The observable universe is believed to contain 10 trillion galaxies. And in just one of those galaxies, there are an estimated 400 billion stars. 11 billion of those stars are classified as “yellow dwarves.” Roughly one in five of those stars have planets orbiting them inside a ring called “the habitable zone” — a region which may be able to support life. And within those billions of planets, we have only been able to find one able to do so: Earth. Where you’re listening to us now. But could there be more intelligent life out there? And if so, why haven’t we seen any signs of them? Nobel prize winning physicist Enrico Fermi pondered this very idea, and asked an electrifying question that would inspire scientists the world over to try and answer this strange phenomenon.
Enrico Fermi was born September 21, 1901 in Italy. In 1926, at just 24 years old, he was elected Professor of Theoretical Physics at the University of Rome. During his time there, he discovered that nearly every element would undergo a transformation when subjected to nuclear bombardment.
This work ended up being used as the basis for the discovery of nuclear fission, and led to him being awarded the Nobel Prize for Physics in 1938. He became one of the chief architects of the Manhattan Project, which would lead to the development of the first nuclear bombs. During a lunch in 1950 with Emil Knopinski, Edward Teller, and Herbert York, Fermi asked a simple question that would spark discussion for decades to come.
“The discussion had nothing to do with astronomy or with extraterrestrial beings,” said Teller, recalling the event years later. “I think it was some down-to-earth topic. Then, in the middle of this conversation, Fermi came out with the quite unexpected question, ‘Where is everybody?’ The result of his question was general laughter because, in spite of Fermi’s question coming from the clear blue, everybody around the table seemed to understand at once that he was talking about extraterrestrial life.”
York recalls that Fermi followed up with a series of calculations on the probability of Earth-like planets, the probability of life given on Earth, the probability of humans given life, and the likely rise and duration of high technology. He concluded on the basis of such calculations that we ought to have been visited long ago and many times over.
If the universe is teeming with intelligent life, as the calculations suggested, then due to the sheer age of the universe one of those alien races should have happened on our solar system by this point or left some evidence of their existence. This is the paradox.
But it was the question, not the conclusion, that has sparked debate ever since, causing a flurry of theories to support and disprove Fermi’s lunchtime ponderings. Doug Vakoch, president of the Messaging Extraterrestrials Institute, also known as METI, a nonprofit dedicated to sending signals into space in order to establish communication with extraterrestrials, had this to say about the Fermi Paradox:
“And so it’s really kind of puzzling because in SETI we look for radio signals. Radio signals could travel at the speed of light. We’ve been doing it on and off for over 50 years and we haven’t detected anything. Does that mean they’re not out there? Even though there are some people who think the aliens who come to Earth, there’s no concrete scientific evidence of that. But even if spacecraft travel at a fraction of the speed of light they should be able to get here if they’ve been out there trying to travel for millions of years. The Fermi paradox is this puzzle about if in fact there is intelligent life in the cosmos, why haven’t we yet made contact? The critical thing is to imagine possible explanations that let you do something different to try to find contact.”
You’ll hear more from Doug as we dig into four prevailing theories that aim to answer the question: where are they?
Could it be possible that we haven’t heard from intelligent extraterrestrial life because there are none? Are we the only intelligent life amongst the unfathomable vastness of space? Some say yes.
Michael Hart, a popular astrophysicist, examined in his 1975 paper, “An Explanation for the Absence of Extraterrestrials,” why we might be the only intelligent life in the galaxy. Hart believed that if intelligent alien life is not unique to Earth, then at some point in the 13.5 billion years of the Milky Way’s lifetime, some of those civilizations should have developed interstellar travel. Humanity is relatively new on the scene. If life is not unique to Earth, then there has been plenty of time for other civilizations to leave traces throughout the galaxy.
Once a civilization made it off planet, it is assumed that species would start expanding outwards. If their spaceships traveled one-tenth the speed of light, and the colony started building more ships as soon as they arrived on a planet, then the entire galaxy could be colonized in a mere 650,000 years.
On this point, Carl Sagan mused, “If colonization is the rule, then even one spacefaring civilization would rapidly spread in a time much shorter than the age of the galaxy throughout the Milky Way. There would be colonies of colonies of colonies.”
In Michael Hart’s view, all it would require is one intelligent species to become interstellar in the last 13.5 billion years for us to see some evidence of its existence nearby. The fact that we haven’t seen any sign, he argued, proves they do not exist.
“Decade after decade, century after century, we find absolutely nothing. Again, I think slowly it’s going to dawn on us that because we’ve committed to this search, something more ambitious than anything humanity has ever taken on before, we have in fact become that long-lived stable civilization we’ve been looking for out there all along. So it’s a profound search whether we find something or whether we find nothing out there. As long as we’re able and willing to do the search.”
What if the distances between stars hosting habitable planets are so great that it isn’t possible for life to travel between star systems?
Frank Tipler, a mathematical physicist and cosmologist, believed that at some point in their evolution an advanced civilization would develop self-replicating probes that could explore the galaxy for them. So, even if the distance between stars were too great for intelligent life to traverse, they would send drones in their place. Voyager 1 and Voyager 2 could be seen as early precursors to intergalactic probes that other species would already have achieved.
If this is true, space should be teeming with intelligent self-replicating drones, and yet we have no physical proof that our solar system has been visited by even one.
On Earth it took 3.5 billion years for intelligent life to form under just the right conditions. Life might just be incredibly rare, and intelligent radio-signal-producing life even rarer. But some scientists say that doesn’t mean it doesn’t exist in a universe as expansive as ours.
The problem could be that we’re not looking in the right place or for the right signals. Jill Tarter’s thinking is that we’ve scanned so little of the sky and may not be looking for the right type of signal, or for long enough, to find them. This problem is compounded by requiring our telescopes to be aimed at just the right spot at just the right time to detect a signal.
The search thus far has been likened to having searched a swimming pool, when there’s still an ocean’s worth left to explore.
Shubham Kanodia, graduate student at Penn State’s Department of Astronomy and Astrophysics, has co-authored a research paper titled “How Much SETI Has Been Done? Finding Needles in the n-Dimensional Cosmic Haystack.” In the paper, Shubham and his colleagues attempt to quantify the amount of searching that has taken place to give perspective on whether we should have found something by now. He likens the search to having searched a drinking glass’s worth of seawater for evidence of fish in all of Earth’s oceans.
Frank Drake, founder of SETI, aimed to predict the amount of communicable intelligence in our galaxy based on both known and speculative factors. With this in mind, he created what is now known as the Drake Equation.
“He realized that they were trying to get at this question: How many civilizations are out there right now trying to transmit? So he said, what do we have to know in order to estimate that number? He wrote out a series of seven terms. If you multiply them together you get what’s now called the Drake equation. The seven terms start with the astronomical and move up to the more societal.
“The astronomical terms: the first thing you have to know is how many stars there are in our galaxy. The first term is the rate of star formation. Then, for those stars, you need to know what fraction have planetary systems. Of those star systems, how many Earth-like or potentially habitable planets are there per star system? Then, once you have a planet that’s potentially habitable, what fraction go on to develop life? Once you have life, maybe microbial life, how likely is it that this leads to the evolution of intelligence? Then, of those planets with intelligent beings, what fraction develop the technology that lets them communicate at interstellar distances like radio and laser? And the seventh and final term, perhaps the greatest unknown, is longevity: how long have they been sending out signals, in terms of a lifetime that we can detect?”
“And so you multiply all of those terms together and you get some approximation of how many civilizations are out there right now.”
Through this estimation it is believed there could be anywhere from 10,000 to 100,000 civilizations. If that’s the case, where are they? One possibility is that intelligent species only exist for a very short period of time. They succumb to their own intelligence through warfare and environmental damage.
Doug Vakoch added to this thought: “If that’s the norm, if they’re as old as we are, then it’s incredibly unlikely that they’re a hundred years and our hundred years are going to coincide. Given that the galaxy is 13 billion years old, it’s as unlikely as if two fireflies each flick on for a single moment over the course of a dark night. What’s the chance that’s going to be exactly the same time? Virtually zero.
“The only way we make contact is if the other civilization has been out there doing this, either listening or transmitting, much longer than we have. In essence we’re trying to look into our future. We’re trying to find a civilization that has a stability of its own, a capacity to search far beyond anything we can imagine.”
The Drake equation is commonly referenced when discussing possible life in the universe, but many of the variables cannot be reliably filled in.
“It’s an important formula because it gives us a ballpark sense that this is a plausible search. Over the decades since that was formulated, we’ve gone from having a pretty good estimate of one of those terms, the rate of star formation, to a much better appreciation of how many planets are around each star. Now we know there are billions of them. There are a lot of places to look, a lot of real estate. The big question is in those later terms of the Drake equation that are harder to quantify. Does life appear, does it take a form that we can communicate with often enough to make contact, and do they keep at it long enough?”
Regardless, we have detected a few anomalous signals in our short time searching. There’s the “WTF Star” from 2016, an atypical light pattern from the dimming of a star between the Cygnus and Lyra constellations. While no radio signals were detected from this patch of the sky, the anomalous dimming hasn’t been explained. One theory is that something artificial is blocking the light. Currently, the Allen Telescope Array is pointed in the area of the anomaly in hopes of locating non-natural radio signals.
From July to August of 2018, the Canadian Hydrogen Intensity Mapping Experiment radio telescope in British Columbia recorded a number of fast radio bursts, including one that repeated six times. Fast radio bursts have become more common and have remained a mystery as to their exact origins. Some have theorized these bursts could be artificial and originate from alien civilizations. Other theories include young neutron stars exploding and supermassive black holes spitting out radiation.
However, we shouldn’t immediately label any anomalous space signal as evidence of an alien race. There is still so much we don’t understand about our own universe, and, taken out of context, a signal could be misinterpreted. For example, there is the infamous “WOW” signal from 1977, a 72-second burst of radio signals from a group of stars called Chi Sagittarii. It’s now widely believed that the signal was caused by a pair of comets passing near the patch of sky that was scanned. The comets carried a cloud of hydrogen gas millions of kilometers in diameter that could have caused the signal.
While anomalous signals from space are exciting, extraordinary claims require extraordinary evidence. We have so much to learn about the universe that it is more likely a rogue radio signal comes from a natural process rather than extraterrestrial origin. But listening to space for signals from other civilizations is some of the most exciting and promising research in the search for extraterrestrial life. We’ve only been actively searching for intelligent alien life for 60 years. Who knows what we will find in the next few hundred?
The third theory argues that alien life could be so utterly foreign to our conception of biology that we do not know what to look for.
It has been speculated that life might use silicon for cells rather than carbon. Like carbon, silicon can form bonds with up to four other atoms simultaneously, making it well suited to forming long chains of molecules conducive to life as we know it. Because silicon bonds differently than carbon, these cells would look and act very differently from our conception of a cell. It has even been theorized that Titan, one of Saturn’s moons, could potentially host silicon-based life, using hydrocarbon as a solvent instead of water.
Any life in our solar system would most likely be simple, microbial forms of life. But what would a silicon-based intelligent society look like? Would it follow the same technological evolution as carbon-based life, or develop in ways we can’t predict and therefore can’t even begin searching for?
Astrophysicist Nikolai Kardashev developed a scale to help rate intelligent life by its technological development. It is worth noting that human beings haven’t even completed the first tier.
A Type I civilization has figured out how to harness all the energy on its planet. Humans are getting close to achieving this. Type II civilizations have figured out how to harness all the energy of their star. Type III civilizations have harnessed all the energy available in the galaxy.
How can we expect to understand civilizations that have made it to Type III when we haven’t even fully grown into a Type I civilization? What forms of communication might these races employ, and how would we know what to listen for?
Think about how much humans have achieved in our short time on this planet. Imagine an alien race that has surpassed our own by a million years. Its technology would be so far beyond anything we could comprehend that we may not even know what to look for. Arthur C. Clarke said it best: any sufficiently advanced technology would appear as magic to us.
MIT radio astronomer John Bell wrote that extraterrestrials “may be beyond our intellectual horizon; some of the phenomena that we already see may be associated with ET, but we don’t understand.” The comparison he draws is to an ant wandering through New York City: the ant has no conception of the humans and their intellectual capacities operating all around them.
Is it possible alien life just hasn’t wanted us to find it? John Bell was also the first author to write about the concept of the zoo hypothesis. It theorizes that there are intelligent alien lifeforms in the galaxy, but they are purposefully avoiding contact with the human race. They may be observing from a safe distance, not wishing to interfere with our technological and societal development.
Doug Vakoch had this to say about the theory: “Maybe there are other beings out there watching this much like we watch animals in the zoo. But imagine you and I go to the zoo and see a bunch of zebras. What if one of those zebras turns toward us, looks us in the eye, and starts pounding out a series of prime numbers? That would give us a radically different relationship.”
“Let’s say that the zoo hypothesis in its strongest form is true: the extraterrestrials out there are watching us, disciplined zookeepers. It’s almost like in Star Trek terms of the Prime Directive. We’re not going to let you know we’re here. We’ll just watch you. If that’s happening in the strongest form, there’s no way to distinguish that response from there being no extraterrestrials at all.
“The only thing we can do is find variations of that strong zoo hypothesis. A weaker zoo hypothesis might mean that some responses and some attempts to make contact will yield a response. According to the federation you’re not supposed to mess around with another civilization, but when those civilizations reach out and start attempting contact, all bets are off and it’s okay to welcome them into the cosmic conversation. It could be that sustainable civilizations are the ones that learn to work within their means.
“Maybe the civilizations that keep a steady program going for thousands or millions of years are more modest in the signals they’re looking for. Those civilizations may not have the technology to come to Earth, but maybe they’ve been looking for a signal. We could make a difference by sending an intentional signal.”
It could be possible that we offer no value to them and pose no threat, so why should they bother communicating with humans? You have to wonder, if this is the case, would they remain silent once humans gain the technology to leave our own solar system.
What if they’re already watching us? What if we can’t recognize them? What if we haven’t looked in the right spot? What if we’re alone? Sometimes it may seem that all that has popped up since Fermi asked his infamous question nearly 70 years ago are more questions. But there are people looking for the answers. In June of 2019, NASA’s Curiosity rover’s tunable laser spectrometer, called SAM, discovered the most methane ever measured during its mission.
The main source of methane on Earth is microbial life. While NASA cannot conclude whether or not it is biological, it has poured more fuel on the fire. If there is life within one solar system, could there be more around the septillion stars that fill this universe? And if there are, where is everybody?