Is there life elsewhere in the Milky Way
to occur. Another theory is that micro-organisms were sent here by a technological civilization on a long-range space craft, in this case Earth would not be an example of a habitable planet fromwhich life could evolve.
The final hypothesis I shall discuss is that life seems to defy the Universal Laws of Thermodynamics, which states that there is an inevitable tendency toward increasing disorder, however life seems to go against these laws, as life is the appearance of order from disorder, hence why life forming may be rare and unique to Earth. This is all speculation of course, and hence our value for the probability of life developing if the conditions are favourable is one of the most uncertain, yet we shall assign a value of 0.5. Intelligent life is a vague phrase, as a dog, for instance, could be considered intelligent, when compared to a plant. However, intelligent life in this essay refers to a species that can make use of its planets resources and can assert dominance over the planet, as humans have done. Initially, unicellular organisms must have a Carbon backbone to evolve intomulti-cellular organisms, as it could be possible for primitive life to formwith a Silicon backbone, then for intelligent life to develop, aerobic respiration must occur which requires oxygen from the atmosphere, which is made by photosynthesis. This is important as through aerobic respiration, the cell makes 18 times as much energy from each glucose molecule as anaerobic respiration does, essential for increasing brain capacity. Photosynthesis began around 2.4 billion years ago, meaning intelligent life had 2.4 billion years to develop. One approach to calculating f i is that out of the billions of species that have lived on Earth, only humans have managed to become intelligent, implying our value for f i is low. However, there is a trend showing increasing complexity of organisms over time, due to mass extinctions and evolution, indicating that intelligent life was bound to occur. 10 Therefore, our value for f i is 0.8. Next is the calculation of f c . A problem that develops is that other civilizations may use other forms of communication which humans cannot detect, this is a valid argument, however Hydrogen is the most abundant element in the universe, and it is assumed any extra-terrestrial astronomer would be tuned to the ‘21 cm line’ which is the exact frequency of radio waves th at Hydrogen atoms emit. 11 There is also the problem that civilizations may be too far away for us to communicate with, which is a likely reason why we have had no contact with extra-terrestrial life so far. Humans have only been releasing detectable signals to space for the last 80 years, so only intelligent life within an 80 light year radius of Earth could have received these signals. It could be that they simply are untrusting, and do not wish to communicate with civilizations on other planets. Therefore, I shall assign a value to f c of 0.2. The lifetime that a civilization releases detectable signals into space depends on how rapidly they become extinct, whether by meteorite impact, or lack of self-preservation. Using humans as an example, with the emission of greenhouse gases from the burning of fossil fuels, temperatures may rise so high as to end the human species, in which case the lifetime of communication available would be approximately 150 years. However, if humans do become more ecological, there is no reason why we couldn't live for millions of years more. Consequently, I will state the value of L to be 10 6 .
10 Langmuir 1985. 11 Cox 2014.
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