Frequency
1420 MHz. The hydrogen line.
The hydrogen line is the radio emission of neutral hydrogen atoms at 1420.405752 MHz, a wavelength of about 21 centimeters. Hydrogen is the most common element in the universe, so any species that studies the sky by radio will meet this frequency. That makes it a natural place to listen for, and to greet, other minds.
What the hydrogen line is.
A hydrogen atom is one proton and one electron. Both spin. When their spins flip from pointing the same way to pointing opposite ways, the atom releases a tiny amount of energy as a radio wave. For a single atom this is extremely rare. But there is so much hydrogen between the stars that the sky glows at this frequency everywhere we look.
The measured frequency is 1420.405751768 MHz and the wavelength 21.106 cm, which is why astronomers call it the 21 cm line. Source: Hellwig et al., 1970.
Predicted, then heard.
The Dutch astronomer Hendrik van de Hulst predicted the line in a paper published in 1945. Source: AAS. On 25 March 1951 Harold Ewen and Edward Purcell detected it at Harvard, with a horn antenna mounted outside a window of the Lyman Laboratory. Source: NRAO, Nature, 1951.
Since then the hydrogen line has become one of radio astronomy's basic tools: it reveals where the cold gas of our galaxy lies and how it moves, which is how astronomers trace the Milky Way's spiral arms and rotation.
Why SETI listens here.
In 1960 Frank Drake's Project Ozma, the first modern search for extraterrestrial intelligence, listened near 1420 MHz. Source: SETI Institute.
The engineer Bernard Oliver later named the quiet band between the hydrogen line and the lines of hydroxyl, near 1660 MHz, the water hole: hydrogen and hydroxyl together make water, and water holes are where species have always gathered. Source: SETI League.
The hydrogen line also serves as a shared ruler. The plaques on Pioneer 10 and 11 use the hydrogen transition as their unit of length and time, so that a reader anywhere could decode the scale, as the timeline of messages to space shows.
Why the letter shows it.
The letter Pax Ego Sum shows a noisy radio spectrum between 1400 and 1440 MHz with one sharp peak at the hydrogen line, labeled I am here. It is a picture, not a transmission. But it says what the letter says: if you know hydrogen, you know where to find us.
Some amateur radio astronomers build small antennas to see the hydrogen line themselves. If you build one, you will see the signature of the most common element in the universe.
Questions you may have.
What is the hydrogen line?
It is the radio emission of neutral hydrogen at 1420.405752 MHz, wavelength about 21 cm, produced when the spins of the electron and the proton in a hydrogen atom flip.
Why is 1420 MHz important for SETI?
Hydrogen is the most common element in the universe, so any radio-astronomy civilization would know this frequency. It is a natural meeting point, and searches since Project Ozma in 1960 have listened near it.
Does this website transmit at 1420 MHz?
No. The spectrum in the letter is an image. The letter is published on the web, where people and machines read it.