Listen to the sound of aliens!
There are currently various efforts going on around the world to receive signals from alien intelligences. These go by the general name of SETI (Search for Extraterrestrial Intelligence). The basic idea is that sensitive radio telescopes are pointed at likely sources and record any radio signals they receive. These signals are then analysed to see if there is any indication that they might be artificially generated.
Such signals would tell us that the aliens are out there!

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