# 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?

• depends how big your empire is. a few nearby solar systems, not much of problem. galaxies upon galaxies, it gets rather crazy. – Serban Tanasa Mar 4 '15 at 18:38
• "some return just after they left, while others come back decades or even centuries behind schedule" Is it really what would happen? Or is it more something like "The ship arrives 20 years later, but only one year has passed according to the pilot"? Which would also make things complicated, but not really in the same way... – Maxime Lucas Mar 5 '15 at 10:11
• @skysurf3000: Normally, not. The solar systems move at pretty deterministic pace and calculating the time (including reference frames) in each would be quite easy (comparing to the travel). Still, you can write a universe where this is the case: someone had developed a warp drive, which folded the space allowing to make many light years of travel in matter of minutes. It was used for a time, until someone noticed the space doesn't return to previous shape after such folding and there's no way to smooth it out again. Currently, the drives are banned and spacetime of the universe is a mess. – SF. Mar 5 '15 at 10:41
• It's Stardate 3456.3. – Dronz Mar 5 '15 at 22:24
• – MichaelHouse Mar 6 '15 at 19:17

This wouldn't actually be a huge problem.

a friendly scientist stops by and explains to you that time is relative, and it moves differently in different reference frames

That's true, what's also true is most reference frames your civilization will move in are very nearly experiencing the same time flow.

You have to get moving very fast in order to notice much difference.

At most you're going to be a few fractions of a second off here and there. We already adjust for this in GPS satellites, so the equations are not difficult and our computers can easily figure them out and adjust automatically. If you know how fast you're going, then you can adjust for the time difference.

Too much work?

Just broadcast the local time for the system along with the location of the broadcast. If your ship receives the signal and knows where it is, it can determine the light distance from the reference signal and set the ship's microwave clock accordingly. You can even use multiple transmitting stations with known locations and set up a solar system positioning system, an SSPS. Using trilateration, your ship can then get location and local time from the same set of signals.

• There's also gravitational time dilation to think about, but that's also going to be quite a small effect (and pretty similar for all planets). – David Z Mar 5 '15 at 10:11
• @DavidZ Yes, that's a good point. It'll matter for the precision of a positioning system, but not for accuracy of dinner plans :) Thanks. – Samuel Mar 5 '15 at 17:26
• Warp is definitely going to crash time. Afar from that, don't worry. – Mast Mar 6 '15 at 10:37
• @Mast So you're saying there's going to be a Time Crash? – KSmarts Mar 6 '15 at 22:54

You've hit the nail on the head: in general, everyone's clocks will run at different rates, depending on their speed and position relative to gravitational fields. For example, we on Earth experience a different rate of time relative to satellites in orbit.

However, just because time is relative doesn't mean that we can't know when a ship will arrive at its destination. We just need everyone to agree on one specific inertial frame that times will be measured in. Once everyone is agreed, you can just convert your own spacetime coordinate system into the reference frame, and then convert into someone else's coordinate system.

We already do this in a simplistic fashion with time zones. I can convert 1:45 pm EST (UTC-5) to 6:45 pm UTC, then convert to 2:45 am (+ 1 day) Beijing time (UTC+8). In your case the math will be more complex, since the time rate depends on relative speed, among other factors. The conversion will likely be handled by computer: you'll just ask your helmsman when you'll arrive at Alpha Centauri, and he'll read off his navigation display, "5 weeks, 3 days, 8 hours, and 31 minutes, and it'll be 6:56 on Tuesday the 22nd when we get there."

The important thing to remember is that in any coordinate system, every point in spacetime will be mapped one-to-one to a unique set of coordinates. As long as you specify which frame your coordinates are in, there won't be any confusion.

## Exactly as we do now

While clocks at two locations will diverge, it will happen (a) slowly and (b) predictably. So you can treat them in the same manner as we treat time zones now, and possibly pick any of them as "privileged" - e.g. GMT or UTC. If you want to use any non-local time, it will not tick forward at the same speed (as on earth) and the offsets between different places will be variable, unlike Earth time zones (unless you simply discard all physics); so the calculations won't be feasible to make in your head - but they're trivial for computers.

In the same way as now you can see currently (on e.g. hotel walls) multiple clocks that show 'London time', 'New York time' and 'Tokyo time', it would be trivial to have an (electronic) clock that would show 'Earth time' and 'Sirius time'; and it would be trivial to have your scheduling systems automatically convert everything to whatever time scale is most convenient for you and show it that way - again, as the current calendaring software does for meetings when participants are in different time zones.

• GMT and UTC aren't the same. I live in the UTC zone, but on summer, the clock is GMT+01:00. But still UTC time. This makes a huge difference. Iy's just a tiny detail. I actually like your answer, but it doesn't answer, for example, the time schedule for a space station. For example. But that is REALLY hard with my current mindset. – Ismael Miguel Mar 4 '15 at 22:17
• Time is just a number. Places like bases in Antarctica or ISS just pick an arbitrary time zone and stick with that. A spaceship in the middle of nowhere can use whatever "galaxy time zone" they want for official/scheduling purposes , simply if they move fast enough they have to plan that during a normal 8-hour sleep the "official" clock may move 9, 19 or 90 hours depending on their speed; so you if you're moving with fast relativistic speeds you may want to use one "time zone" for internal scheduling and another for coordinating with outside people. – Peteris Mar 4 '15 at 22:24
• GMT and UTC are not the same (GMT is a timezone, UTC is a time standard), but they both have the same time (perhaps differing by a small fraction of a second). Neither UTC nor GMT ever change for Daylight Saving Time "DST". However, some of the countries that use GMT switch to different time zones during their DST period. For example, the United Kingdom is not on GMT all year, it uses British Summer Time "BST", which is one hour ahead of GMT, during the summer months. – Johnny Mar 5 '15 at 5:22
• @Ismael: No, it's the other way around. His link is correct. UTC is never GMT+01:00. The UK switches from GMT to BST, which is both GMT+01:00 and UTC+01:00. UTC is the same across the world and does not change in the summer. – Chris Burt-Brown Mar 5 '15 at 10:23
• @Ismael: No. Sorry, you are wrong. UTC is never GMT+01:00. UTC does not observe Summer Time. I live in the UK. When the UK goes onto Summer Time, we are UTC+01:00. – Chris Burt-Brown Mar 5 '15 at 11:31

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.

• I was specifically going to post "pulsars" as the answer. This is the absolute correct answer. – dotancohen Mar 5 '15 at 16:56

If you can have some way of transerring data at decent speeds across planets/solar systems/galaxys you could have a clock somewhere (maybe a satelite) that acts as a base time to which all clocks can ask what "Universal Standard Time" is it.

If not, you could have a series of satelites that are kept in zones of similar gravity conditions so that time flows similarly for each of them and sync them every so often. Then every "clock" will check the UST against the closest satelite thus needing a less powerful data transfer.

If your empire is really big so that it actually spans multiple galaxies, you might want to base the standard clock on the cosmic microwave background.

See for example physics stackexchange

Having chosen a standard clock, obviously you must then set a base time. As a galaxy spanning civilisation, you should have the technology to be able to use the start of the universe for that!

Alternatively, a civilisation with FTL would likely use that technology to manage clocks. Special relativity considers how light signals are passed between observers to come to its conclusions regarding clocks. So it will depend on how break the law of the speed of light being limiting that will determine how to sychronise clocks.

• +1. Go *nix style and set an Epoch. Count from there. – Mast Mar 6 '15 at 10:44

Yes! Just "Make it so!"

For storytelling that is really what generally happens.

However, the only way there is a chance is if there is a instantaneous or nearly instantaneous form of communication. If it is known how long it takes to travel from one point to another, then knowing when the ship leaves, will give you a very good idea when it will arrive. It doesn't matter if the crew experience 1 week while there is a 4 week transit time for everyone else waiting, predicting when it should arrive local time should be 'relatively' easy (pun intended).

Without near instantaneous communication, it is going to be a lot of calculations and assumptions.

• Communication does not need to be instantaneous; the delay just needs to be deterministic. For example if a laser beam was used and transmitted from upper orbit, then light in the vaccum of space travels at C and any delay is deterministic and can be compensated for. – Drunken Code Monkey May 8 '15 at 4:05

You have to distinguish two notions: Local time and Global time.

You only need to agree on the global time, and in a super-tech world where everything is done by computer, this can be as ridiculous number as "the number of seconds since 1st of January 1970".

If two people who use different local time want to meet, they'll agree on the global time value, but the mindset will be their local time anyways, in a similar way as if you set a meeting with your friend overseas: you agree on a UTC time but note down the local time in your schedulers.

Now to the technical part of the question:

What would be the UGC (universal galactic clock)? The idea is the same as now: you fix a rotating object X that is stable enough, and you count its periods of rotation. This can be a galaxy (so in the end the typical lifetime of a human would be very small, 10-8 or even small, but who cares, it's just a number), group of galaxies, or whatever.

How would you refer to the time if relativity has to be considered? Well, the time in any place in the space is measured in the following well-defined way:

1. You change your speed so that you are not moving relative to X.
2. You measure the distance d to X.
3. You get the timestamp t from X.
4. The value of UGC in your location is then T = t - c d, where c is the speed of light.

Of course, in reality you keep track of your local time yourself to high precision (that's what you do now if you watch your clock), you know your movements so you can calculate the relativistic corrections. From time to time, you calibrate your clock with UGC.

There is no time in space because simultaneity is relative. http://en.wikipedia.org/wiki/Simultaneity

In different frames of reference, a pair of events A and B can be seen to have happened A before B, or B before A, or simultaneously. It is only because our frames of reference on earth are so alike that we all agree on the order of events.

This is not to say that events do not have causal relations. San Francisco could not be rebuilt until after the earthquake. I wonder if this would provide a basis for a tree-like, partially ordered notion of universal time?

• There is time in space, but simultaneity is different for different observers. – HDE 226868 Mar 5 '15 at 1:38

If you know how much faster a clock is on planet A than on planet B, just compensate locally.

You could also take whatever magic you use for non-causality breaking faster-than-light and adapt it to beam a time signal throughout the whole empire. Do you have FTL radio, or does information travel at the speed of the fastest courier ships? In which case a "crook" could keep ahead of the authorities by constantly moving as fast as the courier carrying news of his crimes.

Or you could just have 'hyperspace' be filled with weird currents so that travel time is partly random. We coped with that during the Age of Sail.

You don't have to have the time stay the same for everywhere you go. It doesn't have to be 1AM on every planet and on every ship. That's funky and weird, and if there is some interesting orbital or rotational mechanism that would make that no longer feasible then here is what you do. You keep the units of time the same.

Maybe one planet has the same number of hours as Earth. But there might be ones that have more or less. But you can give a travel time, not an arrival time. Since time is relative, and you're not setting up a literal Universal Standard Time, you can just use equivalent units of time measurement.

How I figure this would work in your case is this: Ship leaves port at a certain time on one planet, will take some number of hours, days, weeks or what have you to reach its destination. If there is long-distance communication, this can be communicated to the people expecting the arrival of the ship. Now, the thing is that for us here on earth a day is 24 hours, but it also deals with the positions of the sun and the earth. So you'd have to strictly separate the meanings of the passage of time and the rotation of the planet in orbit.

• The problem is that even the units cannot be the same for all observers. Look up special relativity for the space ships, and general relativity for the planets. – dotancohen Mar 5 '15 at 16:58

In such an empire, surely you could define one pulsar visible from your capital planet as your "standard clock", and broadcast a tick count across your empire. This way everyone is synced to the same extremely precise clock corrected for communications delay. You can also sync up more than one observatories for extra precision. This way it is galactic day 102927 on every planet at the same time.

• You'll need to know the exact location of the planet sending the signal on the time it's send and the location of the destination the moment it arrives, but I guess a decent mathematician can solve that. But your distance won't be constant. – Mast Mar 6 '15 at 10:42
• It doesn't need to, the routing protocols of the future for such a network would certainly handle it. One could also define concentric spherical time zones around the capital planet. Same as earth time zones but calibrated to compensate for light hours (or light days, etc...), and three-dimensional. That would put some systems a few hours behind or ahead others, according to the distance between them and if they are further away or closer to the capital planet. – Drunken Code Monkey Mar 20 '15 at 2:48

If your question was meant to be "What time zone is used aboard the International Space Station" the answer is UTC time. Other countries tend to use the timezone of the site from which their space craft were launched. Have assumed your question relates to actual space craft and not some of the answers offered so far.

• I think the question assumes larger chunks of space -- interstellar rather than just earth orbit. – Monica Cellio Mar 6 '15 at 4:22