Thus far, astronomical observation has not shown evidence of extraterrestrial civilizations. This leads us to the famous question: Where is everybody?
This apparent lack of life outside Earth seems to be at odds with the data we have found so far. Originally, it was unknown how likely a star is to have planets. Now, it looks like planets are common. Similarly, experiments in chemistry suggest that organic compounds self-assemble under relatively mild conditions. Furthermore, there are seasonal methane shifts in the Martian atmosphere, similarly to Earth, where it is a byproduct of bacteria. Finally, there’s evidence of an ocean under Europa’s crust, and hydrothermal vents in the ocean of Enceladus. This means that at least 3 planets/moons other than Earth in our own solar system are prime candidates for life. If life is found on any of them, it will mean that bacterial life is likely to be extremely common.
Even if life is common, maybe the jump from simple bacteria to complex life never happened before? Why haven’t we found anything? Many people have tried to explain this apparent paradox, with varying success. The event that makes our galaxy seemingly lifeless is called “The Great Filter” - if abiogenesis or complex life is rare, then perhaps we are extremely lucky to exist.
There is also the possibility that we have not yet encountered the great filter. In this situation, technological civilizations might develop fairly frequently in our galaxy, but they all encounter something that keeps them silent: either they kill themselves, or perhaps they all hide. But even then, it is hard to believe that everyone would behave identically. Shouldn’t the great filter be triggered by something all technological civilizations have in common?
In order for a civilization to send out radio waves, they must have developed science. There is no way to send radio waves without first discovering that electricity flowing through wires creates magnetic fields!
So let’s suppose that the very thing which is a prerequisite for technological development also starts the doomsday clock: science.
All around Earth, scientists are probing the limits of our understanding of physics - what if somewhere in extreme conditions there is an undiscovered quirk in the laws of nature? Perhaps something weird happens deep within stars, but we ourselves cannot see it until we accidentally recreate the necessary conditions in a lab?
I call such a quirk the “Doomsday Discovery”. There are two ways I see such a discovery destroying life of Earth:
What is worse, since we will relatively soon be technologically capable of creating a self-sustaining colony on Mars, if this “Doomsday Discovery” is real, it means that we must either be on the verge of discovering it, or it sterilizes our entire solar system when triggered: perhaps it would make our sun go supernova.
This quirk in nature would need to have several properties:
While these may seem fairly restrictive, I can immediately give two examples that get quite close: one from reality, and one from science fiction!
It turns out that even when the underlying theory is known, mistakes can happen by those unaware of its consequences. In “Surely You’re Joking Mr. Feynman”, Feynman recounts the story of a uranium enrichment facility:
The people in Oak Ridge didn’t know anything about what it was to be used for; they just knew what they were trying to do. I mean the higher people knew they were separating uranium, but they didn’t know how powerful the bomb was, or exactly how it worked or anything. The people underneath didn’t know at all what they were doing. And the army wanted to keep it that way. There was no information going back and forth. But Segre insisted they’d never get the assays right, and the whole thing would go up in smoke. So he finally went down to see what they were doing, and as he was walking through he saw them wheeling a tank carboy of water, green water which is uranium nitrate solution.
He said, “Uh, you’re going to handle it like that when it’s purified too? Is that what you’re going to do?”
They said, “Sure why not?”
“Won’t it explode?” he said.
Then the army said, “You see! We shouldn’t have let any information get to them! Now they are all upset.”
It turned out that the army had realized how much stuff we needed to make a bomb twenty kilograms or whatever it was and they realized that this much material, purified, would never be in the plant, so there was no danger. But they did not know that the neutrons were enormously more effective when they are slowed down in water. In water it takes less than a tenth no, a hundredth as much material to make a reaction that makes radioactivity. It kills people around and so on. It was very dangerous, and they had not paid any attention to the safety at all.
This same effect, increased effectiveness of neutrons in water, is exploited in nuclear reactors. A reactor meltdown is worldwide news - it is a danger for everyone within hundreds of miles.
Imagine a similar scenario, but with much greater consequences: a material is prepared for some unrelated experiment, and unbeknownst to anyone involved, some unexpected subatomic property is triggered that causes massive energy release.
In “Ender’s Game” (spoiler alert), the military had a weapon called the Molecular Disruption Device, which worked through a chain reaction, and had the effect of scrambling up molecules it comes in contact with. This device was used to rid an entire planet of life:
While all is well if this device was first postulated by theory, so that scientists could be extremely careful when building it, what would have happened if the chain reaction was discovered by accident in a lab on the surface of Earth? What if we are on the verge of accidentally stumbling on something similar in a lab somewhere?
The next question that needs to be asked, is whether astronomical observations show high energy events coming from stars in our galaxy. If the doomsday experiment is the great filter, then depending on the mechanism of destruction, we might be able to observe random stars in our galaxy briefly flash a bit brighter than expected, as civilization after civilization makes its last discovery.
Fast Radio Bursts are quick, extremely high energy blasts of radio waves, which come from random directions (ie: do not seem to come from our galaxy). Their causes are unknown at this time, but they are thought to come from extreme events. Suppose for a moment that these bursts are the outcome of experiments of technological civilizations in our universe. What can we conclude?
If these are the effects of our doomsday experiment, then technological life is probably extremely common in the universe - estimates of detectable FRBs are in the thousands per day. This suggests that while the universe is teeming with life, it is actually fairly rare for a star to have advanced civilizations, since there is no preference for our galaxy.
The main weakness of this hypothesis is the extreme power of these bursts. They would require a good portion of our planet to be immediately converted into pure energy to give the correct readings! I would expect astronomical evidence of a doomsday experiment to be much less pronounced than FRBs: a small, brief increase in the detected energy output of a star would be most likely given technological development of civilizations at the point when this experiment must happen.
The above thought experiment proposes a cute possible explanation for the lack of visible large-scale civilizations, assuming that technological life is actually common in the universe. While it was fun to think about, it is also important to see where it falls short.
The “Doomsday Experiment” has a very simple weakness. It requires civilizations to do something that very rarely happens by itself on planets, and it requires that they do it at a specific point in their technological development: before they expand to other planets or stars. This means that given current technological progress, we should have the ability to perform this “Doomsday Experiment” within the next few centuries.
Here’s the thing: Every quirk of physics that we have explored so far happens countless times all around us in space. Our greatest particle accelerators are no match for the power of cosmic rays that daily pummel the earth. There is even evidence there there was a natural fission reactor on earth - and that’s just Earth! Extreme conditions exist all over our solar system, and yet we are supposed to do something that didn’t happen in our entire solar system for at least the last couple million years?
I could imagine us doing some intense physics if we had our entire sun’s energy output, but at that point we would have been able to at least send von-neumann probes to adjacent stars! Remember, the doomsday discovery would need to happen before we have a chance of escape.
It is for this reason that given current observations, the most likely conclusion is that either the great filter is already behind us, or life is rare enough, and space travel hard enough that it will take us more time to make contact - after all, we’ve only been listening to small patches of sky for the last 50 years!
In astronomical timescales, we’ve barely started the search.