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Worked covector and vector component transformation under a beta = 3/5 boost

Example

Worked computation. In an inertial frame with coordinates (x0,x1,x2,x3) consider the passive boost Λ:=B(35e1) (The passive Lorentz boost with parameter beta), so that on the (x0,x1) plane Λ=(5/4−3/4−3/45/4) and Λ−1=B(−35e1)=(5/43/43/45/4), and let x′ be an inertial chart with x′=Λx+b (Inertial charts are related by proper orthochronous affine Lorentz maps, and every such map is a chart transition). Take the covector k with components (k0,k1)=(−1,1) and the vector w with components (w0,w1)=(1,1), with transverse components zero. A covector k≠0 is called null here when η−1(k,k)=0, where the inverse form is defined by η−1(k,l):=η(k♯,l♯) and η(k♯,v)=k(v) for every v∈V. In this orthonormal basis the unique vector k♯ has coordinates (−k0,k1,k2,k3), verifying both existence and uniqueness directly; hence η−1(k,k)=−k02+k12+k22+k32. This uses the nondegenerate Lorentz form of Lorentz forms of inertia (3,1,0) on a 4-dimensional real vector space. Then:

(i) ka′=(Λ−1)bakb gives k0′=(5/4)(−1)+(3/4)(1)=−1/2 and k1′=(3/4)(−1)+(5/4)(1)=1/2, so k′=(−1/2,1/2).

(ii) w′a=Λabwb gives w′0=(5/4)(1−(3/5)⋅1)=1/2 and w′1=(5/4)(1−(3/5)⋅1)=1/2, so w′=(1/2,1/2).

(iii) k is null for the inverse form and w is null for η0: −1+1=0, and so are k′ and w′: −1/4+1/4=0.

(iv) The contraction is invariant: ka′w′a=(−1/2)(1/2)+(1/2)(1/2)=0=(−1)(1)+(1)(1)=kawa.

The example shows all three features of the component law of Transformation of tensor, covector, derivative and current components under affine Lorentz maps: the covariant slot transforms with Λ−1, the contravariant slot with Λ, and the contraction is unchanged. The common factor 1/2 on the null covector is the numerical form of the transformation law; no frequency or Doppler interpretation is attached to it here (that belongs to the light-and-doppler page).

Facts & Assumptions

Given: An inertial frame with coordinates (x0,x1,x2,x3) and η0=diag⁡(−1,1,1,1), the passive boost Λ=B(35e1) on the (x0,x1) plane, an inertial chart x′ with x′=Λx+b, the covector k with components (−1,1,0,0) and the vector w with components (1,1,0,0).

[L1]

For ∣β∣<1 the boost B(βe1) acts on the (x0,x1) coordinates by x′0=γ(x0−βx1), x′1=γ(x1−βx0), with γ=(1−β2)−1/2>0, and its matrix on (e0,e1) is (γ−γβ−γβγ); B(βe1)−1=B(−βe1) (The passive Lorentz boost with parameter beta).

[L2]

For every proper orthochronous linear Lorentz map L and every event c there is an inertial frame whose chart xL satisfies xL(p)=ML−1(x(p)−x(c)), where ML=E−1∘L∘E; the transition x′∘x−1 of inertial charts x,x′ is affine with linear part a proper orthochronous Lorentz matrix (Inertial charts are related by proper orthochronous affine Lorentz maps, and every such map is a chart transition).

[L3]

Let x,x′ be inertial charts with transition x′=Λx+b, Ω open, and let J, k, T be Cℓ vector, covector and type-(r,s) tensor fields with components Ja, ka, Ta1…arb1…bs in x and primed components in x′. Then (i) J′a=ΛabJb; (ii) ka′=(Λ−1)bakb; (iii) every contraction of one contravariant with one covariant index is invariant (Transformation of tensor, covector, derivative and current components under affine Lorentz maps, A type (r,s) tensor on a finite-dimensional vector space).

[L4]

A nonzero vector is null when η(v,v)=0, and in the frame η(v,v)=−(v0)2+(v1)2+(v2)2+(v3)2 (Timelike, null, spacelike and causal vectors of a Lorentz vector space, Inertial frames and inertial charts of the affine Minkowski model).

Proof

technique · direct
1.1L1L2algebra

By [L1] with β=3/5, γ=(1−9/25)−1/2=5/4, so the matrix of Λ=B(35e1) on the (x0,x1) plane is (5/4−3/4−3/45/4) and its inverse is the matrix of B(−35e1), namely (5/43/43/45/4); the existence of the chart x′ with this linear part is [L2].

2.1L3step 1.1algebra

Clause (i): by L3, ka′=(Λ−1)bakb, where (Λ−1)ba is the matrix entry in row b and column a of Λ−1 from step 1.1. Hence k0′=(5/4)(−1)+(3/4)(1)=−5/4+3/4=−1/2 and k1′=(3/4)(−1)+(5/4)(1)=−3/4+5/4=1/2.

2.2L3step 1.1algebra

Clause (ii): by L3, w′a=Λabwb, so w′0=(5/4)(1)+(−3/4)(1)=1/2 and w′1=(−3/4)(1)+(5/4)(1)=1/2, which agrees with the displayed forms (5/4)(1−3/5)=1/2; the components in the transverse directions are unaffected.

3.1L3L4step 2.1step 2.2algebra

Clause (iii): the inverse form defined in the Example gives η−1(k,k)=−(−1)2+12=0, while [L4] gives η(w,w)=−(1)2+12=0. Both objects are nonzero, so the covector and vector are null in their respective spaces. In the primed orthonormal frame the inverse metric matrix is again diag⁡(−1,1,1,1), the inverse of its metric matrix; the computed components give η−1(k,k)=−(−1/2)2+(1/2)2=0 and η(w,w)=−(1/2)2+(1/2)2=0. Thus the primed components describe the same null covector and null vector; no covector is inserted into the form on V×V.

3.2L3step 2.1step 2.2algebra

Clause (iv): by L3 the contraction kawa is invariant under the chart change, so ka′w′a=kawa; directly, ka′w′a=(−1/2)(1/2)+(1/2)(1/2)=−1/4+1/4=0 and kawa=(−1)(1)+(1)(1)=0, consistent with the invariant statement.

4.1step 1.1step 2.1step 2.2step 3.1step 3.2∎

Steps 1.1-2.2 prove clauses (i)-(ii), step 3.1 proves clause (iii) and step 3.2 proves clause (iv); every entry is an exact rational matrix computation from the displayed law, and no frequency, wave or Doppler interpretation is attached to the null covector.

Depends on

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