 Similarly, inertial observer Bob can assign coordinates formula_71 to the same event by transmitting a radar pulse at time formula_72 and receiving its echo at time formula_73, as measured by his clock.However, as the diagram shows, it is not necessary for Bob to generate his own radar signal, as he can simply take the timings from Alice's signal instead.Now, applying the "k"-calculus method to the signal that travels from Alice to Bob

Similarly, applying the "k"-calculus method to the signal that travels from Bob to Alice

Equating the two expressions for formula_1 and rearranging,

This establishes that the quantity formula_78 is an invariant