Let us image a spacecraft in fast orbit. Since it took off its clock has lost several minutes relative to Earth time. From the ground we can speak to the astronauts, although there is 1 second radio delay. Aside from the signal delay we can say we are coexistent, talking, albeit in different frames and quite different clock time. So is this coexistence an existential, experiential construct disconnected from the mathematics of physics? i.e. Physical processes on the ground are happening at a different rate, so in what way are we coexistent with the astronauts?
In physics the tangible reality is the radio signal, so ground crew and astronauts need coinciding future causal/light cones, and the signals take time to travel. If the delays are known signals could be sent to multiple spacecraft to arrive 'simultaneously' - We can know they are receiving the signal 'right now' - in the present. Also, there is a horizon of the present beyond the spacecraft, but physics is only concerned with what can be causally connected. So physics ignores the 'now', but it metaphysically exists, doesn't it? And is it not really the only thing that truly exists, (unlike memories of the past)?
The astronomer Van Flandern contributes a highly readable essay on "Global Positioning System and the twins' paradox", arguing that GPS is "a practical realization of Lorentz's 'universal time', wherein all clocks remain synchronized despite being in many different frames with high relative speeds". In essence, the GPS system considers one frame, the local gravitational field (read, Lorentzian ether), to be privileged: here alone clocks tick at universal time, lending a new, technology-infused meaning to Lorentzian relativity.