Science Forums 2

[CraigD]

Originally Posted by lemit

From what I've heard, it seems the search might be fairly restrictive in its verifying patterns. I have got an impression they have been listening for a very basic pattern of a single frequency that is repeated for hours at a time.

SETI assumes that some – not necessarily all or many, but at least a few – ET civilizations want to be discovered by listeners like us, so purposefully make an effort to make it easy for us.

An obvious way to do this is to send a strong signal at a frequency where natural sources are weak. On such range of frequencies is about 1420 MHz to 1640 MHz, known as the “water hole” because it falls between the emission line of unexcited hydrogen and OH. It’s not the only “quiet band”, but an easy one for radiotelescopes to monitor, so SETI focuses most of its listening on frequencies in this band.

Originally Posted by lemit

If my impression is correct, wouldn't that basic, simple, repeated pattern be more indicative of a natural phenomenon?

SETI doesn’t look only for repetition of changes in amplitude at a given frequency – an initial indication EM radiation isn’t naturally generated noise – but for the amplitudes to be mostly of two values, indicating binary data, and also that the signal originates from a single location.

Once such a signal is found, again assuming it was sent with the intention that we be able to read it without too much, SETI looks for sequences such as counting, lists of the primes in order, and similar data that would be almost certain evidence of an artificial, intelligent source. A signal consisting of a number of bits that’s the sum of two or three prime numbers is another strong indication of an artificial signal, and additionally that the signal is a 2 or 3-D pixilated image with dimensions of its prime factors.

SETI occasionally sends signals of the same kind it’s looking for. The 1974 “Arecibo message” is a famous example. It’s 73*23=1679 bits long.

Originally Posted by lemit

Wouldn't sophisticated communication be more varied?

Yes. Since sending EM signals over long distances is energetically costly, everyday “real” communication is likely to be both directional and compressed. Compressed data appears nearly random, and would be difficult to distinguish from noise.

Again, SETI assumes that we’d be receiving a signal intentionally created to allow us to detect and decode it, not overhearing everyday communication.