Standardisation of Time in a FTL Universe

Question

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?

Answer 1

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.

Answer 2

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...).

Answer 3

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

Answer 4

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.

Answer 5

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?

Answer 6

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.

(Lots) more information on this on my website.

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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).

Answer 7

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.

Answer 8

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.

Answer 9

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.

Answer 10

"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.

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*Who should be more famous.

Answer 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.

Answer 12

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

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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

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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).

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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.

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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

Answer 13

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.

Answer 14

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.

Answer 15

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.

Answer 16

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.

Answer 17

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.

Answer 18

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.

Answer 19

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