# Standardisation of Time in a FTL Universe

Here on Earth, time standardisation was enabled by the invention of accurate time pieces which allowed us to create a global constant (GMT/Zulu time) and regional offsets. If we travel outside our time-zone for a meeting, we can be sure to know when to leave to arrive there at the right time.

In an FTL equipped universe, this becomes more difficult as an FTL route A might take longer to reach a destination than FTL route B with the result that clocks on both ships would not be in synchronisation.

Is there a theoretical method of inferring a "universal" time that FTL travellers can use for their clocks to maintain a constant time?

• For clarification you're saying that time dilation will be different based on different speeds that are both faster than light, so the resulting times will be different? Oct 17, 2016 at 8:01
• If you have the effects of time-dilation, then there's no way to have a universal time, as everybody's personal timeline will be different.. You could solve the issue by doing sth similar to what I did and have everyone use Solar-Time as a means of communicating time, and then use local time for each planet/system/etc. based on revolutions, etc.; then you could additionally have every thing (ship, human, etc.) have a universal age - but that doesn't answer the question, thus comment Oct 17, 2016 at 8:37
• I think you mean STL travel, not FTL. STL has a time issue causes time to pass for the different ships to go at different speeds due to relativity. FTL is a term to describe ships that are going FTL using a way to get around relativity and don't have that issue... They have a different time related issue, but not what you're thinking. Oct 17, 2016 at 12:46
• If in your context, you can travel Faster Than Light (super fiction) Then what is preventing you from introducing other super fictions like absolute time and/or instantaneous communication (that would also allow absolute time)? Oct 17, 2016 at 13:48
• A challenge with making this science-based is that the particulars of the FTL system matter. For instance, most FTL systems involve bludgeoning Einstein over the head and stealing his wallet. Our current systems of causality are all based on what he proposed, so when he's unconscious who knows what happens! Oct 17, 2016 at 16:31

# Triangulation from X-ray Pulsars

Timing and navigation are inextricably linked. The mechanical clock enabled the first calculation of longitude. GPS navigation depends on comparing arrival times (and thus distances) of different satellite signals.

In a FTL future on a galactic scale, ships would want to use a 'GPS' system to determine their location in the galaxy. Building a satellite cluster with enough transmission power to be detected around the galaxy with is expensive, but fortunately nature provides one for free. Here is a method for using x-ray pulsars to determine position in near-Earth space. Extending the system is as simple as mapping more suitable x-ray pulsars farther away.

Another paper here (page 57) talks about timekeeping adjustments needed to adjust for the relativistic effects on satellites. Here (page 100) is some more general information on about the algorithms needed for time calculation.

The big difference between these methods and the FTL future is the method of FTL travel. These methods assume that you can continually collect information from pulsars, calculate your own velocity and make necessary timing adjustments along the way. That might not be possible depending on the way that FTL travel works; if FTL travel requires a wormhole or some sort of concept like subspace, then you might not be able to interact with or observe the outside world while in flight. In that case you will need another method to re-synch your clock after an FTL jump.

In that case, and now I'm thinking on my own, you would use radio galaxies to determine your absolute orientation in an extra-galactic coordinate system, and then use the locations of x-ray pulsars to determine how far the galaxy has 'spun' while you've been in subspace or the wormhole or whatever. From that you can determine how much time has passed, and then re-synch your clock and go back to using the x-ray pulsars to keep precise time.

• I have a feeling the ideas behind UTC (Universal Time) are backed up by just such an idea you propose. Much like the definition of the meter and kilogram have been updated over time we would probably refine what UTC meant. I am sure there was some suggestion of how you would at least define galactic time based on your suggestion then inter-galic based on observing notable galaxies. Where am I and when am I get linked in space as they did on earth.
– TafT
Oct 18, 2016 at 13:35
• This is what is implemented in the Culture series, in particular "Excession" has one of the Ship AIs needing to do this after being "teleported" to a random location in the universe, which totally scrambles its ability to track its location. theculture.wikia.com/wiki/Excession_(novel) Oct 24, 2016 at 23:37

Is there a theoretical method of inferring a "universal" time that FTL travellers can use for their clocks to maintain a constant time?

Generally, no. There is no universal time, period. Relativity tells us that it is impossible to put a time ordering relationship between spacetime events that are not in their respective light cones, or in other words, depending on the observer, events A and B can (in the general case) be considered to have happened "A before B", "B before A" or "simultaneously" by three different observers, unless A is in the cone of B or vice versa.

Furthermore, it is unclear how time would pass when you do travel FTL. Time slows as you approach c and stops when you reach c (if you happen to be a photon), you cannot just scale it up past c, there is no canonical way to handle it. So you have to arbitrarily define how your FTL works and how time passes during FTL, in your particular universe, before the question makes sense. For example, if you have a warp gate based universe, where you step into a gate and just step out instantly, on the other side, then no time at all passes during the transition. On the other hand, if you enter some kind of "subspace" where you can move FTL but still have the actual "flying" metaphor, you do have a flight path and time that you need to compare with real time. If you can go through wormholes, that's a whole other can of worms (length of the hole/tunnel, location of attachment on either side of normal space, movement of the tunnel etc.).

Of course, there's the saying "everybody said it's impossible, but then someone came who did not know, and just did it". So...

In practice, in an universe where our current understanding of relativity stays as is, but some kind of FTL travel still exists, you would likely just invent a static convention. For example, you simply (and arbitrarily) decide that Alpha Centauri is "Earth Standard Time +1", or however long traveling from Earth to Alpha Centauri may take. It then does not matter which route your ship takes. Ship clocks will usually be wrong, but the first act upon entering the target system in sub-FTL speeds would be to ask them what their time is and set your clock to theirs. Note that there still is not much hope to have any meaningful discussions about what time it is in "universal time" at a specific planet right now, unless you let go of relativity completely.

If you have warp gates, you just take a clock from earth, throw it into the gate, and voilá - you have your universal time/date. (And have violated one of the core laws of our universe, all in one go...).

• "If you have warp gates, you just take a clock from earth, throw it into the gate, and voilá - you have your universal time/date." I wish I'd thought of this. Oct 17, 2016 at 12:07
• The mathematics that model space compression and time dilation would imply that beyond the speed of light, time and space would become imaginary. What that would mean physically is anybody's guess, but I would suggest that strictly speaking, it doesn't mean anything because it's excluded from the domain of the problem. More importantly, our models are only models and we can not theoretically determine what would happen in reality based on some radical in an equation. Oct 17, 2016 at 12:50
• @Devsman, that what was I wanted to point out. There is no way to extrapolate our current models into the realm of FTL, so the auther of a work that includes FTL must make a decision and invent something.
– AnoE
Oct 17, 2016 at 13:18
• Enter ansibles. ...
– AnoE
Oct 17, 2016 at 15:07
• Relativity also tells us that FTL travel is impossible, so whether there might be a universal time depends how the fictional universe violates relativity. In a universe where FTL travel is possible but time travel is impossible, not all reference frames can be equivalent, so there might be some preferred "more absolute" reference frame that gives the "true" time of an event.
– armb
Oct 17, 2016 at 17:19

Let's pretend the whole universe uses Earth years, days, etc., for the sake of this example. However, any universal system would work for this.

Before the ship begins to travel, it decides that your destination is 0.5 light-years away. Next, it decides that you'll be traveling at exactly twice the speed of light. For the sake of simplicity, your ship's technology accelerates to this and decelerates from this instantly; the feasibility of this is a different discussion.

Finally, it calculates that (0.5 light years)/(2 times the speed of light) = 0.25 years

During the journey, the ship's computer

A) Repeats this formula based on the current distance and speed to give passengers an estimate of the current day, week, month etc. (This is only an estimate as distance can be hard to tell at this speed, and slight dilation can occur in the computer's cycle of repeating the formula. It's still more accurate than a clock counting for you, however)

So at a distance of (0.1 light years)/(2 times the speed of light) 0.05 years have passed, or 18 Earth days and 6 hours

B) Does not actually record time, but after the journey adds projected elapsed time to time before the journey to get an accurate, non-dilated current time.

In conclusion

• There is more than just a theoretical method for inferring time during and after FTL travel: there is math!
• There is always room for dilation so it's good to stop at planets and sync just in case
• Since it's just math it will work for any universal system you choose
• No need to have a universal time. Just have each destination have a published reference point and create a database of differences. When you plot your course, the computer calculates what the time will be at your destination when you arrive, and displays the time relative to that on board. Just like updating your watch on an aeroplane when you fly between time zones. You probably also want a second display showing the time relative to your departure point/instant. Oct 17, 2016 at 9:26
• @BillMichell: Oh! Just like timezones! Now each planet is free to switch its time reference around and maybe we'll want adjustments for leap seconds and... (if you haven't guess, I am one of the poor slobs who has had the great honour to develop programs around timezones). Oct 17, 2016 at 13:32
• @BillMichell Does travel between two points always take the exact same amount of time? It doesn't for trains and airplanes today. How can you base a timekeeping system off of that? Oct 17, 2016 at 14:41
• You seem to be using newtonian/classical mechanics of the form displacement = velocity * time when dealing with FTL travel. Mathematics of this form doesn't hold when travelling at near-light speeds, why would it be true for faster than light?
– Luke
Oct 17, 2016 at 17:11
• I just re-read this again, and want to stress, as Luke points out, that this system really doesn't work. Oct 17, 2016 at 17:52

You have it backwards — FTL makes it easier to keep time, not harder.

The challenge behind synchronizing clocks is not about having accurate time pieces: it's about creating a sufficiently well-defined convention that defines the time standard and the communication to carry it out.

The ease* of timekeeping today on Earth is because we have high speed communications — the time it takes to transmit a time signal is much, much shorter than the time scales we actually care about, so there is a fairly straightforward way for anyone to adequately synchronize their clocks to UTC: you simply receive a time signal from an more authoritative source. E.g. before the internet, you could set your clocks simply by turning on the TV news to get the current time.

With light speed communications, galactic timekeeping is hard because communication is slow, and you'd need some sort of fancy convention and protocol to define and keep time everywhere.

But the faster communication becomes, the closer you get to where you can just do the really simple thing and have authorities broadcast a time signal.

*: high precision timekeeping is still quite complicated even on small scales.

• +1, exactly that. FTL means that your world either has an absolute time - the order in which causality happens - or you have all kinds of causality paradoxes. Oct 18, 2016 at 6:31
• @Luaan: That's my point. If there is FTL, then relativity is wrong, and there is one "true" frame of reference, and we can all set out clocks by that one. Problem solved. Oct 18, 2016 at 11:19
• @Luaan: Isn't that what I wrote? If FTL is possible, you either get causal paradoxes or relativity is wrong. And nobody really believes the causal paradoxes are possible. I think you're saying the same thing I'm saying... Oct 18, 2016 at 12:18
• @nikie Well, I guess it's close enough not to bother, really. Oct 18, 2016 at 15:04
• "E.g. before the internet, you could set your clocks simply by turning on the TV news to get the current time." Don't forget the time/temperature phone number. =) Oct 19, 2016 at 1:53

Our Universe has an age

General Relativity might provide what you are looking for. Our Universe has an age, right (at least that is what the $\Lambda$CDM model says)? How can we determine the age of the universe? How can we talk about an age if time is not universal? Well this age is the "cosmic time" that has passed since the Big Bang and cosmic time is the time coordinate for so called fundamental observers (it is the same for all fundamental observers).

So I would say that this is the perfect reference time for you travelers. The problem is how do we determine the cosmic time? Well the best way of doing so that we know of is using the CMB, which is not very practical as (with todays technology) it is a gigantic effort (Planck) and we can not do it very accurately (21 million years of uncertainty). But if your people can travel FTL, that might not be a problem. So you can just say "let's meet on Sunday the 13th of November of the year 3,141,592,653" and then each of the attendees of the meeting has a way of finding out when they need to leave to arrive on time. Assuming they can compute how long it takes them to get to the meeting point in the frame of reference of a fundamental observer.

This does of course assume that you have a $\Lambda$CDM universe, which is the only kind of universe we know but not the only kind of universe we can imagine.

EDIT:

After getting some comments, let me clarify: The point of my answer is really, that our universe has an age, so you can define something like an "absolute" time. And giving the OPs universe an age is very plausible.

Now whether this is practical is a completely different question, but the OP explicitly asked for a theoretical method.

I see two problems with practicality: do you have a precise enough model of your universe? And can you measure something akin to the CMB that allows you to determine at what point in the history of your universe you are with enough precision from aboard a spaceship?

• Funny thing about relativity and FTL, you might get the RSPVs to the meeting before you ever send out the invitiations. As for the Lambda-CDM model, it is based on a lot of assumptions, and, as you say, not precise enough for timekeeping system, but as the basis of such a system in fiction it could do pretty well. Oct 17, 2016 at 16:45
• Except that the idea of a "fundamental observer" rests on the assumption that the universe has homogeneous density, right? No galaxies, no stars, no planets, just equally distributed matter. Close enough for cosmology (the universe is very homogeneous at large scales), but not very practical for everyday timekeeping... Oct 18, 2016 at 6:37
• @nikie Sure, but what limits the error you make when computing the age of the universe? Is it the fact that the Lambda-CDM model is an approximation? Or something else like measurement uncertainties? I'm not familiar enough with the topic to answer those questions. Oct 18, 2016 at 11:11
• The point of my answer really is, I think, that our universe has an age and ergo that you can define a "universal" time. I'm not sure how important an argument practicality with todays knowledge and technology is if you have FTL travel. If you have an exact enough model of the universe you live in, you might be able to bring age determination down to an uncertainty level that makes it useful. Hm, although if you look at navigation applications that's many orders of magnitude in time that you have to cover (billions of years to fractions of seconds). Oct 18, 2016 at 11:28
• I find this answer unrealistic because November 13, 3141592653 is going to be a Sunday Oct 19, 2016 at 14:03

One simply defines proper time at a specific reference point to be the standard. Every spaceship then simply calculates the appropriate deviation based on the path they take, so everyone agrees.

We do this in real life. Earth's UTC is derived from TT(TAI), which is a dynamical time frame. Real-world spaceships use TCB(TAI) as a common reference point. Due to relativity, they're currently ~20 seconds different, yet everyone can calculate it and agree on the time.

The larger problem here is that FTL is impossible, and also literally equivalent to time travel. The scheme above still works, since hopefully you can still figure out a time deviation. But the math with be complicated by all the FTL-ness (magic).

• To elaborate on this, let's assume that we have two observers, A and B who are in motion relative to each other. Their clocks will not agree, and there will exist events in the universe c and d such that c happens first from A's inertial reference frame and d happens first from B's frame. But either one of those observers can calculate what the other one observes. So by establishing a fixed reference frame (e.g. Earth) as the standard, everyone will agree on when an event happened according to Earth Standard Time. (to be continued...)
– Ray
Oct 18, 2016 at 23:16
• (...continued) Adding FTL to the mix may break things, since the equations we're using here break down at v >= c. But as long as it is possible for an observer to calculate the relative rate at which time passes for other reference frames, the general approach still works.
– Ray
Oct 18, 2016 at 23:21
• Right, the question is better phrased as a navigational one. When you're talking about comparing times in different places you're really asking about the space-time interval between them. If you use relativity, you can work out an answer. If you use FTL then we all have to play the game of "guess what imaginary reality I have in mind" to figure out what the answer might be, and ultimately the issue is that the poster doesn't understand the subject well enough to speculate. Space is inconveniently large for storytellers, but you can't throw out Einstein and talk about space/time meaningfully. Oct 25, 2016 at 15:16

Ship located computers are supposedly capable to calculate time differences if they are capable to manage FTL traveling. So calculation will be based on astro-navigation and travelling details at the moment of entering FTL speed. However, by current computer science they will need synchronisation after shorter-longer periods to stay accurate.

The first question to ask is "what is a time standard?" We take them for granted so deeply that we oft don't even think about what they mean.

A time standard typically provides two fundamental concepts, a duration and a time point. The intuitive meanings for those worlds are quite reasonable. A time point is, well, a single point in time. The time points are ordered, so we can say "time point A occurred before time point B". A duration is the time that passes between two time points, which is a scalar value with units of time. With duration we can say "time point A occurred 3 seconds before time point B."*

Time points, by their nature, are very hard to pin down. They're a little on the ephemeral side. To solve this, we pick one privileged time point, the "epoch" for the system, and we refer to all time points with respect to this one. So we might pick time point A to be our epoch, and say "time point B occurred 3 seconds after point A." If we also say "time point C occurred 4 seconds after time point A," we can do simple subtraction to say "time point C occurred 1 second after time point B."

So for our time system, we need two key things. We need a way of measuring the duration between time points, and we need a privileged "epoch" time point. It turns out that this is a challenge, even without FTL. If you do a google search for time standards, you come across all sorts of interesting ones. The history is fascinating enough to work with.

Universal time is a time standard based on the "solar time" at 0 degrees longitude. This is the prime meridian, itself an arbitrary line developed by the British between 1721 and 1851. It is "special" because everyone agreed to let it be special. We all agreed to call it 0 degrees longitude (well... mostly). It defines an epoch (time point 0) of Julian date 2541545.0, itself an arbitrary line in the sand made in 1583 which was literally chosen because its epoch (4713BC) was before any historical record! Talk about arbitrary!

There were a few variants. UT0 was based on measurements of distant quasars and such, but it did not handle polar motion correctly, so it was deprecated for UT1. UT1 is still technically in use, but it has a key limitation because it's time scale is dependent on the motion of the Earth, and that motion is slowly changing. Thus, we developed UTC, which is an atomic time scale (meaning the definitions of durations are defined via atomic clocks) which is kept within 0.9 seconds of UT1 by adding leap seconds.

Now there's a lot of people who don't like leap seconds, so we have other time standards which do not have them. TAI has no leap seconds, so it slowly drifts away from UTC (It is currently exactly 36 seconds ahead of UTC). TAI is a great reference source for your system because it went through growing pains. In the 1970s, they realized that gravitational time dilation was causing the different clocks (at different altitudes) to go at different rates. To resolve this, we re-defined TAI to be corrected such that all atomic clocks appeared to be at mean sea level (slowing down TAI by about a trillionth!).

TAI also has an even more pathological friend, Barycentric Coordinate Time (TCB). TCB accounts for movement of the solar system by applying corrections to put the "clock" at the barycenter of our solar system. It is also corrected to offset any gravitational effects. These effects are small, but they add up to about 490 milliseconds every year, which is quite a lot for atomic clock people!

So we've seen the solutions that have been done with real time systems, and they show how you would probably do it in your FTL system.

• Assign an epoch - This can be anywhere/anytime in the universe, all that matters is that people agree upon it.
• Assign a unit of time - This should be well defined at the location you used for your epoch, and it should be possible to calculate correction factors for elsewhere (just like we do for TAI or TCB)

Your correction factors are going to be more complicated with FTL, because you're going to need to sidestep general relativity without violating causality (if you violate causality, all bets on "time" are off). However, they'll just be correction factors.

And, along AnoE's solution, you could continuously update your correction factors if you can receive a time stamp from the source of your epoch (a.k.a. throwing a clock through a wormhole).

* This exact formalization is used in the <chrono> library of C++, a popular programming language. I find it to be one of the better ways to formalize time, so I use it outside of programming.

Time is relative. Any event in space-time can be labeled relative to another event as any number of different coordinate systems involving different time axes.

However, there is an invarient value, the interval, that is unchanged with respect to coordinate choice.

So I propose that you won’t label everything with a universal time alone, but will keep track of a full 4 dimensional coordinate that can be projected to any desired time as needed. Time per se will not be seen as a universal measurement, any more than our maps show “left/right”. We intuitively understand that how far left point A is from me depends on my current bearing, so maps show spherical coordinates and are oriented as needed for each leg of the journey.

So it is with time. Label an event P, and how far in the future that is depends on my current course and velocity. But the position of P is plotted in 4D and I can navigate to it.

• This confuses me since we aren't able to travel the time dimension, while we are able to travel left or right. I don't see how you can navigate to point P in time if it is outside your event horizon (i.e. in the past, or in the future by farther than your possible rate of travel) Oct 17, 2016 at 20:02
• You mean outside your light cone? That’s what FTL travel does. See my big posts here and here. And even without FTL the principle holds since different observers will be in different relativistic reference frames. Observers will disagree on the time difference between A and B. Oct 18, 2016 at 1:50
• Even in the realtworld case where B is in the future light cone of A, observers will have different ideas of the amount of space between A and B and the amount of time between A and B, with no preferred answer. But each will agree on the result of distance²−time² . That’s the essential part of special relativity. Oct 18, 2016 at 1:56
• I see what you are getting at, although that takes us to the problem that the assumptions (FTL travel) violate special relativity themselves. This is kind of a chicken-egg situation and I see why you say that FTL travel is implicit time travel. Oct 18, 2016 at 3:16
• @kingledion If relativity holds, and FTL travel is possible, then causality doesn't hold and time travel is possible. But it isn't something obvious unless you really know your relativity - until you really understand what it means that there's no absolute time or space. Oct 18, 2016 at 11:57

"In an FTL equipped universe, this becomes more difficult as an FTL route A might take longer to reach a destination than FTL route B with the result that clocks on both ships would not be in synchronisation.

"Is there a theoretical method of inferring a "universal" time that FTL travellers can use for their clocks to maintain a constant time?"

If you have a universe that allows faster than light travel at all, you assume special relativity does not apply. This will be a very different universe to ours. Among the differences will be that the FTL-permitting universe will allow a universal time to exist at all, which the real universe does not.

(A brief digression about the real universe:

In our universe any object travelling faster than light will be seen by some observers as arriving before it left. In other words, a universe that allows faster than light travel also allows travel backwards in time, effects to precede causes, and so on. This link to Richard Baker's site "Sharp Blue" explains further: Sharp Blue: Relativity, FTL and Causality

For more detail, see this link to an answer by JDługosz to a previous question here on Worldbuilding Stack Exchange.

Here is another way of putting the same thing: one of the basic ideas of special relativity is that there is no "reference time". Events that are simultaneous to one observer are not simultaneous to another. It's the speed of light that is universal; time and space can be seen as squashed or stretched depending on your point of view. Mathematically this is equivalent to saying that any object traveling FTL will be seen by some observers as arriving before it left.

In real life you can't have a universal time because you can't have FTL. Equivalently you can't have FTL because you can't have universal time. End of digression.)

Getting back to a fictional "FTL-equipped" universe:

If your fictional universe is "FTL-equipped" that means it does not follow special relativity and it can have absolute time built in. In other words physics could really be the way it seemed to be for most of human history. Your use of the term FTL suggests that you wish to keep light as having a finite speed. But why should you keep it as being 3 x 10^8 m/s? There seems no good worldbuilding reason to say that the results of the Michelson Morley experiment don't apply in your universe but the results of the experiment carried out by Ole Rømer* in 1676 (that demonstrated that light had a finite speed and made a decent estimate of what it was) do apply.

So, just set the speed of light as high as is convenient for the story. Then people all over known space can do as is done on Earth in real life and use clocks synchronized to radio time signals.

Later edit: I have belatedly remembered that having a much higher c would have some awkward effects on Maxwell's equations causing electric and magnetic fields to be different which would in turn cause all the charged particles in the universe to be different in ways that it is utterly beyond me to calculate. Like I said, awkward.

*Who should be more famous.

• If your idea about FTL travel is to "set the speed of light as high as is convenient for the story", then it's not FTL travel. It would be slower than light in a fictional universe where lightspeed is faster than c. There are fictional hyperspaces where lightspeed is significantly higher than in our sublight universe. Most forms of science-fictional FTL travel require getting over the lightspeed barrier or taking shortcuts in spacetime. Agree with your argument about decoupling time from relativistic spacetime. Rømer does deserve better. Oct 17, 2016 at 12:22
• @a4android, I do see that you could have a universe c was very much higher than in reality but special relativity still held. That would indeed allow a common time to be set easily, as well as interstellar empires and all the traditional aspects of space opera that we love so much. But even in a high-c universe it might still be more convenient to go faster yet, by teleportation or wormholes, which would require special relativity to be dropped. FTL plus universal time is what the OP asked for, so I strove to give it to him ;-) Oct 17, 2016 at 12:47
• Correction to my earlier comment: "a common time" should be "a reasonable approximation to a common time". Oct 17, 2016 at 13:23
• You don't need fast-c to have FTL. You can just introduce an ether against which your FTL speed is limited or measured. This also imposes an absolute clock on the universe. Our current physics becomes a lower-dimensional (4-10) solution to the more general physics. Or is there something that makes this not work?
– Yakk
Oct 17, 2016 at 13:27
• @Lostinfrance: Agree with you on faster-c plus special relativity. You did well giving the answer the OP wanted. Tricky business reconfiguring special relativity to permit universal time, if only fictionally. Oct 18, 2016 at 3:11

FTL travels will ALWAYS be very messy, because it is equivalent to time travel (for at least some observers, depending on there velocity).

By changing your velocity by just 1 m/s you change what moment that is "now" in a distance galaxy by hundreds of year.

So, if A and B travels to that galaxy, but B initiate the FTL drive from a 1 m/s lower speed than A, he may arrive 100 years earlier. If the travel takes much lesser time, A can accelerate a bit and then return before he went away!

So to have a universal time, it is very important to have a fixed clock with a fixed velocity, and define the rest with basis of that.

Ehi, We don't know what happens to time when we travel faster than light. But we do know time slow down when reaching speeds that are fractions of C. So I assume you want Almost Fast As Light traveling.

System one: Forward time With Zulu at Earth

We set ZULU at Earth, we send a signal from Earth to nearest space stations/planets. So in example:

• we Colonize Proxima centauri
• we send a signal from Earth to Proxima centauri
• when proxima centauri receive the signal we set whatever date Proxima receive as the local Proxima Time
• Proxima is 4,243 light years away

A simple example, suppose we can travel at 0.99C which can (not) be rounded to C (assume the travel for us will last somewhat few seconds because our time is going very slow).

• We depart from Earth in 2100
• We colonize Proxima Centauri in 2104,243 + (colonization time delta)
• We in the meantime started to send a signal from Earth
• After our receiver is operative in Proxima Centauri's space station we start to receive the signal
• We set the date in Proxima Centauri to be 2100 while in reality on Earth we are in 2104,243
• When we go back on Earth we will find the date is 2108,5

System two: Forward offsetted time With Zulu at Earth

• The date we set on the planet is increased by the distance in Light years
• We departed in 2100 from Earth, and we arrived at Proxima Centauri in 2100+4,243
• When we go back on Earth we will find the calendar is 2108 again.

The system 2 is my preferred because when we travel from one place to another we keep the calendar consistenly increasing with the date, however if we are receiving TV transmissions we will se that date is mismatched with the calendar (we are still receiving signals of 2100).

However the second system is subject to an error:

• we have to measure the distance from Earth exactly

While the first system do not require to measure anything (you set date on received signal).

Both methods are subject to time drift (in example 1 station orbiting near a big star have a slower time). If the drift is not excessive, we can just resynchronize clocks, but we have to prevent colonization of places that have a excessive time drift (hard to synchronize clocks).

And any station/planet MUST have its own local time drift and drift of nearby stations, so that each visitor know in advance how much time is going to lose if he decide to stop in other places.

Another usefull standard for AFAL voyages is the meeting procedure.

• Assume you want to meet someone, you travel straight from A to B
• Someone you want to meet take another route going first to C
• Now You will have to wait YEARS before Someone can reach you unless
• You leave a ticket him: "Meet me withing 10 years"

2 things can happen:

1. Someone reach B before 10 years in that case he can program a short travel in order to make time go on forward by 10-X years
2. Someone have to arrive when you come back after 10 years.
3. Repeat until Someone arrive and will wait for you, or just leave a ticked "Bored after waiting 320 years, we'll not meet anymore (probably)".

In the first case you will meet someone within 10 years, however you don't know how long will take for him to come to B. so the next time you double the time, this time you will leave a ticket "Meet me within 20 years".. and so on.

I would suggest to make public all "meeting plans". So that everyone can join, I would guess in such a future, even with big populations everyone would just get lost "in time". You could have billions of people traveling at same time, but maybe only a bunch of people alive

Not even in our universe

As has been pointed out by others, special relativity states that there is no fixed frame of reference, and, as such, both space and time are experienced differently depending on the observer.

What this means is that different regions of space, for example different parts of a galaxy, have a different frame of reference, and time and space flows differently from one to the other, so right now, as we read this site, what we perceive as a few seconds will be perceived as years in some other parts of our own galaxy (relatively speaking).

In a galaxy (or even a solar system, although it won't be as noticeable) this is mainly due to differences in angular acceleration (velocity depends on the distance to the center of a spinning disc).

And then you have to factor that galaxies are moving with respect to each other, thus each has its own frame of reference, and so on.

So if you're being realistic, it's pretty much impossible to have a reliable "universal" time.

• so right now, as we read this site, what we perceive as a few seconds will be perceived as years in some other parts of our own galaxy - This is not accurate. Other locations in the universe are not Narnia, where time passes at drastically different rates. Unless you are travelling near the speed of light or very very close to a black hole, time passes pretty similarly to the way it does here on Earth.
– Mar
Oct 17, 2016 at 15:41
• GR might provide an answer. See my comment on the question. The universe has an age so you can define a universal time. Oct 17, 2016 at 15:55
• @Martin Carney A far as I understand it, other regions of space will be moving at different velocities relative to our own, thus resulting in vastly different frames of reference which in turn results in time/space dilation. In fact, time/space dilation even takes place between Earth's surface and orbiting satellites, although at a smaller scale. Heck, GPS, for example, has to account for relativistic effects to work properly! Oct 17, 2016 at 16:00
• Relativistic effects are quite slow to kick in. To get Lorentz factor of 2, thus to have reduced the time 1/2, you need to get to 87% of the speed of the light. At 50% of the speed of light you would experience difference of 15%. Our solar system has relative time dilation of 0.2ppm. This is better than most clocks. DS3231, a very accurate microchip for time keeping has 2ppm error rate and I find it excellent. Oct 17, 2016 at 18:18
• Oskuro - Different, yes. Vastly different? Not so much. As @CemKalyoncu points out, you have to be more then halfway to light speed before "drastically different" is a fair description.
– Mar
Oct 18, 2016 at 20:01

Since the setting allows for super science (FTL travel), is there are way to plausibly allow for a absolute time?

I believe that any form of instantaneous communication will allow for the establishment of an absolute time.

So, how might we establish such? It seems reasonable to me that in some super science setting quantum entanglement could be used to provide the foundation for the establishment of an absolute time.

If one did not want to allow instantaneous communication as well, it seems reasonable to also say that this quantum based communication has not been perfected to the point of allowing more than just the maintenance of the absolute time.

• No, entanglement is outside of time. There is no way it could be used to coordinate timekeeping! Oct 17, 2016 at 18:45
• @JDługosz, entanglement in a super science setting might plausibily allow for instant information exchange over any distance. One might use such a capability to broadcast an absolute time beacon that allows all ships, planets, colonies, whatever to know the absolute universal time even when traveling at super relativistic speeds. Oct 17, 2016 at 21:51
• one matching pair of entangled such devices per location to keep in sync with the master Oct 17, 2016 at 22:04
• Not plausible, because enganglement doesn’t do anything like what yiu’re saying! «One might use such a capability to broadcast» huh? What does that have anything to do with entanlement? Use the ftl mechanism you’re using for travel instead. «one matching pair of entangled such devices per location to keep in sync with the master» you keep repeating that (“use it”) without explaining: what prey tell does that have to do with entanglement? So I repeat my initial comment, and claim that modern sci-fi fans would know that. Oct 18, 2016 at 1:11

Ignoring physics since you have stated FTL is to be used.

If you have FTL systems why would it not be possible to have pulse clock at a stationary position that all time can be synced to? This to ad extent had been done in the US with digital clocks that would sync to an atomic clock pulse in Colorado. Taking that you have FTL pulsing bits at FTL would be far less energy than putting one ship into FTL. Thus, if in FTL clocks can remain synced to the stationary clock pulse allowing multiple ships to have a consistent time to reference from.

• FTL does not always mean instant travel. FTL pulses would take time and you would need to know the distance to compansate. Oct 17, 2016 at 18:05
• While not instant would be considerably faster as only having to transfer by example BCD (binary converted decimal) time data. And since it is data can this can be sent also as FTL and can be sent faster than a ship due to the extremely reduced mass (electrons vs millions of kg) ships would get the updated times while in FTL. Oct 17, 2016 at 19:16
• Depending on FTL method, reduced mass would probably mean less energy required. For instance, if FTL method is Alcubierre drive, there is no way to send information alone. If it is a wormhole, the length of the wormhole should be the same, just the entrance would be smaller. I don't know any other scientifically sound FTL methods. Oct 17, 2016 at 20:57

You just need a giantic clock with a date, that is visible from each point of interest. The time of the clock itself is the universal time. The observed time is the offset. To calculate the offset, quantum-entaglement-based datatransfer will do the work.

• if you are a light hour away from it though the observable time will be an hour behind wouldn't it? depending on how it observed Oct 18, 2016 at 13:58
• yes, you have local time, with a negative offset, depending on distance. when you fly towards the clock, local time speeds up, otherwise slows down. when you fly FTL away from clock, local time runs backwards (as long as there are photons fired since the construction of the clock). I think with distance of destination from clock and speed, you could calculate your arrival time and local time during flight. Oct 19, 2016 at 7:36

Let me start by ignoring FTL for a moment.

There is a colony around Alpha Centuri that want have its clocks synchronized to Earths TAI time (or some other time standard without DST, leap seconds and similar ugliness)

Once the colony has established a radio transmitter, it sends a message to Earth:

What is the time at the beep?  **BEEP**


Earth is on the ball and immediately returns

**BEEP** That beep was synchronized with your beep on 2116-10-18 11:52:39.316


That message reaches the colony about 8.734 years after the first is sent. The clever colonists decide that the Earth is 4.367 light years away, so they add that much to the time signal, getting 2121-02-27 21:56:33.432 (or so)

After this they let their local clocks tick time forwards, on Earth time.

They will probably want to repeat the procedure periodically.

This method works because the two stars are almost stationary compared to each other. Any speed they have in common is irrelevant.

If the two ends are moving relative to each other with a relativistically relevant speed, the method breaks down. So space ships, both slower and faster than light, need to use more complex methods.

The nice thing about FTL ships is that they can speed up the whole procedure. A ship leaves AC with the first message, immediately returns with the second message and you can get the whole thing done in days instead of years.

• I know I have ignored some practical problems with this: The movement of the end points around their respective stars, gravitational time dilation and probably more. Oct 18, 2016 at 10:36

Variations on measuring galactic spin and constellation drift would do the trick if you have some method of measuring micro arc seconds of star movement. (Not too complicated for a civilization who already figured out superluminal travel.)

You have a single map that indicates the state of the galaxy at the moment of the epoch and then count microseconds from that point forward. The state of the galaxy becomes your universal reference point.

• Very similar to @kingledion pulsar triangulation method. Oct 18, 2016 at 13:30

You could measure the distance between the Milky Way galaxy and the Andromeda galaxy. They are moving toward each other at a rate of about 110 kilometers per second (68 mi/s). They will collide in about 4 billion years. That would make today be 4 billion B.A. (Before Andromeda). laughs