IN THE last two articles, I described how it seems almost certain that life has arisen elsewhere in the universe beyond our solar system. Estimates have been made of the number of advanced civilisations in our Milky Way galaxy. Searches for extraterrestrial intelligence (SETI), are in operation, listening for messages from such civilisations.
We have sent messages into space giving details about the Earth and life on Earth, hoping the information will be intercepted by alien intelligence.
The 1957 launch of the Russian satellite Sputnik I revolutionised our thinking about space and opened floodgates of creative thinking about the investigation of the universe. In 1959, two scientists, Giuseppe Cocconi and Philip Morrison, published an article in the science journal Nature, proposing that advanced civilisations elsewhere in the universe could well be trying to contact us by using radio signals.
They proposed that a systematic programme of listening for such radio signals should be established. Few will deny the profound importance, practical and philosophical, which the detection of interstellar communication would have," they said. Shortly afterwards, SETI programmes were established in the USA and elsewhere.
If life exists elsewhere it is probably on a planet in another solar system. The vastness of interstellar space presents a problem for communication between solar systems. The fastest speed allowed in the universe by the laws of physics is the speed of light - 186,000 miles per second.
The easiest and cheapest option for interstellar communications would be to send signals by radio wave. Even then, communication still takes a considerable time. Distance in space is measured in terms of lightyears, the distance travelled by light in a year (six trillion miles). Our Milky Way galaxy is about 100,000 light years in diameter. The nearest star to the Earth is Alpha Centauri, which is four light years away. In other words, it will take four years for a radio signal to reach Alpha Centauri.
Searching the skies for radio frequency signals sounds simple, but in fact there are great practical difficulties. The signals could be coming in at any frequency, from any direction, and at any strength. A comprehensive search would therefore have to aim at every tiny piece of sky and successively tune through an almost infinite spectrum of frequencies. However, Cocconi and Morrison pointed out that one frequency in the universe stands out from all the others. This is the fundamental radio signal that hydrogen atoms broadcast.
The galaxy is filled with hydrogen atoms; the signal from an individual atom is tiny, but there are so many atoms that the signals add up to stand out boldly from the background. Hydrogen broadcasts at a frequency of 1,420 million cycles per second. Cocconi and Morrison suggested that an alien civilisation trying to attract attention to itself would be likely to broadcast at or near the hydrogen frequency.
How many extraterrestrial civilisations are likely to be in our galaxy? Frank Drake, who established the first SETI programme in America, developed a method to estimate this number. He broke the question down into seven questions.
Question 1: How many stars are in the Milky Way galaxy? Answer: About 400 billion.
Question 2: How many stars have planets? Answer: Perhaps one in 10 - around 40 billion.
Question 3: How many planets are suitable for life?
Answer: Perhaps one planet per solar system. i.e. 40 billion planets.
Question 4: How many of these planets actually develop life? Answer: Again, perhaps one in 10 - four billion planets with life.
Question 5: How many of these planets develop intelligent life? Answer: Let's be pessimistic and say that only one in 100 planets with life evolves intelligence - 40 million civilisations.
Question 6: How many of these civilisations develop SETI programmes? Answer: Again let us assume that one in 10 will do so, giving us about four million technological civilisations.
Question 7: How long do such civilisations last? Answer: As soon as a civilisation develops the technology to communicate over interstellar distances, it also has sufficient technological power to destroy itself.
Let us assume that the average age of a planet is around 10 billion years, and that a civilisation is communicative for a thousandth of the age of its home world - 10 million years. This would mean that 1,000 of the four million technological civilisations that arose - 4,000 worlds - could be out there now waiting for us to detect them.
We haven't been satisfied just to sit and listen for sounds of advanced life in the cosmos, we have also sent out our own messages. The first deliberate message was sent on NASA's Pioneer 10 spacecraft in 1972. Pioneer 10 was designed to be the first vessel to fly by the planet Jupiter. However, in giving the craft enough of a boost to get to Jupiter, it also had enough energy to escape from the sun after passing by Jupiter and to leave the solar system forever.
IT was decided to send a message on the craft to any alien civilisation that might find it, even millions of years in the future. Frank Drake and Carl Sagan designed a plaque that was fixed to the spacecraft. The naked figures on the plaque, showing the type of creatures who built it, prompted some paragons of virtue to complain that NASA was sending out space pornography.
In 1974, scientists at Cornell University broadcast the first deliberate message into space. The coded message contained a crude picture of human beings and some basic information about our biochemistry. The message was sent to a group of stars 25,000 light years away. If the message is received, and if a reply is sent immediately, it will reach the Earth in 50,000 years time.
Trying to contact alien civilisations may not be quite the unadulterated good idea that it seems to be at first glance. Suppose we attract the attention of a nearby civilisation that is far more advanced than ourselves. By nearby and advanced I mean sufficiently close and sufficiently technologically sophisticated to visit us.
Relative to such a civilisation we would be in a state of aboriginal development. Our whole experience of what happens when developed culture meets aboriginal culture should make us very wary of such a visit. When these two cultures have met on planet Earth it has invariably been to the disadvantage of the aborigines.
We automatically think that our first encounter with aliens will be something like that depicted in the film Close Encounters of the Third Kind. But what if our alien visitors behaved like those in the film Independence Day?