# How to measure time outside of Earth? [duplicate]

In pop culture, whenever an alien species or someone outside of Earth talks about time, they always refer to time using the words "year(s)", "month(s)", "hour(s)", and "minute(s)" etc which are measures of time with respect to Earth. It always used to bug me on how this detail got missed by the creators of those movies and TV shows.

So, is there a way to measure time outside of Earth which will be measured the same in every part of the universe?

• The speed of light in a vacuum is constant and there are some isotopes that release radiation at very specific intervals which can then be used to convert it into distance, similar to our SI units. "The meter is the length of the path travelled by light in vacuum during a time interval of 1/299 792 458 of a second" and "One second is the time that elapses during 9,192,631,770 (9.192631770 x 10 9 ) cycles of the radiation produced by the transition between two levels of the cesium 133 atom" So as long as an alien has something similar, you could measure it in the same way. – Shadowzee May 21 '18 at 6:23
• @Shadowzee you should make that an answer – Erik May 21 '18 at 6:42
• I always assumed that the "universal translator" was advanced enough to give rough estimates of unit conversion – Chromane May 21 '18 at 6:45
• Shadowzee, understanding the decay rate is dependant on them using Base 10 math... if they use Base 5 or 17 or something else then the decay rate could be off by vast amounts – Blade Wraith May 21 '18 at 11:04
• Sorry, but as a fellow world builder, I have to defend the pros: This detail wasn't missed by creators, it's often used so that the audience can relate. Also check out xkcd.com/483 – Raditz_35 May 21 '18 at 11:53

Time is simple, once you've managed to explain distance, because once you have that then distance traveled by light in a vacuum in whatever time frame you or they use. so realistically its all about distance...

Atmospheric Pressure

Pressure would likely be a major factor if not the "translator". its highly likely that an alien race advanced enough to have space travel, would likely have a system comparable to the metric system, where almost all units of measurement are connected based around water. Grams, Metres, Celsius and Litres are all relative to amounts of water. Celsius takes 100 increments between the freezing point and boiling point of water at 1 Atmosphere of pressure 1 Cubic Metre of water at 1 Atmosphere of pressure weighs 1 Metric Tonne. etc

The reason they would do this is because having a system such as this makes building space craft, plotting orbital maneuvers and various other space related activities easier. simply put, everything comes down to water:

life (as far as we know it) cannot exist without water

Fuel, in space split water into hydrogen and oxygen and you have fuel for rocket engines.

So if the alien race met humans, and managed to translate enough of each others languages to talk but needed to discuss measurements, then if they could translate the difference in atmospheric pressure then this would acutally become slightly easier...

Maybe they could simply show a graph as too how each of their units are interconnected through water, and then place a weight onto a scale on earth, then so still complex but easier math could translate this... "ah i see your 1 QZ weight weights 946 grams that means that the temperature range of liquid water would be lower therefore etc etc...

And before anyone points out the old adage

There are two types of countries in the world, those with the metric system and those who have landed on the moon

Perhaps need to rephrase it too:

There are two types of countries in the world, those with the metric system and those who don't... but had a space agency that did use the metric system in order to land on the moon

I admit, its not as catchy, but still very very true

The next issues to consider is if the alien race has a Base 10 math system, where the number increments by multiples of 10, humans use this because we have 10 digits on our hands, if for whatever plot reasons its Base 8 or Base 17 then matter becomes even more complex...

They final issue actually comes from how are they going to talk in the first place, many scientists believe it will begin simply by starting with an atom of hydrgen, and stating its name and atmoic number, then working up from there, if they can understand chemistry together then they can start getting even more complex language after that

• I just want to point out that time (and often units in general) is one of the cases where we don't have a system that's based on 10 but on 60 because the ancient Sumerians apparently had 60 fingers? Also there are the units the Americans use that are completely insane and not based on any amount of fingers, like 3 stones are one rock (sorry, I don't know any of their units, but it's something like that, here I googled one: A mile is 1,760 yards) – Raditz_35 May 21 '18 at 12:16
• our system of time was constructed that way a long long time ago, but it still relies on base 10 Math, Base 10 refers to the numbering system in common use that uses decimal numbers. In base 10, each digit in a position of a number can have an integer value ranging from 0 to 9 (10 possibilities) the imperial system used by Americas was the point i was making, any race capable of space flight would most likely have a system of measurement for distance, similar to the Metric system, once you can quantify distance, then time is measured by 299792m by light in vacuum takes 1 second – Blade Wraith May 21 '18 at 12:28
• Where I'm from, you learn how to convert between systems in the 5th grade. It's a problem that's been solved since forever and there are many applications for systems that are not base 10, units are one example, I wouldn't even mention it as an issue. – Raditz_35 May 21 '18 at 12:42
• @Raditz_35 Its not an insurmountable issue, but humans would need an ability to understand what base math is used by the alien race. Without knowing this information, any information from aliens could work out horrendously wrong, therefore it is a valid issue. I never said converting 1 system to another was really difficult, just that it was an issue that would need accounting for – Blade Wraith May 21 '18 at 13:12

You are asking the same question that scientists have been asking themselves for a while: how can we define fundamental units in a way that is not dependent on us, but bound to fundamental quantities?

Take the unity of length, the meter: originally it was defined as a fraction of the length of the meridian, but then, as such definition is not based on constants, it was changed to the current definition

The meter is defined as the length of the path traveled by light in a vacuum in $1/299 792 458$ second.

Same holds for the measure of time, the second, which most recent definition is

the second has been defined as exactly 9,192,631,770 times the period of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium-133 atom.

For the multiple of the seconds it is convenience to define them in terms of stellar parameters like the year (time to complete an orbit around the main star), but other fictional works refer also to SI-like units, like the kilosecond or megasecond, which is probably less arbitrary.

The SI units as defined today can easily be understood by alien civilizations under the strong assumption that physics laws are the same everywhere in the Universe and provided that they have the proper translation of the concepts, and they do not depend on the human body or surrounding environment (i.e. no foot or palm).

Pulsars give off electromagnetical bursts in precise periods. The periods range from very short (milliseconds) to quite reasonable (seconds). Aliens might have agreed to use a specific pulsar to establish a universal time constant that does not reflect the movement of any specific home world.

• Pulsars are not very handy as standards.. and also, their precision is way worse than atomic transitions – L.Dutch - Reinstate Monica May 21 '18 at 7:50