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Comment by martinkallstrom | Hacker News Reader

"Also, the 1420 MHz signal is problematic in itself in that it is "protected spectrum": it is bandwidth in which terrestrial transmitters are forbidden to transmit due to it being reserved for astronomical purposes."

Wait... what?? If this has been enforced on Earth, then clearly other civilizations might make the same choice and not broadcast anything on that frequency.

Does anyone know if we are broadcasting anything into space on this frequency?

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This frequency is protected because there is a gap in the absorption spectrum of our atmosphere. This is not the same for every atmosphere, so other planets will have different gaps (unless the composition of gases is similar enough), and thus it won't make too much sense right now to single out this frequency for other civilisations. AFAIK we don't broadcast, because it would interfere with astronomic measurements.

Not true. While some of the radio spectrum has been reserved specifically because of the lack of absorption in the atmosphere, the 1420MHz line is the famous 21cm line frequenctly measured by cosmologists to look at the epoch of reionization. It is due to the hyperfine structure of hydrogen, and would not be of unique importance to our civilization.

However, we don't broadcast anything. Any broadcasts from earth would hugely overshadow any small signal we're trying to read.

Ups, thanks for the correction.

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