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Worked affine transition between two inertial charts
Example
Worked computation of the transition between two inertial charts. Let be an oriented time-oriented affine Minkowski space with inertial frames and and charts , , and write with the change-of-basis matrix. Then for every event ,
so the transition is an affine map of with invertible linear part .
Restriction to the plane of . Fix an inertial frame with the origin of used in the charts, and a real parameter , and put
with . Then is again an inertial frame: the matrix of the change of basis is the block matrix with , and
Since has determinant , the two bases are positively oriented together; the new timelike vector satisfies with , so it is future timelike; hence the primed tuple is an inertial frame. The transition values are and . Inertial frames exist at every event by Existence of future-oriented inertial frames at every event, so the computation applies with any chosen origin.
Facts & Assumptions
Given: The inertial frames and charts of the Example, with , and the restricted basis with the hyperbolic parameter .
An inertial frame's ordered basis is -orthonormal, future and positively oriented; its chart is , so ; and for another inertial frame with chart the transition law holds (Inertial frames and inertial charts of the affine Minkowski model).
In an -orthonormal basis the matrix of is , so (Inertial frames and inertial charts of the affine Minkowski model).
The change-of-basis map between two ordered bases has exactly when the bases lie in the same positive class; a positive class is an orientation and a basis in it is positively oriented (Orientation of a finite-dimensional real vector space).
The change-of-basis matrix between two ordered bases is invertible (Invertible linear maps, linear isomorphisms, and inverse linear maps), and an expression is an affine map of (Affine spaces over a real vector space and displacement vectors).
Verification
For a general pair of inertial frames, substituting , equivalently , into and using gives , an affine map with invertible linear part by [L1] and [L6].
For the restricted basis, and by [L3] and [L2], while ; also for and the mixed entries vanish. Hence is -orthonormal by [L1].
The matrix of the restricted change of basis is the displayed block matrix, and by [L2]; its inverse is the displayed block matrix, since multiplying the two blocks gives the identity using .
By and one has for every real ; with from step 1.2, the vector is future timelike by [L4]. Since by step 1.3, the primed basis lies in the same positive class as the positively oriented original basis by [L5]; hence is an inertial frame by [L1].
With and by [L1], the transition law of step 1.1 gives , and for the event , whose coordinates in the first chart are , it gives by the inverse computed in step 1.3. This completes the worked computation of the transition and its two displayed values.
Depends on
Affine spaces over a real vector space and displacement vectors
Oriented time-oriented affine Minkowski space (the spacetime primitives)
Inertial frames and inertial charts of the affine Minkowski model
Existence of future-oriented inertial frames at every event
Invertible linear maps, linear isomorphisms, and inverse linear maps
The six hyperbolic functions and their natural domains
Addition formulas, identities, parity, and derivatives of the hyperbolic functions
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 timelike set has exactly two convex components, described in orthonormal coordinates
Orientation of a finite-dimensional real vector space