Recently I attended a public lecture at UCC, part of a series commemorating the 150th anniversary of the first student intake to the college. Prof Jill Tarter described the search for extraterrestrial intelligence (SETI). Prof Tarter is director of research for the SETI Institute.
SETI has yet to uncover signs of intelligent life beyond Earth, but the programme is growing in confidence and in sophistication.
The philosophy behind SETI rests on our understanding of the origin and nature of life on Earth and on our understanding of the nature of the universe.
Our Earth was formed about 5 billion years ago. It is believed that conditions on the early Earth favoured the formation of simple organic (based on carbon) molecules that dissolved in the oceans to form a rich organic soup.
These molecules combined and recombined with each other. New forms arose in a process of chemical evolution that lasted for millions of years. Classes of molecules gradually arose capable of replicating themselves and directing the formation of other molecules, and eventually (about 3.5 billion years ago) these various molecules organised themselves into the first simple living cell.
Biological evolution now began and a diverse range of life forms, based on the same basic biochemical plan, developed from that first simple form of life. All life now on Earth is descended from that first simple life-form.
The universe is vast, containing hundreds of billions of galaxies, and each galaxy contains hundreds of billions of stars. Many stars have orbiting planets. We also know that simple organic molecules are formed on a widespread basis throughout the universe.
There is every reason to think that life could begin and evolve on planets elsewhere in the universe once initial favourable conditions prevailed on these planets.
Given the vastness of the universe it appears very likely that life has evolved on many planets to a stage of intelligence that is actively trying to communicate with others.
Distances between stars in the same galaxy are huge. Distances between galaxies are unimaginably large. These distances are measured in units called light-years - the distance travelled by light in one year equal to 186,000 miles a second. This is the fastest speed allowed in the universe.
The closest star to our solar system is four light years away. The nearest major galaxy to our Milky Way galaxy is two million light years away.
It is reckoned that if an alien civilisation is trying to communicate with the rest of the universe, it would choose to send the fastest possible signal, i.e., moving at the speed of light.
For various reasons it is thought that the message would be broadcast as a microwave signal. A large part of the SETI programme therefore searches the skies with large radio-telescopes hoping to receive a message from extra-terrestrial intelligence.
How many alien civilisations might be out there in the Milky Way galaxy trying to contact us? Frank Drake, who began the modern SETI programme, has developed a way of estimating the number of extra-terrestrial civilisations. He broke the question down into seven steps out of which he constructed an equation, the Drake Equation.
It is a calculation based on assessing a selection of variables. These include: the number of advanced technical civilisations in the galaxy; the number of stars in the Milky Way; the fraction of stars with planets; the number of planets ecologically suitable for life; the fraction of those planets where life evolves; the fraction that develops intelligent life; the fraction that communicates; and the fraction of a planet's life occupied by the communicating civilisation.
The first one is easy. There are 400 billion stars in our Milky Way galaxy. Planets are probably common. Let us guess that one star in 10 has planets so that gives 40 billion stars with planets in total.
Let us say that, just as in our solar system, whenever planets form around a star about 10 planets occur.
This would give us 400 billion planets. How many are suitable for life? Let us assume that, again as in our solar system, one in 10 is suitable for life. This would give us 40 billion suitable for life.
How many of the suitable planets actually develop life? Let us suppose that one planet in 10 will develop life. This would give us 4 billion planets with life in the galaxy.
How many of these planets develop intelligent life? Let's be conservative and say that only one in 100 planets with life evolves intelligence. This would give us 40 million civilisations. How many of the civilisations develop technology capable of interstellar communication? Let us say that one in 10 does this, which would give us 4 million technological civilisations.
How long do these civilisations last? When technological developments reach a certain stage, civilisation becomes capable of destroying itself through nuclear war. Thus an advanced civilisation might send out signals for only a short time before blowing itself up.
Let us be cautious and assume that a civilisation is communicative for one thousandth of the age of its home world. This would mean that there are 4,000 worlds out there now waiting for us to detect them.
The probabilities I have put on each of the seven steps are personal, optimistic choices. A pessimist would put much smaller numbers on each of the steps and consequently get a much smaller number of civilisations, even fewer than one. If the pessimists are right, we may be alone in this galaxy.
But there is at least 100 billion other galaxies out there. Surely we can't be the only civilisation among all that lot? If we are, it would be a gross understatement to say that the responsibility on us to succeed is awesome.
(William Reville is a senior lecturer in biochemistry and director of microscopy at UCC)