How do we make contact?

One other problem related to interstellar communication is coding. How would you arrange a signal so that some intelligent being hundreds of light years away will be able to unscramble and decode it?

At last, think of the word  light-year and what it implies: one light year is the distance light travels in one year. If some being on Alpha Centauri is listening, it will take 4 years for our message to get there, since Alpha Centauri is 4 ly away from us. Supposing the being understands and replies, it will take 4 more years to get the answer. Can you imagine how frustrating a long distance relationship with a Centaurian woman/man would be? 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 overhear transmissions that have either leaked away from another planet or have been deliberately beamed into space in order to attract attention.

Electromagnetic radiation (photons) travels 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. 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.

We can think that extraterrestrials will adopt some rational and economical rules in broadcasting their signals. Firts, they will likely choose a window in the electromagnetic spectrum that is free from natural interference. In other words, they will send off signals with a certain range of frequencies where there are few other natural signals. Second, sending out electromagnetic waves cost energy; the electrons in the antennas of the transmitters have to be set in oscillatory motion, and this costs energy. The energy of an electromagnetic wave is linearly proportional to its frequency, i.e

It turns out that a range of the electromagnetic spectrum that satisfies these two criteria is the range of frequencies between 1,000 and 10,000 MHz (1 MHz is one million hertz, or one million oscillations per second).

A targeted search works in the following way. A certain spot in the sky is chosen, for example, a close-by star. The range of frequencies that one wants to explore is then divided in many channels an deach channel is listend for a certain amount of time, for example, for 30 seconds. This is similar to the procedure of turning the dial of a radio and listening to each station for a short period of time, except that is usually done in a different frequency range and the detectors are large radiotelescopes. As a search of a station on the radio dial can be time consuming, so is a search for extraterrestrial communications. Millions of channels, or "stations", are searched simultaneously for thrity seconds at the time. Such a search requires to be highly automated since one cannot employ easily millions of people scanning charts day and night!

Obviously we don't know what kind of signal extraterrestrial civilizations might beam to aliens. Several strategies have ben envisioned. Since a contact between two civilizations speaking different languages and using different tchmologies is trying to be established, it is likely that the first communciations will deal with things that both observer are likely to see or know. For example, atomic hydrogen is the most obiquitous element in the Universe. Hydorgen atoms make a transition between two states and in doing so they emit electromagnetic radiation at the wavelength of 21 cm, which is in the microwave range. Extraterrestrials would argue that other civilizations in the galaxy would know of atomic hydrogen and its transition. Thus, they would send signals around that wavelenght or with a wavelngth that is related to that through some universal constant.

The high-sensitivity targeted search, originally conceived by NASA and now being proposed for implementation by the SETI Institute, looks for weak signals originating near solar-type stars. Life and technology evolved around one such star, so these targets are a priori the most favorable candidates. The objective is to see if any civilizations in the vicinity of the target stars are transmitting signals that our present-day radio telescopes, outfitted with SETI- specific instrumentation, are sensitive enough to detect.
 

To learn about specific searches going on right now, go to the section Current.