x0 and momentum p0=(p01,p02,p03) at t=0, define

S0=m2c2+∣p0∣2,γ0=S0/(mc),Ω=qB0γ0m.

Here S0 has momentum units, γ0 is dimensionless, and the signed angular frequency Ω has units of inverse seconds; all displayed positions are in metres and E is in joules.

For Ω≠0, the exact momentum and energy are

p1(t)=p01cos⁡(Ωt)+p02sin⁡(Ωt),p2(t)=p02cos⁡(Ωt)−p01sin⁡(Ωt),p3(t)=p03,E(t)=cS0,

and the exact position is

x1(t)=x01+cS0Ω(p01sin⁡(Ωt)+p02(1−cos⁡(Ωt))), x2(t)=x02+cS0Ω(p02sin⁡(Ωt)−p01(1−cos⁡(Ωt))),x3(t)=x03+cp03S0t.

The transverse projection is a circle of radius ∣p⊥∣/∣qB0∣ when p⊥=(p01,p02)≠0; it is a point when p⊥=0. In that case the full spatial orbit is a straight line parallel to e3 if p03≠0, and a stationary point if p03=0. If p⊥≠0 and p03=0, the circular orbit is planar. The field does no work. If Ω=0 (including q=0 or B0=0), the solution is free motion p(t)=p0, E=cS0, x(t)=x0+cp0t/S0. 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, m>0, constant q, c>0, the uniform field and explicit potential above, the adopted local action and flat massive-particle models, and ACω for the canonical tangent-bundle smooth structure.

[F1]

Charged and spinning test-body quantities: the charged test-body field is an alternating two-form, P=mU, m>0, and in the flat SI chart F0i=−Ei/c, Fij=ϵijkBk; local C2 potentials represent F=dA.

[A1]

External-field charged-particle action: model trajectories are stationary for the fixed-endpoint external-charge action in a local potential domain.

[F2]

Stationary charged action and Lorentz force: under the local hypotheses and ACω, stationarity is equivalent to m∇UU=qF♯(U).

[F3]

Charged test-body mass shell and laboratory work: in the flat inertial specialization, dp/dt=q(E+v×B) and dE/dt=qE⋅v.

[A2]

Adopted massive-particle momentum, observer measurement and isolated-collision conservation: a flat-model massive particle has constant m>0 and kinetic four-momentum P=mU.

[A3]

The Axiom of Countable Choice (ACω): assume only countable choice ACω for the canonical tangent-bundle smooth structure, as stated by the current RPM suppliers.

[F4]

Energy, momentum, invariant mass and four-force of relativistic particles, and necessary collision thresholds: in an inertial frame, E=γmc2, p=γmv, and E2=m2c4+c2∣p∣2.

[F5]
[F6]

The derivatives of sine and cosine are cosine and minus sine: (sin⁡s)′=cos⁡s and (cos⁡s)′=−sin⁡s.

[F8]

The cross product in R3: the right-handed coordinate convention gives e2×e3=e1 and e1×e3=−e2.

Proof

1.1givenF1A1A3F2F3algebra

The potential components satisfy F12=∂1A2−∂2A1=B0 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].

2.1step 1.1A2A3F3F4F6F8F9algebra

With E=0, the actual current carrier [F3] under [A3] gives E˙=0. The adopted massive-particle model [A2] and inertial relations [F4] give E2=c2(m2c2+∣p∣2) and v=c2p/E. Thus E=cS0 and v=cp/S0. Using [F3], [F8], and v=cp/S0, one has v×B0e3=B0(v2e1−v1e2), so p˙1=Ωp2, p˙2=−Ωp1, p˙3=0 because qB0c/S0=qB0/(γ0m)=Ω. 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 ddt(p12+p22)=2Ωp1p2−2Ωp1p2=0, so ∣p(t)∣=∣p0∣ and the displayed constant E=cS0 obeys the shell and E˙=0; when Ω=0, the force is zero and momentum is constant.

3.1step 2.1F6algebra

For Ω≠0, differentiating the proposed x1 and x2 with [F6] gives x˙1=cp1/S0 and x˙2=cp2/S0; differentiating x3 gives cp3/S0. These are exactly the velocity components from step 2.1, and all increments vanish at t=0. For Ω=0, the stated free position has constant derivative cp0/S0=v.

4.1step 2.1step 3.1F3algebra

For Ω≠0, set A=cp01/(S0Ω) and B=cp02/(S0Ω). The transverse coordinates relative to the fixed centre (x01+B,x02−A) are (Asin⁡Ωt−Bcos⁡Ωt, Bsin⁡Ωt+Acos⁡Ωt); their squared distance from that centre is A2+B2. Since S0=γ0mc and ∣Ω∣=∣qB0∣/(γ0m), the radius is c∣p⊥∣/(S0∣Ω∣)=∣p⊥∣/∣qB0∣. If p⊥=0, x1=x01 and x2=x02, so the transverse projection is a point. Then x3=x03+cp03t/S0 makes the full spatial orbit an axial straight line when p03≠0 and a stationary point when p03=0. If p⊥≠0 and p03=0, x3 is constant and the circle is planar. The energy constancy in step 2.1 proves zero magnetic work.

5.1step 2.1step 3.1F3F4F5F7F9algebra∎

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 E=cS0 by [F3], so step 2.1 puts its momentum in the same constant-coefficient system p˙1=Ωp2, p˙2=−Ωp1, p˙3=0. For the difference d of two such momenta, the product rule [F9] gives ddt(d12+d22)=2Ωd1d2−2Ωd2d1=0, and d˙3=0. With zero initial difference, [F7] makes d12+d22=0 and d3=0 on every common connected interval, hence both momenta agree, including Ω=0. 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 t; S0≥mc>0, and in the rotating case the denominator Ω is nonzero. The resulting worldline is timelike by [F5].

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