Updated: Aug 24, 2020
Where is Everybody?
There were errors in the original posting and and I got back some good comments, so I revised a few numbers (doesn't change any of the conclusions).
As I discussed last week humans have been asking the question “Are we alone universe?” probably as far back as we had languages. We still don’t have an answer, but a great deal of work has been done and published. This week I’m going to review what we covered last week and discuss what I think is the most logical answer.
Last week I discussed the huge amount of resources required for an interstellar mission and how that made it unlikely aliens would visit and then depart without leaving a colony or at least the evidence of the resources they gathered to depart the solar system. The only scenario I found reasonable was that an unmanned probe might have been sent and that it still might be out in the asteroid belt observing us and reporting back. No other previous visitation scenarios made sense.
This week I want to assume we have not been visited and discuss the reasons why not. The best way to explain the lack of aliens is the Drake Equation. The Drake Equation was first written by Radio-Astronomer Frank Drake in 1961. The Drake equation is:
N = R* . fp . ne . f1 . fi . fc . L
Where N = number of civilizations in our galaxy which might be traveling or communicating
and R* = the average rate of star formation in our galaxy, stars per year
fp = the fraction of those stars who have planets
f1 = the fraction of planets that could support life and go to develop long term life
fi = the fraction of planets with life that go on to develop intelligent life
fc = the fraction of civilizations that the capability to travel or communicate
L = the effective life of civilizations capable of travel or communicating, in years.
Based on the analyses and books I’ve been reading the inputs are roughly as follows:
R* = ~ 2, fp = 0.9, f1 = 0.01, fi = 0.001, fc = 0.3, L = 10000
This gives the number of civilizations in our galaxy we might interact with as 0.05, which explains the Fermi Paradox. If they were out there, they stopped travelling or talking long ago. I realize some of you might object to limiting the effective life of an advanced civilization to 10,000 years but we have come uncomfortable close to annihilating ourselves once of twice in the last sixty years. I’m afraid a superbomb, an environmental collapse, an asteroid impact, or an engineered pandemic might be the end of civilization as we know it, and before we expect it.
The other questionable numbers might be f1 and fi. If you have read “Rare Earth” by Ward and Brownlee you know Earth’s climate, especially the low variation in temperatures over four billion years is the consequence of Earth colliding with a Mars-sized planetesimal approximately four billion years ago. This collision was not dead on but glancing enough that debris spun off to form a sizable moon while the iron cores merged to give Earth a very strong magnetic field ensuring low radiation at the surface and keeping the solar wind from carrying off the hydrogen critical to large oceans. The large, but not too large, moon was critical to long term stability in Earth’s obliquity (axial tilt). If the moon were much smaller, of slightly larger we would not have the small temperature variations we have enjoyed for billions of years. We were exceedingly lucky when the Earth-Moon double planet formed, and I doubt other habitable planets would have done so well. Hence the low scores for f1 and fi.
A bit of early Earth history might make fi more understandable. Primitive one cell life formed on Earth about 3.8-billion years ago, not long after the oceans formed about 4.4 billion years ago. The earliest direct evidence of life are microfossils of microorganisms in the 3.5-billion-year-old Australian Apex Chert rocks. These are fossils of Prokaryotes, a small single-cell creature lacking a nucleus. The first Prokaryotes were archaea followed by bacteria and cyanobacteria (oxygen-producing bacteria). So, single-celled creature has been around since soon after the Earth cooled, but it took almost 3-billion years for evolution to produce complex multi-celled creatures. These were Eukaryotes (much larger single cell creatures with a nucleus). Eukaryotes eventually formed multicellular organisms, leading to plants, fungi, and animals. During the Cambrian explosion, about 540 million years ago, the first fossils of complex creatures with hard body parts appeared. There very long period between simple one cell creatures and the more complex more advanced multicell creatures implies a need for extended period of stable climate and this should be lacking on many planets in the “Goldilocks Zone”. Hence, even though single-cell life can develop early, complex multi-cell life capable of developing intelligence requires an extended period of stable conditions.
For these reasons I think intelligent life in the galaxy is very scarce and I am not at all surprised we hear nothing from SETI. Based on my work I know how much energy is required for interstellar travel and we ourselves will not be ready for about one hundred years (until we occupy and exploit the resources of the inner solar system). Of course, if someone invents a working Warp Drive tomorrow, I will happy to be wrong, but don’t hold your breath.
Finally, there is timing. The Universe is 13.799 billion years old and Sol, our star, is 4.6 billion years old. The first-generation stars in the Universe burned hydrogen and helium because that was all there was from the big Bang. After hundreds to billions of years the larger of those first stars supernovaed and the first metals were formed. Billions of years later next generation stars agglomerated out nearby gases in a solar nebula and the cycle repeated. The point is that it could easily take eight or more billion years for a 2nd or 3rd generation star to form with metal content capable of supporting life as we know it. Hence, we might not be all that behind the aliens in development, and even if they existed, they have not had the time to reach us yet.
Thanks for your attention.