The Milky Way galaxy has 200 billion stars and perhaps 100 billion planets. If even a small fraction of those planets harbored life, and even if only a pathetic scattering of those planets had lifeforms which became intelligent, our galaxy would be teeming with alien civilizations, some of whom would be either looking for us or discoverable for at least a little while.
The number of alien civilizations the galaxy should have can be determined by an equation, the Drake equation, that turns the above factors into variables. When you plug them into the formula, you find that there should be at least 20 civilizations in our cosmic neighborhood. This makes the fact that we have yet to find any other life in the cosmos almost shocking when you think about it.
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The seeming discord between how many advanced civilizations ought to be in space and the lack of evidence for any is known as the Fermi paradox. It has lead to dozens of hypotheses and potential solutions over the last few decades.
Many of the solutions aim at one of the variables in the Drake equation and try to make the supposed number of civilizations lower so it is more reasonable for us to not have met anybody yet.
Some propose that life starting at all is rare, others suggest that the development of intelligence is the bottleneck, others still posit that most civilizations would live for a short time before blowing themselves up or, conversely, never even manage to invent the radio.
One solution, however, is a bit darker than the others
The Dark Forest solution explains why we haven't heard from aliens by positing that they are purposefully keeping quiet.
The reasoning is laid out best in the science fiction novel The Dark Forest, by Liu Cixin. The plot of the book, the second in a series, concerns questions of how to best interact with potentially hostile alien life.
In the novel, the argument is laid out like this:
- All life desires to stay alive.
- There is no way to know if other lifeforms can or will destroy you if given a chance.
- Lacking assurances, the safest option for any species is to annihilate other life forms before they have a chance to do the same.
Since all other lifeforms in the novel are risk-averse and willing to do anything to save themselves, contact of any kind is dangerous, as it almost assuredly would lead to the contacted race wiping out whoever was foolish enough to give away their location. This leads to all civilizations attempting to hide in radio silence.
The reasoning behind the paranoia is explained in this paragraph from the novel:
The universe is a dark forest. Every civilization is an armed hunter stalking through the trees like a ghost, gently pushing aside branches that block the path and trying to tread without sound. Even breathing is done with care. The hunter has to be careful, because everywhere in the forest are stealthy hunters like him. If he finds another life—another hunter, angel, or a demon, a delicate infant to tottering old man, a fairy or demigod—there's only one thing he can do: open fire and eliminate them.
It's a bit like the prisoner's dilemma really, and the concept is based on applied game theory.
Is there a non-literary approach to this solution? Or is it just an idea that is good for a story?
It was also put forth by scientist David Brin as a potential solution to the lack of radio evidence for alien life. While the variant he describes relies on robotic probes carrying out the task of killing off civilizations other than the one that created it, the core concept remains the same. In this excerpt, he explains why this solution an attractive one for scientific purposes and terrifying for existential reasons:
“It is consistent with all of the facts and philosophical principles described in the first part of this article. There is no need to struggle to suppress the elements of the Drake equation in order to explain the Great Silence, nor need we suggest that no ETIS anywhere would bear the cost of interstellar travel. It need only happen once for the results of this scenario to become the equilibrium condition in the Galaxy. We would not have detected extraterrestrial radio traffic- nor would any ETIS have ever settled on Earth- because all were killed shortly after discovering radio."
He then reminds us that broadcasts of I Love Lucy are racing across the cosmos, ready to reveal our location and sense of humor to anybody who can pick them up.
How plausible is this theory?
This theory has the advantage of only affecting one of the variables in the Drake equation and affecting the one that is the most open to speculation. It also doesn't require us to make broad assumptions about how all alien civilizations behave; a single advanced race that acts this way would be enough to cause the observed situation.
This would also explain why we haven't found any mundane alien radio signals despite a century of being able to pick them up. Just as we accidentally send our radio signals, meant for us, out into space, another civilization would be likely to as well. One possible reason for this is that other civilizations are so fearful of being detected that they purposely avoid sending out any radio evidence of their existence.
It does, however, assume that other species have a similar risk aversion level and reasoning process as we do or that there really is one civilization out there killing off anybody they think can harm them. This is a big assumption.
Why is this theory dark?
We've been screaming our existence to the cosmos for almost one hundred years now. Any aliens within a one hundred light year radius of us would be receiving a barrage of radio signals from our direction. If we had reason to avoid letting aliens know about us, as Stephen Hawking thought we did, we might have a problem.
Why haven't we heard from aliens yet? If this solution is correct, they are purposely hiding in the darkness of space for fear of death. Should we stop broadcasting our existence to the universe too then? Or would alien life be a little nicer than we've been in our history?
- So. There are 36 alien civilizations in the Milky Way. - Big Think ›
- The biology of aliens: How much do we know? - Big Think ›
Our hearts beat at a resting rate of 60 to 100 beats per minute. Lots of other things pulse, too. The colors we see and the pitches we hear, for example, are due to the different wave frequencies ("pulses") of light and sound waves.
Now, a study in the journal Geoscience Frontiers finds that Earth itself has a pulse, with one "beat" every 27.5 million years. That's the rate at which major geological events have been occurring as far back as geologists can tell.
A planetary calendar has 10 dates in red
Credit: Jagoush / Adobe Stock
According to lead author and geologist Michael Rampino of New York University's Department of Biology, "Many geologists believe that geological events are random over time. But our study provides statistical evidence for a common cycle, suggesting that these geologic events are correlated and not random."
The new study is not the first time that there's been a suggestion of a planetary geologic cycle, but it's only with recent refinements in radioisotopic dating techniques that there's evidence supporting the theory. The authors of the study collected the latest, best dating for 89 known geologic events over the last 260 million years:
- 29 sea level fluctuations
- 12 marine extinctions
- 9 land-based extinctions
- 10 periods of low ocean oxygenation
- 13 gigantic flood basalt volcanic eruptions
- 8 changes in the rate of seafloor spread
- 8 times there were global pulsations in interplate magmatism
The dates provided the scientists a new timetable of Earth's geologic history.
Tick, tick, boom
Credit: New York University
Putting all the events together, the scientists performed a series of statistical analyses that revealed that events tend to cluster around 10 different dates, with peak activity occurring every 27.5 million years. Between the ten busy periods, the number of events dropped sharply, approaching zero.
Perhaps the most fascinating question that remains unanswered for now is exactly why this is happening. The authors of the study suggest two possibilities:
"The correlations and cyclicity seen in the geologic episodes may be entirely a function of global internal Earth dynamics affecting global tectonics and climate, but similar cycles in the Earth's orbit in the Solar System and in the Galaxy might be pacing these events. Whatever the origins of these cyclical episodes, their occurrences support the case for a largely periodic, coordinated, and intermittently catastrophic geologic record, which is quite different from the views held by most geologists."
Assuming the researchers' calculations are at least roughly correct — the authors note that different statistical formulas may result in further refinement of their conclusions — there's no need to worry that we're about to be thumped by another planetary heartbeat. The last occurred some seven million years ago, meaning the next won't happen for about another 20 million years.
When I was a junior in college, my electromagnetism professor had an awesome idea. Apart from the usual homework and exams, we were to give a seminar to the class on a topic of our choosing. The idea was to gauge which area of physics we would be interested in following professionally.
Professor Gilson Carneiro knew I was interested in cosmology and suggested a book by Nobel Prize Laureate Steven Weinberg: The First Three Minutes: A Modern View of the Origin of the Universe. I still have my original copy in Portuguese, from 1979, that emanates a musty tropical smell, sitting on my bookshelf side-by-side with the American version, a Bantam edition from 1979.
Inspired by Steven Weinberg
Books can change lives. They can illuminate the path ahead. In my case, there is no question that Weinberg's book blew my teenage mind. I decided, then and there, that I would become a cosmologist working on the physics of the early universe. The first three minutes of cosmic existence — what could be more exciting for a young physicist than trying to uncover the mystery of creation itself and the origin of the universe, matter, and stars? Weinberg quickly became my modern physics hero, the one I wanted to emulate professionally. Sadly, he passed away July 23rd, leaving a huge void for a generation of physicists.
What excited my young imagination was that science could actually make sense of the very early universe, meaning that theories could be validated and ideas could be tested against real data. Cosmology, as a science, only really took off after Einstein published his paper on the shape of the universe in 1917, two years after his groundbreaking paper on the theory of general relativity, the one explaining how we can interpret gravity as the curvature of spacetime. Matter doesn't "bend" time, but it affects how quickly it flows. (See last week's essay on what happens when you fall into a black hole).
The Big Bang Theory
For most of the 20th century, cosmology lived in the realm of theoretical speculation. One model proposed that the universe started from a small, hot, dense plasma billions of years ago and has been expanding ever since — the Big Bang model; another suggested that the cosmos stands still and that the changes astronomers see are mostly local — the steady state model.
Competing models are essential to science but so is data to help us discriminate among them. In the mid 1960s, a decisive discovery changed the game forever. Arno Penzias and Robert Wilson accidentally discovered the cosmic microwave background radiation (CMB), a fossil from the early universe predicted to exist by George Gamow, Ralph Alpher, and Robert Herman in their Big Bang model. (Alpher and Herman published a lovely account of the history here.) The CMB is a bath of microwave photons that permeates the whole of space, a remnant from the epoch when the first hydrogen atoms were forged, some 400,000 years after the bang.
The existence of the CMB was the smoking gun confirming the Big Bang model. From that moment on, a series of spectacular observatories and detectors, both on land and in space, have extracted huge amounts of information from the properties of the CMB, a bit like paleontologists that excavate the remains of dinosaurs and dig for more bones to get details of a past long gone.
How far back can we go?
Confirming the general outline of the Big Bang model changed our cosmic view. The universe, like you and me, has a history, a past waiting to be explored. How far back in time could we dig? Was there some ultimate wall we cannot pass?
Because matter gets hot as it gets squeezed, going back in time meant looking at matter and radiation at higher and higher temperatures. There is a simple relation that connects the age of the universe and its temperature, measured in terms of the temperature of photons (the particles of visible light and other forms of invisible radiation). The fun thing is that matter breaks down as the temperature increases. So, going back in time means looking at matter at more and more primitive states of organization. After the CMB formed 400,000 years after the bang, there were hydrogen atoms. Before, there weren't. The universe was filled with a primordial soup of particles: protons, neutrons, electrons, photons, and neutrinos, the ghostly particles that cross planets and people unscathed. Also, there were very light atomic nuclei, such as deuterium and tritium (both heavier cousins of hydrogen), helium, and lithium.
Cosmic alchemy
So, to study the universe after 400,000 years, we need to use atomic physics, at least until large clumps of matter aggregate due to gravity and start to collapse to form the first stars, a few millions of years after. What about earlier on? The cosmic history is broken down into chunks of time, each the realm of different kinds of physics. Before atoms form, all the way to about a second after the Big Bang, it's nuclear physics time. That's why Weinberg brilliantly titled his book The First Three Minutes. It is during the interval between one-hundredth of a second and three minutes that the light atomic nuclei (made of protons and neutrons) formed, a process called, with poetic flair, primordial nucleosynthesis. Protons collided with neutrons and, sometimes, stuck together due to the attractive strong nuclear force. Why did only a few light nuclei form then? Because the expansion of the universe made it hard for the particles to find each other.
What about the nuclei of heavier elements, like carbon, oxygen, calcium, gold? The answer is beautiful: all the elements of the periodic table after lithium were made and continue to be made in stars, the true cosmic alchemists. Hydrogen eventually becomes people if you wait long enough. At least in this universe.
In this article, we got all the way up to nucleosynthesis, the forging of the first atomic nuclei when the universe was a minute old. What about earlier on? How close to the beginning, to t = 0, can science get? Stay tuned, and we will continue next week.
To Steven Weinberg, with gratitude, for all that you taught us about the universe.
