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Worked longitudinal Doppler shift at beta = 3/5

Physical scope

Worked longitudinal Doppler computation of the ideal plane-wave model at relative speed (3/5)c in the two propagation directions along the boost axis. Conditional on the model; no measurement, statistical inference or empirical claim.

Example

Worked example in the model of sr-light-and-doppler, on the page sr-light-and-doppler-examples. Let x and x′ be inertial charts with x′=B(β)x+b, β=(3/5)e1, so that γ=5/4 and the frame x′ moves with velocity (3/5)c e1 relative to x. Let an ideal plane wave have frame-x data (ω,s) with ω>0 and (a) s=e1, (b) s=−e1. Then Doppler shift and aberration for an ideal plane wave between inertial descriptions gives:

(a) ω′=γ(1−3/5)ω=(5/4)(2/5)ω=ω/2, ν′=ν/2, λ′=2λ and s′=e1: an observer at rest in x′ moving in the direction of propagation reads half the frequency and twice the wavelength.

(b) ω′=γ(1+3/5)ω=(5/4)(8/5)ω=2ω, ν′=2ν, λ′=λ/2 and s′=−e1: an observer at rest in x′ moving opposite to the propagation direction reads twice the frequency and half the wavelength.

In both cases ∣s′∣=1 and ω′>0, in agreement with the general theorem; the verification uses only the exact rational values γ=5/4, 1−3/5=2/5 and 1+3/5=8/5. Scope: a conditional computation in the ideal plane-wave model; no measurement is performed and no empirical claim is made.

Facts & Assumptions

Given: Inertial charts x,x′ with x′=B(β)x+b, β=(3/5)e1; an ideal plane wave with frame-x data (ω,s), ω>0, ∣s∣=1, and s=±e1.

[A1]

Adopted ideal plane-wave model: one ideal monochromatic plane wave with future null wave covector; the rest observers of x and x′ read the frequencies ω and ω′ (Adopted ideal plane-wave and frequency-reading model, Null wave covectors, frame frequency data and the observer frequency of an ideal plane wave).

[L1]

For the chart pair of the Given the theorem gives ω′=γ(1−β⋅s)ω>0, ν′=γ(1−β⋅s)ν, λ′=λ/[γ(1−β⋅s)], ∣s′∣=1, and in the axis case β=be1 the direction formulas s1′=(s1−b)/(1−bs1), s2′=s2/[γ(1−bs1)], s3′=s3/[γ(1−bs1)] (Doppler shift and aberration for an ideal plane wave between inertial descriptions, clauses (i) and (ii)).

[L2]

Longitudinal clause: for s=βhat the factor is γ(1−∣β∣)=(1−∣β∣)/(1+∣β∣) and for s=−βhat it is γ(1+∣β∣)=(1+∣β∣)/(1−∣β∣); here βhat=e1 (Doppler shift and aberration for an ideal plane wave between inertial descriptions, clause (iii)).

[L3]

γ=(1−∣β∣2)−1/2, so for β=(3/5)e1: γ=(1−9/25)−1/2=5/4; and the boost transformation of the frame data is the one of Boost transformation of a future null wave covector and of its frame data (The passive Lorentz boost with parameter beta).

Verification

technique · direct
1.1A1L3given

Setup. Here βhat=e1, ∣β∣=3/5 and γ=(1−9/25)−1/2=(16/25)−1/2=5/4 by [L3]; both witness directions s=±e1 have ∣s∣=1, so the wave is in the model [A1].

2.1step 1.1L1L2

Case (a), s=e1. Then β⋅s=3/5, so [L1] gives ω′=γ(1−3/5)ω=(5/4)(2/5)ω=ω/2 and hence ν′=ω′/(2π)=ν/2 and λ′=c/ν′=2c/ν=2λ; the axis formulas of [L1] with b=3/5, s1=1, s2=s3=0 give s1′=(1−3/5)/(1−3/5)=1 and s2′=s3′=0, so s′=e1; the frequency factor agrees with the longitudinal clause [L2], ω′/ω=(1−3/5)/(1+3/5)=(2/5)/(8/5)=1/2, since s=βhat.

3.1step 1.1L1L2∎

Case (b), s=−e1. Then β⋅s=−3/5, so [L1] gives ω′=γ(1+3/5)ω=(5/4)(8/5)ω=2ω, ν′=2ν and λ′=c/ν′=λ/2; the axis formulas of [L1] with b=3/5, s1=−1 give s1′=(−1−3/5)/(1+3/5)=−1 and s2′=s3′=0, so s′=−e1; the frequency factor agrees with the longitudinal clause [L2], ω′/ω=(1+3/5)/(1−3/5)=(8/5)/(2/5)=2, since s=−βhat. In both cases ω′>0 and ∣s′∣=1 as the theorem asserts. This proves (a) and (b) by exact rational arithmetic; the computation is conditional on the ideal plane-wave model, no measurement is performed and no empirical claim is made; no choice principle is used.

Depends on

Dependency tree · 2 levels

Sources