Alien civilisations, including advanced ones, are significantly more likely to have existed out there in the Universe than not, that is, if you make the calculation of likelihood starting right from the Big Bang when everything started, say two scientists from the University of Rochester and the University of Washington.
We have long been wondering whether we are unique and alone in our vast Universe. The question, summed up in the famous Drake equation, has for the past five decades been one of the most uncertain and intractable in science.
Adam Frank, professor of physics and astronomy at the University of Rochester, and Woodruff Sullivan, professor in the astronomy department and astrobiology program at the University of Washington, explained in the journal Astrobiology (citation below) that recent discoveries of *exoplanets combined with a broader approach to that question, have made it possible to assign a new empirically valid probability to whether any other advanced technological civilization has ever existed.
Even if there are alien civilisations out there, the distances are so vast that having a conversation with them would be virtually impossible. If one was 2,000 light years away, for a message to get there and a response to return to us would take 4,000 years.
* An exoplanet is a planet outside our Solar System.
And it shows that unless the chances of advanced, intelligent life evolving on a planet in the habitable zone, also known as the Goldilocks zone, are astoundingly low, then we are not the first technological or advanced civilization in the Universe.
Pessimists and Optimists regarding alien civilisations
The authors say that their study also shows, for the first time, exactly what ‘pessimism’ or ‘optimism’ mean when it comes to estimating the probability of advanced alien life.
Prof. Frank said, regarding the question of whether smart aliens ever existed elsewhere, said:
“The question of whether advanced civilizations exist elsewhere in the universe has always been vexed with three large uncertainties in the Drake equation.”
Drake’s equation (top) and Professors Frank and Sullivan’s equation (bottom). (Image: rochester.edu)
“We’ve known for a long time approximately how many stars exist. We didn’t know how many of those stars had planets that could potentially harbor life, how often life might evolve and lead to intelligent beings, and how long any civilizations might last before becoming extinct.”
“Thanks to NASA’s Kepler satellite and other searches, we now know that roughly one-fifth of stars have planets in “habitable zones,” where temperatures could support life as we know it. So one of the three big uncertainties has now been constrained.”
Prof. Frank said that the third big question – how long any civilization in our Universe might survive – is still something we are unable to calculate.
Humans have had rudimentary technology for approximately 10,000 years. However, this fact does not really tell us whether other societies would last that long, or perhaps much longer, the authors wrote.
However, the scientists found that they could eliminate that question altogether if they expanded the question.
Widen the question to include a longer time-span
Prof. Sullivan said:
“Rather than asking how many civilizations may exist now, we ask ‘Are we the only technological species that has ever arisen?”
“This shifted focus eliminates the uncertainty of the civilization lifetime question and allows us to address what we call the ‘cosmic archaeological question’ – how often in the history of the universe has life evolved to an advanced state?”
According to NASA, the ‘Habitable Zone'(Goldilocks Zone) is: “The range of distance from a star where liquid water might pool on the surface of an orbiting planet.” Life as we know is more likely to exist in the Habitable Zone. There is no evidence, however, ruling out other types of life forms that don’t depend on liquid water, sunlight, etc. (Image: keckobservatory.org)
That still leaves enormous uncertainties in calculating the chances of advanced life evolving on habitable planets or exoplanets.
Turn the question the other way round
It is here that the authors turned the tables on the question. Rather than guessing what the odds are for advanced life developing, they calculated the odds against it occurring in order for humankind to be the only advanced civilization in the entire history of the observable Universe.
With that, they then calculated the line between a Universe where we have been the sole experiment in civilization and one where we have been preceded by others.
Prof. Frank said:
“Of course, we have no idea how likely it is that an intelligent technological species will evolve on a given habitable planet. But using our method we can tell exactly how low that probability would have to be for us to be the ONLY civilization the Universe has produced.”
“We call that the pessimism line. If the actual probability is greater than the pessimism line, then a technological species and civilization has likely happened before.”
The researchers used this approach to calculate how unlikely advanced life must be if among the Universe’s ten billion trillion stars there has never been another example, or even among the hundred million in our own galaxy the Milky Way.
What is the result? By applying the new exoplanet data to the number of stars estimated to be in our Universe – 2 x 10 to the 22nd power – the authors found that human civilization is likely to be unique only if the odds of a civilization developing on a planet in the Goldilocks zone are less than about one in 10 billion trillion, i.e. one part in 10 to the 22nd power.
Smart aliens likely evolved before us
Prof. Frank said:
“One in 10 billion trillion is incredibly small. To me, this implies that other intelligent, technology producing species very likely have evolved before us. Think of it this way, before our result you’d be considered a pessimist if you imagined the probability of evolving a civilization on a habitable planet were, say, one in a trillion.”
“But even that guess, one chance in a trillion, implies that what has happened here on Earth with humanity has in fact happened about a 10 billion other times over cosmic history!”
For smaller volumes the numbers are not so extreme. For example, another technologically-advanced species likely has evolved on a habitable planet in our own galaxy if the odds against it evolving on any one habitable planet are greater than one chance in 60 billion.
However, if those numbers seem to support the optimists regarding the existence of extraterrestrial civilisations, Prof. Sullivan points out that the full Drake equation – which calculates the probability of other civilisations existing today – may encourage the pessimists in their argument.
In 1961, American astronomer and astrophysicist, Frank Drake, developed an equation to estimate how many (if any) advanced civilisations are likely to exist in the Milky Way.
The Drake equation
The Drake equation is a popular and proven framework for researchers, and space technology has advanced mathematicians and physicists’ knowledge of such variables. However, all we can do is guess at such variables, such as ‘L’, the probable longevity of other advanced civilisations.
In their latest study, Professors Frank and Sullivan offer a new equation to address a slightly different question: ‘What is the number of advanced civilizations likely to have developed over the history of the observable universe?’ While still drawing on Drake’s equation, theirs eliminates the need for ‘L’.
The authors’ argument hinges upon the recent discovery of how many exoplanets exist, and how many of them lie in the habitable zone – planets with the right temperature for liquid water to exist.
This allows Professors Sullivan and Frank to define a number they call Nast. Nast is the product of N*, the total number of stars out there; fp, the fraction of those stars that form planets; and np, how many of those planets (average) are in the habitable zone of their parent star.
They then set out what they called the Archaelogical-form of Drake’s equation, which defines ‘A’ as the “number of technological species that have ever formed over the history of the observable Universe.”
Professors Frank and Sullivan’s equation, A=Naast*fbt, describes A as the product of Nast – the number of habitable exoplanets in a given volume of our Universe – multiplied by fbt – the likelihood of a technologically-advanced species existing on one of these planets.
The volume considered could be just within the Milky Way or the entire Universe.
Most alien civilisations could be extinct
Prof. Sullivan said:
“The universe is more than 13 billion years old. That means that even if there have been a thousand civilizations in our own galaxy, if they live only as long as we have been around—roughly ten thousand years—then all of them are likely already extinct.”
“And others won’t evolve until we are long gone. For us to have much chance of success in finding another “contemporary” active technological civilization, on average they must last much longer than our present lifetime.”
Regarding the chances of talking to another advanced civilization, Prof. Frank said:
“Given the vast distances between stars and the fixed speed of light we might never really be able to have a conversation with another civilization anyway. “If they were 20,000 light years away then every exchange would take 40,000 years to go back and forth.”
Even if there are no other civilisations in the Milky Way to communicate with right now, the authors point out that the new result still has a profound philosophical and scientific importance.
Prof. Frank said:
“From a fundamental perspective the question is ‘has it ever happened anywhere before?’ Our result is the first time anyone has been able to set any empirical answer for that question and it is astonishingly likely that we are not the only time and place that an advance civilization has evolved.”
The researchers say their result also has a practical application. As humankind faces its crisis in climate change and sustainability, we can wonder whether other civilization-building species on exoplanets have experienced a similar bottleneck and survived through it.
We don’t even know whether a technologically-advanced civilization can prevail for more than a few centuries, Prof. Frank pointed out.
With Professors Sullivan and Frank’s new result, researchers can start using everything they know about exoplanets and climate to begin modelling the interactions of an energy-intensive alien species with their home world, knowing that a large sample of such cases has already existed in the Universe.
Prof. Sullivan said:
“Our results imply that our evolution has not been unique and has probably happened many times before. The other cases are likely to include many energy intensive civilizations dealing with their feedbacks onto their planets as their civilizations grow.”
“That means we can begin exploring the problem using simulations to get a sense of what leads to long lived civilizations and what doesn’t.”
Citation: “A New Empirical Constraint on the Prevalence of Technological Species in the Universe,” Frank A. and Sullivan W. T. Astrobiology. April 2016. DOI: 10.1089/ast.2015.1418.
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