Hydrogen · Science
The Pioneer plaque and hydrogen
The plaques on Pioneer 10 and 11 and the Voyager golden record use hydrogen as a yardstick: 21 cm in length and 0.7 billionths of a second.
How would you explain the size of a person to someone who doesn't know the meter, the hand span, the foot, or any of the measures we invented here?
In 1972, a small group of scientists had to answer that question for real. And the answer was hydrogen.
A message in a bottle
The Pioneer 10 probe was launched on March 2, 1972, and Pioneer 11 on April 5, 1973. Both were sent to study Jupiter and Saturn and then travel on out of the Solar System, never to return.
Carl Sagan and Frank Drake realized those probes would be among the first human-made objects headed out of the Solar System. They proposed attaching to each probe a gold-anodized aluminum plaque, about 23 by 15 centimeters, carrying a message. The artwork was drawn by Linda Salzman Sagan.
The plaque shows a man and a woman, the silhouette of the probe, the Sun and the planets, and a map of Earth's position relative to fourteen pulsars. But, before all that, in the upper left corner, there's a drawing that looks like a dumbbell: two small circles joined by a line.
They are two hydrogen atoms. In one, the spins of the electron and the proton point the same way. In the other, they point opposite ways. The line between them is the hyperfine transition, the flip that emits the 21 cm wave.
Hydrogen as a yardstick
The choice was ingenious. Any civilization capable of finding the probe would know hydrogen, because it's the most abundant element in the universe. And any that did radio astronomy would know its line.
So the transition became a unit of measure. For length, the wave's 21 cm. For time, its period, about 0.7 billionths of a second.
Next to the figure of the woman, between two marks indicating her height, the number 8 appears written in binary. Eight times 21 cm: 1.68 meters. The height of a human being, measured with the hydrogen yardstick.
The golden record
In 1977, the Voyager 1 and 2 probes set out carrying a gold-plated copper record, with sounds and images of Earth. On the cover that protects it, there are instructions for playing it.
There, once again, is the drawing of hydrogen in its two lowest energy states, indicating that the time interval of this transition is the fundamental unit. The time of one rotation of the record, 3.6 seconds, is written in units of 0.70 billionths of a second, the period associated with the hydrogen transition. The cover also carries the same pulsar map as the Pioneers.
The Voyagers keep traveling, already beyond the region dominated by the Sun's wind. The yardstick goes with them.
Sources: Pioneer plaque · The Planetary Society, the Pioneer plaque · Voyager Golden Record · Hydrogen line
In Bairo's view
This is where the expression Bairo uses was born: the 21 cm yardstick. When humanity wanted to introduce itself to the universe, it measured its own body with hydrogen. Eight 21 cm yardsticks: one whole woman.
For him, that yardstick measures more than height. Where physics sees a proton, an electron and the flip between them, Bairo sees body, soul and spirit. It's his spiritual reading, never a scientific claim, and it is told in full in Body, soul and spirit in the hydrogen yardstick.
There's also the image of a letter sent without knowing whether anyone will read it. The Pioneers and the Voyagers were sent with a scientist's faith: with no assured reply, with the care of someone writing to a reader who may never exist. Bairo recognizes in that gesture the logic that guides his project, told in Manifesto 14:20: write it first in the stars, so as to have a guide to navigate by.
For you
If someone had to measure your life, what yardstick would they use?
Society tends to offer short yardsticks: salary, job title, followers, other people's opinions. The Pioneer plaque suggests something else. To speak of the human being to the universe, the scientists chose the most universal measure they knew, the one that holds anywhere and for anyone.
Perhaps it's worth looking for a yardstick like that for your life: one that doesn't change depending on who's looking. And perhaps it's worth writing, as they did, a message to a future you don't yet know whether you'll see.