A closer look
Sometimes, when you shout into the cosmic void, it waits an uncomfortably long time before shouting back. The long delayed echo phenomenon occurs when radio operators send a signal into the sky and receive the exact same transmission back 3 to 40 seconds later. Because light travels at 186,000 miles per second, a 40-second delay means the signal traveled millions of miles into space and bounced off... something. Theories range from plasma clouds to lunar ionospheres, but the universe’s favorite unresolved radio glitch remains mostly a mystery.
