We reside on a random mote we call Earth orbiting an average sun in a galaxy that is one out of potentially trillions in an expanding universe.[1] The observable universe’s size is so huge it’s essentially an abstraction, something like ten to the power of 23 (1023) kilometers in diameter. About 13.8 billion years ago our universe was smaller than a millimeter in diameter when space and time began rushing outward faster than the speed of light to reach its current size.
Sometime after the big bang, the first stars formed out of gas and dust and merged to form billions upon billions of galaxies. Inhabiting each galaxy are billions of stars of different types, white dwarfs, red giants, yellow suns, neutron stars et al., and black holes, gas and dust nebulae, and planetary systems. Our own solar system orbits around the middle to the outer edge of a galaxy we call the Milky Way.
There are anywhere from 150 billion to 2 trillion galaxies[2] in the universe. If each galaxy is assigned on average 100 billion stars, the universe has between ~10**19 to 2 X 10**23 stars. Recent evidence provided by the Kepler Telescope and other surveys suggests that stars like our sun with rocky planets are common. About a fifth of these rocky planets are estimated to be orbiting their stars in a region of distance known as the habitable “Goldilocks” zone.
If there are many stars like our on sun, and many of these stars have rocky planets orbiting at the right distance, and many of these planets have liquid water necessary for life, there should be many planets harboring intelligent life in our Milky Way galaxy alone. So far, after over 50 years of looking, no evidence of intelligent life has been found. This apparent absence of advanced civilizations is known as the Fermi paradox, named after the Physicist Enrico Fermi, who asked over a lunchtime conversation in 1950, “where are they?”
There have been many attempts to answer Fermi’s question. To list a few, 1) The aliens are either hiding or sleeping. 2) Transmission beams carrying extraterrestrial messages are too weak to detect unless we are in their direct path. We can't detect a message beamed from a civilization 100,000 light years away on the opposite side of our galaxy. 3) We are the only advanced civilization in the Milky Way. 4) We are living in a computer simulation.[3]
A seminal paper on this subject was published in 1975 by Michael Hart, "An Explanation for the Absence of Extraterrestrials on Earth.” Hart dismisses the first and second above possibilities and argues persuasively for the third, that intelligent extraterrestrial civilizations besides our own don’t exist.
If intelligent extraterrestrials don’t exist, it may be that at some stage in their development they wipe themselves out because of nuclear war, global climate change, or plastic pollution, or get wiped out because of gamma-ray burst radiation, or some other catastrophe outside their control. Hart dismissed these explanations as well. The numbers favor many civilizations avoiding these fates.
“A single ear of wheat in a large field is as strange as a single world in infinite space.”
— Metrodorus of Chios, 4th Century BC [4]
Extraterrestrial civilizations may not exist because intelligent life depends on a set of physical and environmental factors to come together at the right time and occur in the right order, and this is extremely rare. Only a uniquely rare planet like Earth can shelter the abiogenesis of life out of a soup of organic chemicals and its slow but not inevitable evolution into us.[5]
We don’t know how common abiogenesis is throughout the galaxy and it may be very common. But life can’t advance beyond microbes and goes extinct on all but the rarest of rare rocky planets, according to the “Gaian Bottleneck,” or great filter, hypothesis. Life can’t evolve fast enough to take over the regulation of its environment and feedback mechanisms on a planetary scale.
Not long after the Earth formed out of the dust and gas encircling the young sun it collided with a planet about the size of mars. The Moon formed out of the debris thrown out by collision and Earth emerged with a relatively rapid rotation, a moderate axial tilt, and an iron inner core spinning within a liquid outer core. The spin generates a magnetic field that has protected the Earth from solar wind and storm radiation for billions of years.
The collision may also have fragmented the Earth’s crust, creating plate tectonics. The movement and subduction of these plates into the Earth’s mantle helped regulate temperature for eons by removing CO2 from the atmosphere. Nutrients put into the oceans by subduction were present at the time of the Cambrian explosion.
Life may have originated in tidal pools containing a mix of complex organic chemicals and nutrients. There were more tidal pools, increasing the likelihood of success, because of our relatively large Moon’s strong gravitational pull.
Planets the size and mass of Jupiter are rare. Jupiter has guarded Earth by absorbing a lot of the debris left over from the formation of the solar system, and its massive gravity has absorbed or thrown out of the solar system comets and asteroids that may have been headed towards Earth.
At least one out of the five major extinction events which nearly wiped out all life on Earth was fortuitous. The last event, famous because it wiped out all the dinosaurs, occurred about 66 million years ago after an asteroid the size of Manhattan struck the Earth near the Yucatan peninsula.[6] This event allowed the small furtive mammals that survived to thrive and evolve into the elephants, tigers, rabbits, and human beings we see today.
The Rare Earth and Gaian bottleneck hypotheses make a compelling argument for why we don’t have evidence of extraterrestrial civilizations. If it is true now, it's likely true for the foreseeable future that we’re alone. If we someday build spaceships and journey to the stars, we may never find, no matter how far we travel, a planet in our galaxy as comfortable to live on as Earth. We’ll have to either transform each planet we find into a copy of Earth, or build a self-sustaining Earth-like environment on or under the surface.
Experimenting with terraforming, or para-terraforming world-cities, could begin with our dead neighbor Mars. Before we do, we better fully understand the social, geological, biological, ecological, astronomical mechanisms which sustain life on Earth. This is important because Earth may be all we’ll ever have,[7] and we better understand and agree on what to do to keep it.[8]
Notes
- What I'm describing as true is my understanding.
- Cosmology has a lot of estimates that aren't yet nailed down. The estimate of two trillion galaxies is new.
- Briefly, we live within a simulation run by a very large, possibly planet-size, quantum computer. It's like the Matrix movie except that, according to Philosopher Nick Bostrom, everything down to your neural synapses are simulated. I imagine, as a primitive analogy, a set of boundary conditions which are applied when the computer is turned on. It makes sense then that in order to conserve computing resources the conditions exclude detecting, and meeting with, extraterrestrials.
- Quoted from Webb, Stephen. "If the Universe Is Teeming with Aliens ... WHERE IS EVERYBODY?" (Science and Fiction) (p. 2). Springer International Publishing. Kindle Edition.
- I owe the PBS Space Time YouTube video series, hosted by Matt O'Dowd a lot of the information summarized here. I highly recommend subscribing and helping them out if you can.
- A long time ago I had this crazy thought. The asteroid arrived when it did because after hundreds of millions of years someone (God?) got tired of watching dinosaurs and wanted something new.
- About a billion years or so from now if we're still around we better have figured something out.
- I'm arguing for a science of unintended consequences. This could be the already invented Complexity Science.
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