Some scientists have suggested that the reason why we haven't heard from the ETs by now is because the number of alien civilisations in the Milky Way (and, by implications, the rest of the universe) is few and far between. Is this true?
Are there enough technological civilizations in our galaxy to communicate with?
There is a famous equation developed by Dr Frank D. Drake known, naturally enough, as Drake's Equation. This mathematical equation gives scientists an estimate on the number of civilizations there are likely to be in our galaxy (or the entire Universe if you like; however scientists prefer them to be close by for easier communication) at this moment in time.
The equation that determines the number of technically-advanced civilizations (i.e. the ones that can communicate with us) that have evolved on a planet with Earth-like conditions at this moment in time is given by:
N = R.fs.fm.fp.ne.fl.fi.ft.L
To work out roughly how many civilizations there are in our Milky Way, let us substitute a few figures into the above terms.
The factor R
The average rate of star formation over the lifetime of the galaxy
When we evaluate the age of the oldest stars—located in what are known as globular clusters strewn about the halo of our Milky Way galaxy—we are also determining an approximate age of the galaxy. Astronomers have estimated this age to be at least 18,000 million years. Therefore, given that there are roughly 400,000 million stars in the Milky Way, the average rate of star formation is:
R = 400,000 million / 18,000 million, or about 22 stars per year.
The factor fs
The fraction of stars in the Milky Way that are single and sun-like
Although scientists have argued that it might be possible to find intelligent, technological life around a solar system with more than one star, we shall assume life can only arise on a single star like our Sun. Therefore, astronomers estimate than the percentage of stars in the Milky Way that are single stars is about 25 percent. And another 25 percent of those stars are said to be sun-like.
fs = 0.25 x 0.25 = 0.0625
The factor fm
The fraction of single sun-like stars in the Milky Way that are metal rich
To estimate the number of stars likely to lead to the development of a technology for communicating with other civilizations, a single sun-like star should have a reasonable abundance of metals in its chemical composition. This means planets associated with these stars will almost certainly be rich with metals.
Although some sun-like stars can be richer in metals than others, we will assume virtually all single sun-like stars will have enough metals to help a fledging civilization develop a technology.
fm = 0.9
The factor fp
The fraction of stars with a planetary system
Planetary systems appear to be common throughout the Milky Way galaxy. But to be on the safe side, let us assume a little over half the stars develop a planetary system of their own, or:
fp = 0.6
The factor ne
The average number of planets per star that have evolved a suitable ecosphere
Earth is our only fine example of a planet possessing a suitable ecosphere at this moment in time, so set:
ne = 1
The factor fl
The fraction of those planets that eventually lead to life
Dr Stanley Miller's famous experiment on the origin of life, a study of those meteorites containing organic substances that resemble the tarry oils found on Earth (i.e. Carbonaceous Chondrites), and the fossilised remains of animals and plants that appeared on Earth over the past 4,500 million years suggests there is good indication for life to arise very quickly. Hence let:
fl = 0.9
The factors fi and ft
The fraction of those planets that eventually lead to intelligent, technological life
Whenever there is a highly competitive, harsh and unforgiving environment for life to develop and grow, the propensity for intelligence will arise given enough time. On Earth, one type of intelligence has seen the development of creatures capable of affecting all other forms of intelligent life through its technology. Although this may suggest an inevitable trend on all other life-bearing worlds, the hundreds of millions of years it has taken for humans to evolve suggests the road to a technically-advanced society can be somewhat long and precarious. Of particular concern in this regard are those extraterrestrial threats of large asteroids or comets colliding with a life-bearing planet which could extinguish all forms of life. Consequently, some evolutionary biologists have given a conservative estimate for the product fi.ft to be about 0.02.
The factor L
The mean lifetime in years of technically-advanced civilizations
The factor L has been given great discussion by the scientists. This factor depends on how well a technically-advanced civilization has learned to live with itself, other lifeforms, and its technology. Let us define technology as the application of science to fashioning implements, practising manual arts and skills, and collecting materials. With only humans to go by, possibly the Industrial Revolution in England could be considered the beginnings of a truly technical society, so let L = 250 years.
Multiplying all the numbers
Multiplying all these numbers together gives us a figure of n=3.34 or roughly three technical civilizations (maybe four if we are optimistic) scattered somewhere in the galaxy.
This means every year there are likely to be three technical civilizations (one of which has to be ourselves). Two of those civilizations must exist somewhere in the Milky Way. In other words, the closest one to us could be very far away indeed (and it assumes the civilization is not preoccupied like we are in making money and fighting wars instead of trying to peacefully communicate with other civilizations in a more co-operative manner).
The results really do speak for themselves. It would appear the number of civilizations in our galaxy is an absolute pittance and may well explain why we haven't heard from them. The chances of us communicating with ETs would have to be astonishingly small.
Improving the odds
Yet we must remember, the number we have entered are rough estimates. Furthermore, the factor L is probably the single biggest factor for determining whether plentiful alien civilisations exist, or very few.
Assuming all other estimates are fairly accurate, there are two ways to dramatically increase the factor L: 1. Learn to be peaceful for all times including looking after life on this planet; and/or 2. Build the technology to take us to the stars. However, we should make it clear that even if we fail to apply point 1 properly and go for point 2 and later discover at least one advanced alien civilisation with the capability of reaching the stars as we can, there is another factor we should add to this equation. Let us call it factor Z. Its value is either 0 or 1. In other words, we will either find out there are no alien civilisations in our galaxy, or there are potentially many civilisations living out there. Which value we take depends entirely on our attitude toward all living things, both here on Earth and on other planets. To put it simply, if our attitude to life is one where we feel we can conquer and use up what we would describe as "resources" to help keep us alive without any consideration of the recycling requirements and searching for alternative and less harmful solutions and this results in an imbalance in the intricate web of life, then the factor Z is zero meaning that we will not survive for much longer and certainly we would not know precise how many alien civilisations exist. And if we should meet just one alien civilisation in space with this attitude using our technology, this civilisation will quickly realise what we are doing and will deal with us and solve the problem of our attitude by ensuring we are extinct to avoid being taken over by us or destroying their means of survival. A simple introduction of a deadly alien virus on our planet and we will disappear before we know precisely how many alien civilisations there are out there. However, if our attitude is one that shows compassion and love to all living things and we make active efforts to preserve our planet and look after each other, then an encounter with any number of alien civilisations advanced enough to reach us through their own technology will reveal the answer on the number of alien civilisations in our galaxy (and it is likely to be in phenomenal numbers). And at the same time we will understand that all these alien civilisations must have the same positive and loving attitude to life in order for them to guarantee their own survival in this universe, especially among other star-faring alien civilisations, barring any natural catastrophe, of course! So what attitude do you have for life on Earth?
Therefore, if alien civilisations achieve both things mentioned above (i.e. show love and have the technology to visit the stars), it means the Milky Way should be literally teeming with intelligent life.
Is there any evidence to support both approaches being taken by ETs? And have people already observed them to give us an indication that both of these two approaches are taking place in reality?