# How to describe a well defined “zero moment” in time

Suppose you have to specify the moment in time when a given event occurred, a "zero time". The record must be accurate to the minute, and be obtainable even after thousands of years. All the measures of time we currently have are relative to a well defined zero, but the zero is not easy to backtrack exactly.

One possibility would be to take a sample of Carbon with a well defined, very accurate amount of 14C, and say: the event occurred when the 14C was x%. At any time, measuring the rate of decay, you would know when the event occurred. This however, requires a physical entity to measure, which may be lost.

Another way would be to give the time lapsed after a well defined series of solar eclipses. In order to define precisely the context, you would say a list of (say) five consecutive eclipses and the places on Earth where they were total, and then a time gap from the last of the set. At any time in the future, you can backtrack the specified conditions into a celestial mechanics program and find when the event occurred.

Is there a standardized or well recognized method to do so?

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Perhaps some stellar observations (like position and redshift of all the stars)? I am not sure how precise they are though. – Marek Jan 11 '11 at 18:48
I'm not sure I understand the question. Are you asking about ways to specify a particular moment in time that will still be unambiguous thousands of years in the future? – David Z Jan 11 '11 at 18:48
@David : exactly. – Stefano Borini Jan 11 '11 at 18:56
In that case, I think this should be a community wiki question, because you're asking to assemble a list, and there isn't going to be one canonical right answer. (Anyone want to argue otherwise before I change it?) – David Z Jan 11 '11 at 19:59
@Stefano: it doesn't matter how many there are; the point is that no one of them would be the canonical correct answer. Although I suppose that's not necessarily the case anymore after your edits. – David Z Jan 11 '11 at 20:55

I think the standard method is UTC.

Perhaps you are asking how to translate UTC into a form that people thousands of years from now will understand. Here is a method off the top of my head: If you specify the relative positions of all of the planets and the moon, you should get uniquely defined precision to much less than a day over many millenia. Then you can use local solar time with respect to some specified locations (e.g., summits of recognizable mountains) to get within a fraction of a second.

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Certainly not to fractions of a second. Leap seconds have to be added or subtracted every few years to keep the sidereal time in agreement with the actual behavior of the planet. I don't know how long you use the solar system a clock for when you want "How long until Leno starts?" timing. Certainly a long time if you want "What year is it?" timing. – dmckee Jan 11 '11 at 23:58
Yes, but UTC does not define a unique, unambiguous and exactly traceable "zero" in its scale. – Stefano Borini Jan 12 '11 at 9:29

You might be interested in this resource: The Clock of the Long Now.

The Clock of the Long Now, also called the 10,000-year clock, is a proposed mechanical clock designed to keep time for 10,000 years.

In building it they discuss relevant topics such as:

1. Longevity: The clock should be accurate even after 10,000 years, and must not contain valuable parts (such as jewels, expensive metals, or special alloys) that might be looted.
2. Maintainability: Future generations should be able to keep the clock working, if necessary, with nothing more advanced than Bronze Age tools and materials.
3. Transparency: The clock should be understandable without stopping or disassembling it; no functionality should be opaque.
4. Evolvability: It should be possible to improve the clock over time.
5. Scalability: To ensure that the final large clock will work properly, smaller prototypes must be built and tested.

The solution they are building is far from trivial.

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I do not completely understand what you want, so I'll try to address three possibilities.

1. If you need the standard notation for a time measurement with (reasonably) large scales, then you are looking for Julian Day (JD). It is standard for scientific/astronomical communities.

2. If you are looking for "one and only" experimental procedure for precise measurement of large time intervals, then I don't think that there is one. There are a lot of methods with own good and band sides.

3. Finally. You are emphasising the question of "zero time". And we know that the laws of physics are invariant under time translation -- there is no "selected" moment in time. Therefore any moment can be chosen as the reference point -- it's just a matter of convention. For the JD the reference point is the astronomical noon in Alexandria on the 1st of January 4713 year BC. This choice is mainly due to historical reasons.

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the special theory of relativity implies that there is no unique "the timing system" in the Universe. Each inertial system, depending on its velocity, has a different answer to the question whether two events occurred simultaneously, and if they didn't, what was the delay in between them.

So to be able to quantify the delay between two events, you have to choose a reference frame. Most typically, you may associate your preferred reference frame with a particular pointlike object - such as the small golden seed that sits at the center of the Earth, or the tip of the Big Ben, or anything else. There is clearly no God-given preferred choice.

Once you have such an object, you may measure the proper time along its world line in spacetime by some accurate clocks, e.g. by atomic clocks. You may also associate time with events away from the world line of your benchmark objects - e.g. as the time when the photons that got to the event were emitted from your benchmark object. Such a definition of time is nicely well-defined but is constant along "light cones" and for practical purposes, one may make subtractions.

To summarize, there are many clocks and methods how to measure and coordinates how to describe events in spacetime but there is no privileged one. Quite on the contrary, relativity implies that the right labeling of "time of an event" depends not only on the preferred "time zero" - which is ambiguous due to the time-translational symmetry - but also because of the velocity that changes the notion of simultaneity and expands the duration of processes by the time dilation.

Cheers, LM

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## protected by Qmechanic♦Feb 12 '13 at 8:50

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