PDA

View Full Version : what would be used to measure "universal time"

mungoid
2010-Jan-11, 06:05 AM
What would be the best way to measure time in some form of universal standard?
We measure time according to our local standard which isn't all that accurate anyway.

1 day = 1 revolution of the Earth on its axis = (roughly) 24 hrs
1 year = 1 revolution of the Earth around the Sun = (roughly) 365 days

These (rough) estimates of time mean than the large distance measurement we use such as the light year (read 1 light Earth year) while probably close enough for practical use cannot be an exact measurement because it is based on our own inaccurate local standard.

Say in the future we have colonised a planet in another star system. Presumably the colonists would use (eventually) a time measurement that meant more to them locally (one year represents one revolution around their star etc.
There would be, I think, a need for some universal time measurement.

So, leaving aside time dilation effects at high speeds and other anomolies, what could be used as an accurate universal standard measurement of time? Maybe some form of atomic clock? (I think we use the vibrations of a Caesiam atom as a standard but would this necessarily be the best clock to use for a universal standard)

Van Rijn
2010-Jan-11, 06:39 AM
What would be the best way to measure time in some form of universal standard?
We measure time according to our local standard which isn't all that accurate anyway.

1 day = 1 revolution of the Earth on its axis = (roughly) 24 hrs
1 year = 1 revolution of the Earth around the Sun = (roughly) 365 days

I'm not sure what you mean by time measurement not being accurate on Earth. Using the right tools, time measurement on Earth can be as accurate as anywhere else.

So, leaving aside time dilation effects at high speeds and other anomolies, what could be used as an accurate universal standard measurement of time?

Well, that's exactly why you can't have a universal standard measurement of time. Clocks in different reference frames will not agree on elapsed time. You can have a reference clock that can be used to compare against, though. That's already done with GPS.

01101001
2010-Jan-11, 07:06 AM
Say in the future we have colonised a planet in another star system. Presumably the colonists would use (eventually) a time measurement that meant more to them locally (one year represents one revolution around their star etc.
There would be, I think, a need for some universal time measurement.

The second.

Wikipedia: Second (http://en.wikipedia.org/wiki/Second):

[...] nineteenth- and twentieth-century astronomical observations revealed that this average time is lengthening, and thus the motion of the earth is no longer considered a suitable standard for definition. With the advent of atomic clocks, it became feasible to define the second based on fundamental properties of nature. Since 1967, the second has been defined to be: 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.

Then they, like us, could group seconds into comfortable locally-based clumps of large numbers of seconds, the sort of things we call minutes, hours, days and so forth.

Jeff Root
2010-Jan-11, 08:45 AM
I, too, do not understand what you mean when you call our own local
standard "inaccurate". In what way do you consider it to be inaccurate?

If what you want is a precise time measuring technique that can be used
anywhere, atomic clocks are excellent. They keep time very well.

If what you want is a standard unit that humans can use wherever they go,
the second is fine. It has the drawback of not being an exact, even division
of the length of the day or year for any planet, and only a close approximation
for Earth. But that drawback holds for any standard. The lengths of the days
and years of different planets are not precise multiples of one another.

If what you want is a clock that people everywhere in the Universe can
read and get the same reading, the time since the Big Bang is probably as
good as you can get. But the reading is very approximate, useless for short
time periods, and takes a lot of effort.

What is it that you want?

-- Jeff, in Minneapolis

astromark
2010-Jan-11, 09:21 AM
So you think our ability to be accurate when talking of time... That is wrong.
As has been said above the ability to measure the passage of time is very accurate. I do understand how you see some confusion however. Without actually knowing this, I will blame the British empire for this obvious and often confusing manner in which we record times motion... yes, sixty seconds to a minute and then sixty minutes to the hour. To go to smaller increments than the second we seem to revert to the metric normality of the numeric system. Counting as time elapsed a year or even a month as terms of measure are clumsy at best and could be deemed as inaccurate. That we should find a better unit of time as a standard is well reasoned. but what is the alternate. This is going to be introduced across in the against the mainstream thread... lets see if it gets any comments there...?
Mark in a seemingly ever clear Wanganui.

ZaphodBeeblebrox
2010-Jan-11, 09:47 AM
The second.

Wikipedia: Second (http://en.wikipedia.org/wiki/Second):

Then they, like us, could group seconds into comfortable locally-based clumps of large numbers of seconds, the sort of things we call minutes, hours, days and so forth.
Saw this Thread, couldn't Resist a Comment ...

This is an Issue I've Looked into Myself, Many Times, Over The Years ...

The Problem in a Nutshell; Humanity's Constants Simply have NO Place in a Truly Universal Measurement System, at Least, Not Unless we use The Unitless Ones!

Planck's Constant (h) Provides The Easiest Answer for a Both Distance and Time Measurements, Kinda Fitting for a Universe in Which The Two are Merely Different Aspects of The Same Hyper-Spacial Form ...

Unfortunately, The Resulting Units are ONLY Really Workable in a Base-60 Numbering System, But does Anyone Else have a Better Idea?

Marauder
2010-Jan-11, 11:54 AM
Time isn't a universal constant. Even atomic clocks will run at different times when on board fast moving space ships or even when near a large gravitational field. The atomic clocks on board our GPS satellites are constantly adjusted as they run slower than the same atomic clocks on the earths surface.

Veeger
2010-Jan-11, 12:08 PM
Does the rate of decay of an atomic element remain constant if the sample is moving near the speed of light?

Hornblower
2010-Jan-11, 12:15 PM
Does the rate of decay of an atomic element remain constant if the sample is moving near the speed of light?

If I am not mistaken, no. As observed in high-energy particle accelerators, the observed decay rate of unstable particles slows down as their velocity with respect to the laboratory increases.

DrRocket
2010-Jan-11, 03:19 PM
What would be the best way to measure time in some form of universal standard?
We measure time according to our local standard which isn't all that accurate anyway.

1 day = 1 revolution of the Earth on its axis = (roughly) 24 hrs
1 year = 1 revolution of the Earth around the Sun = (roughly) 365 days

These (rough) estimates of time mean than the large distance measurement we use such as the light year (read 1 light Earth year) while probably close enough for practical use cannot be an exact measurement because it is based on our own inaccurate local standard.

Say in the future we have colonised a planet in another star system. Presumably the colonists would use (eventually) a time measurement that meant more to them locally (one year represents one revolution around their star etc.
There would be, I think, a need for some universal time measurement.

So, leaving aside time dilation effects at high speeds and other anomolies, what could be used as an accurate universal standard measurement of time? Maybe some form of atomic clock? (I think we use the vibrations of a Caesiam atom as a standard but would this necessarily be the best clock to use for a universal standard)

According to general relativity time is a local concept and there is no meaning to "universal time".

On the other hand, for tiime on earth there are agreed upon standards and a definition of "coordinated universal time".

http://en.wikipedia.org/wiki/Universal_Time

http://www.usno.navy.mil/USNO/astronomical-applications/astronomical-information-center/universal-time

2010-Jan-11, 04:21 PM
According to general relativity time is a local concept and there is no meaning to "universal time".

Cool, If I'm late for work one day, I'm going to use that exact quote on my boss. :)

-- Dennis

NorthernBoy
2010-Jan-11, 05:17 PM
Does the rate of decay of an atomic element remain constant if the sample is moving near the speed of light?

No, it does not. Time slows down for the sample, and so it decays more slowly than if it were stationary.

In fact this is one of the normal schoolbook examples of a test of relativity. You test the muon flux high in the atmosphere, where they are created as cosmic rays arrive from space, and look at it again at seal level. Given the rate of decay of muons, the expected number reaching ground level is tiny, but in practice many more are seen, showing that their rate of decay is much reduced.

From the muons point of view, the atmosphere is actually only a few metres thick, so although they see themselves as decaying at their normal rate, they think that the time it takes to get to the bottom is small enough that many make it.

Aristocrates
2010-Jan-11, 06:09 PM
No, it does not. Time slows down for the sample, and so it decays more slowly than if it were stationary.

In fact this is one of the normal schoolbook examples of a test of relativity. You test the muon flux high in the atmosphere, where they are created as cosmic rays arrive from space, and look at it again at seal level. Given the rate of decay of muons, the expected number reaching ground level is tiny, but in practice many more are seen, showing that their rate of decay is much reduced.

From the muons point of view, the atmosphere is actually only a few metres thick, so although they see themselves as decaying at their normal rate, they think that the time it takes to get to the bottom is small enough that many make it.

Don't forget that for an observer in the same inertial frame as the particle in question, the decay rate does remain constant.

Mungoid, the definitions of day and year, as far as measuring events scientifically, are not based on Earth's periods of revolution and orbit any more. Now they are multiples of the second. As earlier posters mentioned, the second is now a universal measurement of time.

DrRocket
2010-Jan-11, 07:07 PM
Cool, If I'm late for work one day, I'm going to use that exact quote on my boss. :)

-- Dennis

Try it. But polish up your resume first.

There is still local time, and he still has a clock in the office.

korjik
2010-Jan-11, 08:32 PM
Cool, If I'm late for work one day, I'm going to use that exact quote on my boss. :)

-- Dennis

If he is really clever, he would ask you why you think your local time is important in this case.

:)

uncommonsense
2010-Jan-11, 08:39 PM
And if you're really clever you will answer that just before you left your "local" home, you were thinking of how much you wanted to get done at work, which means the universe to you........

DrRocket
2010-Jan-11, 09:17 PM
And if you're really clever you will answer that just before you left your "local" home, you were thinking of how much you wanted to get done at work, which means the universe to you........

And if he is smart and not just clever he won't buy the **. Best get to work on time.

WayneFrancis
2010-Jan-12, 02:57 AM
Does the rate of decay of an atomic element remain constant if the sample is moving near the speed of light?

Depends on the observer.

The decay rate of an atomic element remains constant to that chunk of that element. To an external observer the the rate seems to change and is correlated to the amount of acceleration it has undergone, relative to the external observer, and what point the element is with a gravity well relative to the external observer.

Matthias
2010-Jan-12, 03:33 AM
Suppose we wanted to use the Planck second as the basis for a "universal second". Because the Planck second is so small, we would have to choose some kind of huge value as a multiplier to produce a length of time that better fits our frame of reference. What would be a large constant or formula to use which is not a derivative of an arbitrary human unit? For example, we could not use Avogadro's number or a formula which contains it because Avogadro's number is derived from the gram.

Jens
2010-Jan-12, 04:00 AM
The Problem in a Nutshell; Humanity's Constants Simply have NO Place in a Truly Universal Measurement System, at Least, Not Unless we use The Unitless Ones!

To be honest, even if you capitalize all the words, I think this is still a non-problem. I can't really see any need for a "universal time." If human colonists are living in another star system, they can use their own local time, and when communication is carried out, the people receiving the transmissions can just keep a radio dish pointed in the right direction. Since it will take tens or hundreds of years for light to go from one to another, there isn't much point coordinating transmissions. For all intents and purposes, they will be isolated.

forrest noble
2010-Jan-12, 05:30 AM
mungoid,

What would be the best way to measure time in some form of universal standard? We measure time according to our local standard which isn't all that accurate anyway.

As was pointed out, a "second" is the unit of time standard in physics. As Einstein explained, time is relative to the time frame (relative motion) being evaluated. We certainly could set up a universal time frame(s) such as Earth Mean Time, Solar Mean Time, being specific about where exactly on Earth or otherwise we are referring to where a time keeping system is in place.

The present system on Earth uses synchronized cesium clocks which define and keep tract of time by the second supposedly to an accuracy of 1 Second in a billion years. Pretty good I would say.

http://www.timezone.com/library/archives/archives0072

traceur
2010-Jan-12, 01:33 PM
what are the chances that Google calendar can't handle different local time measurements by the time we would otherwise need universal measurements? a human space faring civilization whose computing capacity is less then what we have today doesn't seem very probable to me unless we're talking about a Dune scenario (which... still doesn't seem very probable).

Veeger
2010-Jan-12, 02:58 PM
Since measuring time by atomic decay, is relative, does the relative change in mass have any bearing on the rate of decay?

Grey
2010-Jan-12, 07:00 PM
Suppose we wanted to use the Planck second as the basis for a "universal second". Because the Planck second is so small, we would have to choose some kind of huge value as a multiplier to produce a length of time that better fits our frame of reference. What would be a large constant or formula to use which is not a derivative of an arbitrary human unit? For example, we could not use Avogadro's number or a formula which contains it because Avogadro's number is derived from the gram.I can't think of any choice that would satisfy this and still be particularly useful. For example, if you want to avoid all humanocentricity, you can't even use 10 or 100, because those are only interesting numbers because we happen to have 10 fingers. I suppose you could do something like divide the Planck time by some power of the fine structure constant or one of the other dimensionless constants, but that seems astonishingly complex, and doesn't serve any useful purpose. Moreover, it now ties our units of measurement to values that we can't measure all that precisely. I'd say just accept that our units of time are arbitrary, and move on. :)

henriquefd
2010-Jan-12, 08:07 PM
What would be the best way to measure time in some form of universal standard?
We measure time according to our local standard which isn't all that accurate anyway.

1 day = 1 revolution of the Earth on its axis = (roughly) 24 hrs
1 year = 1 revolution of the Earth around the Sun = (roughly) 365 days

These (rough) estimates of time mean than the large distance measurement we use such as the light year (read 1 light Earth year) while probably close enough for practical use cannot be an exact measurement because it is based on our own inaccurate local standard.

Say in the future we have colonised a planet in another star system. Presumably the colonists would use (eventually) a time measurement that meant more to them locally (one year represents one revolution around their star etc.
There would be, I think, a need for some universal time measurement.

With that in mind, if we start a colony in another planet, the colony would have TWO callendars, probably. One from the Mother Planet, since it's important that they follow OUR time. And then they would have their own callendar, because it wouldnt work for them to divide a day in 24 hours if their planet takes 12 hours to rotate on its axis. So, to them, one day would take 12 hours and not 24.

There will always be a local reference of time. Even inside our own planet we have Time Zones. And some civilizations have their own callendar as well, like the Chinese.

So I dont really think you can have only ONE time measurement to be used anywhere, because it is not practical. But there could be a time of reference and in the case of the colonies, both the Mother Planet and the Colony Planet would have to follow each other's times.

Jens
2010-Jan-13, 05:18 AM
With that in mind, if we start a colony in another planet, the colony would have TWO calendars, probably. One from the Mother Planet, since it's important that they follow OUR time.

Why so? I don't see how it would be important. Perhaps so they could avoid sending a signal in the middle of the night and waking people up? I don't think it's a real concern. AFAIK, the Mars rovers or Voyager probes are totally oblivious to our sleep needs when they beam back messages.

There will always be a local reference of time. Even inside our own planet we have Time Zones. And some civilizations have their own calendar as well, like the Chinese.

I think that many or most civilizations had their own calendars. But they are not just made up. They are I think invariably tied to things like the rotation of the earth and the orbit of the moon and our orbit around the sun.

henriquefd
2010-Jan-13, 09:22 AM
Why so? I don't see how it would be important. Perhaps so they could avoid sending a signal in the middle of the night and waking people up? I don't think it's a real concern. AFAIK, the Mars rovers or Voyager probes are totally oblivious to our sleep needs when they beam back messages.

If you dont see the importance, then there is nothing I can do. Either you see it or you don't. Maybe you really don't see it, since your example has nothing to do with it.

If the other planet is a colony, that means there is interaction between the Colony and the Mother planet. Do you think we would just start a colony somewhere and forget about it? Do you think we wouldnt want to keep track of time? we are so obsessed with time, but in that which would be one of the most important endevours of our civilization when it actually happens, we would just dont care?

We even keep track of the rovers and the probes. Since they are machines, they dont have to CARE about our time, but they KNOW our time, since it is important to know when they found something. Any picture taken is dated according to OUR time. That is information they give to us, together with the picture. So, even those machines you mentioned were programed to know our time. And we are certainly monitoring them and we certainly know how long they have been up there. They are part of a mission. Missions have schedulles. Schedules have time.

I think that many or most civilizations had their own calendars. But they are not just made up. They are I think invariably tied to things like the rotation of the earth and the orbit of the moon and our orbit around the sun.

Who said that a callendar is just made up? The Chinese callendar wasnt made up for fun. Of course it is based on observation. But it is different than the western callendar. They follow both callendars. Callendars also have a cultural quality. BEcause they are also used to mark important holidays, which change locally, from city to city.

The new colony will have its own holidays. If you dont keep track of the new colony's time or callendar, you would miss a lot of information. Which is not good diplomatically. Since holidays are also a reflect of a local culture, denying that information is not trying to understand who you are dealing with. Yes, at first, they will be nothing but our brothers living in another planet. But give it a generation or so, and they will become much different, in the same way that we have our own differences here between our cultures.

Jens
2010-Jan-13, 09:28 AM
If the other planet is a colony, that means there is interaction between the Colony and the Mother planet. Do you think we would just start a colony somewhere and forget about it?

I think the disagreement may be here. The original post suggested the idea of a colony in another star system. I would say that if we started a colony in another star system, then no, there would be no interaction between the colony and the mother planet. Or no meaningful interaction. Light signals themselves would take tens or hundreds of years, and material exchange would be essentially impossible. So it would sort of be like two groups of people in a prison, with different rooms but no way to get from one to another, but with the ability to communicate by tapping on a pipe.

henriquefd
2010-Jan-13, 09:52 AM
I think the disagreement may be here. The original post suggested the idea of a colony in another star system. I would say that if we started a colony in another star system, then no, there would be no interaction between the colony and the mother planet. Or no meaningful interaction. Light signals themselves would take tens or hundreds of years, and material exchange would be essentially impossible. So it would sort of be like two groups of people in a prison, with different rooms but no way to get from one to another, but with the ability to communicate by tapping on a pipe.

How did we get there? We can't get back? We can't go there again? We can't communicate with them?

For the sake of argument and to give more strength to yours, lets suppose that when we start a colony in another star system, we do so adventurously. Meaning it is so expensive it will be a one way trip and they wont have any chance of coming back. And the next spaceship going there would do so in a long time and it would also be one day.

So far so good. Who cares if its day or night on the other side. Still, do you think we would track time of how long the colony exists? We would have families, friends and celebrities in the colony. Wouldnt we want to knwo how they are doing? IF the colony planet has a year of 412 days. Do you think we would ignore it? Or do you think we would track time of that? Noooobody would care here back home?

And the people who have arrived there and left families and friends behind. Do you think they will want to continue to keep track from the time in their original planet? Put yourself in that situtation. Would you keep track of the time from your Mother Planet? Or to the heck with them, we're starting fresh!

Maybe we just have different points of view, or different values. I see us tracking time and trying to observe as much as POSSIBLE. If what is possible is only what it can be measured through a distance, then so be it. But we would hardly ignore it. IMHO.

Jens
2010-Jan-13, 10:09 AM
So far so good. Who cares if its day or night on the other side. Still, do you think we would track time of how long the colony exists? We would have families, friends and celebrities in the colony. Wouldn't we want to know how they are doing? IF the colony planet has a year of 412 days. Do you think we would ignore it? Or do you think we would track time of that? Noooobody would care here back home?

And the people who have arrived there and left families and friends behind. Do you think they will want to continue to keep track from the time in their original planet? Put yourself in that situation. Would you keep track of the time from your Mother Planet? Or to the heck with them, we're starting fresh!

I agree with most of what you say. I agree that information would be transmitted. Why not? I suppose the the people on the colony would be interested in what is happening on earth, and vice versa. I was more arguing about the need for coordinating time. I am sure that the colonists would want to remember how long they had been gone in earth terms, but it would be more of a calculation that recorders somewhere could remember, and it wouldn't really matter what it was based on. Some kind of atomic clock would be fine for the purpose. But there would be little point in saying what day it was on the earth calendar. Likewise, the earthlings would certainly be interested in the length of the day, composition of the atmosphere, how people were doing, but knowing whether it was day or night at a specific time when radio transmissions take 20 years to arrive is just not an important consideration. That's all I'm saying.

dhd40
2010-Jan-13, 05:06 PM
I can't really see any need for a "universal time."

Maybe, there's no need for a universal time. But wouldn't it be fantastic? With c=infinite we could see any part of the universe "at the same time". We would know a lot more about our universe if we could have taken snapshots at 1, 100, 100000, 1 billion, ... years after the Big Bang, and, of course, NOW
And it would be easy to define a "universal time". No time dilation, no length contraction, ... alas ...

Jens
2010-Jan-14, 03:14 AM
With c=infinite we could see any part of the universe "at the same time". We would know a lot more about our universe if we could have taken snapshots at 1, 100, 100000, 1 billion, ... years after the Big Bang, and, of course, NOW

If C were infinite, then we couldn't take those snapshots. We would see everything now, and would not be able to see into the past at all. So it is actually thanks to the speed of light being limited that we can get snapshots from the past. So ironically, having it your way would make it less possible for us to understand the history of the universe.

dhd40
2010-Jan-14, 01:14 PM
If C were infinite, then we couldn't take those snapshots. We would see everything now, and would not be able to see into the past at all. So it is actually thanks to the speed of light being limited that we can get snapshots from the past. So ironically, having it your way would make it less possible for us to understand the history of the universe.

Absolutely correct. That's why I said "if we could have taken snapshots at 1, 100, 100000, 1 billion, ... years after the Big Bang"

eburacum45
2010-Jan-15, 03:26 AM
The problem is, two colonies separated by several light years can never synchronise their clocks accurately with each other. You could calculate the time elapsed back on Earth when you arrive at Alpha Centauri, since you know your speed and the duration of your flight; but as Alpha Centauri is continually moving at many kilometers per second with respect to the Earth, that time will soon become inaccurate. The exact proper motion of Alpha Centauri depends on the gravitational pull of all the other stars in the Galaxy, and that results in a very slight randomisation of the relative positions of the two stars.

Since you can't know exactly where you are with respect to the Earth at any particular point in time, you can't know exactly when you are either.

Jens
2010-Jan-15, 03:35 AM
Absolutely correct. That's why I said "if we could have taken snapshots at 1, 100, 100000, 1 billion, ... years after the Big Bang"

I don't understand what you mean. We weren't around 1 year after the big bang. How could we take a picture of it?

a1call
2010-Jan-15, 04:33 AM
So, leaving aside time dilation effects at high speeds and other anomolies, what could be used as an accurate universal standard measurement of time? Maybe some form of atomic clock?

Pulsars are the most regular signals known in the universe.

Their pulses are so consistent that they can be used to tell time with an accuracy much greater than atomic clocks, which are currently the most accurate timekeeping devices on Earth. The fastest know pulsar, known as PSR1937+21, has a pulse period of over 1.56 milliseconds, or 640 times per second.Source (http://www.seasky.org/celestial-objects/pulsars.html)

Wished I knew the methodology behind this determination, but haven't got a clue.

ETA: I assume two atomic clocks will/may fall out of relative-synch to a higher degree in a given period than two pulsars.

dhd40
2010-Jan-15, 04:08 PM
I don't understand what you mean. We weren't around 1 year after the big bang. How could we take a picture of it?

Sorry for the confusion. It was just a hypothetical idea: 'If we had been around at these times', or 'If we will still be around in 1 billion, ... years'