Science: The Truth About... Extraterrestrials and the `Hydrogen Band'
Friday 18 September 1998
The project will listen in a specific frequency range - 1,000MHz to 3,000MHz, in the microwave part of the electromagnetic spectrum - for signals that might be artificially produced. The system will be able to search in steps of 1Hz.
Quite apart from the question of how they might determine whether a signal is "artificial", one might ask: why those frequencies particularly? If ET wants to let us know that he, she or it is out there, why not any arbitrarily- chosen frequency?
It turns out that there's a very good reason. Within the microwave band is a particular frequency, 1,420MHz (equivalent to a wavelength of 21 centimetres). Known as the "hydrogen band", it corresponds to the energy of a photon emitted from a hydrogen atom when it undergoes the "spin-flip" transition. That happens when the atom's single electron spontaneously flips so that its spin value is the opposite of that of the single proton in the atom's nucleus. The spin-flip transition is chosen because it is the most common transition for an atom of free hydrogen, and that is the most plentiful element in the universe. For that reason, 1420MHz was first suggested as a channel for interstellar communication in 1959; the very next year, Frank Drake, now head of SETI, set up an experiment looking for messages on that wavelength from nearby stars.
Furthermore, there is relatively little background static from galaxies, quasars, and other cosmic noisemakers in the microwave part of the spectrum. This also helps by making faint signals easier to pick out.
As Professor Drake says: "Every radio astronomer - including extraterrestrial ones - will know about this hydrogen emission. It may serve as a universal `marker' on the radio dial. Consequently, it makes sense to use this, or nearby frequencies, for interstellar `hailing' signals."
Signals originating from outer space will be checked for signs of encoding, or being limited in bandwidth, which is a sure sign that the signal is being carefully controlled.
Some critics have suggested that this approach is too limited, and insisted that other frequencies - related, say, to carbon dioxide's emission spectrum - should be used instead. (Perhaps, as oxygen breathers, we should use oxygen.)
But the SETI logic does at least mean that we will find minds that think like ours, and which know the basics, such as the composition of the universe, the composition of atoms, and how to build radio transmitters. That certainly covers a lot of physics, only some of which (the transmitters) was feasible before this century, even though humans have been walking the Earth for millions of years.
SETI and similar projects have never found any confirmed contact from the stars, but perhaps it's just because our alien counterparts are still in their equivalent of Victorian times; statistically, there are so many stars that intelligent life must be out there somewhere. Meanwhile, the search goes on.
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