What Time is it IN SPACE?
So, let's assume you've cobbled together an interplanetary empire without using transportation methods that violate causality or have the power to blow up stars. You may feel pretty proud of yourself, until you realize a big problem: ship aren't arriving when you think they should; some return just after they left, while others come back decades or even centuries behind schedule. You're about to order someone executed when a friendly scientist stops by and explains to you that time is relative, and it moves differently in different reference frames. That's all well and good, you say, but how is anyone supposed to keep to a schedule when no two planets have clocks that tick at the same speed?
I realize that most planets would keep time their own way, and that should work pretty well for local schedules, but I'm more interested in how one could come up with an empirical measurement of time that everyone on all planets could agree on and use effectively. I'd really like it if both a source and a destination had one time that they could say a shipment would arrive, and that they could both be right without either side having to do too many calculations. Is there a way to make this possible?
This post was sourced from https://worldbuilding.stackexchange.com/q/11348. It is licensed under CC BY-SA 3.0.
1 answer
The main problem is to establish a common time standard to base your local clocks off. Then it doesn't matter if your local time system differs from that (for example, you will want to base your planetary time scales on the rotation of that planet), since you can convert between whatever your local time is and "universal" time. Note that "universal" time also doesn't need to go at a rate of one physical second per universal second, so even things like planets close to heavy black holes (a la Interstellar) are not a problem for your time keeping.
So what remains is to find a good source of a base time. One possibility for that might be pulsars. Pulsars are astronomical objects which basically provide a natural clocking. A disadvantage is that you only see the same pulsars if you are sufficiently close to the earth because you only see them if you're swept by their "light fire ray". So this method only works if your empire is not too large. But then, if you're bound by relativity, your empire likely won't be too large anyway. Anything inside the solar system definitely is close enough, and also the neighbouring stars are.
Another possibility is to have a set of time signal senders, on/near different planets, and calculate the time from their signals by a GPS-like calculation. In other words, build up an interplanetary GPS (IPPS?) and use that both for determining your position and your canonical universal time.
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