The plaques on the Pioneer and Voyager spacecraft, designed by Carl Sagan, are intended to describe Earth and the human race to any alien race that encounters it. It has some familiar features, like a naked couple and a map of our solar system, but also a weird explosion-like diagram which I never understood, until now. This Wikipedia article expounds the details and makes you think - just how do you communicate with aliens, and convey basic things like distance and time.

The plaque uses the frequency of the hyperfine transition of hydrogen for the unit of time (explained by a diagram), and binary notation to express numbers, which should be interpretable by aliens. The explosion diagram is the distance of the Sun from 14 known pulsars, annotated with their current frequencies, which change over time, so hopefully they can work out when the probe was launched as well as where from.

It’s exceedingly unlikely that the plaques will ever be found, and even with such a universal encoding it will still be very hard to decipher (and some of the symbology, like the arrowhead, is definitely anthroprocentric), but it’s still a very humbling experience reading about it. Great care has been taken to make sure there as few barriers as possible that will prevent understanding it, but it is still mind-boggling to try and think about what logical and visual assumptions, which we take for granted, that any alien race might not understand. We may not be physically alone in the universe, but our entire system of thought may be so different that we are metaphysically stranded forever.

As a footnote, the copyright notice for the Voyager Gold Record is quite amusing.