One day a new asteroid appears out beyond the orbit of Mars. A football field sized chunk of rock and ice, it is remarkable only in that we had never noticed it before. In the course of a regular survey of near neighbors in solar system, astronomers turn a radio telescope towards the asteroid and notice that it is emitting a clear and regular electromagnetic signal seemingly aimed in the direction of Earth. The signal consists of two half-second bursts of energy separated by a one-second pause that repeats over and over. Played through a set of speakers it sounds like, “Beep, beep.”
Physicists and astronomers spend the next year publishing a raft of papers speculating on what natural processes could give rise to this phenomenon. Late one night, a bored post-doc manning a radio telescope station decides to amuse himself by hacking into the control system for a transmitter and echoing the signal back to the asteroid. About twenty minutes later (or, if you take the speed of light into account, immediately) the signal changes. Now it’s three short bursts: “Beep, beep, beep.” This provokes another raft of papers speculating on what feedback mechanism is at work. Finally, there’s nothing else to do but try again. Astronomers send back a signal with three beeps. The asteroid signal changes again–to five beeps.
Barely daring to hope, astronomers change their outgoing signal to five beeps. The asteroid sends back seven beeps. The dialog continues: 2, 3, 5, 7, 11, 13, 17. When they get back nineteen beeps, shivers of wonder run up the astronomers’ spines. You might expect some oddball natural phenomenon to give rise to a Fibonacci sequence, but not the prime numbers. This cannot be anything but a sign of intelligent life.
So say the astronomers send nineteen beeps to the asteroid, and the asteroid responds with…twenty beeps. Now what?