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 overhear transmissions that have either leaked away from another planet or have been deliberately beamed into space in order to attract attention. 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.
A transmitter at microwave frequencies requires only modest power to produce a signal detectable above the natural noise background. It is here that we shall listen because it is likely that these frequencies will be used by other technological civilizations, just as we do, for transmission of communication signals over great distances. Nearly perfect radio telescopes and receivers now exist for the frequency range 1,000 to 10,000 MHz, the quietest part of the microwave spectrum for any telescopes that must work from within the Earth's atmosphere. Recent advances in microelectronics have allowed SETI engineers to develop signal processing equipment with tens of millions of simultaneous, narrowband channels. The same electronics technology also permits real-time computer analysis to find narrowband signals even if the frequency changes in time or if they pulse on and off.