The distances between the stars are vast! To cross these distances in any kind of spacecraft requires either prodigious energy or prodigious time, or both. There is little chance of discovering extraterrestrial life via direct contact, but we can attempt to contact aliens with transmissions beamed into outer space with hopes of attracting attention.

Electromagnetic radiation (photons) travel at the fastest velocity possible, the speed of light. The Pioneer 10 spacecraft left the Solar System after traveling for nine years, but its radio signal takes only 6 hours to get back to Earth. Photons can carry information, are easily generated and detected, are undeflected by the galactic magnetic field, and at many frequencies have a very small probability of being scattered or absorbed. Photons are thus the ideal particles for transmitting information over long distances.

Only in the microwave region of the electromagnetic spectrum (1,000 to 100,000 MHz) is the Universe fairly quiet; here there is only a faint whisper from the remains of the Big Bang explosion. A transmitter at microwave frequencies requires only modest power to produce a signal detectable above the natural noise background.