Radio signals travel at the speed of light. There are scenarios where our first human-made electromagnetic radiations are sweeping out into the galaxy and life on distant planets are envisioned hearing spark-gap Morse code, the sign-on of KDKA, Bing Crosby, Arthur Godfrey or Sonny and Cher.
It is not far fetched as long as you keep things in perspective. Those alien ears will not be in another of the estimated one hundred billion galaxies because our broadcasts have not yet reached them. Nor have they been heard on the other side of our own Milky Way galaxy. If they have been heard at all the listeners are quite nearby in cosmological terms because even at the speed of light our broadcasts have not traveled very far at all.
The first picture makes the reach of our human-made electromagnetic waves seem huge – and it is. The second picture gives a different perspective where the blue dot represents the extent of all our radio emissions to date.
