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“Where are they?” was a question of a physicist named Enrico Fermi during a lunch break with his colleagues back in the 1950’s. He realized that given the size and age of the Universe, extraterrestrial civilizations should have already developed a sophisticated enough technology to concur other galaxies and ultimately made their presence know to us. The title of this blog post refers to the absence of alien life in the Universe, also known as the Fermi Paradox. If we were able to answer his question, it would give us some information about the fate for intelligent life in the Universe. That’s important for us because we humans are, believe it or not, included in the spectra of intelligent life.

To start things off, there is two basic things we need to consider when we are talking about the possibilities of encounter intelligent life. The quantity of planets which has the right conditions to support life and that there is enough time to develop intelligent life on these planets. You have to remember that when we are talking about life, we are talking about carbon-based life, which is the only life we know about. There could very well exist non-carbon-based life somewhere else in the Universe, but it's easier to look for something that we at least could recognize.

So, let’s crunch some astronomical numbers!

There is approximately 100 to 400 billion stars in our galaxy, you can see a very small sample of them in the poster above. To narrow down our search for life a bit, we’ll only look for stars that are similar to our own sun in terms of mass and temperature. As it turns out, about 20% of every star in the Milky Way is similar to our sun. Furthermore, every fifth of the Sun-like stars has an Earth-like planet orbiting in the habitable zone. The habitable zone is where a planet can host liquid water, which means it’s neither too hot or too cold — just like Earth. This will get us a number of 4 to 16 billion Earth-like planets theoretically able to harbor life, and this is in our galaxy alone! If only 0.1% of these actually were able to support life we would have 4 to 16 million of these planets. Furthermore, estimates from astrophysicists says that there is roughly the same amount of galaxies in the Universe that there are stars in our galaxy, which makes it highly unlikely that we are the only planet that has ever hosted life.

The Milky Way Our galaxy, the Milky Way seen from above. You can find the location of the Sun in one of the spirals called Orion Spur. Credit: NASA/Adler/U. Chicago/Wesleyan/JPL-Caltech

Earth has been around for about 4.5 billions years and that could be consider a relative young planet compared to the age of its galaxy, which is about 11.4 billion years old. The universe itself is calculated to be about 13.8 billion years old.

Timeline of the Universe An illustration of the creation of the Universe. Credit: NASA / WMAP Science Team

To put things in perspective let’s image that the entire lifespan of our universe has the length 1 year, or, 365 days. This will mean that 1 month is (roughly) equal to 1.15 billion years, 1 week is 265 million years, 1 day is 38 million years, 1 hour is 1.6 million years, 1 minute is 26 thousand years and finally, 1 second is 440 years. The calculations are derived from how long one day is (13.8/365 billion years) and the result is also rounded up or down to make the result cleaner without getting to far from the real value.

With our new timescale the Milky Way was created just past midnight of March 6 and Earth itself on September 4. Life begun to emerge on Earth almost one month later, on the last day of September. (But it could have occurred even earlier than so.) The fifth and last extinction in Earth’s history, the one that wiped out the dinosaurs, happened just one and a half days ago. The anatomically modern humans (homo sapiens sapiens), i.e. us, is believed to has its origin in Africa 200 thousand years ago. However, on our new timescale that would be just short of 8 minutes before New Years Eve. The industrial revolution that made major technological developments possible happened just 0.6 seconds ago. Mankind's giant leap, the moon landing, occurred just 0.1 seconds ago.

Think about that for a moment, we humans have been here just for a snapshot of the history of the Universe.

In theory, this means that there has been plenty of time for other civilizations to evolve and spread through out the galaxy. According to the Kardashev scale there is at least three types of civilizations based on their technological development and energy consumption. Energy consumption is a good indicator for how advance a species are. You need to reach a high enough civilization type to concur a galaxy and ultimately the Universe. According to Carl Sagan we are currently a type 0.7 civilization and we’ll maybe reach a type 1 civilization in the next hundred years or so.

Let’s get to know the three different kinds of civilization types, and as you probably already figure; this is just a thought experiment and for now, only science fiction.

Type 1 Civilization

A civilization of this type is a planetary society (just like we are on planet Earth) that harness all the energy of its planet. This civilization has an understanding of the laws of physics and has developed space and nuclear technology as well as electromagnetic communications and transmission of information (i.e. radio waves, microwaves, infrared radiation and light). Their technological development has made it possible for them to do space flights and interplanetary travel followed by settlement of space. As the civilization has reached this point they’ve started to push the limits of its planets resources.

Type 2 Civilization

When a civilization has reached this level they’ve moved on from being only a planetary society to being a stellar system society, i.e. their civilization stretches out through the whole solar system. In order to do so they need to create space habitats (either on other planets or on something man made) and be able of harness all the energy that its star(s) produces. The energy conversations is theoretical possible with a Dyson sphere. A Dyson sphere could for example be a swarm of solar panels that encompasses the star in order to harvest its energy. So, what would the Dyson sphere be made of? In theory they could use self-replicating probes to harvest other planets and moons in their solar system for its raw material. The Dyson sphere will be the ultimate limit for how much energy this civilization can obtain, kind of like how the type 1 civilization's limit is their planets resources. They’ve also probably begun their search for intelligent life in the Universe as their lifespan has increased within 1 thousand to 100 thousand years. Their increased lifespan makes it easier for interstellar travel and colonization, because obviously they have more time on their hands.

Dyson Sphere An artists illustration of the creation of a Dyson sphere where two probes mine a planet for its resources. Credit: Shield World Construction by Adam Burn.

Type 3 Civilization

A civilization of this type has conquered their whole galaxy and can harness all of its energy, this may even include the super-massive black holes which seems to exist in the center of most galaxies. Their lifespan has consequentially increased to an astounding 100 million to 1 billion years and the technological development of this kind of civilization is just unimaginable. It’s probable that they use more sophisticated and exotic ways of communication and information transfer that we could ever imagine, maybe they take advantages of neutrinos or dark energy somehow. An other interesting point of view is that the former NSA employee and whistle-blower, Edward Snowden, has recently said that proper use of encrypted communication will make it impossible for an outsider to distinguish conversations from cosmic background noise.

von Neumann Probes

So how would a type 3 civilization conquer a whole galaxy? One theory is the use of von Neumann probes, which essentially is self-replicating automatic robots driven by artificial intelligence. This probes spreads through out the galaxy similar to how viruses spreads in nature. Up on finding a new host they replicate themselves over and over again until they gets launched in various directions. A host in this case could be a planet, a moon or even an asteroid. It would mine the host for its raw material to create new copies of itself to further conquer the galaxy.

If this was to happened in our galaxy, and the probes was traveling at 10% of the speed of light every planet would have been colonized in only a couple of million years. If the speed was reduced to 1% of the speed of light, it would take about 20 million years instead. It may sound a lot to go from a couple of million years to 20 million years, but in the grand scheme of things it doesn’t make a difference. Do you remember the timescale we created earlier? Let’s say that some civilizations von Neumann probes were traveling even slower, like 0.5% of the speed of light, then it still would only take one day to colonize the entire galaxy.

If this seems a lot like science fiction, it’s because it is. But keep in mind that something that has been done in nature can, with enough time, be replicated and perfected by us in a laboratory. E. coli bacteria and seeds from trees are both examples of von Neumann probes created by nature.

So, Why Would Anyone Want to Conquer a Galaxy?

There are numerous arguments to why a civilization would like to do this. For once, subgroups of a civilization may want to expand or escape their territory. Consequently, a civilization may like to prevent the same subgroup from expansion or escaping — and this can only be done if you're in control of this territories already. You can compare it to the early stages of the modern human race, when we still only roamed in Africa. A subgroup of the early homo sapiens sapiens decided to leave Africa and after many years westerners conquered the world and came back to Africa to enslave people still living there. In this metaphor does Africa represent the Earth and the whole planet represents the Milky Way. Consequentially; the best way to escape something, is to get there first.

A different argument to why someone would like to expand in to its galaxy is to ultimately prevent your civilization from collapse. So far in the history of Earth there have been five mass extinctions, the most severe one killed about 95% of every living species at the time. This is probably nothing unique to our planet so we can assume that this happens on most planets in the Universe were there is life. To secure your civilizations future you have to create a backup of it and locate it somewhere else, essentially how you create backups of your important pictures, documents etc. on your computer. (You do keep backups of your files, right?) In case of a disaster and your computer stops working or gets stolen you still would’ve access to your data. So if a planet with an intelligent civilization gets destroyed or a new mass extinction event takes place, that civilization would still be around if they were multi-planetary.

But the argument I find most compelling is if you look at the problem as a prisoners dilemma. If you don’t know what the prisoners dilemma is you can read more about it here. But what it basically means in this situation is that you, as a civilizations would like to grab the galaxy first, to prevent other, possible evil civilizations from doing it before you. If an evil civilization took control of your galaxy it probably means the end of yours, or at best, you become enslaved before you get wiped out. (This do require you to be any use as a slave for them, which you probably aren’t.)

Possible Explanations to the Fermi Paradox

If no other civilization exists in the Universe, there might be some kind of great filter out there making the Universe a hostile place. Either we are past this filter or it’s ahead of us. This filter can for example be between the formation of a planet and the state of our civilization today or it could be between a civilization like ours and a much more advanced space civilization. We don’t really know where we are at this stage, either of them are just as likely. But there have been some exciting news lately that scientist have found water on Mars, which makes it more probable that life at some point has existed on there. However, Nick Bostrom, a Swedish philosopher at the University of Oxford, argues that if would be very bad news for us if we will find evidence of life on Mars since that would indicate that the great filter is ahead of us and that we will soon encounter it. If the findings is of only simple cells that could still mean that the great filter is between those simple cells and where we are now. But that gap is probably much smaller than the gap between where we are now and to this advanced space civilization.

If we assume that the great filter is behind us and we are the only ones that miraculous made it through it, what critical steps in our evolution could be part of this filter? We know that our planet itself is not unique in any way, so it must have something to be with how life arise or evolves. It took many hundred million years before a random interaction between molecules formed something that were able to replicate itself. The criterion Nick Bostrom use for a step to be a valid part of this filter is that it should only occurred once and that is highly improbably to even happened at all. The very first self replication life form certainly matches this criteria. Additional filter candidates could be the transition from simple cells to complicated cells, i.e. from prokaryote to eukaryota cells, this single step took about 1.8 billion years of evolution.

Furthermore we have candidates like the rise of multi-cellular organisms, sexual reproduction and the ability to pass down information and knowledge from generation to generation. Maybe even our opposable thumb is a key difference that made it possible for us to use our intelligence in such a profound way. If you have the intelligence but lack the physical attributes to be able to develop technology, then your journey probably ends there. Or maybe, the great filter is just time and we have had enough of it.

If this filter is not behind us, then it’s in our future. So what kind of challenges do we face? The most obvious one is probably climate change, climate change is such a real issue that environmental scientist even face depression from doing their research about it. Even if we we’re to overcome climate change there is a couple of more possible events that can wipe us out. Some people have raised concerns about asteroid impacts, like the one who was a major contribution to the extinction of dinosaurs 65 million years ago. But I think that’s a low risk issue since first of all; they didn’t have the technology that we have today to protect themselves from asteroids, a small nudge is all it takes to deflect an asteroid from hitting Earth. And secondly, life ending asteroids are not that common. Of course it’s a lot more complicated than that, but I still think it’s the least of our concerns, but I may certainly be wrong about this.

James Annis, an astrophysicist at the Fermi National Accelerator Laboratory, has an interesting take on the Fermi Paradox; what if explosions of dying stars, also called supernovas, can wipe out intelligent land based life on a galactic scale? Supernova’s produces gamma ray bursts that is so energy dense that it may disturb the evolution of life on planets that it reaches. It’s believed that supernova’s were quite common in the early stages of our universe but has slowly become less frequent as time has passed by making it possible for intelligent life to arise. Maybe we are in the transition phase where the time between supernovas are longer then the time it takes for intelligent life to evolve.

Supernova On the left is Supernova 1987A after the star has exploded. On the right is the star before it exploded. Credit: Anglo-Australian Observatory

We have just begun our journey to become a multi-planetary species with our plans to put humans on Mars. This is the first step we’d expect other species had done before us, maybe this is part of the great filter. If you don’t have a habitable planet in your imitate surroundings you’ll fail as a spices to become multi-planetary because there is to big of a step to migrate to an other solar system. If you don’t have a planet like Mars which you could theoretically terraform, then maybe you’ll eventually destroy your own planet, and consequentially have nowhere to go. There is also this possibility that we humans and other technological advanced civilizations ends up destroying ourselves through wars, terrorists attacks or nuclear explosions before we even get the chance to be a multi-planetary species. Maybe it’s just in our nature to be destructive, I refuse to believe so, but it’s still possible and could act as a filter for reaching interstellar travels.

An other hot topic right now is AGI, or artificial general intelligence, with unintentional destructive goals. AGI is basically about optimizing a solution to a certain problem. Nick Bostrom argues that AGI could be the most important (and maybe the last) invention we humans ever have to make since it would conduct scientific research and technological development at an exponential rate, thus outperform even the most intelligent human minds with ease. Because of an intelligence far greater than ours, it may lead to unexpected results when solving a problem. There is a classic thought experiment called the paperclip maximizer that raises concerns about the development and use of AGI. It may sound silly at first, but it is a very real issue.

Imagine that you for some weird reason likes to collect paperclips, and a lot of them, in all sizes and colors. But you’re to lazy to collect them by yourself so instead you come up with this brilliant idea to use AGI to do this for you. AGI is just software so you decide to code this new masterpiece that will make you the number one paperclip collector in the world. In its early stages it might physically collect paperclips around the world or earn money to buy them. So far you could say that things are going pretty well! But since this is AGI it will realize that this is not the most efficient way of doing things so instead it starts manufacturing paperclips. This is where things turns sour. As its intelligence will increase exponentially your paperclip maximizer will soon figure out how to convert all mass around it into paperclips by rearranging the atoms. (Yes, this will include you and all of the people you know.)

Or let’s say that you want to eliminate cancer in human beings, who wouldn’t like to do that? If we’re not careful enough, though, an AGI machine could just wipe out all of humanity to solve this problem, because that would be much more energy efficient than to actually cure all of the different types of cancer that exists in our world today. Would this really happened? Yes, if we not make sure that AGI’s share the same values as we humans it may be the end of us. In general there is a big uncertainty of what a super intelligence entity would do when it’s released in the wild since there probably won’t exist an off button. People like Stephen Hawking, Elon Musk and Bill Gates has also publicly expressed their concerns about this issue.

Moving on to some biological threats that may act as a great filter, we have something as terrifying as engineered superbugs or viruses. What this refers to is bacteria or viruses that has, essentially through natural selection, grown resistant to antibiotics. If this was done purposely in a lab, and not through people taking antibiotics when they don’t absolutely need to, it is engineered and could be used in bioterrorism causing a pandemic killing million or billions of people. According to Terri Rebmann, from the Institute of Biosecurity at Saint Louis University, there is a high likelihood that we will encounter a pandemic of some kind in the near future.

There is of course also the possibility that alien life do exist, but that we just haven’t heard from them yet. Maybe the most plausible explanation is that the Universe is such a vast place where everything is so, so far away from each other, that we might just haven’t had the time to explore enough of it or that they haven’t had to time reach us, physically or through radio signals. Maybe we use a to primitive way of communicating that we just can't understand each other, even if both ends tried. We do have to keep in mind that this require alien civilizations to actually try to find us, which is absolutely not certain that they will, even if they could.

One thrilling theory is the zoo scenario with was proposed by John Ball in 1973. In this scenario the Earth has been isolated by a much more technological advanced civilization than us to simply be observed. Maybe we are so rare that they want to keep us “safe” from evil spirits, kind of how we try to keep endangered spices from extinction through enclosed environments. The reason why we can’t detect them, is because they have the technological ability to make themselves hidden from us. They could also be too alien for us to even be recognized, i.e. we don’t know what to look for or they are here without our knowledge. Perhaps these advanced civilizations have gotten so information dense that we are completely uninteresting from their perspective and thus have nothing to offer them. We are essentially not worthy either interaction or elimination.

Maybe many of us are a bit optimistic with our exceptions of finding intelligent life in the Universe? As we know, evolution and natural selection doesn’t have a goal in mind, it’s simply about adaptation to an environment. If you look at the earth today, out of all the billion of the species that has ever existed on this planet, only one has evolved intelligence to develop science and technology. I think that life in the Universe is fairly probable, but that life evolving civilizations like ours, with technology and consciousness like ours, is super rare. It maybe is as unlikely to find intelligent life in the Universe as it is to find it on Earth. If it turns out that we are the only technological advanced civilization in the Universe (or that we are the first) that would give us enormous responsibility to not screw things up. We should embrace this opportunity and go out there and explore all of the possibilities there are, and I’m sure we will make great discoveries.

But for now, I think the Universe will stay silent just for a little bit longer.

References and Further Reading