What is SETI and why do people do it?
SETI is the Search for Extra Terrestrial Life, such a search was first envisaged in 1959 by a pair of scientists Giuseppe Cocconi and Phillip Morrison who published an article in Nature entitled ‘Searching for Interstellar Communications’. The article puts forward the suggestion the likelihood that more advanced civilisations will be sending out signals to attempt to contact us. By doing so they would be sending some sort of advanced communications in the direction of our sun (because they would be aware of there being a good chance of life developing near such a star) which we have yet to detect. This led to the beginning of several SETI projects to detect such a signal, one of which in 1960 was conducted by Frank Drake, ‘Project Ozma’ almost fifty years ago. Attempts to find life have evolved considerably since Drakes attempt almost fifty years ago but they remain based on the same principles.
The article published by Cocconi and Morrison states that the most likely way an extraterrestrial civilisation would try and contact us would be via a radio signal with an emission line with a wavelength of 21cm (1420Mhz) because it would be the easiest signal for a developing civilisation to receive (as it is the spectral line associated with the change in state of hydrogen). Work has continued along this vein for the most part, scanning the skies and more recently scanning specific stars that could be host to life for radio signals. One issue with this method is the huge amount of data received and the need to examine this data for an extraterrestrial radio signal the parameters of which are unknown. Therefore the more sensitivity required the more computing power is needed, one project to overcome this is SETI@Home. SETI@Home uses a ‘distributed grid concept’ to analyse large chunks of data collected from the Arecibo radio telescope in Puerto Rico for signs of extra terrestrial life.
Whilst radio has been the preferred method in the search for extra terrestrial life, there has been the suggestion that extra terrestrial life forms may communicate using high powered lasers. Whilst not a new idea Optical SETI (OSETI) is becoming more credible and some scientists are moving away from radio in favour of optical means as they believe it is a more likely method of communication for a civilisation more advanced than our own. However OSETI is not without its problems, unlike radio frequency where there is an ‘obvious’ frequency choice, this is not the case with lasers, there is no obvious starting point when it comes to choosing what to search for. This has been overcome in some by using the uncertainly principle which dictates if the pulse time of the laser is sufficiently small the frequency bandwidth will become larger making it easier to detect. Lasers are also highly directional requiring a line of sight with the receiver, though this could be overcome by using an optical system incorporating a mirror to create a more dispersive pulse.
There are various reasons for believing in SETI, many scientists believe that complex life forms other than ourselves should be ‘common’ throughout our galaxy and that it is inevitable we will one day find them (or they one day find us). The Drake equation is a famous formula derived by Frank Drake in 1961 written after the aforementioned article by Cocconi and Morrison, shown below:
Whereby the final answer N denotes how many civilisations in our galaxy we might be able to communicate with. The other factors relate to probabilities of various factors that contribute to the chances of forming extra terrestrial life.N’ is the number of stars in our galaxy, ‘fp’ is the percentage of stars that form planets,’ne’ is the average number of planets suitable to sustain life,’fl’ is the fraction of planets where life evolves,’fi’ is the fraction of these that develops intelligent life,’fc’ is the fraction that communicates and ‘fL’ is the fraction of a planets life occupied by a communicating civilisation.,
The problem with the parameters of the drake equation is they are all estimates to varying degrees of accuracy, some being merely educated guesswork. Whilst values like the number of stars in the galaxy can be narrowed down to a high degree of accuracy ( Others such as the percentage of stars that form planets is a hot topic, especially considering the continuing advancement in the technologies and discoveries in the field of exo-planetary science making any estimate unreliable.
SETI therefore is an ambitious project as there are no definitive estimate to how many intelligent civilisations are indeed out there, and even if they are there whether they’d be trying to contact us. However assuming extra terrestrial life is anything like us then we are certainly making positive steps to finding any signals they may be sending.
 Cocconi. G. Morrison. P. Searching For Interstellar Communications. Nature Vol. 184 p844-846
 “SETI@home — Massively Distributed Computing for SETI”. Computing in Science & Engineering 3: 78–83.
 Nature, Vol. 190, No. 4772, pp. 205-208, April 15, 1961
 ‘ The Search for Extraterrestrial Intelligence: Listening for Life in the Cosmos’ Thomas R McDonough p114-117
 Extra Terrestrial Intelligence: The First encounter edited by James L. Christian p53-65
 Communication with extraterrestrial intelligence Edited by Carl Sagan p1-8
 ‘The Search for Extraterrestrial Intelligence: A philosophical inquiry’ David Lamb (2001) p39-59