At the Los Alamos National Laboratory in 1950, four physicists were walking to lunch one day. They were discussing the recent string of UFO reports and the possibility of extraterrestrial life, life from another planet. One physicist asked another, “What do you think? How probable is it that within the next ten years we shall have clear evidence of a material object moving faster than light?” “One in a million,” the other physicist said. “That is much too low. The probability is more like ten percent.”
Later at lunch with that conversation long gone, that same physicist exclaimed, “Where are they?” That physicist was Enrico Fermi, the Italian-American physicist often called the “architect of the atomic bomb,” and he just asked the question that would bear his name: the Fermi paradox.
The basis of the argument goes as such. There are billions of stars in the galaxy that are similar to the Sun, and many of these stars are billions of years older than the Solar System. With high probability, some of these stars have Earth-like planets, and if the Earth is typical, some may have already developed intelligent life. Some of these civilizations may have also developed interstellar travel, which the Earth is developing now. Even at the slow pace of current Earth-based travel, the galaxy could be traversed in a few million years.
Therefore, the Earth should have already been visited by extraterrestrial aliens, or at least their probes. Where, then, is everybody?
To put numbers to this question, in 1961, the American astronomer Frank Drake invented the Drake equation to elucidate this question. This relatively simple and empirical equation combines many factors to find the number of civilizations in our galaxy with which communication might be possible: the average rate of star formation in our galaxy, the fraction of those stars with planets, the average number of planets that can potentially support life per star, the fraction of planets that could support life at some point, the fraction of planets that actually develop intelligent life, the fraction of civilizations that develop technology that allows them to be detected, and the length of time they release these detectable signals.
As you might guess, except for the first three variables, most of the variables are not known to any degree and very difficult to estimate. Therefore, you can arrive at a wide range of answers; this makes the Drake equation untestable, but it is still fun to think about.
To give two wildly contradictory examples, consider first a conservative estimate. We will hold that the star formation rate in the Milky Way is somewhere between 1.5-3 stars per year.
The proponents of the “rare Earth hypothesis” propose additional constraints on habitability for planets. For instance, the planet not only needs to be in the habitable zone of their host star (the region where liquid water can exist on the surface), but the star itself needs to be in the region of the galaxy with low radiation, the star needs to have a high enough metal content, and low asteroid bombardment rates, among other things. They give for the next three variables a combined value of 0.00001.
Biologists and astrobiologists may infer for the next value, the fraction of planets that develop intelligent life, an extremely low value such as 0.00000001. Their reasoning: there are billions of species on Earth and only one has become intelligent. We will tack on Drake’s original estimate that 10-20% of these civilizations will be communicating. And finally, analyzing the average length of time a civilization has existed on Earth since the Roman Empire, we arrive at a final value of 304 years.
Multiplying these values together we arrive at 0.000000000091 being the number of civilizations we can communicate. This indicates that we are probably alone in this galaxy, and possibly in the observable universe.
On the other hand, we can create a more liberal estimate. Keeping the star formation rate the same, we have found that almost every star has a planet orbiting it, many of them are in their stars’ habitable zones, and life seems to arise as soon as favorable conditions arise. This gives a factor of 0.026 for the next three variables combined.
Contrary to some biologists and astrobiologists, others have noted the generally increasing complexity of life over time. On Earth, life started out as single-celled organisms and slowly evolved into planets, mushrooms, dogs, cats, and humans. This seems to imply a value approaching one. We will keep Drake’s original estimate that 20% of these intelligent lifeforms will be communicating. Finally, David Grinspoon has argued that once a civilization advances enough, it might overcome all threats to its survival. It will then last for an indefinite period of time, making the final value possibly billions of years.
Multiplying these values together we arrive at 15,600,000 civilizations. This indicates the opposite to the conservative estimate: that we are undoubtedly not the only civilization in the galaxy.
|Star Formation Rate||1.5 – 3||1.5 – 3|
|No. Supporting Life||0.00001||0.2|
|Fraction Actually Supporting Life||0.13|
|Length of Time Broadcasting||304||1,000,000,000|
|Number of Civilizations||0.000000000091||15,600,000|
You can see how difficult the Drake equation would be to test, at least until we find other life in the universe. So it remains to hypothesize: where is everyone?
In astronomy, two principles are usually implicitly assumed: the Copernican principle and the mediocrity principle. The Copernican principle states that humans are not privileged observers in the universe; the Earth is not special. The mediocrity principle states that life is not special.
When it comes to the first hypothesis, the “rare Earth hypothesis,” its proponents reject the mediocrity principle. Life arrived by chance, they say. So many things had to happen for complex life to arise: the Earth had to be in the Galactic habitable zone, it needed to have a central star and the requisite material to form, the Earth had to be in the stellar habitable zone, it needed to be a terrestrial planet, we needed to have a giant planet like Jupiter to deflect or absorb asteroids, it needed to have large natural satellite for the same reason, a magnetosphere to block harmful rays from the Sun, plate tectonics, atmospheric chemistry, oceans, evolutionary “pumps,” and whatever led to eukaryotic cells, sexual reproduction, and Cambrian explosion. Clearly life arose just by chance. If you were to tweak any one of these steps, humans might not evolve!
It is also possible that no other intelligent life has arisen. This is the “we’re first” hypothesis. Current technology can detect exoplanets with high amounts of oxygen in their atmospheres. While oxygen is normally created and replenished by life, we have no way of knowing if that life is plants or intelligent animals. Similarly, it may be intelligent life, but they haven’t reached the proper level of technology to communicate. Earth certainly didn’t have that ability until about 100 years ago.
In 1966, Carl Sagan proposed that there would be a test all civilizations go through. He speculated that technological civilizations will either destroy themselves within a century of developing interstellar communicative ability or master their self-destructive tendencies and survive for billions of years. Destruction methods range from war to resource depletion and climate change. (We certainly wouldn’t know of anything like happening right now, would we?)
Or perhaps rather than a civilization destroying itself, civilizations destroy other civilizations. If meet another civilization, the worst human qualities might take over and greed, paranoia, or aggression might set in. Perhaps it would be an act of prudence: an intelligent species that has overcome its own self-destructive tendencies might view another species bent on galactic expansion as a threat.
New life might also die out all the time due to natural effects. On Earth, there have been tons of extinction events that have destroyed the majority of complex life. The dinosaurs were the most complex life on Earth until an asteroid decided they weren’t. Other extinction-level events might be volcanic eruptions, or gamma-ray bursts from outer space. A carefully timed natural extinction could stop an advancing civilization in its tracks.
It may be that non-colonizing civilizations exist, but that they are too far apart for any meaningful communication. One or both cultures might go extinct before they can actually communicate. In the Star Wars universe at least, it took thousands of years just for humans to leave Coruscant near the center of the galaxy and to arrive at the nearby planets via generation ships. Furthermore, constructing enough ships to colonize other worlds might be too resource-intensive. There might be a resource-delimited boundary past which colonization would be too expensive.
There might be civilizations out there, but humans haven’t existed long enough. Our ability to detect intelligent life has only existed since 1937, since the invention of the radio telescope. Plus, radio transmissions by humans only started in 1895. Thus, we may have just not existed long enough nor made ourselves detectable enough to be found by others.
That implicit assumption I made about radio telescopes being the method of detection brings up another point: maybe we’re not listening properly. Maybe extraterrestrials are transmitting signals that employ weird frequencies, making them hard to discern from the everyday background noise. Our TV and radio broadcasts are only detectable at distances up to 0.3 light years, which doesn’t even make it to the nearest star. If we wanted to make sure those broadcasts made it across large tracts of space, we would need to broadcast in either specific frequencies or at a specific region of space. If other civilizations are doing this, that makes it highly improbable that we are listening in the right ranges.
Maybe we’re thinking about this all too human. Aliens are alien after all. They may be psychologically unwilling to attempt to communicate with us. Maybe mathematics, the language we assume is universal, is actually a human invention. Aliens might be physiologically different too; they might have thought processes magnitudes slower or faster than ours.
Scarier options exist too. Earth may be deliberately not contacted so that natural evolution may take place. However, all it takes is one civilization to break this imperative and the hypothesis is broken. The more civilizations there are, the less probable this hypothesis is true. Perhaps Star Trek got it right with the General Order Number One: “No starship may interfere with the normal development of any alien life or society.”
Unbeknownst to us, it may be dangerous to communicate, either for us or them. After all, when different civilizations have met on Earth, the results have been disastrous for one side or the other. This might apply to interstellar contact. Prudent civilizations may isolate themselves not only from Earth but from everyone, out of fear of a superpredator. Then again, probabilistically, for any civilization, it would be unlikely for them to be the first ones to make contact with another. Therefore, it is likely that other civilizations faced fatal problems with first contact and doing so should be avoided.
Finally, a significant fraction of the population believes that at least some UFOs were actually piloted by aliens and that they are already here among us. Maybe governments or scientists across the world are hiding contact from the general public. But once again, it falls to the same problem as before: the more people involved, the less likely the information remains hidden.
Well, what do you believe? Do you believe that alien life exists out there somewhere in the galaxy? What’s your favorite hypothesis?