I thought about this the other day. We often send messages to outer space hoping that someone picks them up.

How could you explain to aliens that we sent this satellite about 13.8 billion years after the big bang? Aliens don't know our concept of years.

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    $\begingroup$ Note: there is a special stack called Worldbuilding (worldbuilding.stackexchange.com) where we do nothing but discuss hypothetical situations like this. This question may already have an answer there, and if not, you can always ask one, provided you can formulate it as a worldbuilding-related question (this question might be too related to an individual's actions in the current style). $\endgroup$ – Nzall Apr 19 '17 at 14:21
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    $\begingroup$ How is this a Worldbuilding question? $\endgroup$ – Russell Borogove Apr 19 '17 at 14:41
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    $\begingroup$ @RussellBorogove I think the idea is that on Worldbuilding, we have plenty of questions about communication with aliens, so we presumably are better equipped to answer this kind of question. Though it looks like that's not really an issue here. $\endgroup$ – DaaaahWhoosh Apr 19 '17 at 21:04
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    $\begingroup$ Are you sure the timing (or even existence) of the Big Bang is defined well enough to serve as an epoch? $\endgroup$ – chepner Apr 19 '17 at 21:06
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    $\begingroup$ The moral of the story is: forget about communicating with aliens. We cannot even reliably communicate with humans. $\endgroup$ – Jörg W Mittag Apr 19 '17 at 23:27

The approach taken by Voyager Golden Record is through use of pulsars. Unlike the insane exponents needed to utilize microscopic physical phenomena or planck time, pulsars operate on ~1 second timescale, which makes them quite convenient. A map specifiying location of Earth in relation to a number of pulsars identified through ratios between their frequencies provides a unique location. Picking frequency of any selected pulsar on that map provides a very decent time base too.

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    $\begingroup$ I think this is a very elegant approach as the pulsars tell you the position in not only space, but time as well at the same time. I really like this answer and I think it's the best one so far, but I think you should elaborate a bit on how to reconstruct the date from the pulsar's frequencies. $\endgroup$ – JohnEye Apr 19 '17 at 17:19
  • $\begingroup$ @JohnEye: I would if I knew :-] I can imagine several ways but I don't know which would be preferable or optimal. $\endgroup$ – SF. Apr 19 '17 at 17:53
  • $\begingroup$ Then just mention the possible ways, for example the steady decline in frequency might be reasonably precise. $\endgroup$ – JohnEye Apr 19 '17 at 18:20
  • $\begingroup$ Looks like someone else had the same idea: en.wikipedia.org/wiki/Pulsar_clock $\endgroup$ – JohnEye Apr 19 '17 at 18:27
  • $\begingroup$ But how is that pulsar map to be formulated or drawn? Since we only have a subset of astronomy and math knowledge, and they maybe focus on active dark matter or something and don't care much about pulsars, we really need to find some common starting point. Scratches in a golden disk might be completely overlooked by them. $\endgroup$ – LocalFluff Apr 21 '17 at 9:16

Our basic unit of time, the second, is officially defined in terms of physical phenomena, so our messages have to get that across.

If the aliens and ourselves have a compatible understanding of mathematics, chemistry and physics, which we hope are universal, it's possible to construct a message which builds up the necessary vocabulary from first principles.

As an example, the Cosmic Call message designed by Dutil and Dumas does in fact include an estimate for the age of the universe near the end. Here's a good blog series describing the messages in detail.


One value for time is common for all species, the planck time. It depends only on mathemathical/physical concepts.

But you will have to explain to them the powers of 10 as well because else you would have to draw something like 8*10^60 ticks on the calendar.

(Age of the universe is 4.32 × 10^17s and planck time is 5.4 × 10^-44s)

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    $\begingroup$ Powers of ten are only particularly meaningful because we use a decimal (base-10) system of counting. It's easy to imagine that an alien species with, say, six digits per appendage might find base-12 more natural, assuming two "hands" (which isn't a given at all!) $\endgroup$ – user Apr 19 '17 at 13:33
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    $\begingroup$ Yeah, but explaining them to another species seems trivial... But just maybe because I didn't try. $\endgroup$ – Nygael Apr 19 '17 at 14:52
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    $\begingroup$ Explaining something to another species with no common language at all is a lot more difficult than trivial. $\endgroup$ – Uwe Apr 19 '17 at 15:28
  • $\begingroup$ @Uwe Assuming the aliens have a similar grasp of mathematics, you could send an increasing sequence of numbers 0,1,2,3,4,5,6,7,8,9,10,11,12... to demonstrate what base is used. If the aliens don't have a similar grasp of mathematics, the chances for any meaningful communication pretty much vanish. Of course, I don't see why we would use base-10 when we could just use binary. You can see the pattern in binary a lot quicker: 0, 1, 10, 11, 100... $\endgroup$ – called2voyage Apr 19 '17 at 15:40
  • $\begingroup$ It's pretty trivial if they have enough math to build radio receivers. If you look at the message described in my answer, you'll see that the first page shows a mapping of unary, binary, and decimal representations of numbers; the decimals are used thereafter, telling the aliens that that's our preferred base. Page three introduces the vocabulary for exponentiation and scientific notation. Dutil & Dumas used base 10 throughout their message for two reasons: it's more compact than binary and it's easier to proofread by the message composers. $\endgroup$ – Russell Borogove Apr 19 '17 at 17:40

One option nobody else mentioned is cosmic microwave background. If you can encode the current temperature of the universe in a way the receiving aliens understand, they will either have a record of its past development or at least a pretty good model.

CMB is the same everywhere in the universe and since the fluctuations are very small and it's relatively simple to measure with high precision, this method may be surprisingly precise.


Well, the speed of light is never going to change. That being the case, one method could be to use light years as a means to calculate time. 'x light years from now' could be the way to specify a relative date.

You could also include how much time a second is (via a gap in the audio) and specify that 60 seconds make a minute and 60 minutes make an hour and so on. Using that you can explain our concept of the passing of time and use the same to explain what you need….

if they can pick up the signal, they should be able to find where it came from. so you can use the speed of light and the distance between us and them to give them an idea of how long ago the message was sent to them.

EDIT: I seem to have made a mistake: light year can only be used to measure distance and not time. so my first option can be discarded.

once i went through the question, i realised one fact: our concept of the big bang and when it happened might not necessary be what they believe in. so using the big bang as the reference might not be the way to go.

We could measure the distance travelled by the satellite and use the speed of light as reference to convey the time taken by the satellite to get to the aliens with reference to the launch of the satellite.

that is to say, as long as the satellite had good enough sensors, the satellite could find the distance between local astro bodies and specify that the (satellite) was launched x units of the time taken to go from one astro body to the other.

  • $\begingroup$ That would require that "they" know Earth's orbital period, the length of our year. Which actually could be assumed for communication with stars in the plane of the ecliptic. They would see Earth transiting the Sun regularly. This period could indeed become a common unit for communication, because it is pure nature and doesn't rest on any of our hidden cultural assumptions. I +1 you for the one Earth year (light year) idea, although you could phrase it more pointedly. Their transit period maybe should be used by us as basis for SETI-transmissions to other stars. $\endgroup$ – LocalFluff Apr 21 '17 at 10:53

Time-keeping is a particularly interesting subject for me. There are multiple ways you can communicate a date, and some sort of system of keeping time to reference that to. Some constants in nature can be used to keep time outside our solar system where the relative motion of our planet to our sun become irrelevant. The challenge here is to help this life form understand our definition of time - the second using the various constants in nature.

Quantum oscillations:

Defining a second (or some unit of time) to keep time using oscillations of atoms between a ground and excited state when bombarded with microwave radiation. Since atoms of Cesium or Rubidium are found throughout the universe. I should see no reason for this not to be a viable option.


Radioactive decay can be used to define a unit of time. Decay rates are constant for various radioactive atoms. As thegreatemu pointed out, you could alternatively send radioactive isotopes along with the Voyager (or some spacecraft) and point out the constants like rates of decay and the ratio of decayed to undecayed atoms in that sample at the time of launch.

Speed of light:

This should be pretty obvious for anyone with high-school level physics. We simply define our unit of time (second) using the distance light travels during its interval and expect the life form to understand it.

Cosmic Microwave Background (CMB):

CMB is universal and redshifts at some rate allowing us to communicate a universal definition of time with any extraterrestrial. We could note the z value at the time of launch and expect the life form to read the current z and work out the math to figure out how much time had passed.

Once the life form is smart enough to keep time and know how much time has passed (since some cosmological event such as the Big Bang), it should be possible for us to convey some point in time to the life form and expect it to reference it to the same (or equivalent) time scale.


Use radioactive isotopes

If sending a physical probe that you expect to be found like Voyager, you could create 2 disks, one with say 1 mCi of potassium-40, and one with 0.5 mCi. This is 1.251 billion years independent of any reference frame. Then as long as you can communicate multiplication, you can represent any amount of time. Something like tritium might be more recognizable as it is more abundant universally, and can more easily represent shorter time periods.

If broadcasting, you would need to find a way to transmit which isotope you are using as your base unit of time. The tritium absorption spectrum is shifted enough from deuterium and hydrogen that it might be possible.

  • $\begingroup$ What do you mean by the second paragraph? I mean, you have no idea when and where the signal is going to be intercepted. $\endgroup$ – JohnEye Apr 19 '17 at 18:26
  • $\begingroup$ Well, the original Voyager used the ratio of pulsar frequencies. You could for example use the ratio of amplitudes and frequencies of the first N lines in the tritium absorption spectrum. I won't pretend to know how unique that sequence is, or how to communicate that you are talking about spectral lines. $\endgroup$ – thegreatemu Apr 19 '17 at 18:58
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    $\begingroup$ Well, yeah, but then you require a physical token to measure, and in such case you can simply use radiocarbon dating. If you just transmit a signal, you need to encode something into it which the receiving party can take and compare to something they can observe. Or I did not understand the second paragraph correctly. $\endgroup$ – JohnEye Apr 19 '17 at 19:34

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