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?