For weeks, astrophysicists, planetary-defense specialists, SETI researchers, and even theologians have been quietly wrestling with a singular question that has no precedent in recorded human history: Why would an interstellar object produce a narrow-band absorption signature at 1665 and 1667 MHz—two frequencies so scientifically significant that SETI researchers have considered them a prime channel for interstellar communication for nearly half a century? And why would that signature appear not during a quiet phase of the object’s journey, but five days before perihelion, at the exact moment it should have been radiating maximum natural emission?
The answer requires us to take an evidence-driven look at the physics behind those numbers. The frequencies 1665.4018 and 1667.3590 MHz correspond to the 18-centimeter microwave transitions of the hydroxyl radical (OH). These lines were discovered in the mid-20th century and quickly became one of the most important tools in radio astronomy.
OH emission is common in star-forming regions, molecular clouds, and occasionally in comets when the Sun breaks apart water molecules in the coma. But the crucial part of the science is this: every comet ever observed in radio wavelengths has produced emission, never absorption, at those lines. Heating creates population inversion, inversion produces maser activity, masers emit radio energy. That is the natural physics.
