NOTICE the previous result

NOTICE the previous result:

Δt=γ(Δt₁-Δx₁u/c²)=-γΔx₁u/c²

^{Let us consider that
the left system sees}Δx₁^{optically,
and in the mean time moves with velocity u:}

Δt⁼(Δx₁ -Δtu)/c=Δx₁/c -Δtu/c: Δt(1+u/c)=Δx₁/c:

Δt=Δx₁/c(1+u/c)=Δx₁/(c+u): so there is a delay, due to seeing simultaneously by receiving from both ends of Δx_1, signals at time difference Δt=Δx₁/(c+u): So the above result in the box, is not due to time delay of light propagation with velocity c.

It is the result of Lorentz transformations, which are though relative, an objective reality for left system, not only based to the finite velocity c, but mainly to its invariance: c+u=c.