I'm working on a storyline where humanity has spread across a handful of star systems via a device that basically teleports a ship from one system to the next. I have worked out some scientifically possible ways this might work, but the question I have is how these systems might keep track of time. Interstellar Trade and Banking makes me think that a "universal" time keeping system would be needed. I am probably over complicating this a bit, but since time flows differently in a gravity well, and differently at various speeds, how would different star systems establish and maintain such a universal time? Could a pulsar that all can see act as an "atomic clock"? My understanding is that the pulses will change over time, so may not be the best option. With people living in orbiting space stations, traveling on ships, and going to different star systems, it is mind boggling how, from a current day understanding of physics, that we could even get a standard time system. I'm open to some ideas on this. Added note, there is no FTL travel or FTL communications, just these interesting devices that "teleport" the ship between systems. More about that at a later time. ;-)
The same way we keep GPS satellites synchronized
While it is true that different planets will have different time scales due to gravity, those differences are easily calculated. GPS satellites have clocks that work just like the clocks we have here on Earth, but they have built into their software the ability to automatically adjust how they count time because they experience time marginally faster than we do here on Earth.
The second thing you do to standardize time over a long period of time is periodic time-synchronization to a master clock. Your average clock is not perfectly accurate. Most computers/cellphones for example drift by several seconds a year; so, your operating system is designed to automatically sync up with one of a small handful of time servers that are all themselves syncing up to a master atomic clock. While atomic clocks are accurate enough that you could keep separate master clocks running on each world and just mathematically adjust them based on local gravity and velocity and you would be fine for most practical purposes, this does not guarantee that there would not be any techs where atomic level accuracy would come into play. The more advanced our society becomes, the more we invent new technologies that rely on highly precise timing of things; so, by the time we get to teleportation technology, we will probably already have a number of technologies where time variations in the 10e-18s range will be significant enough to need to worry about.
Since you have instantaneous travel, whenever a data courier ship is leaving Earth, it can sync its own atomic clock to Earth's Master Clock, jump to another planet, and then transmit the updated time. In this way, as long as you have a data courier ship jumping from Earth to all the other planets on a somewhat regular basis, they will be able to correct any minor time variances that might emerge over time.
If you measure a set of regular pulsar signals they will have a set beat at a given point in space. If you know where you are to a reasonable degree of accuracy and can see the selected stars you can tell the time by the rhythm you see in the pulse of those stars. It's a clock and calendar that works anywhere in the universe. It's not perfect though, on a long enough timeline the beat does eventually loop. In fact if you aren't using a large enough sample set it loops quite regularly which could lead to errors so more stars is better.
Ignore it and use Escrow.
The financial community is notoriously conservative when it comes to adoption of technologies and financial risks. Rewrite your interstellar trading and banking communities to place their trust in Escrow companies. All trade and banking happens locally within the system. The Escrow agent is responsible for moving money between star systems, personally and physically.
This way your transactions will not take place at synchronised times, which are subject to vagaries in the UST (Universal Standard Time) computation, but are simple chronologically related events. After I get the money, I release the goods.
Everything works on local calendars, if you're ordering something from another gravity well, the social custom is that you specify the local date of the supplier for delivery. Any goods or monies that are transferred get a "leaving gravity well" timestamp applied that is valid against your contract. You as the purchaser are responsible for co-ordinating deliver dates that work against your local clocks.
If I'm reading https://astronomy.stackexchange.com/questions/6217/what-is-the-relative-time-difference-between-us-and-a-star-system-in-outer-layer correctly, we're talking about "clocks will eventually fall out of sync", not something human-noticeable.
I think the elegant option is many time systems: one for each star system and one for the galaxy as a whole. Each time system is always computed in the frame of reference it's designed for. But they also have convenient sized units (e.g. an integer number of hours in a local day). As long as you're doing units conversions anyway, the relativistic conversion comes free.
This means that if you move a clock to a new star system, you'll have to re-calibrate it. Good clocks do this automatically based on standard network protocols. Bad clocks aren't worth the cost to ship.
Similarly, a device whose performance is measured with respect to time (e.g. a computer than can do x quadrillion floating point operations per second) will have slightly different performance in a new place. If you advertise such a devise in local units, it is understood that this means that performance in that system. If in galactic, you should specify where it achieves that.
You cannot use a common pulsar, nor even a common clock. You need the difference in both velocity and mass proximity, throughout history and into the future.
The passage of time is described as 'relative' to others. Its passage depends on:
- The relative speed between two points you are measuring (Special Relativity)
- The proximity of these points to mass (including how much mass) (General Relativity)
If you know both of these things, then you do not need a separate '3rd' object to calculate time. In fact, using a pulsar may not be effective, as its frame of reference is dynamic and might be different to both points you are trying to synchronise, creating an unnecessary complication. For instance, a pulsar may be moving away from Point A faster than from Point B, and thus have a different frame of reference, making you have to perform multiple calculations. Or a massive object may affect this rate of time (or at any point in-between distorted by mass) changing its pulse.
So the best way is to not use an external reference point. Simpler to use your own ticking clock (and calendar), and calculate the different rate of time given the two requirements above, to obtain the the other points time.
Once the different rates of time are established, you need to calculate this at all points past and future, as your rate of time might be different at different times. This creates a dynamic 'exchange rate' of times between all star systems, in order to coordinate calendars, meeting times, dates etc. you need to state the reference you are using. (eg. 10.30am 29/11/2021 SystemA ST = 8.30am 12/1/2900 SystemB ST).
After this, you may realise there is no 'universal time'. Like economic exchange rates, time and dates change in history and the rate for each point in space throughout time is going to be different. You need to find the differing rates of time throughout time in order to match up dates, however eventually each civilisation will develop its own 'worldline' standard in spacetime relative to each other civilisation, so you don't have to do the whole calculation every single instance you mention a date.
As some others pointed out, the best way would be to synchronise everything according to a single regular timekeeper, such as a pulsar. This would work fine, since any relativistic effects should be consistent and interactions and exchanges would still abide by the rules too. Everything should work out, causality-wise, allowing everything to be in sync, or have at least consistent time exchange rates.
Until you bring in teleportation. This is where the whole system falls apart, since teleportation implies FTL travel. I know you specifically state that your world doesn't have FTL, but unless the time it takes to teleport information / matter is longer than the time light takes to travel the same distance, you have FTL. Which kinda defeats the point of teleportation.
Of course, once you bring that into the mix you open up a whole world of pain in terms of keeping everything in sync. If FTL travel is possible in your universe, by whatever mechanism, it must automatically also be possible to see effects before causes, and to time travel. There are some excellent YouTube videos explaining this topic further, my favourite being Superluminal Time Travel by PBS Spacetime
I'm not sure what the best solution to this might be, but perhaps there might be some inter-solar laws preventing causality breaking effects from being abused in financial systems, similar to insider trading laws. In terms of timekeeping, perhaps each star-system or galaxy is in it's own independently synchronised time-block, with inter-system travel going through sync-gates that update the on-board systems to the new time on arrival. I'm really out of my depth here in terms of knowing how this would all pan out and what solutions could work - just thought I'd weigh in and open that can of worms for you 😁
Your answer may rely on your method of FTL travel. You only mention teleportation.
If taken literally its instantaneous without any time loss between points.
If so a time keeping network is your only answer. A 'standard' agreed upon central source for time keeping is updated throughout the network. You see this in modern computer networks. If your method of teleportation involves gates, or static points. time keeping drones could be used to regularly patrol the teleportation lanes and maintain the network. Other maintenance duties could be assigned to these drones that would be required for the upkeep of such a network as well.
Perhaps keeping onboard a entangled molecule or particle as a sort of 'parity bit' to ensure the correct time is in sync with its mate on the other side of the gate?
Define a purpose, define a calendar to agree upon
Take into account any galactic common time is only interesting when there are meaningful common events. This is relevant, because events at time X will be at time Xo (observer time) for everyone involved. Now, suppose you'd like to send a congratulation email on behalf of a member planet, to celebrate the anniversary of the Federation. You'll have to agree what an 'anniversary' is, what year length is used. The email will move with the speed of light. To let the email arrive at a meaningful moment on the federation's main planet, the time point of sending the email should be (long) before the event occurs.
Just take some observable planet orbit to be your clock
With static observers, there will be no issue at all. Just agree upon a certain solar system in a certain galaxy that can be seen and measured by all observers involved, e.g. take the orbit phase of the largest gas giant in a certain known system. The reading must be corrected for distance, for both observers: the time measuring device for each observer will output the actual orbit phase for a local observer in the gas giant's system, not the apparent orbit phase, as seen from afar.
Even if you move fast, it can work
For now let's assume there is no teleportation. Local time frames can differ, but when a civilization is able to fly at near light speed, it can also calculate relativistic time corrections. When an observer is flying at a relevant speed irt light speed, the clock's distance correction will show a relativistic calculation error, because of the time frame difference. On a moving ship, the clock shows the time in a slower, local time frame, because the observation is done by the ship. The observation is then translated into delta-time irt departure, as it would count for time frame of the planet of departure. Assuming the distances are known, the distance to the "clock planet" for a static observer on the location of the ship can be calculated. Then, the time found is calculated back into ship time, to decide when to send the email. With sufficient precision in the measurements and calculations, the email will arrive in time.