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Uniform low-speed expansion of the observer energy, momentum and kinetic energy of a massive four-momentum

Statement

Let (A,V,η,C+) be the oriented time-oriented affine Minkowski model (Oriented time-oriented affine Minkowski space (the spacetime primitives)), c>0, let n∈V be future unit with η(n,n)=−1, and let P=mU with m>0 and U∈V future timelike, η(U,U)=−c2. Let En(P), qn(P), Tn(P) and m(P) be the observer energy, spatial momentum, kinetic energy and invariant mass of Observer energy, spatial momentum, invariant mass, rest energy and kinetic energy of a four-momentum, put h(v):=v+η(n,v)n as in Norms of four-velocity and four-acceleration, and the observer rest-space decomposition,(c), and let γ:=−η(U/c,n)≥1 and w:=h(U)/γ=c h(U/c)/γ∈n⊥ be the Lorentz factor and relative velocity of Norms of four-velocity and four-acceleration, and the observer rest-space decomposition,(d); then u:=U/c satisfies u=γ(n+w/c), ∣w∣2=c2(1−γ−2) and γ=(1−∣w∣2/c2)−1/2, and Positivity, mass shell and kinetic energy of the observer quantities of a four-momentum,(b) gives En(P)=mc2γ, qn(P)=mγw, Tn(P)=mc2(γ−1) and m(P)=m.

Let z0∈[0,1), suppose z:=∣w∣2/c2≤z0, put K(z0):=38(1−z0)−5/2, and let f,R be as in The uniform second-order Taylor remainder of z↦(1−z)−1/2 on [0,z0], so that γ=f(z) and γ=1+z2+R(z) with ∣R(z)∣≤K(z0)z2. Then:

(a) En(P)=mc2+12m∣w∣2+RE and Tn(P)=12m∣w∣2+RE for the common real number RE:=mc2R(z), and ∣RE∣≤K(z0) z m∣w∣2=K(z0) m∣w∣4c2,∣Tn(P)−12m∣w∣2∣≤2K(z0) z 12m∣w∣2.

(b) qn(P)=mw+Rq with Rq:=m(γ−1)w, and ∣Rq∣≤m∣w∣(z2+K(z0)z2)≤(12+K(z0)z0)z m∣w∣; moreover ∣qn(P)∣=mcγ2−1=mγ∣w∣ and m(P)=m.

(c) Every constant above depends only on z0, so the estimates are uniform over all future unit n, all m>0 and all P=mU with ∣w∣2≤z0c2; if z=0 (equivalently w=0, U=cn) then RE=Rq=0, En(P)=mc2, qn(P)=0 and Tn(P)=0. The lemma is a statement about the mathematical model only; it asserts no measured quantity, no approximation of a physical theory and no error estimate for any experiment.

Facts & Assumptions

Given: The affine Minkowski model (A,V,η,C+), c>0, a future unit n, P=mU with m>0 and U future timelike with η(U,U)=−c2, the projection h(v)=v+η(n,v)n, the numbers γ=−η(U/c,n) and w=h(U)/γ, a bound z0∈[0,1) with z:=∣w∣2/c2≤z0, and K(z0)=38(1−z0)−5/2.

[L1]

The observer quantities: En(P)=−cη(P,n), qn(P)=h(P)∈n⊥, m(P)=−η(P,P)/c for nonzero future causal P, and Tn(P)=En(P)−mc2 after the constant m>0 is fixed; P=(En(P)/c)n+qn(P) is the unique decomposition of P along n and n⊥ (Observer energy, spatial momentum, invariant mass, rest energy and kinetic energy of a four-momentum).

[L2]

Projection and relative velocity: h is linear with image n⊥, kernel span(n), h(n)=0, and the restriction of η to n⊥ is positive definite with norm ∣w∣=η(w,w); for future unit u one has γ=−η(n,u)≥1, w=c h(u)/γ∈n⊥, u=γ(n+w/c), ∣w∣2=c2(1−γ−2), γ=(1−∣w∣2/c2)−1/2, and γ=1 if and only if u=n (Norms of four-velocity and four-acceleration, and the observer rest-space decomposition,(c),(d)).

[L3]

Observer identities: for P=mU with m>0 constant and U future timelike, η(U,U)=−c2, and γ,w as above, one has En(P)=mc2γ, Tn(P)=mc2(γ−1)≥0, qn(P)=mγw, m(P)=m and ∣qn(P)∣=mcγ2−1=mγ∣w∣ (Positivity, mass shell and kinetic energy of the observer quantities of a four-momentum,(b)).

[L4]

Uniform Taylor remainder: for z0∈[0,1) and 0≤z≤z0, f(z)=(1−z)−1/2=1+z2+R(z) with ∣R(z)∣≤K(z0)z2; in particular f(0)=1 and R(0)=0 (The uniform second-order Taylor remainder of z↦(1−z)−1/2 on [0,z0]).

[L5]

Model and powers: the model data are those of Oriented time-oriented affine Minkowski space (the spacetime primitives); ∣w∣2=η(w,w)≥0 on n⊥ and z=∣w∣2/c2≥0; rational powers such as (1−z)−1/2 are those of Rational powers ar of a positive base and integer powers those of Integer powers am.

[L6]

Scalar absolute values satisfy ∣ab∣=∣a∣ ∣b∣, ∣a∣≥0, and ∣a∣=a for a≥0 (Absolute value in an ordered field, Basic properties of the absolute value). The rest-space norm supplied by [L2] (Norms of four-velocity and four-acceleration, and the observer rest-space decomposition,(c)) satisfies ∣λv∣=∣λ∣ ∣v∣ for v∈n⊥.

Proof

technique · direct
1.1givenL2L4L5algebra

Relative velocity and the Taylor form of γ. The map h is linear by [L2], so h(U)=c h(U/c) and w=h(U)/γ=c h(U/c)/γ; the vector u:=U/c is future unit and γ=−η(n,u)≥1, so [L2],(d) gives w∈n⊥, u=γ(n+w/c), ∣w∣2=c2(1−γ−2) and γ=(1−∣w∣2/c2)−1/2=(1−z)−1/2=f(z). By [L4] applied to z≤z0 we have γ=1+z2+R(z) with ∣R(z)∣≤K(z0)z2; define RE:=mc2R(z).

1.2givenL3algebra

The observer identities. By [L3] at these data, En(P)=mc2γ, Tn(P)=mc2(γ−1), qn(P)=mγw, m(P)=m and ∣qn(P)∣=mcγ2−1=mγ∣w∣; define Rq:=qn(P)−mw=m(γ−1)w.

1.3givenL5L6algebra

Translating z into norms. Since z=∣w∣2/c2, we have mc2z=m∣w∣2, and also K(z0)z2mc2=K(z0)z m∣w∣2=K(z0)m∣w∣4/c2. Both expressions are nonnegative because m>0, K(z0)>0 and ∣w∣2≥0.

2.1step 1.1step 1.2step 1.3L1algebra

Part (a), the expansions. By steps 1.1 and 1.2, En(P)=mc2(1+z2+R(z))=mc2+12m∣w∣2+RE using mc2z2=12m∣w∣2 from step 1.3, and Tn(P)=En(P)−mc2=12m∣w∣2+RE by the definition of Tn in [L1].

2.2step 1.1step 1.3L4L6algebra

Part (a), the bounds. By step 1.1, ∣RE∣=mc2∣R(z)∣≤mc2K(z0)z2=K(z0)zm∣w∣2=K(z0)m∣w∣4/c2 using step 1.3; the second displayed bound is ∣Tn(P)−12m∣w∣2∣=∣RE∣≤K(z0)zm∣w∣2=2K(z0)z⋅12m∣w∣2.

2.3step 1.1step 1.2L4L6algebra

Part (b). By step 1.2, qn(P)=mw+Rq with Rq=m(γ−1)w. Norm homogeneity [L6] gives ∣Rq∣=m∣γ−1∣ ∣w∣; since γ=f(z), the bound on ∣f(z)−1∣ of The uniform second-order Taylor remainder of z↦(1−z)−1/2 on [0,z0],(b) gives ∣Rq∣≤m∣w∣(z2+K(z0)z2)≤m∣w∣ z(12+K(z0)z0), where the final inequality uses z≤z0 and K(z0)>0. The norm identity ∣qn(P)∣=mcγ2−1=mγ∣w∣ and m(P)=m are those of step 1.2.

3.1step 2.1step 2.2step 2.3L1L2L4algebra

Uniformity and the case z=0. The remainders RE=mc2R(z) and Rq=m(γ−1)w and the bounds of steps 2.2 and 2.3 depend on the state only through z, m and ∣w∣, and their constants depend on z0 alone, so the estimates hold uniformly over all future unit n, all m>0 and all P=mU with ∣w∣2≤z0c2. If z=0 then γ=(1−z)−1/2=1 by step 1.1, so u=n and w=0 by [L2],(d), whence R(0)=0 by [L4], RE=0, Rq=0, and steps 1.2 and 2.1 give En(P)=mc2, qn(P)=0, Tn(P)=0; conversely w=0 gives z=0, and U=cn is the same case.

4.1step 3.1given∎

Conclusion. Steps 2.1 and 2.2 are clauses (a), step 2.3 is clause (b) including the norm identity and m(P)=m, and step 3.1 is clause (c) with the uniformity statement and the z=0 case; no step used more than the stated hypotheses z≤z0<1, m>0 and the model's normalization, and the statement concerns the mathematical model only.

Depends on

Dependency tree · 2 levels

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