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Worked aberration and Doppler shift at beta = 3/5 and cos theta = 4/5

Physical scope

Worked numerical aberration and Doppler computation of the ideal plane-wave model at relative speed (3/5)c and source angle cosine 4/5. 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 and γ=5/4, and let an ideal plane wave have frame-x propagation direction s=(4/5,3/5,0), so cos⁡θ=4/5 and sin⁡θ=3/5 with θ∈[0,π] the angle between s and the boost axis. Then Doppler shift and aberration for an ideal plane wave between inertial descriptions gives s′=(5/13,12/13,0), hence cos⁡θ′=5/13, and the angular frequency ratio ω′/ω=γ(1−βs1)=(5/4)(1−12/25)=(5/4)(13/25)=13/20, so ν′=(13/20)ν and λ′=(20/13)λ. Define the apparent source direction here to mean the incoming unit direction −s (and −s′ in x′); this is a direction convention, not a source or detector model. Since 5/13<4/5 (as 25<52), its axis component moves from −s1=−4/5 to −s1′=−5/13 measured along the boost axis, that is, toward the direction of motion of the observer's frame: this is the sign of stellar aberration in this configuration. The example records the exact direction transformation and leaves any observational interpretation to the framework's experimental work. Scope: a conditional computation in the ideal plane-wave model; no measurement and no empirical claim.

Facts & Assumptions

Given: Inertial charts x,x′ with x′=B(β)x+b, β=(3/5)e1; an ideal plane wave with frame-x direction s=(4/5,3/5,0) and angular frequency ω>0.

[A1]

Wave data and readings: the rest observers of x and x′ read the frequencies ω and ω′, and the linear frequency and wavelength satisfy ν=ω/(2π), λ=c/ν (Null wave covectors, frame frequency data and the observer frequency of an ideal plane wave); the direction and frequency transformation used below is the theorem's (Doppler shift and aberration for an ideal plane wave between inertial descriptions).

[L1]

For the chart pair of the Given the theorem gives ω′=γ(1−β⋅s)ω, 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]

Aberration clause: for β≠0, with θ,θ′ the angles between s,s′ and the boost axis, cos⁡θ′=(cos⁡θ−∣β∣)/(1−∣β∣cos⁡θ) (Doppler shift and aberration for an ideal plane wave between inertial descriptions, clause (v)).

[L3]

γ=(1−∣β∣2)−1/2, so for β=(3/5)e1: γ=(16/25)−1/2=5/4 and ∣β∣=3/5; 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
1.1A1L1L2L3

The transformed direction. Here s=(4/5,3/5,0) has ∣s∣2=16/25+9/25=1 and s1=4/5; the axis formulas of [L1] with b=3/5 and γ=5/4 [L3] give s1′=(4/5−3/5)/(1−(3/5)(4/5))=(1/5)/(13/25)=5/13, s2′=(3/5)/[(5/4)(13/25)]=(3/5)/(13/20)=12/13 and s3′=0, so s′=(5/13,12/13,0) with ∣s′∣2=25/169+144/169=1; the same value follows from the aberration clause [L2], cos⁡θ′=(4/5−3/5)/(1−(3/5)(4/5))=(1/5)/(13/25)=5/13.

1.2A1L1L3

Frequency and wavelength. By [L1] with β⋅s=(3/5)(4/5)=12/25: ω′/ω=γ(1−12/25)=(5/4)(13/25)=13/20, so ν′=ω′/(2π)=(13/20)ν and λ′=c/ν′=(20/13)λ by [A1].

2.1step 1.1step 1.2L2∎

The apparent direction. By the incoming-direction convention in the Example, the apparent source direction is −s in x and −s′ in x′; its components along the boost axis βhat=e1 are −s1=−4/5 and −s1′=−5/13 by step 1.1. Since 5/13<4/5 (as 25<52), also −5/13>−4/5: the axis component of the apparent direction increases, so the apparent direction moves toward +e1, the direction of motion of the frame x′ relative to x, which is the sign of stellar aberration in this configuration. The computation is conditional on the ideal plane-wave model, records the exact direction transformation, and makes no measurement or empirical claim; no choice principle is used.

Depends on

Dependency tree · 2 levels

Sources