At the start of the 20th century an Italian by the name of Marconi began experimenting with wireless telegraphy. Soon after, wireless telephony and radio broadcasting were developed. Since those early days, humans have been pumping out radio, television and communications signals from our planet in an ever increasing quantity and strength.
Like a wave that travels outward when a stone is thrown into water, radio and TV signals travel by electromagnetic wave at the speed of light in an ever expanding sphere away from the earth. Theoretically, anyone with a powerful enough receiver on another planet would be able to eavesdrop on us. So how likely is this, and if they are listening, are they coming to visit?
Our earliest communications have been traveling at the speed of light away from earth for over 100 years. Within that time, they would have passed in the order of 15,000 stars, about 1000 of which are similar to our own sun. Our best guess is that 1 in 5 of all sun like stars have earth like planets in orbit around them, at the correct distance from their sun to have liquid water and therefore be capable of supporting life. So that is a possible 200 earth like planets, capable of hosting life, that could have heard our signals.
Since we have only one example of life in the universe, we don’t know if it’s common for life to arise. It could be that earth hosts the sole sample of life in the universe, and we are the best the universe can come up with.
Either way, predicting how common life is on earth like planets, and how often Intelligent civilisations develop, is guess work. But being optimistic and picking figures at random, let’s say life arises on one out of every 500 earth like planets and intelligent life arises once from every 2000 examples of life. This means that intelligent life arises just once in every million of those earth like planets capable of supporting life. Given that our estimate is just 200 earth like planets capable of hosting life, I wouldn’t be rolling out the red carpet just yet in preparation from an eavesdropping E.T.. On the off chance that someone has heard our chatter, it would take an eternity to get here, assuming of course, they haven’t developed the warp drive.!!!