The SETI project – Search for Extra-Terrestrial Intelligence – has been in existence in one form or another for several decades, dating back to American astronomer Frank Drake’s first SETI experiment named Project Ozma. SETI is basically the search for intelligence through listening for radio waves of another civilization. For Drake back in the 1960’s, this was the sign of a technologically prevalent society, and the smartest means to search for life.
The Drake equation states:
N = Ns x fp x ne x fl x fi x fc x fL N = number of alien civilizations in the Milky Way
Ns = estimated number of stars in the Milky Way;
fp = fraction or percentage of these stars with planets on its orbits; ne = average number of these planets with potential to host life as we know it;
fl = percentage of these planets that actually develop life; fi = percentage of these planets that actually develop intelligence on human level;
fc = percentage of these civilizations that actually develop electromagnetic radiation emitting technologies;
fL = percentage of these civilizations that keep emitting electromagnetic signals to space. This factor is extremely dependent on the lifetime a civilization remains electromagnetic communicative.
Looking at the Drake equation factors, says Astrobio.net, it is obvious that none can be precisely determined by modern science. More than that, as we move from the left to right in the equation, estimating each factor becomes more controversial. The later terms are highly speculative, and the values one may attribute to each of them might tell more about a person’s beliefs than about scientific facts.
But the Drake equation must not be evaluated only by the numerical values it produces. Some say the Drake equation is a way to organize our ignorance. By exposing the extraterrestrial intelligence hypothesis mathematically, we limit the real possibilities to each term and approach the final answer: how many alien civilizations are there?
The L term is considered the most important one in Drake equation. We have no idea how long a technological civilization can last. Even if only one extraterrestrial civilization lasts for billions of years, or becomes immortal, the L factor would be enough to reduce Drake’s equation to N = L. Actually, Frank Drake recognizes this in his license plate: “ NEQLSL ”
However, one can only look at the decision to search for intelligence through listening for radio waves of another civilization and see it as a mistake. At least, that is what some scientists and others connected to the field of extra-terrestrial search believe, such as George Dvorsky, who serves on the Board of Directors for the Institute for Ethics and Emerging Technologies. Recent insights in such fields as cosmology, astrobiology have changed our perception of the cosmos and the ways in which advanced life might develop.
In the early days of SETI its astronomers predicted a steady rise in radio traffic, as populations and technology advanced. But the reverse has happened. Point to pint communications have become dominated by low-powered satellites directing their signals Earthward while the bulk of telecommiunications shifted away from radio to buried fiber optics for cable TV and Internet traffic. In another hundred years there will be no substantial radio output from Earth.
Another key weakness is Fi -the fraction of those Earthlike planets (10,000 was Drakes estimate in the Milky Way) on which intelligence evolves. To date, there isero evidence that there is a "life principle" directing primal chemical soups towards the glory of a homo sapiens-like species. Until we find strong evidence for life on an exo-solar planet, Fi remains moot.
The Drake Equation does not take into consideration such factors as the age of the Galaxy, when intelligence first emerged, or the presence of physio-chemical variables such as the presence of metals necessary for the presence of life and the formation of planets. The equation, Dvorsky emphasizes, assumes "a sort of cosmological uniformity rather than a dynamic and ever changing universe."
The equation asks us to guess the number of Earth-like planets, but it does not ask us to estimate when Earth-like planets evolve advanced life forms. The Milky Way's extreme age and the potential for intelligence, which may have been present as long as 2 to 4.5 billion years ago, to have emerged at disparate points in time leaves an absurdly narrow window for detecting radio signals.
The Drake Equation, Dvorsky believes, does not tell us about exponential civilizational growth on account of Von Neumann probe disbursement. "It does not tell us where advanced ETI’s may be dwelling or what they’re up to (are they outside the Galaxy? Do they live inside Jupiter Brains? Do they phase shift outside of what we regard as habitable space? ).
This is a serious shortcoming because the answers to these questions should help us determine not just where we should be looking, but they can also provide us with insight as to the makeup of advanced intelligence life and our own potential trajectory."
In other words, Dvorsky concludes, post-Singularity machine-based intelligence may represent the most common mode of existence for late-stage civilizations. And that’s who we should be looking for rather than radio transmitting civilizations.
Since 1992 astronomers have been finding more and more exoplanets and as of today over to 2000 exoplanets are confirmed. The number of Sun-like stars with planets is believed to be around 40% or higher. Currently most of the planets found are massive and orbit very close to their stars (they’re called Hot Jupiters), but as detection techniques improve scientists think many more planets will be found of different sizes and orbits.
Research of the past two decades have shown that literally billions of planets in the Milky Way might have niches that would support at least a level of life represented by Earth's extremophiles.
Yet, in 2012, Drake Equation is of still of seminal importance because it orders our thinking. This one equation formed the backbone of astrobiology as a science. Carl Sagan was inspired that the Drake Equation showed the chances of intelligent alien life were high but he also added that extraordinary claims require extraordinary evidence.
In 2010, the Italian astronomer Claudio Maccone published in the journal Acta Astronautica the Statistical Drake Equation (SDE). It is mathematically more complex and robust than the Classical Drake Equation (CDE).
The SDE is based on the Central Limit Theorem, which states that given the enough number of independent random variables with finite mean and variance, those variables will be normally distributed as represented by a Gaussian or bell curve in a plot. In this way, each of the seven factors of the Drake Equation become independent positive random variables. In his paper, Maccone tested his SDE using values usually accepted by the SETI community, and the results may be good news for the “alien hunters”.
Although the numerical results were not his objective, Maccone estimated with his SDE that our galaxy may harbor 4,590 extraterrestrial civilizations. Assuming the same values for each term the Classical Drake Equation estimates only 3,500. So the SDE adds more than 1,000 civilizations to the previous estimate.
The image below is the Gaussian or bell curve showing the probability of finding the nearest extra terrestrial civilization from Earth.
Another SDE advantage is to incorporate the standard variation concept, which shows how much variation exists from the average value. In this case the standard variation concept is pretty high: 11,195. In other words, besides human society, zero to 15,785 advanced technological societies could exist in the Milky Way.
If those galactic societies were equally spaced, they could be at an average distance of 28,845 light-years apart. That’s too far to have a dialogue with them, even through electromagnetic radiation traveling in the speed of light. So, even with such a potentially high number of advanced civilizations, interstellar communication would still be a major technological challenge.
Still, according to SDE, reports Astrobio.net, the average distance we should expect to find any alien intelligent life form may be 2,670 light-years from Earth. There is a 75% chance we could find ET between 1,361 and 3,979 light-years away.
Beyond 500 light-years away, the chance of detecting any signal from an advanced civilization approaches zero. And that is exactly the range in which our present technology is searching for extraterrestrial radio signals. So, the “Great Silence” detected by our radio telescopes is not discouraging at all. Our signals just need to travel a little farther – at least 900 light years more – before they have a high chance of coming across an advanced alien civilization.
Image credit: With thanks to Maccone (2010) and astrobioloblog.wordpress.com