On September 2nd 2022, Frank Drake, a pioneer in the search for extraterrestrial intelligence, passed away. He started his pursuit to answer the question "are we alone in the universe?" in 1961 when neither the technical capabilities nor the mindset was conducive to this search. He was ridiculed at the time but he stayed calm and carried on. As the creator of SETI (search for extra terrestrial intelligence) and one of the founders of the astrobiology discipline, he's been a hero for many stargazers. He made humanity a more curious species and among many of his accomplishments gifted us his famous Drake Equation.
DRAKE EQUATION
The Drake Equation was meant as a method to stimulate scientific conversation about the existence of alien life. It's variables represent the different factors that are important to consider, in oder to approximate the number of civilisations with which we could potentially communicate. Here it is in its full glory (don't freak out, it's pretty straightforward!):
Let's go though each variable:
N = number of civilisations out there for us to communicate with
R* = is the rate of star formation per year (no stars, no life)
fp = is the fraction of stars with planets
ne = is the number of Earth-like (or habitable) planets per system with planets
fl = is the fraction of such planets with life
fi = is the fraction with life that develop intelligence
fc = is the fraction of intelligent civilizations that are detectable/contactable
L = is the average longevity of such detectable civilizations in years
Drake never meant for his equation to be solved. It's a tool for contemplation, so let's contemplate briefly shall we? R* (star formation) is a variable that we have somewhat of a grasp on. Stars are "easy" to track and as such we have a good sense at which rate they spawn in our galaxy (rates differ widely based on circumstance). We have made massive strides in Drake's lifetime to get a better understanding of fp (planets per star) and ne (potential for life on a planet) through the immense progress in telescope technology (most notably the Hubble and recently the James Webb Telescope).
As one would expect when looking at the night sky, many of the stars up there actually have planets orbiting them. We have identified about 5000 so far. Planets are hard to observe as they are not a source of light. They reflect light but that's hard to spot, given the glare from their star. The most reliable way to "see" a planet is by observing the light of a star and registering a drop in the intensity of said light. That's a planet passing between us and its star and thereby obstructing the light from it (nearly 4000 planets have been discovered this way). Based on these transit observations we have some basis to extrapolate fp (NASA has a really cool site if you want to geek out on this).
When planets orbit between their star and us, the light emitted by the star passes through the planet's atmosphere (imagine holding a snow globe between you and a lamp). That light changes its frequency based on the molecules it interacts with as it's passing through. Scientists here on Earth can then deduce what the planet's atmosphere is made up of using spectroscopy (how freaking cool is that for a bunch of primates). So for instance if we detect light passing through an atmosphere that arrives with the wavelength of 15 microns, we know it bounced off a CO2 molecule. With these data points we have a basis to speculate about ne (habitability of a planet).
All variables that follow after are somewhat of a crapshoot. Guessing fl (how many develop life), fi (intelligent life), fc (civilisations with radio technology) and L (how long they will stick around) is a very imprecise affair. We have quite sophisticated models about how life could have formed (Nick Lane, The Vital Question is my favourite biochemistry read) but we have no clue of its likelihood. Obviously, the chances for intelligent life are even harder to guess, knowing how many times life on earth nearly went extinct before we developed general intelligence required for civilisations.
That hasn't stopped people from taking a punt. As a matter of fact, all of us have an intuition about this, however ill founded right? According to polls, a majority of people in the US and Europe believe that aliens exist (7% of Britons claim to have seen them?!). When you consider all those stars out there, how could it be possible for life not to exist on other planets? In other words, if you multiply really big numbers as part of the Drake Equation the result must be a really big value for N (number of alien civilisations we can booty call). "But where is everybody?"
FERMI PARADOX
This is the question that Enrico Fermi (an Italian scientist - Nobel Prize winner, father of the first nuclear reactor and co-inventor of the atom bomb under Oppenheimer) asked his colleagues while on a walk to grab lunch in 1950. There are many great resources on this topic so if I do a bad job at explaining, I suggest you check out these Youtube videos. The paradox is why haven't we found anyone if the likelihood for life to exist is seemingly so high?
Let me boggle your mind. Technically the universe had all the required elements present to form life when it was 1 billion years old (A). Our planet formed 4.5 billion years (B) ago. Since we're here now we can assume it takes that long for a civilisation to emerge. So say from age 5.5 billion (A+B) the universe could have had civilisations. The universe is currently 13.8 billion years old. So we've had 8.3 billion years for potential civilisations to emerge. Where is everyone? Consider for a moment humanity (if we don't blow ourselves up) in 50,000 years. Now imagine a civilisation that's millions or billion years older. Wouldn't they be gods? A simulation by NASA's Jet Propulsion Lab suggests you could colonise large swaths of the Milky Way galaxy in 90 million years. No biggie for billion year old alien gods! So where the HELL is everyone?
Robin Hanson (an economist and brilliant thought experimenter) proposed the idea of Great Filters to provide an answer. A Great Filter is a blind spot in our seemingly instinctive assumption about the high probability of life in the universe. Maybe it's really hard for single celled life to form? Or multicellular is really hard? Or intelligence is ultra rare? All these could be Great Filters. Thankfully, we have already overcome those and everyone else who's not here seems to have failed to pass through. But wait...what if the Great Filter is yet to come? Maybe all civilisations blow themselves up? Well that would mean Xenoarchaeology could be a great career, because the universe is littered with remnants of extinct alien civilisations. It would also mean that we're most likely doomed.
(I’ll deal with some alternative explanations of the Fermi Paradox in the Addendum at the end of the post)
OVERCONFIDENCE
To decide whether we have passed the Great Filter or are still facing a Great Filter, we need to revisit the Drake Equation. Remember what's at stake here. If we still have a Great Filter ahead of us and also can't spot anyone out there, then it seems very likely that civilisations just don't make it very far. If the Great Filter is behind us and we can't spot anyone out there, then we may be the first consciousness to emerge from this universe (what a waste of space and time!).
Oxford University's Future of Humanity Institute to the rescue! A few years ago Anders Sandberg (Ph.D. in computational neuroscience), Eric Drexler (the inventor of molecular nanotechnology), and Toby Ord (philosopher and leading figure in the effective altruism movement) wrote a paper called Dissolving the Fermi Paradox. Their conclusion is that our intuition about a teeming universe is off. The paper is statistics heavy, so let me summarise some of its conclusions.
The main problem leading to the Fermi Paradox is that we don't account for uncertainties properly when using the Drake Equation. Using an overconfident estimate or average for any of the fractions can create a 100 orders of magnitude difference in N (number of civilisations) given the nature of big numbers we're dealing with. To account for uncertainties we need to plug in a variation of possible values for the Drake parameters. This is particularly true for fl, fi and fc given the lack of our current scientific knowledge about the likelihood for life to emerge. To determine this probability distribution they used the parameter ranges in the existing science literature as a proxy. In other words, if 20 scientists used 1/2 for fl and 1000 used 1/100000, they would adjust the range and weight of the input parameters in their simulations accordingly.
Let me quote their conclusions here directly:
"When we take account of realistic uncertainty, replacing point estimates by probability distributions that reflect current scientific understanding, we find no reason to be highly confident that the galaxy (or observable universe) contains other civilisations, and thus no longer find our observations in conflict with our prior probabilities. We found qualitatively similar results through two different methods: using the authors’ assessments of current scientific knowledge bearing on key parameters, and using the divergent estimates of these parameters in the astrobiology literature as a proxy for current scientific uncertainty.
When we update this prior in light of the Fermi observation, we find a substantial probability that we are alone in our galaxy, and perhaps even in our observable universe (53%–99.6% and 39%–85% respectively). ’Where are they?’ — probably extremely far away, and quite possibly beyond the cosmological horizon and forever unreachable."
To be clear, they are not saying that there is certainty about us being alone, just that it is very scientifically plausible and should not surprise us. It is a statement about our state of knowledge. Also given our uncertainty about life formation and current observations of an "empty" universe our assumption should be that the Great Filter is behind us, rather than thinking all civilisations fail.
BACK ON EARTH
Let's bring all this back to earth. The universe through billions of years of cosmic evolution of physics and chemistry has given rise to biology. Biology through billions of years of natural selection and many mishaps has given rise us. This seems to be a very rare event, given we can't find cosmic pen pals despite the billions of stars and billions of years that life could have formed elsewhere. Not that humanity isn't too full of itself already, but it's plausible that life in general and humans are VERY special.
Considering all this makes me look at life and people around me in awe. We are a work in progress attempt by this universe to create something ultra rare. We really have to make this civilisation thing work and should rise to the occasion to take on this ultimate responsibility. Otherwise, there may be nobody left to ask "where is everybody?".
ADDENDUM
There are some alternative explanations of the Fermi Paradox. Sorry for the brief responses to them but I frankly don’t even think you’re still around to read this:
-The aliens are hiding!
=If the universe would be teeming with life, then only one of them would have to be non-compliant and show themselves.
-We are looking for the wrong signals!
=Unlikely, it would be hard to miss an expanding alien civilisation.
-We haven't been looking for long enough!
=True, but life certainly isn't as abundant as it should be because we've done decent canvassing our galaxy.
-They are too far away!
=Possibly. The universe is expanding faster than the speed of light. Faster than light travel doesn’t seem possible, so we might never meet alien civilisations that are too far away.
-The aliens are expanding very fast!
=Pretty interesting idea posed by Robin Hanson and likely topic of another post. Say aliens would be expanding with close to the speed of light, then you wouldn’t see much happening until right before the moment they arrive.
-We live in a simulation! = Sure we do Elon!