SETI: Science Background

The question “are we alone in the universe?” has fascinated people for centuries.  In 1959 two Cornell University physicists described how we might answer that question through interstellar communication.  Philip Morrison and Guiseppe Cocconi analyzed how two civilizations separated by many light years of space might use electromagnetic radiation to communicate.

Electromagnetic radiation is the general term for the more familiar light and radio waves, but also includes gamma rays, x rays, ultraviolet rays, and infrared.  All of these forms of electromagnetic rays or waves travel at the speed of light, the fastest possible speed.  The only difference between these types of electromagnetic waves is the length or the wave or the “wavelength.”  Since the waves all travel at the speed of light, another way to characterize the wave is by the number of waves received per second.  This is the frequency of the wave.

Morrison and Cocconi considered how well the types of electromagnetic waves passed through interstellar space.  While space is usually described as a vacuum, it does contain some gas and dust.  Over great distances that gas and dust absorb many types of radiation but radio waves pass through nearly unimpeded.  For interstellar communication, a particular range of radio frequencies, “microwaves” from 1 GHz to 10 GHz, are particularly good choices.  At lower frequencies our galaxy emits prodigious amounts of radio waves creating a loud background of noise.  At higher frequencies the Earth’s atmosphere, and presumably the atmosphere of other Earth-like planets, absorbs and emits broad ranges of radio frequencies.  The result is a quiet “Microwave Window” through which efficient radio communication is possible.

The Microwave Window has another interesting feature to recommend it as a place for interstellar communication: the “Water Hole.”  Some atoms and molecules in space emit radio waves at particular frequencies.  Hydrogen atoms emit at 1420 MHz (a wavelength of 21 cm).  Hydroxyl molecules, composed of one atom of hydrogen and one atom of oxygen (OH), emit at four specific radio frequencies ranging from 1612 MHz to 1720 MHz.  When a hydrogen atom combines with a hydroxyl molecule it forms a molecule of water, the most essential molecule for life as we know it.  Thus, the range of frequencies from 1420 to 1720 MHz is called the Water Hole.  It has been a popular frequency range for many SETI programs