A space/time physics thought experiment, of sorts, which I imagined today, along with questions that make it an experiment and an interesting one, in my view, goes as follows.
A pair of advanced and durable satellites, built to communicate with each other across vast distances while exploring space, EXO-1 and EXO-2, head for respective locations greater than several light- minutes apart. They both emit fixed frequency telemetry and can receive it from each other, using it to gauge their distance apart based on frequency shifts, which they can measure accurately. Also, burst signals from EXO-1 receive replies from EXO-2, which the latter sends immediately after receiving the burst signal from the former. Since the signals travel at the speed of light, this also provides them with distance data.
Their programmed routes contain a period where they will hold their distance at a constant one light hour apart. EXO-1 sends a burst signal to EXO-2, expecting to hear back in two hours, the time needed for the burst to reach EXO-2 and the response to come back.
But after EXO-1 sends out its burst signal, only and precisely one hour passes when EXO-1 receives a signal burst, not necessarily the acknowledgment burst, from EXO-2. Then, at the two hour mark, the acknowledgment burst arrives to EXO-1 from EXO-2.
EXO-1 makes and transmits to Earth the following message:
“EXO-2 apparently sent an unscheduled burst signal to me at the exact time I sent the scheduled burst signal to EXO-2.”
Questions: In what physical sense did EXO-1 make a true statement? Would the statement have been equally accurate if the unit had been a light year instead of a light hour? Would its truth value hold across interstellar or intergalactic distance between EXO-1 and EXO-2, provided their function and accuracy could survive and correctly deal with the attainment of such distance between each other?
Why I am asking is simple: The sense in which the answer to these questions is yes is a sense in which we could speak of a universal base time. The sense in which anyone could explain to this layman what, if anything, keeps the answer from being yes will help me understand how time and space work in ways I would not expect.
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