The Arecibo message.
In 1974 the Arecibo Radio Observatory in Puerto Rico transmitted a message to M13, which is a cluster of stars 25,000 light years away. One light year is approximately 6 million, million miles (9 million, million kilometers).
Looking for other transmissions.
If we can transmit messages this powerful, we can also pick up quite weak messages. It is hoped that any other intelligent life in the Galaxy that has the technology is doing the same as us. If this is the case there should be transmissions made by them that we can pick up.
The birth of SETI.
Since then SETI has taken off in a big way. An array was planned to comprise of 1,500 radio telescopes to be called "Project Cyclops" to look for extraterrestrial intelligence. However this would have cost $50 billion and it was decided it was too expensive. Apart from that SETI had now become important and a huge amount of time, work, money and effort is being put into it, in hope of a message. Even our deep space probes carry messages for any alien life that may find them.
Now hundreds of years later, we have SETI consultants on all major arrays and radio telescopes in case of any message being picked up. They all will also check out anything that they or SETI picks up that looks promising.
The main SETI base is in Saha the 60 mile (100 kilomter) crater on the far side of Earth's moon. Since we don't know how aliens may transmit a message Saha has a radio telescope like Arecibo but in the weak luna gravity it is even bigger. Also it has large telescopes round the crater to seek out laser beam signals and infrared (heat) emissions from large, artifical structures in space. There is a buried netriono telescope and laser beams travelling along the surface of the crater should measure gravitational waves created by aliens manipulating large bodies such as black holes. This is all in hope of something turning up.
If we pick up a message it has been decided that we will reply and hope the CETI (Communication with Extra Terrestial Intelligence) can be made. Who knows? If they give us the invitation, maybe we'll make plans to visit them or meet up somewhere out there!
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