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.

Universality alone would only guarantee that the event of destruction happens in every inertial frame, at exactly one (x, y, z, t) tuple. However, it states nothing about the existence of the particle before or after t. That's what causality tells us in addition.

**(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. From universality, t_{1}' = t_{2}' if and only if t_{1} = t_{2}. Thus, 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.

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