Are we alone?
It’s probably one of the, if not the most basic questions of human existence.
People have been trying to answer it for millennia in one form or another, but only recently have we gained the tools and knowledge to start tractably trying to estimate whether we are or not.
Those efforts take the form of famous tools like the Fermi Paradox and the Drake Equation, but there’s always room for a more nuanced understanding.
A new paper in Acta Astronautica from Antal Veres of the Hungarian University of Agriculture introduces a new one – The Solitude Zone.
The Solitude Zone isn’t a place, though, it’s a statistical window where the probability of exactly one lifeform of a given complexity exists is greater than a situation where either multiple lifeforms of that same technological advancement level exist or none at all do.
That second part is critical – it makes the statistical window a bell curve rather than an exponential function. But before we get too far in the statistical weeds, let’s lock down some concepts.
The Fermi Paradox, famously put forth by physicist Enrico Fermi, asks that if there are so many places for life to evolve in the Universe, where are all the aliens?
To date, noone has been able to come up with a satisfactory answer, but there are a number of theories that attempt to answer the question – two of them are critical to understand The Solitude Zone.
First is the idea “Great Filters”, whereby, to reach a certain level of technological development, a lifeform has to pass a statistically improbable event.
The original creation of life on Earth is one classic great filter, as is the jump from single-celled to multi-celled organisms.
Each of these filters are collective, as each life form must pass them in turn – and according to some versions of the theory we might still have more that lay ahead of us in our technological development cycle.
Another key concept is the Kardashev Scale. Originally developed by Soviet physicists Nikolai Kardashev, this scale rates the technological development level of a lifeform by the amount of power its civilization consumes.
A Type I uses the power a typical Earth-sized planet would receive, a Type II uses all of the output of a star, and a Type III would use all of the power of an entire galaxy. Current estimates from a variety of sources put humanity at about a .7 on this scale.
A final key concept is the Drake Equation. Developed by astronomer Francis Drake, this famous equation attempts to calculate the number of civilizations there are in the galaxy that have advanced to a human level of technological sophistication.
Its factors include the rate of star formation, the fraction of those stars with planets, and the fraction of those planets on which life could, potentially, evolve.
Dr. Veres’ Solitude Zone model uses ideas from all three of those concepts, and is build around four key principles.
First is “complexity”, which allows a lifeform to be ranked on a scale of 0 to infinity of how “complex” they are, ranging from simple single-cell organisms to “postbiological intelligence” – this is where the Kardashev scale factor comes in.
Next is the “existence likelihood” – i.e. what probability a civilization of at least a certain level of complexity exists. Another key factor is the “emergency probability” – the chance that a lifeform of at least that level will arise is only one system.
And finally, the total number of potential systems there are in the universe. Taking a modified version of the number from the Drake equation and expanding it out to the whole universe rather than just this galaxy, the paper locks this number in at 1024, the estimated number of terrestrial planets in the observable universe.
From these factors, Dr. Veres calculates how often an emergence probability is in the “solitude zone” by calculating whether it meets two conditions: The probability that only one civilization of that technological level exists is higher than the probability that multiple of them exist. The probability that at least one civilization of that technological level exists is higher than the probability that none of them do.
This second factor might sound obvious, but it’s a critical insight that comes out of this framework. In scenarios that are too pessimistic about the evolution of biological life, its actually more likely that life wouldn’t exist at all than it would in only one instance. To prove this point, Dr. Veres ran four different scenarios to see what the likelihood of a lifeform being in the Solitude Zone was.
For the “Astrobiological Optimism” scenario, which accounts for “easy” evolution on multiple worlds, there is essentially no chance for a civilization of humanity’s technological capacity to be alone in the universe as it would be teeming with other civilizations. On the flip side, the Evolutionary Hard Step scenario, which puts a Great Filter earlier in life on Earth’s historical path, again the probability of being in the Solitude Zone is near 0 because it is more likely that no lifeforms exist rather than just one does.
However, there is one particular answer to the Fermi Paradox that fits this model well – the Rare Earth Hypothesis. In this scenario, complex life is extremely rare, but its not so far as to not be feasible at all.
The conditions on early Earth had to be just right to allow life to evolve, but recreating those conditions on another planet unintentionally is extremely unlikely. That results in a relatively high chance that we are, in fact, in a Solitude Zone – Dr. Veres estimates that chance at around 29.1%. He even takes it a step further and defines another scenario in the “sweet spot” of the framework called the Critical Earth Hypothesis, which maximizes the probability that we are alone at 30.3%.
One big take away from this exercise is that, in no scenario is the probability of us in the Solitude Zone higher than 50%. Its far more likely that either there are multiple lifeforms of our technological levels, or that there would be none at all.
However, as a lifeform’s Kardashev scale estimate rises, the likelihood of ending up in that zone does increase – with really advanced civilizations have over a 50% chance of being alone at any given time.
This model is a useful tool for understanding how to think about that fundamental question – but like all conversations around that fundamental question there’s sure to be debate about it.
Unless something really goes wrong on our civilizational development path, we might one day be able to definitively answer it, but until then humanity at large will continue to have to ask if we are, in fact, in the Solitude Zone.
Written by Andy Tomaswick/Universe Today.
