I propose to estimate a lower bound for the probability of passage through the Gaian bottleneck. The first Gaian bottleneck is converting a wet, rocky planet that forms in the HZ to one that is Gaian regulated in a way that assures habitable stability for billions of years, thus providing an opportunity for intelligent life to evolve. The second Gaian bottleneck is passage of an evolved intelligent species through those dangerous centuries when the intelligent species so dominates the planet's atmosphere that it risks destroying the Gaian regulation of the past billions of years, which is equivalent to humanity surviving its Anthropocene Era. I claim that the probability of passage through the first Gaian bottleneck, G1, is so difficult to predict that its uncertainty dominates any calculation of the abundance of intelligent life in the universe. I claim that a lower limit for it can be derived based on the fact that the Earth passed it. I derive that G1 > 5e-21. Using plausible estimates for the other factors in a revised Drake equation I conclude that "we are alone" in our galaxy.
Classical Drake Equation
In 1960 Frank D. Drake presented the "Drake Equation" for the
purpose of stimulating a conversation about intelligent life in
the galaxy with the hope of encouraging a search for
extraterrestrial intelligent beings.
Ng = R* × fp × ne ×fl × fi × fc × L
where Ng = number of communicating intelligent
civilizations in the galaxy, R* = rate of star
formation, fp = fraction of stars that have
planets, ne = fraction of these planet that have an
environment that could support life, fl = fraction of
those that actually support life, fi = fraction of
those with intelligent life, fc = fraction of those
that are technologically capable of communicating over
interstellar distances, and L = average lifetime of those
civilizations. Notice that the "units" for the answer is simply a
number, since R* time L have units that cancel.
Let's interpret R* to be the rate of star formation
for sun-like stars, ~ 1/year. An optimist would set fp
= 1, ne = 1, fl = 1, fi = 1 and fc
= 1. The equation then reduces to Ng = L. So if L = 100
years (an approximate lower limit for humanity), then there would
be 100 communicating civilizations in the galaxy at any one time.
An optimist might argue for L = 1000 or 10,000 years, but for now
let's just say we don't know L.
Let's ask how many communicating civilizations there might be in
the universe at any given time, Nu. If the number of
galaxies is 5e11, then Nu = 5e11 × L.
Now let's ask how many communicating civilizations have ever
existed during the history of the universe, Ntot. Since
the lifetime of the universe is Lu ~ 20 billion years =
2e10 years, Ntot = 2e10 × 5e11 / L = 1e22 / L.
We have information about Ntot; it's > 1 (i.e.,
We are safe in estimating L > 100 years, so what could be the
meaning of the last equation. We're now in a position to
re-evaluate some of the less certain f-values in the Drake
equation, or place limits on them.
I'm going to take the position that he most uncertain combination
of terms is the product f = ne ×fl × fi
× fc. I'm working up to writing a different Drake
equation, but for now let's evaluate f. The original Drake
equation can be written:
Ng = R* × f × L
Ntot = R* × f × Lu
Solving for f yields:
f = Ntot / (R* × Lu
Substituting values for Ntot, R* and Lu
f > 5e-23 [year-1] × L
and since we're safe is stating that L > 100 years, we have:
f > 5e-21
We now have an evidence-based estimate for ne × fl
× fi × fc.
The Gaian Bottleneck is customarily described as a wet planet in the circumstellar habitable zone (CHZ) evolving life early in the planet's history that produces negative feedback dynamics regulating the atmosphere in a way that overcomes the positive feedback dynamics of an otherwise abiotic planet that would lead to either the runaway greenhouse sterility or albedo ice cube sterility. It is thought that this biotic intervention must occur during the first 1/2 to 1 billion years after the end of the heavy bombardment, which for Earth was 3.8 Gya. However, I am unaware of a derivation of the probability that such a Gaian regulation should occur. I will refer to this probability as G1. As an approximation, G1 = ne × fl.
For any planet that survives this first Gaian Bottleneck there is
a possibility that life will continue to evolve, and eventually
produce an "intelligent" species. Humanity is usually used as an
example of what is meant by an intelligent species. Given that
humans today have had such a profound influence over the Earth's
climate, raising the possibility of an atmosphere that warms to
such a level as to threaten the extinction of humanity, and other
species, it is reasonable to ask if the Gaian regulation of the
Earth's past 3.8 billion years is again under threat. I refer to
this as the Second Gaian Bottleneck.
How long does a communicating species exist before it confronts Second Gaian Bottleneck? Let's call it L1, and assign it with a round number of 100 years. Then, if it succeeds in passing the Second Gaian bottleneck, with probability G2, how long will species endure? Let's call it L2. The Drake equation now has two components, one associated with G1 and the other associated with G2. Substituting G1 = ne × fl:
Ng = R* × (G1
× fi × fc × L1 + G2 ×
where G1 and G2 are the two Gaian
Bottleneck passage probabilities. This equation assumes that after
passage through the Second Gaian Bottleneck a species will remain
communicative for L2 years.
In the previous section I derived that the right side of the
above equation is f > 5e-21. We can therefore begin to place
bounds on the terms on the left side of the equation. It is my
opinion that the least known of the 3 terms on the left side is G1.
Let's set fc =1, L1 = 100 years, and adopt
the following reasonable bounds for fi, G2
1e-6 < fi < 1
1e-3 < G2 < 1
0 < L2 < 1e6 years
Solving for G1 is now possible:
G1 > 5e-21 / (100 × (1e-6 to 1)
+ (0 to 1e-6) × (1e-3 to 1)), or
G1 > 5e-21 / ((1e-4 to 1) + (0
to 1e-6)), or
G1 > 5e-21
This may be the first evidence-based derivation of G1, the probability of a wet planet in the HZ developing life (shortly after its heavy bombardment ends) that happens to produce negative feedback climate dynamics that overcome the normally present abiotic positive climate dynamics leading to runaway greenhouse or high albedo ice cube sterile end states, such that the planet can remain habitable for the billions of years necessary for life to evolve sufficiently long as to produce an intelligent species.
Another Drake Equation
Implicit in the above is the assumption that whenever an
intelligent species evolves, and becomes technologically
"communicative," it has ~ 100 years before it faces the Second
Gaian Bottleneck. I believe G1 is so much more
uncertain than R*, fp, fl, fc
that we can simply set them to 1, and set and L1 =100
years, and rewrite the Drake Equation using both Gaian Bottleneck
Ng = G1 × fi
× 100 + G1 × fi × G2 × L2
Ng = (G1 × fi ) × (100 + G2 × L2)
The right side has two terms, one associated with a "free pass"
100 years before facing the Second Gaian Bottleneck, and the
second term for the possibility that it could pass the bottleneck
and endure for L2 years.
Let's play with some assumptions. Working backwards, if we want
to be somewhat optimistic and believe that we have the company of
one other intelligent and communicating civilization in the
galaxy, at this time, i.e., Ng = 2, what parameter
combinations would allow that? Setting fi= 1, G2
= 0.1, and L2 = 1000 years, this can be achieved with G1
= 0.01. If G1 << 0.01, then we'd have to increase
G2 or L2 to maintain the likely presence of
company in our galaxy.
Let's be wildly optimistic and adopt G1 = 0.1 and G2 = 0.1. With
L2 = 1000 years we calculate Ng = 20. In
order to arrive at greater values for Ng we have to
assume that L2 > 1000 years.
Is it likely that L2 > 1000 years? I don't think
so, based on sampling theory applied to the human situation (as
described in my book Genetic Enslavement, 2014). To
date ~ 6e10 humans have existed (since 50,000 years ago). With
plausible future population scenarios an equal number of humans
will be born during the next 300 years. If humanity disappears at
that time, then we now will be close to the 50 percentile of all
humans in the entire past/present/future sequence. I have shown
that if we require the present generation of humans to reside
within the 25% to 75% range of birth sequences, which can be
stated as encompassing 50% of future scenarios, then humanity's
demise will occur sometime between 2100 AD and 2500 AD. In other
words, sampling theory states that humanity will be
"communicative" for ~ 350 ± 200 years. If humanity's G1
= 1, then humanity's L2 < 450 years. This is just a
suggestion of what values for L2 should be considered.
I conclude that Ng<< 10, and that we are
probably "alone" in the galaxy.
Chopra, A. and C. Lineweaver, 2016, Astrobiology, 16,
Gary, B. L. 2014, Genetic Enslavement: A Call to Arms for
Individual Liberation, Amazon.com
For those who don't want to believe that "we are alone" in the
universe, I remind you that "If intelligent life originated only
once throughout the universe, during the entire lifetime of the
universe, we, of necessity, must be it!"
Another things to remember was stated by Einstein, when a
reporter pressed him for an example of infinity: "The size of the
universe, and human stupidity; but I'm not sure about the first
WebMaster: B. Gary. Nothing on this web page is copyrighted. This site opened: 2016.02.11. Last Update: 2016.02.12.