Actually, the formula doesn’t give an answer as such, for any number generated can only be based on the numbers attributed to each part of the formula. That said, essentially it works like this:
Fp is the fraction of stars that could contain planetary systems and whilst this is much debate about this, a figure of one in ten is not unreasonable. Next Ne signifies the number of these planets that are Earth-like. Based on our own solar system, this could be determined as 1. F1 stands for the fraction of these Earth-like planets on which life could develop. This is fairly straightforward, either life develops or it doesn’t therefore F1 is either 1 or 0. If 0, then the overall formula will always generate a figure of zero as well. We will accept the view expressed by Professor Jesco Puttkamer, former Senior Staff Scientist of Advanced Programmes of Space Flight, NASA, who, when asked "Could you give me a clear mathematical probability of the existence of life in outer space?" replied "One … one is certainty." (1) We also now not only believe their might have been life on Mars, but the moons of Jupiter are also being suggested as probable homes for possible micro-organisms) so F1 will be 1. Fi is the fraction of these planets where life has become intelligent. On Earth there are a number of intelligent species, so applying this to Drake’s formula, a number between 1 and 4 could be used. Conservatively, this will be determined as 1 (representing humanity on Earth.) Fc refers to the number of these species that actually want to communicate with us. We are now purely in the realms of speculation, but for arguments sake, it will be assumed perhaps 1 in 10 of these species would want to talk generating a figure of 0.1 for Fc. Finally L represents the lifetime of a civilisation (in years). The first part of this book demonstrated how civilisations can rise and fall, but we will use a figure of 5000 years – from the founding of Egypt to the present day when we are able to transmit signals to the stars (although hopefully mankind will survive somewhat longer.)
Having calculated the number of races that might be trying to contact us, the next question is, how could they reach us? Albert Einstein advises us that nothing can travel faster than the speed of light (E=MC2) and late twentieth century space missions to the nine known planets of the Solar System have revealed no signs of intelligent life there. Therefore, any one visiting us must be travelling from outside the Solar System, yet the distances involved suggest that such travel is not achievable.
Betty and Barney Hill (left) alleged that they had been taken on board a UFO whilst out driving through the White Mountains in New Hampshire in September 1961. They later sought professional assistance to resolve personal difficulties that later surfaced and during regression, undertaken by psychiatrist and neurologist Dr Benjamin Simon, the Hills revealed that they believed they believed they had been taken on board a space craft where they had undergone a detailed physical examination. Following this examination, Betty Hill claimed that a crew-member of the spacecraft had shown her a star map of what were supposedly trade routes and "paths of exploration". During further hypnosis in 1964 Betty Hill reproduced this alleged ‘star map’ as a drawing (right).
Seriously though, there is no reason to believe that we would ever be able to actually communicate with any such aliens should they manage to reach us. Even on Earth we share the planet with other life forms that are acknowledged as being ‘intelligent’. Yet, to date, we have not been able to achieve any real communication outside Hollywood remakes of 1970s Australian television shows and US science fiction programmes. In any event, once the home star system had been identified, Friedman and Slate set to work. "What this implies," they concluded, "is that the Reticulan crew would not have had to be going faster than the speed of light to pay a visit to our solar system and return facing the prospect of residence in a home for the elderly. Using Einstein’s time-change factor, a one way trip at 80% of the speed of light at a constant velocity would take them 22 years. At 99% of the speed of light, it would take them five years and two months, but at 99.9% of that velocity, the trip could be made in only twenty months." (3)
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Mankind first
transmitted a dedicated radio message to the stars at 5pm on 16th
November 1974 from a radio telescope situated in Arecibo in Puerto Rico (left),
although other radio and television transmissions had been drifting into space
for decades; the inadvertent result of normal terrestrial broadcasting. The
message sent was a three-minute signal targeted towards a group of stars 24,000
light years away and was primarily intended to be a demonstration that
terrestrial radio astronomy had reached a level that would allow interstellar
radio communication over vast distances.
Drake (right)
determined that N signifies the number of civilisations in our galaxy
attempting to make contact. R stands for the average rate of star
formulation and based on observations from the Hubble telescope this is
generally accepted to be around ten stars each year. 
Using these figures,
the formula calculates that N, (the number of
advanced civilisations wanting to make contact) is 10 x 0.1 x 1 x 1 x 0.1 x
5000. This suggests that there are 500 civilisations in this galaxy alone (one
galaxy in a universe of 100 billion) who may be trying to contact us.
Space is so infinite that it is measured in ‘light years’;
that is, the time it would take light to travel any given distance. Light
travels at 186,000 miles a second, meaning that light could travel round the
planet Earth seven times in one second. It takes 2.5 seconds for light to go to
the Moon and back, and eight and a half minutes to travel from the Sun to Earth.
It takes light five hours to travel from the Sun to the furthest planet Pluto
and to the nearest star 4.3 years. Thereafter, the time periods appear to
preclude the possibility of travel, taking 30,000 years for light to reach the
centre of our galaxy; 100,000 years to cross the galaxy, and to reach our
nearest galaxy, Andromeda (right) light takes 2.2 million years.
Finally, for light to travel to the furthest known point in the universe it
would take fifteen billion years (and the universe is still expanding at
an accelerating rate.)
Scientist Stanton B
Friedman, along with co-author B Ann Slate, applied this science to establish
whether or not claims made by two American so-called ‘abductees’ could be of any
substance. 
Unfortunately or
perhaps fortunately for Betty if they story was fabricated, no-one could
recognise the star chart she had drawn and it therefore neither helped prove nor
disprove the story. However an American schoolteacher, Marjorie Fish, undertook
the mammoth task of attempting to identify which stars Betty’s map could have
represented. She constructed three dimensional models of all stars within
approximately sixty light years of our own sun, and in July 1969, some five
years following the original ‘abduction’, Fish came across a nine star pattern
that appeared to represent the star chart drawn by Betty Hill.
Additional support for the Hill claims came in 1969 when a revised
star catalogue was issued. This catalogue containing information that appeared
to confirm the Hill’s original story, for three of the stars shown on Hill and
Fisher’s maps were not known to anybody on Earth at the time of the alleged
abduction. (These stars were called named Gliese 86.1, 95 and
97.) This information suggested that the ‘aliens’ who ‘abducted’ the
Hills were from Zeta 1 Reticuli and Zeta 2 Reticuli (the 12th and
13th nearest stars to our Sun), some 176,340,000m miles away. Quite
why they should travel 37 light years and simply want to subject Betty and
Barney Hill to a medical examination, have a chat around a star map, then dump
them back on Earth, has never been adequately explained, but then, hey, maybe
these aliens have got another set of values. 
Their calculations for the amount of time the journey would have
taken may be correct, but they had forgotten that travelling at the speed of
light brings about its own problems. Stephen Hawking (right) explains: "Because
of the equivalence of energy and mass, the energy which an object has due to its
motion, will add to its mass. In other words, it will make it harder to increase
its speed. This effect is only really significant for objects moving at the
speed of light. For example, at 10% of the speed of light, an object’s mass is
only 0.5% more than normal, while at 90% of the speed of light it would be twice
its normal mass. As an object approaches the speed of light, its mass rises ever
more quickly, so it takes more and more energy to speed it up further. 
