Everyone feels something when they’re in a really good starry place on a really good starry night and they look up and see this:
Some people stick with the traditional, feeling struck by the epic beauty or blown away by the insane scale of the universe. Personally, I go for the old “existential meltdown followed by acting weird for the next half hour.” But everyone feels something.
Physicist Enrico Fermi felt something too—”Where is everybody?”
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A really starry sky seems vast—but all we’re looking at is our very local neighborhood. On the very best nights, we can see up to about 2,500 stars (roughly one hundred-millionth of the stars in our galaxy), and almost all of them are less than 1,000 light years away from us (or 1% of the diameter of the Milky Way). So what we’re really looking at is this:
When confronted with the topic of stars and galaxies, a question that tantalizes most humans is, “Is there other intelligent life out there?”
Let’s put some numbers to it (if you don’t like numbers, just read the bold)—
As many stars as there are in our galaxy (100 – 400 billion), there are roughly an equal number of galaxies in the observable universe—so for every star in the colossal Milky Way, there’s a whole galaxy out there. All together, that comes out to the typically quoted range of between 10^22 and 10^24 total stars in the universe, which means that for every grain of sand on Earth, there are 10,000 stars out there.
The science world isn’t in total agreement about what percentage of those stars are “sun-like” (similar to our sun in size, temperature, and luminosity)—opinions typically range from 5% to 20%. Going with the most conservative side of that (5%), and the lower end for the number of total stars (10^22), gives us 500 quintillion, or 500 billion billion sun-like stars.
There’s also a debate over what percentage of those sun-like stars might be orbited by an Earth-like planet (one with similar temperature conditions that could have liquid water and potentially support life similar to that on Earth). Some say it’s as high as 50%, but let’s go with the more conservative 22% that came out of a recent PNAS study. That suggests that there’s a potentially-habitable Earth-like planet orbiting at least 1% of the total stars in the universe—a total of 100 billion billion Earth-like planets.
So there are 100 Earth-like planets for every grain of sand in the world. Think about that next time you’re on the beach.
Moving forward, we have no choice but to get completely speculative. Let’s imagine that after billions of years in existence, 1% of Earth-like planets develop life (if that’s true, every grain of sand would represent one planet with life on it). And imagine that on 1% of those planets, the life advances to an intelligent level like it did here on Earth. That would mean there were 10 quadrillion, or 10 million billion intelligent civilizations in the observable universe.
Moving back to just our galaxy, and doing the same math on the lowest estimate for stars in the Milky Way (100 billion), we’d estimate that there are 1 billion Earth-like planets and 100,000 intelligent civilizations in our galaxy.[1]
SETI (Search for Extraterrestrial Intelligence) is an organization dedicated to listening for signals from other intelligent life. If we’re right that there are 100,000 or more intelligent civilizations in our galaxy, and even a fraction of them are sending out radio waves or laser beams or other modes of attempting to contact others, shouldn’t SETI’s satellite array pick up all kinds of signals?
But it hasn’t. Not one. Ever.
Where is everybody?
It gets stranger. Our sun is relatively young in the lifespan of the universe. There are far older stars with far older Earth-like planets, which should in theory mean far more advanced civilizations than our own. As an example, let’s compare our 4.54 billion-year-old Earth to a hypothetical 8 billion-year-old Planet X.
If Planet X has a similar story to Earth, let’s look at where their civilization would be today (the orange is a reference to show how long these timelines really are):
The technology and knowledge of a civilization only 1,000 years ahead of us could be as shocking to us as our world would be to a medieval person. A civilization 1 million years ahead of us might be as incomprehensible to us as human culture is to chimpanzees. And Planet X is 3.4 billion years ahead of us…
There’s something called The Kardashev Scale, which helps us group intelligent civilizations into three broad categories by the amount of energy they use:
A Type I Civilization has the ability to use all of the energy on their planet. We’re not quite a Type I Civilization, but we’re close (Carl Sagan created a formula for this scale which puts us at a Type 0.7 Civilization).
A Type II Civilization can harness all of the energy of their host star. Our feeble Type I brains can hardly imagine how someone would do this, but we’ve tried our best, imagining things like a Dyson Sphere.
A Type III Civilization blows the other two away, accessing power comparable to that of the entire Milky Way galaxy.
If this level of advancement sounds hard to believe, remember Planet X above and their 3.4 billionyears of further development (about half a million times as long as the human race has been around). If a civilization on Planet X were similar to ours and were able to survive all the way to Type III level, the natural assumption is that they’d probably have mastered inter-stellar travel by now, possibly even colonizing the entire galaxy.
One hypothesis as to how galactic colonization could happen is by creating machinery that can travel to other planets, spend 500 years or so self-replicating using the raw materials on their new planet, and then send two replicas off to do the same thing. Even without traveling anywhere near the speed of light, this process would colonize the whole galaxy in 3.75 million years, a relative blink of an eye when talking in the scale of billions of years:
Source: Scientific American – Where Are They?
Continuing to speculate, if 1% of intelligent life survives long enough to become a potentially galaxy-colonizing Type III Civilization, our calculations above suggest that there should be at least 1,000 Type III Civilizations in our galaxy alone—and given the power of such a civilization, their presence would likely be pretty noticeable. And yet, we see nothing, hear nothing, and we’re visited by no one.
So where is everybody?
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Welcome to the Fermi Paradox.
We have no answer to the Fermi Paradox—the best we can do is “possible explanations.” And if you ask ten different scientists what their hunch is about the correct one, you’ll get ten different answers. You know when you hear about humans of the past debating whether the Earth was round or if the sun revolved around the Earth or thinking that lightning happened because of Zeus, and they seem so primitive and in the dark? That’s about where we are with this topic.
In taking a look at some of the most-discussed possible explanations for the Fermi Paradox, let’s divide them into two broad categories—those explanations which assume that there’s no sign of Type II and Type III Civilizations because there are none of them out there, and those which assume they’re out there and we’re not seeing or hearing anything for other reasons:
Explanation Group 1: There are no signs of higher (Type II and III) civilizations because there are no higher civilizations in existence.
Those who subscribe to Group 1 explanations point to something called the non-exclusivity problem, which rebuffs any theory that says, “There are higher civilizations, but none of them have made any kind of contact with us because they all _____.” Group 1 people look at the math, which says there should be so many thousands (or millions) of higher civilizations, that at least one of them would be an exception to the rule. Even if a theory held for 99.99% of higher civilizations, the other .001% would behave differently and we’d become aware of their existence.
Therefore, say Group 1 explanations, it must be that there are no super-advanced civilizations. And since the math suggests that there would be thousands of them just in our own galaxy, something else must be going on.
This something else is called The Great Filter.
The Great Filter theory says that at some point from pre-life to Type III intelligence, there’s a wall that all or nearly all attempts at life hit. There’s some stage in that long evolutionary process that is extremely unlikely or impossible for life to get beyond. That stage is The Great Filter.
If this theory is true, the big question is, Where in the timeline does the Great Filter occur?
It turns out that when it comes to the fate of humankind, this question is very important. Depending on where The Great Filter occurs, we’re left with three possible realities: We’re rare, we’re first, or we’re fucked.
1. We’re Rare (The Great Filter is Behind Us)
One hope we have is that The Great Filter is behind us—we managed to surpass it, which would mean it’s extremely rare for life to make it to our level of intelligence. The diagram below shows only two species making it past, and we’re one of them.
This scenario would explain why there are no Type III Civilizations…but it would also mean that we could be one of the few exceptions now that we’ve made it this far. It would mean we have hope…
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