Where are they?

Ah, Halloween! The one day of the year when aliens can walk among us unnoticed… or so some conspiracy theories suggest. They claim extraterrestrial visitors have been interacting with humanity for ages. These theories often depict aliens as green, humanoid, fond of Mexico and the US (especially cornfields), interested in moving large stones (think pyramids and Stonehenge), and prone to creating crop circles. One might expect a bit more sophistication from a civilization capable of interstellar travel and surviving intense radiation.

Jokes aside, the question remains: are there aliens out there? Let’s approach this scientifically. The observable universe contains roughly 10 billion galaxies, each with an average of 100 billion stars. This equates to a staggering 1 billion trillion stars! (Source: University of California).
With so many stars, shouldn’t the universe be teeming with alien life?
Highly advanced civilizations would also require vast amounts of energy, potentially leaving visible megastructures. Yet, we haven’t encountered any extraterrestrials (except perhaps those in the cornfields).

This discrepancy between the expected abundance of alien life and the lack of observed contact is known as the Fermi Paradox.
So, where is everyone? Estimating the number of potentially communicative alien civilizations depends on three main factors:

1. Astrophysical factors: The number of planets with suitable conditions for life.
2. Biological factors: The probability of life developing on those planets.
3. Social factors: The longevity of civilizations and their willingness to communicate.

We can estimate the likelihood of each factor and combine them into a formula.

For astrophysical factors, we can estimate the number of habitable planets (Pl) as:

Pl = R* * fp * ne

Where:

R* = the average rate of star formation in our galaxy
fp = the fraction of stars with planets
ne = the average number of potentially life-supporting planets per star with planets

For biological factors, we estimate the number of intelligent, communicative alien civilizations (IA) as:

IA = Pl * fl * fi

Where:

Pl = the number of habitable planets (calculated above)
fl = the fraction of habitable planets where life actually develops
fi = the fraction of planets with life that develop intelligent civilizations

For social factors, we estimate (Sc) as:

Sc = L * fc

Where:

fc = the fraction of civilizations that develop detectable communication technology
L = the length of time such civilizations transmit detectable signals

Combining these factors gives us the famous Drake Equation:

N = R * fp * ne * fl * fi * fc * L

Our current observations suggest N = 0 because we did not encounter any Alien civilization yet. Given the incredibly high value of R, this implies that at least one of the other variables must be zero or extremely close to it. Which one? In 1961, when the equation was formulated, all these variables were largely guesswork. Today, thanks to telescopes and satellites, we have better estimates, at least for the astrophysical factors.
We’ve discovered thousands of exoplanets, suggesting that nearly every star has planets (fp ≈ 1). NASA estimates around 40 billion potentially habitable planets in the Milky Way alone (ne is also high).

So, astrophysically, the universe seems ripe for life. But what about the biological and social factors?
We’re still in the dark regarding how often life actually emerges. However, we know that either fl, fi, or fc*L must be close to zero.
This means that life either struggles to emerge, or, if it does, it faces significant obstacles to developing into communicative civilizations.

This leads to the concept of the “Great Filter.” We can frame the problem as two possibilities:

1. Life rarely starts: Despite favorable environments, life struggles to evolve. In this case, the biological factor (fl or fi) is near zero.
2. Civilizations are short-lived: Life might be common, but something prevents civilizations from surviving long enough to communicate.
   Here, the social factor (fc*L) is near zero.

If the first scenario is true, it’s good news for us! It means we’re incredibly lucky, perhaps the only successful case.
It also implies that we’ve overcome the biggest hurdle and have the potential to colonize the galaxy. If the second scenario is true, it’s less encouraging. It suggests that countless civilizations have risen and fallen, implying a similar fate might await us. Which scenario is more likely? Historically, the first seemed more plausible. However, recent research has revived the second possibility.
As civilizations advance technologically, they also become more vulnerable to self-destruction, whether accidental or intentional.
Potentially civilization-ending technologies become more accessible, increasing the risk of someone “pushing the red button.”
Examples include engineered viruses, nuclear weapons, and other bioweapons.

We don’t yet know the answer. But if the second scenario is correct, we should be extremely cautious about our own actions.

Finally, for those who believe aliens are already among us, I’ll quote a common scientific refrain: “It’s never aliens until it is.” In other words, always look for simpler explanations first.

See you in the next article!