The Fermi Paradox
Source: Nick Rose (ESA/Hubble & NASA)
Where is everybody?
This was the question asked by the physicist Enrico Fermi, in 1950. It became clear to him that, given the very old age of the galaxy, and given all the possible life-bearing planets that exist in our Milky Way galaxy, why don't we see evidence of alien life? This is a very intriguing question and forms the essence of the Fermi paradox. If you think about it logically, you realize that by now we should
have widespread colonization in the galaxy by some sufficiently intelligent and technologically advanced alien race. This line of reasoning also extends to the (observable) universe at large, which contains more than 170 billion galaxies, and should also exhibit signs of colonization spanning the different galaxies. But for purposes of this discussion let's just focus on the hypothetical colonization of our own galaxy by aliens living within our galaxy.
Given that our galaxy is about 13 billion years old, and given that there are 100-400 billion stars in our galaxy, and given that many of those stars likely have life-bearing planets orbiting them, and given that some of those planets likely have intelligent civilizations on them, and given that some of those civilizations have reached a technological level advanced enough to allow them to attempt galaxy-wide colonization, why don't we see evidence of this? It's estimated that it would take 5-50 million years to colonize the galaxy. This time frame is based on the feasible travel time between stars and the time it takes to colonize habitable planets, with those colonies in turn yielding additional colonization efforts. As a result, a sort of exponential growth occurs and life eventually spreads throughout the galaxy like wildfire.
5-50 million years is certainly a very long time, but relatively speaking it is very short compared to the 13 billion year age of the galaxy. There has been plenty of time for intelligent life forms, throughout the galaxy, to evolve and successfully colonize over the 13 billion year history of the galaxy. To give you some perspective on this, it took life on earth 3.8 billion years to evolve to where it is today.
Even if 99% of all the technologically advanced civilizations failed at colonizing the galaxy, that still means that 1% succeeded. In fact, even if just one
alien civilization succeeded, the galaxy should, by now, be full of alien colonies and technological artifacts belonging to this one successful civilization. Consequently, we should be able to see evidence of this floating all around the earth via what we can detect either in terms of things we can see by telescope, or radio signals we can detect using large receiving dishes. But given that this is not the case and we have not detected anything... where is everybody?
This is indeed a paradox, and after reading ample material online and watching videos in order to inform myself, I've come to certain reasonable conclusions as to why we haven't seen evidence of alien life.
Two fundamental statements by two different people, stood out to me. One expert said, "It's too dangerous and the tickets are too expensive". The other said, "Just because we don't see bears in our backyard doesn't mean there are no bears around".
These are both key view points in my mind, and although they might seem in opposition to each other, upon closer examination they actually complement each other.
My view is basically a hybrid of the two view points. I think that our galaxy has been colonized, by one or more alien civilizations. But one reason that we don't see evidence of this is because the alien colonies are spread out non-uniformly. Due to the dangers of outer space (cosmic radiation, asteroid impacts, extreme cold, vacuum of space, etc.) only certain preferred areas of the galaxy have been colonized. These areas are likely the most suitable in terms of having the highest number of habitable planets. You can think of these areas as oases in the desert, and given that space travel is very risky and very expensive economically, you want to set up base on planets that are most similar to your home world, and which are as close as possible. It's not like Christopher Columbus traveling across the Atlantic Ocean to unknown territory. Everywhere he ends up he knows that he will breath the same air and drink the same water. But traveling to other worlds usually means totally different air, and totally different water, among other things. You must be picky about where you go, and naturally you go for the low hanging fruit. Even if earth has everything an alien species wants, the fact that earth is most likely not unique, and given that many other planets likely have the same raw materials as we have, an alien race would naturally opt for earth-like planets that are much closer to them. Why drill for oil far away when you can drill close to home? Chances are real good that earth is far from unique. In fact, more and more earth-like planets are being discovered, adding credence to this theory.
Before aliens set out to colonize, they most likely first send out advanced probes to map out the galaxy, so they know where to go. These probes could also be self-replicating, using raw materials on asteroids and the energy of stars, to create identical versions of themselves to further explore the galaxy. But the fact that we haven't even seen evidence of ancient probes, or other artifacts drifting in space, goes back to the "no bears in my backyard" analogy. Space is far too vast and the probes, relative to this, are probably too few and too far between for us to ever notice them, except by sheer luck. We are attempting to infer the existence of aliens and their artifacts from a very limited vantage point our galactic backyard so to speak. But what about radio signals? Can't we detect alien radio signals? The fact is that radio signals, unless transmitted directionally, will quickly fade in strength. Our large receiver dishes would have to be pointed directly at an incoming directional signal for us to reasonably hope to detect it, and we have barely probed even a fraction of the night sky. It is estimated that the typical electromagnetic communication signal transmitted on earth (such as by radio and TV transmitters), would fade so as to be virtually undetectable in a distance of 0.3 light years. And given that the galaxy is roughly 100,000 light years across, this should give you some perspective on how likely we are to detect alien radio signals.
There's also the high probability that any alien colony out there would be very distant from neighbouring alien colonies, as in several light years of distance away at least. So given their isolation they couldn't exactly call for help or backup if they need it. No one would be able to come and help them for likely thousands of years. The speed of space travel is limited by the speed of light. Nothing can travel faster than the speed of light, and most likely aliens would travel much slower than the speed of light, hence greatly prolonging the time it takes to travel to neighbouring alien colonies. So when you colonize somewhere, you're on your own! So for an alien race to try to take over our planet (and overpower us in the process!) they would really have to be out of options. We would truly have to be their last resort. Fortunately, everything our planet has which aliens could want, most likely exists somewhere else much closer to them, and with no one else living there which would resist their colonization effort.
It may be that aliens are much more inclined to just observe us from a safe (and convenient) distance, just to satisfy their curiosity, if nothing else (the zoo hypothesis). And if aliens are watching us, then this would likely mean that we have been watched for a very long time, and from somewhere inside our solar system, using a probe. Aliens may have sent out probes designed to investigate every single solar system in the galaxy. Now, given that there are 100-400 billion stars in the galaxy, that's a lot of probes! But if the probes are self-replicating, an exponential replication rate would quickly create enough probes for every single solar system in the galaxy. These probes would reach a solar system, like ours, and "hang out" there indefinitely and monitor for signs of civilization, by searching for radio signals and other evidence of technologically developed civilizations. And if such a civilization is detected on a particular planet in the solar system, the probe would notify the aliens, and they would then invest more resources in monitoring that planet. Such a probe (or probes) in our solar system could hide out in the asteroid belt, using the energy of our sun as a power source, and mining and fabricating the materials needed to maintain themselves and keep themselves operational for many millions of years. It would be very difficult for us to spot such probes in our solar system using even our best telescopes.
Assuming there is a probe (or probes) monitoring earth, it surely has detected our presence by now. And given that we have been "live" for about 200 years, in terms of having signs of civilization that aliens could detect, that may be plenty of time for an alien colony living a few light years away to have sent additional spy equipment to investigate us more closely. Their spy equipment may be disguised as asteroids, or as one of our satellites, and be completely undetectable to us.
Another possibility is that aliens have sent out a limited number of probes, not necessarily self-replicating, to investigate the galaxy at periodic intervals. These probes would "fly by" different solar systems, looking for signs of intelligent civilizations and would notify the aliens if they find anything. Consequently, these probes are much less likely to be discovered since they don't remain in the solar system for long. This strategy would be ideal for aliens who want to keep their existence a secret. Probes such as these would occasionally swing by some galactic neighbourhood (like ours) monitoring for signs of civilization. But given that we have only been "live" for about 200 years, then they probably haven't noticed us yet. In other words, we are a very recent blip on the alien radar, and most likely they just don't know about us yet. Given the sheer vastness of the galaxy and all the possible life-bearing planets in it, it is not unreasonable to imagine that every possible life-bearing planet may only be checked once every few thousand years or so (by a select number of probes), for signs of an intelligent civilization. Perhaps the alien census hasn't arrived yet, and this may be another reason why we haven't seen any evidence of aliens.
Detecting Alien Civilizations
How might we be able to detect alien civilizations on other planets? Chances are that they would be very far away, many light years to be exact, making detection difficult. According to Dr. Jeff Kuhn (University of Hawaii - Institute for Astronomy), if we use special telescopes designed to detect artificially generated heat and light signatures on planet surfaces (which could not be created naturally), we can definitively say that a technologically advanced civilization lives there.
There are some interesting theories on how aliens might satisfy their (likely) very high energy needs. One theory is that they might be advanced enough to capture most or all of the energy of their parent sun, using what is called a Dyson sphere. This would create a heat and light signature around the parent sun, which could also be observable on earth with a suitably designed telescope.
There are some that believe that the reason we haven't detected aliens, or their artifacts, is because we are alone in the galaxy, and possibly the universe. The common reasoning here is that, given the seemingly huge window of time that life on other planets could have evolved to intelligent space-faring life, and propagate itself throughout the galaxy (and universe), we should have seen evidence of this by now. Just by the law of averages it should have happened by now. Now, this is certainly a possibility, and it does explain the lack of evidence of aliens, but I think this conclusion is an over-extrapolation based on what we think
aliens should be able to do, based on our very limited knowledge. The fact is that we just don't know how difficult it is for an alien race to colonize and spread throughout a galaxy (and the universe) in a way that we imagine they should be able to. It may just be too difficult for any
intelligent beings (including us) to spread throughout a galaxy in sufficient numbers to make themselves easily noticeable to others. The ultimate truth may be that outer space is far too difficult and dangerous a place for any
intelligent beings to successfully occupy in very high numbers, even by way of robot ambassadors (or probes). It may be that an alien race can, at best, only limit themselves to the most suitable areas to live which are closest to them. Furthermore, sending out probes on exploratory missions may only be done on a limited basis for practical and common sense reasons that we can't fully envision. That said, there may very well be many, many alien probes out there, but not nearly enough to fill up the galaxy.
This brings us back to the original point: "It's too dangerous and the tickets are too expensive". We have to accept that just because we can't imagine an end point where intelligent life cannot eventually propagate throughout the galaxy (and universe) in high enough numbers for us to notice them, doesn't mean it isn't true. And this may be a clue as to what lies in our future; that our reach beyond earth is limited. If no aliens have become a dominant presence in outer space then neither can we. But rather than deal with that sobering possibility it may be easier for some to just assume altogether than no other intelligent civilizations exist at all, and that we are all alone. It may be easier to accept this than it is to accept that there is an ultimate limit on our ability to venture beyond earth and become a dominant presence in outer space.
Return to Physics Essays page
Return to Real World Physics Problems home page
If you need one-on-one help with physics, get help from a Chegg Tutor! 30 minutes free for new users.