The discussion of the Voyagers made me think about the plaques on the Pioneer probes, which have a schematic of the “hyperfine transition of neutral hydrogen”, which then is used as a unit of measurement for the rest of the plaques.
What is the “hyperfine transition of neutral hydrogen”, and why was it mentioned on the plaque? And, is that schematic really going to be intelligible to other races, if ever found?
The hyperfine transitions in atoms and molecules are intranuclear or intramolecular transitions that are based on the difference in spin states. In other words, when an electron or nucleon goes from spin-up to spin-down, there is a slight energy shift. These aren’t like the energy shifts that occur due to an electron moving between orbitals or a molecular being ionized; these are much lower energy and represent an entire family of electrodynamic “ground states”, called base states. The neutral hydrogen atom is naturally the most simple atomic system (one proton, one electron), and has the fewest number of spin combinations (four), which are bisymmetric (the energy shift in proton spin up and electron spin down is the same as proton spin down and electron spin up). This hyperfine shift in hydrogen is much, much smaller than the shift between the electron ground state and first excited state; about 10[sup]-4[/sup] eV versus 10 eV, giving five orders of magnitude.
The significances of this to an alien species is two-fold. First, it obviously indicates our understanding of the quantized nature of energy and fine structure of the atomic nucleus (“We are smart!”), and more importantly, it serves as a universal base reference for a discrete unit of measurement that should be evident to any species capable of star-faring or astronomy. This hyperfine shift produces the distinct 1420 MHz spectral line used in radio astronomy (often referred to as the 21-centimeter line) to estimate concentration and momentum of atomic hydrogen, which is the most abundant hot (visible) matter in the universe. On the Pioneer plaque, this is used as a base unit to calibrate other measurements of time and distance on the plaque, which serves as a kind of the Rosetta stone.
The plaque itself is symbolic; the odds of an alien species (even if they exist and travel between stars) ever intercepting the probe is infinitesimal. But it may be the most enduring legacy of humanity that could potentially outlive the species and even the planet. When you consider that against our other legacies, such as pyramids or carved mountains that may last a few hundreds of thousands or millions of years at most, it is both incredible (that we could do so much with so primitive technology and understanding) and humbling (that the best we can do with our most advanced technology is send a tiny message that will be utterly lost in the vastness of space).
Stranger