Tuesday, January 1, 2008

Calendars and science

This is the post I promised to write about calendars.
We are all familiar with the Gregorian calendar. It is the most used calendar today. However it is not the only one. In Israel for example we have another calendar. And while it is not used as much as the Gregorian (even in Israel), in many places we have dates written according to both Gregorian and Hebrew calendars. The reason why it is still used is mainly religious. The Gregorian calendar is usually regarded as Christian calendar - after all it begins from Jesus birth. Therefore, it is only natural for the orthodox Jews to be against its use.

The Gregorian calendar is an excellent one, in my opinion. However it and all the other calendars suffer from one major problem - they are not universal. I don't mean by this that they cannot by used in other parts of the universe. This is also true but it is only another aspect of the some fundamental problem.

Lets start from the beginning, taking the Gregorian calendar as an example.
First of all, we have the following - a year is 365 or 366 days, divided into twelve months of varying length. The month is usually considered to have 4 weeks and a week is 7 days. You must have noticed that even this description is not perfect - the year can have different number of days and so are months. We have of course rules according to which all these numbers are set in each year. But still the definition is not perfect - while it is not too difficult to make all of the necessary calculations the fact that the devision is not obvious shows that there is something that causes this.

Why we have 365 days in the year? It is the period in which earth orbits the sun. Thus for every planet we have a different duration of the year. Even the duration of one day is different from planet to planet. Therefore, our calendar would be entirely useless on other planets. If lets say we will colonize Mars one day the Gregorian calendar will be of little use there. But if we will make a different calendar for Mars, we will have problems communicating since we will be living according to different calendars. And this is even more important for computers that for humans. So what can be done?

Lets go deeper. What is time? And how can it be measured? It is very difficult to answer this question. And since according to relativity theory, time, gravitation, space and speed are all connected, it is almost impossible to give a really good definition..
In the Gregorian calendar the shortest period of time is one day. This is its basic time unit. But a day is not a good choice for a measure unit because it length changes. Even on earth the length of the day is different at different seasons. In fact all of the problems with the Gregorian calendar and with other calendars come from this simple fact - a day is not a natural measure unit for time.

But then what is? Well one option is the so called Planck time. It is the time needed for light to cross a distance equal to the Planck length. Since the speed of light is invariant the time will also be always equal. The problem with it - it is too small. It is only about 10^-44 sec.
The only other option is to use a system of calendars. This is one calendar for every scale. In this way there can be one calendar for earth and one for mars - and all the communications between the two planets can be done according to the main calendar of the solar system.
While this brings more complexity, this is probably the best choice if we want a calendar that is naturally connected at least to the planet it is used on..

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