Ask Dr. SETI ®

In the film "Contact", SETI scientists refer to a frequency of "hydrogen times PI". Now if the atomic weight of Hydrogen is 1.00794 u ± 0.00001 u, then wouldn't hydrogen * PI = 3.16672? And what would that 3.16672 GHz mean, anyway?
 
Fentro, Danbury CT USA

The Doctor Responds:

That frequency wouldn't mean anything, Fentro -- because GHz (billions of cycles per second) is related to the arbitrary way we reckon time on this planet. The real answer to your question has to do with how you define "hydrogen."
 
Since we're trying to establish a communications frequency, we should look for a frequency which is related to a natural constant, in this case one associated somehow with hydrogen. This means what we seek is not an atomic weight, but rather a resonance. Whenever a hydrogen atom undergoes a spin-flip transition (which it naturally does), it emits a single photon at a known frequency (which is, in turn, related to a specific energy level, according to Planck's Law).
 
It doesn't matter what language you use, or what units you choose, for defining that frequency -- it is a natural constant. Now, if you multiply that frequency by another known natural constant (such as pi, which is a unitless ratio), you get one of many possible "magic frequencies" for interstellar communications. Other civilizations may call that frequency 2.345 UrmaPlatz, or whatever -- doesn't matter, as it's based upon natural constants, which are physically the same for us as for them, regardless of how we count, or what we call them.
 
Of course, we don't know which of these possible frequencies an alien technological society might choose, so we have to monitor them all. Let's seek ones in the quietest part of the electromagnetic spectrum, the microwave window that extends from what we call 1 to about 10 GHz, and we have any number of likely SETI target frequencies. (Quiet spectrum is in the same place everywhere in the cosmos, no matter what numbering system we use to define it.)
 
Of those many possibilities, Carl Sagan chose the product of pi times the hydrogen spin-flip transition frequency, for use in his novel Contact. In fact, several SETI scientists in Australia had independently arrived at that same target frequency when Carl was writing his novel in 1985, and were already monitoring there. So, great minds think alike (let us hope on other planets, as well as our own!)