# UNITS OF TIME...HYPOTHETICAL QUESTION

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#### SolAlpha

##### Guest
Our(Humans) standard units of time are based on Earths unique relationship with the sun....
One rotation is one day, One orbit around the sun is a year and so on.

My question is.... If you were a civilization that occupied more than one planet ( or even more than one star ) , how would you standardize time?

For example....hydrogen is the most abundant element in the universe....Is there a stable natural process involving hydrogen that requires a set interval of time (ie cesium/atomic clock)?

I am not communicating with aliens or trying to coordinate conquering a star......I have just been lurking here a while and thought I would probe the minds on this forum for my own entertainment.

Thanks for any responces...

R

#### ramparts

##### Guest
The speed of light It's the most easily measurable universal constant* involving time. In fact, about 20 years ago the meter was defined as the distance light travels in free space in 1/299792458 seconds, pretty much guaranteeing the speed of light will forever be 299792458 meters per second

*We think

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#### SolAlpha

##### Guest
I think you misunderstand my question.....

....how do you know how long a second is?

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#### SpeedFreek

##### Guest
You can derive the duration of a second using the speed of light, as ramparts has already said. If you know how far light travels in a second, you can work out what a second is, as long as you can measure the speed of light.

This has already been done, and has been applied to the speed that atoms "vibrate" to give us a standard for atomic clocks.

From wiki:

Under the International System of Units, the second is currently defined as:

The second is the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom.

This would apply wherever you were in the universe, whatever your inertial frame of reference, as long as you had a caesium 133 atom with you that you could use as a clock!

(now waiting for the inevitable follow-up question)

M

#### MeteorWayne

##### Guest
I think the point the OP is aiming for is that it might not be called a second, and their "standard" would probably be a different length of time. The derivation of the second in our system is pretty arbitrary. It's based on a mean solar year/365 days/24 hours/ 60 minutes/ divided into 60 parts = 1 second. THEN it was defined using a arbitrary number of cycles of the hyperfine transitions of Cesium.

Of course it wouldn't make any difference. Our second would be the same length of time. Another species unit might be 1 cycle of the travel time in light over 12 schmerdiffs. It might be called a groklin. Or it might be based on the rotation of a nearby pulsar. Or how often their "Old Faithful" geyser erupts. Whatever.

So the basic measuerement UNIT might be a different length of time, but the amount of time measured using the differing units would measure the same amount of time, whether it's 1 second, 0.145 grokolins, or whatever.

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#### MeteorWayne

##### Guest
BTW Solalpha, welcome to space.com!

We could do without the all CAPITAL letter titles; we generally don't do that here at Space.com.

Interesting question, hope the answers have helped.

Wayne

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#### SolAlpha

##### Guest
Sorry for the Caps....I'll remember for next time.

MeteorWayne is in the ballpark. I'm not concerned with how to determine the length of a second. The unit 'seconds' evolved on Earth and is based on what we have already mentioned.

If you were looking at the universe in a fishbowl ( as an observer without a home planet to derive a second from ) what would your standard unit for time be? Humans decided how long a second should be based on experience and environment...How might another civilization derive a standard unit for time ( maybe their planet does not rotate...or maybe it orbits a binary system )?

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#### drwayne

##### Guest
For a non-technical people, an osciallation of some kind, the day or the year would in all
probability dictate things in much the same way that ours did.

As technology evolves, this departure point will probably be tied to some form of standard oscillator,
but it will still be normalized to whatever the cycles of life are for that planet.

Such a system is transportable, so it would probably be used for a new planet, as we would
do if we were on a planet with a weird "day", like Venus.

Wayne

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#### silylene

##### Guest
The unit of time "grokling" would most likely be defined in a pre-technological time within the culture of the inhabitants of the planet. One key thing about the time definition: it needs to be reasonably reproducible in different locations on the planetary surface, or be measurable with a easily prepared transportable gizmo. At this non-technological period, the definition does not need to be exact or even precise.

If the planet has moon(s), or is a moon that orbits a Jovian, or has multiple suns, than most likely the grokling would be a subdivision of some easily measurable astronomical period.

If the planet has no moon, then something else would be defined as the standard unit of grokling in a pre-technological culture:
- a peridioc motion device, such as a standard pendulum or torsional pendulum
- perhaps the half life of radioactive decay of some element (maybe the LGMs have a sensory organ to detect alpha or beta emission)
- perhaps the time it takes a fixed amount of some material to melt when placed in a fixed temperature bath
- perhaps the time it takes to oxidize or reduce a known amount of a material under some fixed condition.
- perhaps the pulse rate of the heart (Galileo used this measure as a boy to time pendulums while in church!) or the time rate of some other biological clock
- perhaps the time it takes to drum out the meter of Beethoven's 5th

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#### ramparts

##### Guest
Exactly right (all of the above ). A civilization that's really smart and knows fundamental physics from the get-go would probably use the Planck time, a unit of time based on fundamental constants, but most of us civilizations go through a more primitive phase first, so the most natural thing is probably to base it on whatever astronomical periods happen to be in the area.

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#### CalliArcale

##### Guest
Part of the trouble with a multi-planetary civilization is that you need to measure time in a way which is meaningful to the residents. There is a natural diurnal cycle in most species, and it's important to maintain that -- how you choose to reckon time is actually very significant when you start to think about that, because if you choose unwisely, you could end up messing up everybody's circadian rhythms. Also, you need to be able to divide time in a way which is a) not too cumbersome and b) relates well to whatever the natural unit of a useful "day" is going to be.

This problem has already been addressed in the space program. ISS crews have to have a day-night cycle, even though their "day" is about 90 minutes long (the duration of one orbit). Since there's no useful correlation to the cycles of light and darkness outside the station, they just go by what ground controllers are most comfortable with, and that means they have to periodically adjust their internal "clocks" so that they mesh with ground controllers. If there's a Soyuz taxi mission coming up, they want to be fairly close to Moscow time, for instance, though I think mostly they stay about halfway between Moscow and Houston so it's easier to slide one way or the other.

Unmanned missions to Mars have faced this problem as well, but with a different imperative -- while the ISS crew wants to keep their clocks at a cycle that will be useful for ground controllers, Mars landers have to optimize daylight. This is especially true with the modern solar-powered rovers such as Spirit and Opportunity. In order to do this, the missions don't run on Pacific Time, even though they're run from JPL in California (IIRC). They run on local Martian time. Yes, a system of reckoning time has been devised for Mars, complete with time zones. Time is measured not in days but in sols. A sol is the same thing as a day, but with Mars' rotational period -- the different name makes it easy to tell what kind of a "day" you're talking about. Teammembers have had to live on Mars time, which means that their commute shifts a bit every day, given that a Martian sol is just a little bit longer than an Earth day, and many of them wear two watches -- one is an ordinary Earth watch set to Pacific time, and the other is a special custom-made Mars watch set for their rover's time zone. The Spirit crew rise at a different time than the Opportunity crew, since their rovers are at different longitudes and thus different time zones. Interestingly, they also have a 24-hour clock -- but the hours are longer on Mars than on Earth. (I suspect the spacecraft's internal computer doesn't care about that, though, and has gone merrily on with Earth time.)

Basically, they're largely ignoring Earth time because the need for Mars time overrides it. But if you were running a civilization that had lots of activity on both Earth and Mars, I think you'd have to have Mars running on Mars time and Earth running on Earth time, with clocks and calendars designed to simplify the task of correlating the two. Just as a company with business in both India and the US will have to manage two time zones, a company with business on Earth and Mars will need to manage both time zones, even although those time zones don't progress through their calendar at the same rate. There will certainly need to be some larger efforts in standardization of interplanetary time, and I suspect there will be several such experiments before a system actually sticks.

BTW, the Earth/Mars solution is workable largely because the Mars day is pretty close in length to the Earth day. The Moon has a "day" which is about two Earth weeks long; such a system would not be helpful there, and you'd have to find some other means of subdividing time. Perhaps it would simply default back to Earth time.

http://en.wikipedia.org/wiki/Timekeeping_on_Mars
http://en.wikipedia.org/wiki/Darian_calendar
http://www.giss.nasa.gov/tools/mars24/

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#### SolAlpha

##### Guest
Thanks for the responses!

I think Planck time, as ramparts mentioned, best represents my thought process for the original question.

If an advanced civilization was spread over many planets but old enough that there was no longer a singular "home world," what could be fundamental unit of time that would make sense for all?

Or beyond that, no other civilization has knowledge of seconds or meters as standard units...if one were tying to communicate using the "universal language of math," wouldn't units based in nature,on a universal scale (i.e. Planck length, time, etc), be more easily translated?

Again, this was all food for thought... Thanks for playing!

M

#### matthewota

##### Guest
The use of time on another planet is an issue that has already been addressed by the folks at the Jet Propulsion Laboratory. When they sent the Mars Exploration Rovers on opposite sides of the planet Mars, the people controlling the spacecraft went on a martian time, with a day length of 24 hours, 37 minutes, 22.66 seconds. They went to a local watchmaker/jeweler that made custom watches that were slowed down just enough to reflect the length of the Martian day. Note that they still used Hours, Minutes and Seconds as units of time. However, the definition of the Martian day has been designated a 'Sol', instead of a day.

Interestingly, the scientists and engineers that were on this martian clock had trouble with their circadian rhythms as they gradually fell out of synch with sunrise/sunset cycles on the Earth.

Assuredly when we expand our civilization into the Solar System, we will retain our units of measurement of time. I am not sure if colonists on Mars will ever get used to the slightly longer day. As for the Moon, their day/night cycle will be much harder to deal with. as the length of the day and night on the moon take a month to cycle.

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#### ramparts

##### Guest
Yes, a universal like Planck units is the only thing we'd have in common with an alien civilization. But I have a feeling we'd find it easier to just find out what their time unit, the zork, corresponds to in seconds, and just do the conversions when necessary (the Planck time is about .00000000000000000000000000000000000000000001 seconds, very unwieldy ). Besides, that's going to be a piece of cake compared to how hard it will be just to communicate between zorkese and English!

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#### SolAlpha

##### Guest
Mathewota,

Not knowing much about circadian cycles.... Would it be a worthwhile, although impossible, experiment to rasie isolated groups of humans on Earth, Mars, and the Moon and see how their cylces differ? If the cycles are based on genetics and not environment, how many generations would need to pass in order for these cycles to change?
Would an ancient society, spread out over many different planets, but able to readily travel between them, be as tied to the cycles of the "home planet?" Or, would those cycles become less relevent as that society evolved on multiple worlds?

Ramparts,

The cycle of Planck time, seems to me, is no more unwieldy than what we already do with an atomic clock. "Easier" is relative.....If you were dealing with a large number of different time cycles & units (planets, colonies, species, whatever) wouldn't it be easier to have one standard unit instead of many?

How was the metric system derived? Conversions exist for other standards of measure.....At some point, it made sense to have a simplified/unified standard.

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#### ramparts

##### Guest
I mean unwieldly in that Planck units require huge numbers to talk about familiar quantities, and we have a tougher time conceptualizing big numbers than small. Let's take a second (1.855 * 10^43 Planck times) and one minute (1.113 * 10^45). Isn't it much easier to say "in a second I'll get the keys and in a minute I'll leave the house" than "in 1.855 times ten to the 43 Planck times I'll get the keys and in 1.113 times ten to the 45 Planck times I'll leave the house"?

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#### SpeedFreek

##### Guest
SolAlpha":2bujv3sx said:
If you were looking at the universe in a fishbowl ( as an observer without a home planet to derive a second from ) what would your standard unit for time be? Humans decided how long a second should be based on experience and environment...How might another civilization derive a standard unit for time ( maybe their planet does not rotate...or maybe it orbits a binary system )?

SolAlpha":2bujv3sx said:
If an advanced civilization was spread over many planets but old enough that there was no longer a singular "home world," what could be fundamental unit of time that would make sense for all?

Or beyond that, no other civilization has knowledge of seconds or meters as standard units...if one were tying to communicate using the "universal language of math," wouldn't units based in nature,on a universal scale (i.e. Planck length, time, etc), be more easily translated?

One point to note here, although it might not be what you are actually asking (but I think it is related at a fundamental level), is that there is no sense in which time is absolute in this universe.

So, even if everyone in the universe were to somehow be using the exact same units for time (based on a constant like c, some form of planck time perhaps), all their clocks would still be moving at different rates, relative to each other. This greatly complicates how we translate the distance between events, when those events are viewed from different places.

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#### SolAlpha

##### Guest
Ramparts,

This was taken from Wikipedia on the atomic clock:

Since 1967, the International System of Units (SI) has defined the second as the duration of 9,192,631,770 cycles of radiation corresponding to the transition between two energy levels of the caesium-133 atom.

We don't use nanoseconds, milligrams, and millimeters exclusively....we use the most convenient unit for the application.
Think GigaPlancks! :mrgreen:

SpeedFreek,

I assume you are referring to time dilation....Couldn't Einstein synchronization be used in this situation?

M

#### michaelmozina

##### Guest
SpeedFreek":2udp8kiz said:
One point to note here, although it might not be what you are actually asking (but I think it is related at a fundamental level), is that there is no sense in which time is absolute in this universe.

So, even if everyone in the universe were to somehow be using the exact same units for time (based on a constant like c, some form of planck time perhaps), all their clocks would still be moving at different rates, relative to each other. This greatly complicates how we translate the distance between events, when those events are viewed from different places.

That's a really important and often misrepresented point. I also liked Calli's response:

Part of the trouble with a multi-planetary civilization is that you need to measure time in a way which is meaningful to the residents. There is a natural diurnal cycle in most species, and it's important to maintain that -- how you choose to reckon time is actually very significant when you start to think about that, because if you choose unwisely, you could end up messing up everybody's circadian rhythms. Also, you need to be able to divide time in a way which is a) not too cumbersome and b) relates well to whatever the natural unit of a useful "day" is going to be.

The notion of a "natural" circadian rhythm seems to be something that most folks prefer, hence all the different "timezones" right here on Earth. Calli's insights into the "two watch" scenario of Mar's rover teams also made me smile. I can just see someone on the rover team with two watches on their wrist flying from Florida to California trying to figure out what time it is in the local time zone between flights. I can only imagine what kind of confusion would take place while traveling between two different planets.

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#### vividasday

##### Guest
Why is the sun the question here....what about the moon? I appreciate the thought of time.

K

#### kelvinzero

##### Guest
My interpretation of this question is what periods the society would use, rather than the technical aspects of how to define it accurately.

There could be a biological basis, but personally I think our ability to reengineer ourselves will be absolutely startling before we even leave this solar system.

Although biology may be outdated, societies themselves also have rhythms, such as defined by weekends, elections and celebrations. These do not need to be tied to seasons, we would invent them anyway. They are part of keeping a society healthy and coordinated. A race might not need rest or understand celebration yet it would still probably invent rhythms. It is these social rhythms that would become the units by which individuals measured times IMO.

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#### SolAlpha

##### Guest
I like the mars example in that it illustrates how quickly keeping track of time can get complicated.

I think my question ended up going in two different directions;

Mechanically, how would a society design a standardized unit of time that makes sense for multiple locations in space or is a natural rhythm to time on a larger than planetary level (i.e. Planck time).

and

Is it more logical,for the hypothetical society we are talking about, to have a unified way to track time, or is it better to let each part track time as it wishes?

Given that, on Earth, societies have started with individual units( money for example) and gradually simplified those units ( the Euro ), time would eventually be standardized as well ( given enough time zones ).

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#### SpeedFreek

##### Guest
SolAlpha":3q1cio9n said:
SpeedFreek,

I assume you are referring to time dilation....Couldn't Einstein synchronization be used in this situation?

It could be used in certain circumstances where clocks can be synchronised directly between inertial frames but if, for instance, accelerated or rotating frames of reference are involved (as they surely would be), it is not particularly useful. If you have to use a series of clocks between the two frames in question, then it all depends on the path chosen between the clocks, through space-time.

Due to the relativity of simultaneity, any synchronization is merely a convention. So it seems to me that the idea of having a "unified" way to track time for frames of reference separated by large distances is essentially meaningless, if no two observers can agree on the simultaneity of events.

Imagine we somehow knew that we were measuring time here on Earth using exactly the same units as someone in Andromeda. Say these units are based on the speed of light, or the "vibrations" of a caesium atom.

Apart from the fact that, due to the time dilation caused by either relative motion or the difference in gravitational potential, clocks in each location would be running at different speeds relative to each other, we also have to deal with simultaneity issues.

As you sit in your chair, it is "today" on that planet in Andromeda. If you get up walk across your living room towards Andromeda, it would be "tomorrow" there and if you turn and walk in the opposite direction it would be "yesterday". This effect increases with distance, so we end up with the unfortunate situation where as you pace up and down in your room, any notion of simultaneity you might have for a distant galaxy leaps backwards and forwards by centuries!

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#### SolAlpha

##### Guest
If only all events in the universe could be viewed from a single frame of reference :lol:

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#### Shpaget

##### Guest
Why bother with standard unit of time for planets that have significantly different periods of rotation and revolution when you can open Notepad and write couple of lines of code that will display two clocks side by side. One for your time and the other for Martian/Zorkian?
Make the computers do the hard work, that's what they're for.

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