Understand the meaning of numbers when they refer to very large amounts or very small is complicated. This is something that we experience right now with the pandemic of the COVID-19. The combination of a few very large numbers (the total population capable of being infected) along with a few odds trace (associated to the contagious individual and the onset of certain severe symptoms) gives as a result a few amounts of very difficult interpretation. Especially if we ignore the availability of hospital services. Thus, the differences in risk between the individual and the community are intertwined and at times swings between the excessive alarmism and optimism unjustified.
To illustrate this situation in a context more friendly, we can use the formula presented in 1961 by the radioastrónomo Frank Drake to give a quantitative basis to the SETI project (Search for Alien Intelligence) in the second half of the TWENTIETH century. SETI was an attempt by scientist to determine the likelihood of establishing contact with extraterrestrial civilizations in our galaxy, something that many see as mere science-fiction, but that we can address with a spirit of analytical.
How could we determine the number of events of a specified event as improbable as the communication with a technological society, extraterrestrial? The strategy consists in separating the factors that need to be concatenated to achieve a positive. We will assume that all of these events are independent and that, therefore, we can determine the probability of which is given simultaneously with a simple multiplication. Adding factors we can arrive at a reasonable estimate on the probability of the final event: the one in which all events are combined perfectly.
This task covers a number of disciplines ranging from the experimental sciences to speculations of very questionable grounds, passing by areas of knowledge is hardly measurable as the history and the social sciences.
We could begin by taking as the prime factors, the fraction of stars in the galaxy that have planets in your environment and the amount of these that have the appropriate composition and are in the zone of habitability. These are issues that the latest space missions like Kepler, Gaia, and TESS allow us to quantify. We walk on firm ground, or, at least, a ground firmer than that Drake could do at the time.
How do we quantify the probability of the emergence of life on a planet that apparently meets the right conditions? How many of them have developed intelligent life in the form of civilizations? To date, although Mars or Venus to meet habitable conditions in the past, we know only a positive case: the Earth. None of the more than 4 000 extra-solar planets known has shown to of time to sustain a biosphere such as ours.
From here the question is complicated. We would need to quantify the evolution of societies towards technology and desire to communicate with the rest of the universe. It will also influence the time during which they are able to do it: a century, a thousand years or, as suggested by Drake, up to 10 000 years. The experimental data to establish these quantities are very scarce and are based on human history and on the dynamics of the societies that have just started to understand in a quantitative manner.
At the time that Frank Drake was assigned values to all of the terms, found a remarkable result: up to ten civilizations should be detectable by SETI. But, if so, where are you? This is the so-called Fermi Paradox, the opposite of the optimism of Drake. Find the reasons for this disturbing silence, as it has been called, is also a good way of exploring our immediate future and try to guess at the risks that as a civilization we can expect to round the corner and cosmic.
Other authors disagreed with the numbers of Drake from the first instant, obtaining values much lower than demonstrating the improbability of achieving the contact, thanks to the SETI project. Small variations in the terms that are multiplied in that long chain have resulted in notable changes of the final result and, worse still, the uncertainties are propagated exponentially in the result.
Of the Drake equation we can learn that the individual events can be very infrequent or unlikely but, applied to a population large enough, their appearance is inevitable. In addition, when events depend on a long chain of conditions whose probabilities we cannot estimate with complete certainty, our ability to predict future events is obscured. The difference with the epidemiology is that, in this, we seek that the events happen in the smallest possible number, and, to do this, we can act on some of the factors involved.
From a health point of view, the probability of a single event, as that sick with serious symptoms, it can be very low, almost negligible. Applied however on the whole population, it will end up happening. And it will do so more than once. The influencing factors include the biology, physiology and sociology, with a crosswalk similar to that of astrobiology.
The good news is that changing this is in our hands: by altering a few factors we can reduce the number to an amount, if not zero, at least manageable. So we are on it.
This article has been published in The Conversation.