# What is the unit of time on the Voyager Golden Record?

What is the unit of time on the Voyager Golden Record?

Apparently, it has something to do with "a fundamental transition of the hydrogen atom".

A high-level description was provided by NASA, as shown in this answer.

How will the aliens know what this means?

In a neutral hydrogen atom the ground state has the electron and proton spins anti-parallel i.e. lined up with each other but pointing in opposite directions. The state with the spins parallel and pointing in the same direction has a slightly higher energy, and transitions between these two states produce the notorious 21cm hydrogen line.

Since the hydrogen 21cm emission is detectable everywhere the idea is that any aliens will know what it is and will be able to guess that's what the diagram means. The frequency of this oscillation is 1420.40575177 MHz, so the period is 0.704ns and that is the time unit you're referring to.

• I like the idea of alien scientists "guessing" what our message means -- just like in the movie Contact. I wonder, though, why we didn't provide a more straightforward method of checking the guess -- like making the radius of the disc approximately 21.1 cm, or just drawing a ruler. – nobar Jun 10 '14 at 15:51
• I wonder what advantages there are to using that, versus including some artifacts that would produce a frequency, and starting the record with a recording of that frequency. Many frequency-producing artifacts would be fragile or would decay with time, but I would think something like a tuning fork could be pretty stable. – supercat Jun 10 '14 at 16:23
• @nobar centimeters, while standard on Earth, are not standard in the universe. Making the disc 21.1 cm would mean nothing because the aliens will likely be using different units of measurement. Likewise, our rulers may look nothing like theirs, so drawing a ruler probably wouldn't help. Admittedly, though, what he have done isn't much better. Interstellar communications will be a huge problem once we find another civilization "out there". – Dave Coffman Jun 10 '14 at 19:13
• When you say "frequency of this oscillation", are you referring to the energy of an occasionally emitted photon or are you suggesting that every hydrogen atom is constantly transitioning between states at precisely the indicated rate -- as if it were a ticking clock? – nobar Jun 11 '14 at 23:53
• @nobar: it's the energy of an occasionally emitted photon. – John Rennie Jun 12 '14 at 5:56

Because physics is the same for both us and for anything which might be out there.

If we use basic things like the transitions in hydrogen atoms, prime numbers etc. then no matter who or where you are you can interpreted these basic things.

And while hydrogen might seem complicated to us, it is one of the most abundant elements in the universe and its properties will be well known by anyone able to examine a spacecraft.