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The guided-ring Sagnac delay: return times and receiver proper-time difference for counter-propagating guided signals on a rotating ring

Physical scope

Conditional kinematic computation in flat classical special relativity for an ideal rigidly rotating circular ring, an adopted ideal circular guide carrying two counter-propagating guided signals with inertial tangential speed c, and an ideal clock detector on the ring. The guide and detector models are explicit idealizations, not consequences of the special-relativistic postulates; the item excludes real optical fibers, refractive and dispersive media, apparatus, finite resolution, interference and wave optics, gravitation and quantum effects, and it makes no measurement, statistical or empirical claim.

Statement

Adopt Adopted inertial covariance of the model equations and Adopted ideal-clock and ideal-vacuum-light interpretation and the SI conventions of SI units and coordinate conventions of the special-relativity framework (x0=ct). Let x=(x0,x1,x2,x3) be the chart of an inertial frame (Inertial frames and inertial charts of the affine Minkowski model). Fix R>0, a real Ω with ∣Ω∣R<c, a real x03 and a real ϕ0, and put γ:=(1−Ω2R2/c2)−1/2. A rigidly rotating circular ring of radius R and height x03 about the x3-axis, with angular velocity Ω, has material worldlines

Wϕ0(t):=x−1(ct,  Rcos⁡(ϕ0+Ωt),  Rsin⁡(ϕ0+Ωt),  x03),t∈R.

(i) The ring worldline and the ring congruence. Wϕ0 is a C∞ future timelike worldline with unit observer

n=γ (1,  −(ΩR/c)sin⁡(ϕ0+Ωt),  (ΩR/c)cos⁡(ϕ0+Ωt),  0)=(γ/c) ∂t∣φ,

η(n,n)=−1, and four-velocity U=cn=γ ∂t∣φ, η(U,U)=−c2, where φ:=ϕ−Ωt is the rotating angular label of the ring material points; it is an unparametrized flow line of the rigid-rotation congruence n=γ(r) [∂0+(Ω/c)(−x2∂1+x1∂2)] restricted to r=R, and its proper time τ and the chart time t satisfy dτ=dt/γ, so τ=t/γ+τ0 for an arbitrary proper-time origin τ0 on this worldline (Fermi-Walker transport preserves the metric, and explicit rigid and non-rigid observer congruences (c), Proper time along a future timelike worldline, The proper-time function is a strictly increasing C^1 parametrization with C^1 inverse).

(ii) The guided-signal model. Adopt, as an explicit model assumption of an ideal circular guide attached to the ring and of the signal and detector model below, and not as a consequence of the postulates: two ideal signals are emitted at the common emission event Wϕ0(0) into the guide, one in the positive angular direction and one in the negative angular direction (respectively co-rotating and counter-rotating for Ω>0, with these roles reversed for Ω<0), and in the inertial chart they follow the curves C+ and C− given by ϕ+(t)=ϕ0+ct/R and ϕ−(t)=ϕ0−ct/R with radius R and height x03, i.e. each guided signal has chart angular rate +c/R or −c/R along the guide ('inertial tangential speed c along the guide'); the detector is an ideal clock carried by the ring material point labelled φ=ϕ0. Each of C+ and C− is a future null curve of the chart: its tangent has time component c and spatial part of Euclidean length c, so the tangent has η-norm 0 and positive time component (Timelike, null, spacelike and causal vectors of a Lorentz vector space, Time orientation, future and past timelike/null/causal vectors, and the closed cones).

(iii) Return times and differences. The displayed guided curves have all-real mathematical extensions; emitted signals are their restrictions t≥0. On those extensions C+ meets the worldline Wϕ0 exactly at the chart times 2πkR/(c−ΩR), k∈Z, and C− exactly at the chart times 2πkR/(c+ΩR), k∈Z; the smallest positive meeting times are t1:=2πR/(c−ΩR)>0 and t2:=2πR/(c+ΩR)>0. Hence the two counter-propagating signals emitted at the same event reunite with the emitting material point at the reception events Wϕ0(t1) and Wϕ0(t2), each having gained exactly one circuit relative to that material point, and the chart-time difference of the two reunions is t1−t2=4πΩR2/(c2−Ω2R2)>0 for Ω>0. The proper-time difference measured by the detector ideal clock is (t1−t2)/γ=4πΩR2/(c21−Ω2R2/c2). For Ω=0 (inertial ring) t1=t2=2πR/c and both differences vanish; for Ω<0 the sign of the first difference reverses and the roles of C+ and C− are interchanged.

(iv) Relation to the ring rest-synchronization holonomy. On the fixed-height timelike cylinder C={r=R, x3=x03}, material worldlines have constant rotating label. Its rest lines instead vary φ on an angular lift and satisfy dt/dφ=ΩR2/(c2−Ω2R2), being integral curves of ∂φ+[ΩR2/(c2−Ω2R2)] ∂t; for Ω=0 they are t=constant circles; one circuit φ→φ+2π has the chart-time holonomy Δth=2πΩR2/(c2−Ω2R2)=(t1−t2)/2 and the receiver proper-time holonomy Δτh=Δth/γ=2πΩR2/(c21−Ω2R2/c2)=(1/2)(t1−t2)/γ, exactly half of the detector proper-time difference of (iii); for Ω=0 both holonomies vanish. The ambient three-dimensional rest distribution of the rotating congruence is nowhere integrable for Ω≠0, so no ambient rest-simultaneity hypersurface exists, while the one-dimensional rest distribution restricted to the fixed-height cylinder C is locally integrable and has the global holonomy just computed, nonvanishing exactly when Ω≠0; the return-time computation of (iii) uses only the guided null curves and the material worldline and does not use either integrability property (Fermi-Walker transport preserves the metric, and explicit rigid and non-rigid observer congruences (c), Local flow boxes, adapted charts and the Frobenius criterion for rest distributions).

(v) Scope. All quantities are conditional model computations for the specified ring, guide, signal and detector idealizations. The guided-signal curves are an adopted device model: the ideal vacuum light rays of Adopted ideal-clock and ideal-vacuum-light interpretation are straight future null lines and do not circulate around the ring by themselves, and no assertion is made about optical fibers, refractive media, dispersion, absorption, finite resolution, apparatus calibration, interference phase or wave optics, or about one-way synchronization conventions between distinct ring observers; no measurement outcome, statistical inference or empirical claim is made, the values of R and Ω are parameters and not measured quantities, and no choice principle is used.

Facts & Assumptions

Given: c>0; an inertial chart x with x0=ct and η=diag⁡(−1,1,1,1) in the orthonormal frame; R>0, real Ω with ∣Ω∣R<c, real x03,ϕ0, and γ=(1−Ω2R2/c2)−1/2; the ring worldlines Wϕ0(t)=x−1(ct,Rcos⁡(ϕ0+Ωt),Rsin⁡(ϕ0+Ωt),x03); the guided curves C± of clause (ii), adopted as the explicit device model of the statement; and the detector ideal clock carried by the material point φ=ϕ0.

[L1]

Ideal clocks read the proper time of their worldline, and ideal vacuum light rays are the future null straight lines with coordinate speed c in every inertial chart (Adopted ideal-clock and ideal-vacuum-light interpretation).

[L2]

In every inertial chart the coordinates are x0=ct, the matrix of η is diag⁡(−1,1,1,1) with dimensionless entries, and the interval carries m2 (SI units and coordinate conventions of the special-relativity framework, Inertial frames and inertial charts of the affine Minkowski model).

[L3]

Points of A are determined by their inertial coordinates, and vector components are read in the orthonormal basis (ea) (Inertial frames and inertial charts of the affine Minkowski model, Oriented time-oriented affine Minkowski space (the spacetime primitives)).

[L4]

A curve x:I→A is differentiable precisely when its coordinate representation is, its tangent is the corresponding V-vector, and it is Ck when that representation is; at a regular point the causal character and future direction of the curve are those of its tangent (Curves, regular causal curves and future timelike worldlines in affine Minkowski space (a),(b)).

[L5]

Causal classification: timelike means η(v,v)<0, null means v≠0 and η(v,v)=0, and each nonzero causal vector is future or past but not both; C+={v:v0>∣vs∣} and Cˉ+={v:v0≥∣vs∣} in every inertial chart (Timelike, null, spacelike and causal vectors of a Lorentz vector space, Time orientation, future and past timelike/null/causal vectors, and the closed cones, The closed future cone in inertial coordinates: description, finite sums, and the coordinate criterion for causal reachability (a)).

[L6]

Four-velocity: U=x′/τ′=cx′/−η(x′,x′) with η(U,U)=−c2 along a C1 future timelike worldline, and for a congruence flow line with tangent x′=cn the four-velocity is cn (Four-velocity and four-acceleration of a future timelike worldline (a),(c), Norms of four-velocity and four-acceleration, and the observer rest-space decomposition (a)).

[L7]

Proper time: for a C1 future timelike worldline, τ(b)−τ(a)=c−1∫ab−η(x′,x′) du, and the proper-time reparametrization is C1 with tangent x′/τ′ (Proper time along a future timelike worldline, The proper-time function is a strictly increasing C^1 parametrization with C^1 inverse (i),(iii),(iv)).

[L9]

Trigonometric identities: sin⁡2t+cos⁡2t=1, parity, addition formulas, quarter-turn shifts sin⁡(t+π)=−sin⁡t, cos⁡(t+π)=−cos⁡t, and sin⁡t=0 iff t=mπ (m∈Z), with both sine and cosine 2π-periodic (Parity and the Pythagorean identity for sine and cosine, The addition formulas for sine and cosine, Quarter-turn values and shifts by pi/2 and pi, The zero sets of sine and cosine and the least positive common period 2 pi).

[L10]

Rigid rotation and the ring: on the timelike region r2<c2/Ω2 the field n=γ(r)[∂0+(Ω/c)(−x2∂1+x1∂2)] is a smooth future unit timelike Born-rigid congruence with nowhere-zero vorticity for Ω≠0; on the ring cylinder r=R, x3=const, the induced metric is −(c2−Ω2R2)dt2+2ΩR2 dt dφ+R2dφ2, the ring observer ϕ=ϕ0+Ωt has n=γ(R)(1,−(ΩR/c)sin⁡ϕ,(ΩR/c)cos⁡ϕ,0), the one-dimensional rest distribution is locally integrable with dt/dφ=ΩR2/(c2−Ω2R2), one positive circuit has signed chart-time offset Δt=2πΩR2/(c2−Ω2R2), and for Ω=0 the ring is inertial with rest lines t=const (Fermi-Walker transport preserves the metric, and explicit rigid and non-rigid observer congruences (c)).

[L11]

Frobenius: the ambient rest distribution of an observer congruence is integrable on an open set exactly where its vorticity vanishes; a locally integrable rank-one cylinder distribution is unaffected by the ambient obstruction (Local flow boxes, adapted charts and the Frobenius criterion for rest distributions (iii), Kinematics of an observer congruence: acceleration, expansion, shear, vorticity and Born rigidity (c),(d)).

Proof

technique · direct
1.1L2L3L8L9algebra

(Tangent and invariants of the ring worldline.) With ϕ:=ϕ0+Ωt, the coordinate functions of Wϕ0 are t↦ct, t↦Rcos⁡ϕ, t↦Rsin⁡ϕ and the constant x03; differentiating with [L8] gives W˙=(c,−ΩRsin⁡ϕ,ΩRcos⁡ϕ,0), and by [L2] and [L9], η(W˙,W˙)=−c2+Ω2R2(sin⁡2φ+cos⁡2φ)=−c2(1−Ω2R2/c2)=−c2/γ2<0, while the time component is W˙0=c>0 and the spatial part has Euclidean norm ∣Ω∣R<c.

1.2L5L8L9algebra

(The guided curves are future null.) With ϕ±:=ϕ0±ct/R the curves C± have coordinate functions t↦ct, t↦Rcos⁡ϕ±, t↦Rsin⁡ϕ±; differentiating gives tangents (c,∓csin⁡ϕ±,±ccos⁡ϕ±,0), whose η-square is −c2+c2(sin⁡2+cos⁡2)=0 by [L2] and [L9], so the tangent is nonzero null, with time component c equal to the Euclidean norm of its spatial part; hence each tangent lies in Cˉ+∖{0} by [L5], so each of C± is a future null curve.

1.3L10L11algebra

(Rest-synchronization data on the cylinder.) By [L10] the induced metric on the cylinder is −(c2−Ω2R2)dt2+2ΩR2dt dφ+R2dφ2 with c2−Ω2R2>0 by ∣Ω∣R<c, and the one-dimensional rest distribution is locally integrable with dt/dφ=ΩR2/(c2−Ω2R2); one positive circuit φ↦φ+2π has chart-time offset Δth=2πΩR2/(c2−Ω2R2), and for Ω=0 the rest lines are t=const. For Ω≠0 the ambient rest distribution has nowhere-zero vorticity, so by [L11] it is nowhere integrable and admits no local rest-simultaneity hypersurface; the rank-one cylinder distribution is a separate locally integrable object, and no global single-valued rest time on the cylinder exists when Δth≠0.

1.4L9algebra

(Agreement of sine and cosine pairs.) For real angles α,β: if cos⁡α=cos⁡β and sin⁡α=sin⁡β, then the addition formulas give cos⁡(α−β)=cos⁡αcos⁡β+sin⁡αsin⁡β=cos⁡2α+sin⁡2α=1 and sin⁡(α−β)=sin⁡αcos⁡β−cos⁡αsin⁡β=0 by [L9]; the zero-set statement puts α−β∈πZ, and the quarter-turn shifts give cos⁡(mπ)=(−1)m, so cos⁡(α−β)=1 forces m even, i.e. α−β=2πk for some k∈Z. Conversely α−β=2πk implies cos⁡α=cos⁡β and sin⁡α=sin⁡β by the 2π-periodicity of [L9].

2.1L6L7L9L10step 1.1algebra

(Congruence data and proper time on the ring.) By [L10] the field n=γ(R)(1,−(ΩR/c)sin⁡ϕ,(ΩR/c)cos⁡ϕ,0) is the unit observer of the ring material point and Wϕ0 is its flow line; direct substitution with [L9] gives η(n,n)=γ2(−1+(Ω2R2/c2)(sin⁡2+cos⁡2))=γ2(−1+Ω2R2/c2)=−1, and U:=cn=cγ(1,−(ΩR/c)sin⁡ϕ,(ΩR/c)cos⁡ϕ,0)=γW˙, coinciding with γ ∂t∣φ evaluated along the material line and with the four-velocity of [L6]. Since η(W˙,W˙)=−c2/γ2 by step 1.1, the proper-time integrand of [L7] is c−1−η(W˙,W˙)=1/γ, so dτ=dt/γ and τ=t/γ+τ0.

2.2L2step 1.2step 1.4given

(The meeting sets of the guided curves with the material worldline.) The guided signal C+ meets Wϕ0 at chart time t exactly when the material point and the guided point coincide, i.e. exactly when cos⁡(ϕ0+ct/R)=cos⁡(ϕ0+Ωt) and sin⁡(ϕ0+ct/R)=sin⁡(ϕ0+Ωt); by step 1.4, applied to α=ϕ0+ct/R and β=ϕ0+Ωt, this is equivalent to (c/R−Ω)t∈2πZ, i.e. to t=2πkR/(c−ΩR) for some k∈Z. Similarly C− meets Wϕ0 exactly when −(c/R+Ω)t∈2πZ, i.e. t=2πkR/(c+ΩR), k∈Z. The angular labels of the two curves at a common meeting time differ by exactly one winding per unit of k by construction.

3.1L4L5L6step 1.1step 2.1

(The ring worldline is a future timelike worldline and a congruence flow line.) By step 1.1 the coordinate functions are smooth compositions of affine maps with sine and cosine, so Wϕ0 is C∞ by [L4]; its tangent is timelike with positive time component exceeding the spatial norm, so it is future timelike and regular by [L5], and Wϕ0 is a future timelike worldline. By step 2.1 it is a flow line of the unit field n of the rigid-rotation congruence restricted to r=R, with four-velocity U=cn and η(U,U)=−c2 in agreement with [L6], and its proper time is τ=t/γ+τ0 by step 2.1.

4.1L1L2step 2.1step 3.1step 2.2algebra

(Return times and the differences.) Since ∣Ω∣R<c, both c−ΩR>0 and c+ΩR>0; hence the smallest positive members of the two meeting sets of step 2.2 are t1=2πR/(c−ΩR) and t2=2πR/(c+ΩR), both positive, and the emitted signals reunite with the emitting material point at Wϕ0(t1) and Wϕ0(t2) after exactly one circuit relative to it. Their chart-time difference is t1−t2=2πR[(c+ΩR)−(c−ΩR)]/((c−ΩR)(c+ΩR))=4πΩR2/(c2−Ω2R2), which is positive for Ω>0 and negative for Ω<0. By step 3.1 the two receptions correspond to proper times τ=t1/γ+τ0 and τ=t2/γ+τ0 of the detector ideal clock, so the detector reads the proper-time difference (t1−t2)/γ=4πΩR2/(c21−Ω2R2/c2) by [L1] and step 2.1.

5.1L10step 2.1step 1.3step 4.1algebra

(Holonomy and the factor-two relation.) By step 1.3 the chart-time holonomy of the ring rest lines over one circuit is Δth=2πΩR2/(c2−Ω2R2); comparing with step 4.1 gives t1−t2=2Δth, i.e. Δth=(t1−t2)/2. Dividing by γ using dτ=dt/γ of step 2.1 gives the receiver proper-time holonomy Δτh=Δth/γ=(1/2)(t1−t2)/γ, exactly half the detector proper-time difference of step 4.1; explicitly Δτh=2πΩR2/(c21−Ω2R2/c2).

6.1L10step 1.3step 4.1step 5.1algebra

(Inertial limit and reversed rotation.) If Ω=0 then γ=1, the meeting sets of step 2.2 are both 2πkR/c, so t1=t2=2πR/c and both differences of step 4.1 and both holonomies of step 5.1 vanish; the rest lines are the t=const circles of step 1.3. If Ω<0, the factor 4πΩR2/(c2−Ω2R2) of step 4.1 is negative, so the sign of t1−t2 is reversed, and the labels 'co-rotating' and 'counter-rotating' are interchanged because the material angular rate Ω now has the opposite sign to that of C+.

7.1L1L6L10step 2.2step 4.1step 5.1step 6.1∎

(Scope.) The computation used only: the inertial chart and the adopted guided curves and detector of clauses (ii) and (iii); the rigid-rotation data of the ring cylinder; the proper time and four-velocity suppliers; and the trigonometric suppliers. The adopted guided-signal model is not a consequence of the ideal-light postulate, whose rays are straight; no claim is made about media, apparatus, interference or measurement, and no choice principle was used in any step.

Remarks

  • The two differences are distinct quantities. The detector proper-time difference of (iii) is the difference of the two reunion times of the counter-propagating signals at the emitting material point; the holonomy of (iv) is the synchronization offset accumulated around one circuit of the ring rest lines. They differ by exactly a factor two, and the factor-two relation is proved, not assumed.
  • The ambient Frobenius obstruction is not needed for the delay. The return-time computation proceeds along the two guided null curves and the single material worldline and uses neither the nonintegrability of the ambient rest distribution nor the local integrability of the cylinder rest lines; those facts serve only to interpret clause (iv).

Depends on

Dependency tree · 2 levels

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