Dr Tom McDonough, an astrophysicist at the California Institute of Technology, is waiting patiently for along distance call. If it comes, it will have taken years to arrive and will have crossed trillions of kilometres of empty space. It will be the most stupendous event in human history – a message from an alien race among the stars.

At present, several experiments are going on around the world to try to communicate with extraterrestrials using radio signals. Some, like Dr McDonough’s, are attempting to eavesdrop on the stars from alien messages that may be coming from planets orbiting them. Others involve beaming out signals from Earth to let anyone who may be listening know that we are here. How many intelligent races are there ‘out there’ that we might be able to contact? The simple answer is, no one can be certain. Until scientists find just one example of life beyond the Earth, we cannot be sure that there are any extraterrestrial beings.

One of the first people to mount a search for alien radio messages was the American astronomer, Frank Dyke, in 1959. He wrote down a formula for calculating the number of technological civilizations in the galaxy. This took account of factors like the fraction of stars with planets, the fraction of suitable planets per planetary system, and the fraction of planets where life arises. However, the formula gives answers that are only as good as the estimates put in by individual scientists.

Alien broadcasts

Unfortunately, listening for extraterrestrials messages is not as simple as tuning into an ordinary radio station. Not only are there billions of stars to chose from, but there are also billions of different radio frequencies at which an alien race might choose to broadcast.

Today, major advances in electronics have greatly speed up the quest for E.T. Instruments like the giant radio dish Arecibo, in Puerto Rico, have been linked to a special device known as a multichannel spectrum analyzer. While the Arecibo dish is pointed at any given star in its search for alien messages, it tunes in to 27,000 different frequency bands. The analyzer linked to it can break each band into 74,000 channels and, using a computer, decides if each of these is broadcasting static, natural signals, or artificial signals. It also has to work out weather artificial signals come from some nearby man-made source, or from deep space. The prospect to success is improving all the time as new, faster analysis equipment is developed.