Astronomers have been pointing radio telescopes to the heavens above for more than half a century, listening for the signs of little green men.
Unfortunately (or fortunately), in all that time, most of what they have heard has been silence, naturally occurring phenomenon, or false positives from the Earth itself.
We say most, because there is one instance that has no explanation – the “Wow!” signal.
On Aug. 15, 1977 Dr. Jerry R. Ehman, an American astronomer, detected what might very well be a signal from an extraterrestrial intelligence. The Wow! signal was a strong narrowband radio signal detected by Dr. Ehman while working for SETI.
The “Wow!” Signal
It was so startling to see such a strong signal in the band that an extraterrestrial intelligence was expected to use, he wrote “Wow!” on the computer print out right next to the signal.
The signal itself originated from the constellation Sagittarius at a frequency of 1420 MHz. This number is significant for SETI because they reason that hydrogen is the most common element in the universe. Hydrogen resonates at about 1420 MHz, so extraterrestrials may use that frequency to transmit a strong signal.
They attempted to listen for the signal many times after. Most recently, in 1999. They listened for 6, 14 hour intervals and heard nothing.
As for possible explanations, there are a few. Interstellar scintillation of a weaker continuous signal, similar to atmospheric twinkling, could be the reason. This still would not exclude the possibility of the signal being artificial in its nature, however.
Dr. Ehman himself originally believed it to be an earth based signal reflected off space debris, however, he has since recanted that after further research. The new research showed an Earth based signal to be very unlikely due to the requirements of a space-borne reflector being in an unrealistic and unlikely scenario. The band itself is also part of a protected spectrum which transmitters are forbidden to transmit on.
Our Own Messages
We have since broadcasted a few “cosmic calls” of our own, with the latest being sent towards Gliese 581 in 2009 (a digital time capsule containing 501 messages that were selected through a competition on the social networking site Bebo). However, due to the lag, that particular “hello” won’t arrive until 2030. Russia has been ringing up our neighbors since 1999, with a total of 9 ‘cosmic calls‘ broadcasted. The real question is, how will humanity react once we receive a reply?