The following suffices for our purposes:

**(Assumption)** Particles can be destroyed: if a particle gets destroyed, its trajectory is discontinued in every inertial frame. That is, if t_{K} denotes the time of destruction relative to inertial frame K, then the trajectory will not exist for any t > t_{K} in K.

**Note:** backed by the fact that the speed of light is finite, similar can be assumed about signals too.

**(Theorem)** The order of two events e_{1} and e_{2 }that happen directly to a given particle is the same relative to every inertial frame.

**(Proof)** Let K and K' be two inertial frames, and let e_{1} and e_{2} happen at t_{1} and t_{2} in K, respectively, with t_{1} ≤ t_{2}. Let t_{1}' and t_{2}' denote the corresponding time values in K'. Moreover, let the particle get destroyed in K at t_{2}, which corresponds to t_{2}' in K'. Then, since the particle does exist in K' at t_{1}', t_{1}' ≤ t_{2}' must hold. Finally, t_{1}' = t_{2}' if and only if t_{1} = t_{2}, so t_{1}' < t_{2}' if and only if t_{1} < t_{2}.

The argument can be applied to signals too:

**(Theorem)** The order in which a signal meets two given entities is the same relative to every inertial frame.

**Note:** the tip of the signal plays the role of the particle.

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