If you live in a city, you might only rarely see a sight like this. But if you ever get away from the lights, into the darkness of the countryside, you’ve likely looked up and marveled at the number of stars in the Milky Way galaxy. You’ve maybe even tried to count them, wondering at just how many there are. Which is a great question. Just how many stars can you see out there?
If you’re in the darkest area on the planet, devoid of all light from the city, on a moonless night, with clear air and an unobstructed 360-degree view of the horizon, the answer is right around 4500 stars. That’s under the most ideal conditions. In places out in the countryside, the places where many people get out of the city to marvel at the blanket of stars, places where it’s dark enough to just begin to see the stripe of white that makes up an arm of the Milky Way, the answer is between 2000 and 2500 stars.
That’s still a lot of stars, and if you’ve stared up at the sight of all those glimmering points of light and considered that each of them represents a solar system, with many, if not most, having planets that orbit them, you’ve probably wondered about the possibility of alien life. Once you realize that for every star you see, there are approximately 100,000 stars you can’t see, just in the Milky Way galaxy alone, you realize how impossible it must be for us to be alone in the universe.
Think about the numbers here for a minute. Just in the Milky Way galaxy alone there are an estimated 200 billion stars. When we look up at the sky on those dark nights and marvel at all of the stars, we are seeing only a fraction of a percentage of our own galaxy. Like, one hundred-millionth of the stars in our galaxy alone. For every star we can see on a dark, clear, moonless night, there are around one hundred million stars we can’t see, just in the Milky Way galaxy. Think about that the next time you’re looking up at all those stars.
The furthest stars we can see with the naked eye are about 1000 light years away. The Milky Way galaxy is 100,000 light years across, which means we can only see around 1% (with regard to distance, not quantity) of it with the naked eye. All the stars we can see with the naked eye from Earth are the brightest stars within the little red circle in the picture below.
All of those stars in our galaxy, and we’re just one galaxy out of many in the universe. How many? Take a look at this picture taken a few years ago by Nasa using two different telescopes.
Every one of the points of light in this picture is a GALAXY. All different sizes, all different shapes, ranging from an estimated 50 million stars, to 100 trillion stars in size. There are approximately 10,000 visible galaxies in this photo, the most distant, nearly 13 billion light years away. And here’s the most amazing thing:
This picture represents an area of space the width of about 1/10 that of the full moon. A tiny, minuscule, fractional, sliver of space in our view plane and it contains at least 10,000 galaxies! The reason astronomers chose this area of space to train the telescopes, letting them gather all available light for days to get the image, was because it was the darkest area of space visible from Earth, where nothing could be seen with normal telescopes. And they found 10,000 galaxies hiding there. It’s hard to fathom that, and when you extrapolate those figures to the remainder of the universe, which is symmetrical and consistent in all directions, it’s estimated that there are somewhere between 200 billion and 500 billion galaxies in the observable universe. (Some scientists think it might even be as high as 1 trillion galaxies)
How many stars is that?
It’s typically estimated at between 1022 and 1024 stars. 1024 is this number:
That’s 1 septillion stars. A trillion-trillion stars.
To put that in another perspective, the estimated number of grains of sand on every beach on planet Earth is 1021.
That means for every grain of sand on every beach on Earth, there are 1000 stars in the universe. Think about that the next time you’re laying on the beach and decide to wipe the sand off your feet.
Even if we take the lower number of the total star estimate, 1022 stars, that’s still 10 stars for every grain of sand on every beach on Earth. 10 sextillion stars in the universe. That’s not a septillion by any means, but still kind of a lot.
Astronomers’ opinions about the number of Earth-like planets that might orbit these stars varies, with some saying it’s as low as 20% of stars that contain an Earth-like planet, and some saying it could be as high as 50%. In addition, not all stars are even capable of nurturing life as we know it—white dwarfs are small and hot, red giants are huge and cool. Astronomers estimate that the number of stars that are sun-like, with regard to size, temperature, and luminosity, might only be 10%-20% of all stars.
If we take a middle of the road approach…actually, let’s take a low-end approach, we’ll look at the bottom of the range of estimates for all three figures. We’ll say there are 1022 stars and 10% of them (1021) are sun-like, and 20% of those (2×1020) contain planets that are Earth-like, in the habitable zone, or “Goldilocks Zone” as it’s known, (not too hot, not too cold, juuust right!). That leaves the possibility of 200,000,000,000,000,000,000 – 200 quintillion – or 200 billion-billion Earth-like planets in the universe.
Okay, let’s put this in more reasonable numbers by getting rid of all the other hundreds of billions of galaxies and just focusing on our own Milky Way galaxy.
We’ll take the low-end numbers again and we’ll give an estimate of 100 billion stars in our galaxy, which is the lowest estimate I’ve been able to find. (The high range is 400 billion.) If 10% of those are sun-like, that’s 10 billion suns in the Milky Way. If only 20% of those contain Earth-like planets, that’s 2 billion Earths in our galaxy.
How many of those 2 billion Earth-like planets in the Milky Way might support life? Now we have to move from educated estimates to speculation. There is an equation for determining the number of intelligent civilizations that exist in the galaxy, known as the Drake equation, conceived by Dr. Frank Drake. The problem with the equation is it requires a number of pure guesses and speculation to solve, things like the fraction of planets on which life appears, and the fraction of those systems which become intelligent life. These are things we can’t really know, so we have to speculate. We can take some pretty careful and conservative numbers and input them into the equation, but in the end, it’s still nothing but a guess.
So, let’s guess. Of those 2 billion Earth-like planets in the Milky Way, let’s say that only 1% of those planets form any kind of life whatsoever, from single-celled organisms, to simple life forms, to tool-using mammals with big brains, all the way on up to super-intelligent beings. That would be 20 million planets with some type of life on them. Now let’s say that only 1% of those have more than single cellular life-forms, those that have evolved to intelligent, multi-celled life like fish and even dinosaurs or Neanderthals and other tool-users. That would be 200,000 planets like that. And let’s say just 1% of those advance to a civilization that’s at least as powerful and knowledgeable as human-kind, communicating and sending radio signals out into space. That would be 2,000 super-advanced civilizations in the Milky Way galaxy alone. And remember, this is speculating with tiny numbers. Tim Urban over at my favorite website, Waitbutwhy.com, used rather conservative extrapolation that actually came up with an estimate of 100,000 advanced civilizations in the Milky Way.
So where is everybody?
Welcome to the Fermi Paradox.
In the 1950s, while on a lunch break with several other scientists involved in the nuclear program, physicist Enrique Fermi postulated that question. If the math says there should be thousands (maybe tens or even hundreds of thousands) of intelligent civilizations in the galaxy, then where are they all? Because there has never been one single instance of provable contact or signal from an alien civilization.
When you realize that our sun is very young, it gets even stranger. There are stars in our galaxy that should support Earth-like planets that are hundreds of millions, even billions of years older than the sun. Our sun is about 4.5 billion years old, but the Milky Way is about 13.4 billion years old. For 9 billion years before our sun was even formed, stars and Earth-like planets were galivanting about in our galaxy!
In theory, some of those stars and planets should have created life millions or even billions of years before our planet was even born, while our sun was still a cloud of hydrogen gas floating around in space. There should have been hundreds of intelligent civilizations out there when Earth was just beginning to cool, and then millions of years later, when single-celled organisms were just forming in the primordial soup. By now, hundreds of millions of years later, those civilizations should be so advanced that we might not even be able to fathom their intelligence. They should be advanced enough that they’ve spread throughout the galaxy, colonizing other planets, exploring other stars.
Even if you say, “well, okay, but you’re guessing on those numbers. Life might not evolve at anything near the rate you think it does”, remember that there are hundreds of billions of galaxies in the universe. Maybe as many as a trillion galaxies. We can see galaxies out to a distance around 13 billion light years. That means the light from these galaxies has been traveling to us for 13 billion years. Radio signals travel at light speed. Even if you think my speculation is off by a thousand-fold, that would mean there were 2 intelligent civilizations in all the vastness of the Milky Way, but still hundreds of billions of those civilizations in the universe, many that should have been in existence for billions of years longer than humans have been around. Billions of years for those hypothetical radio signals to travel across the vast distances between the galaxies. Where are those signals? We’ve trained our radio telescopes at many of the galaxies nearest to the Milky Way and we’ve heard nothing.
So where are they all?
In the 1970s, the Search for Extraterrestrial Intelligence (SETI), a collective of scientific searches for intelligent life was formed to search for radio signals using deep-space radio telescopes like the Very Large Array shown here.
These types of arrays have been searching over the universe for decades and they’ve heard nothing. Not a peep from anything that couldn’t be explained as natural. (Other than this one mysterious signal known as the Wow! signal that we heard in 1977. It’s still unexplained.) In addition, we’ve sent signals out into space, broadcasting to all who might listen that we’re here and we’re looking for our neighbors. Nothing.
Incidentally, many scientists look at these intentional broadcasts into space as the equivalent of standing on the edge of a dark, unknown, and mysterious forest, and shouting loudly into it, (IS ANYTHING IN THERE? I’M RIGHT HERE AND I’M UNARMED!) They think it’s a pretty dangerous thing to do with the lack of knowledge we have, and, as the new kids on the block with regard to interstellar communications and technology, we should probably listen quietly for a while until we understand what might be out there. Of course, these messages will take hundreds or thousands of years to reach the systems they’re aimed toward, so unless a passing alien spacecraft happens to pick them up, we should be okay. Anyway, no response has ever been received to any message.
So again, where is everybody?
Science has a number of possible explanations for this lack of alien evidence. Some are very scary, such as there is one super-predator species out there that waits until a civilization advances to the ability to travel to other stars and then wipes them out, or assimilates them ala the Borg from Star Trek TNG.
Another theory is that advanced civilizations might have a Star Trek-like Prime Directive that enables them to observe us, but not to make contact or let us know in any way that they exist. (Known as the Zoo Hypothesis. We’re all just animals in a giant galactic zoo, folks.) A civilization that has the technology to travel the hundreds or thousands of light years between their home and ours would almost certainly have the ability to hide their presence from us. As an example, when you walk by an anthill, do the ants know you’re there? Do they have the ability to conceive of your presence, your purpose, your technology and intelligence? Civilizations that are millions or billions of years more advanced than humans might very well look at us the same way we look at ants. Their technology might be so advanced that we can’t even conceive of its purpose or its meaning, much like an ant could never conceive of what a car is, or what an iPhone can do, even if the ant was crawling right over it.
Another theorized solution to the Fermi Paradox is that super-advanced civilizations may have trapped all the energy of their home star in a type of hypothetical megastructure called a Dyson sphere
and that we can’t pick up their radio signals because they’re all trapped within this sphere. This theory, as outlandish and sci-fi in appearance as it might seem, could actually have some merit. Scientists have determined that there’s enough mass within the core of the planet Jupiter to build a sort of modified Dyson sphere, called a Dyson swarm, around the orbit of Mars. If we advanced far enough to develop the ability to dismantle Jupiter, we could build this Dyson swarm. Lining it with solar panels would give us the ability to harvest all the energy of the sun – unlimited free energy, allowing us to create a sort of inner solar system that would be completely controlled by us. If we broke up Mercury too, and built a full Dyson sphere, our sun and our radio signals would be completely undetectable to other civilizations that didn’t already know we were here.
There’s been some speculation that astronomers may have actually detected emissions from stars where Dyson spheres are currently under construction. (Or, more accurately, were under construction whenever the light we’re seeing was emitted from the stars, thousands of years ago most likely.) These stars are acting strangely, their brightness dimming periodically in similar fashion to the dimming that occurs when a planet in orbit passes between them and us, but to an extent that the object causing the dimming is way larger than a planet could actually be. You can read more about this interesting mystery here and here.
However, even if a civilization is advanced enough to build a Dyson sphere and hide all evidence of their presence, we still should be able to pick up signs of their civilization. Humans, for example, have been creating radio transmissions and projectile spewing them out into space for decades. Even if we advanced enough in the next hundred years to create a Dyson sphere (an unlikely rate of advancement), we would still have broadcast signals, both intentional and unintentional, for the previous 200 years. Those signals are traveling at the speed of light out into space, and, much like a bullet and a drunken comment, once it’s out there, it’s out there. You can’t retract it.
So, if a star suddenly disappears from our view because an advanced civilization closes up a Dyson sphere, we still should expect to have received some kind of radio signal from them over the last few decades we’ve been listening for them.
There are other theories and hypotheses that attempt to explain the Fermi Paradox, but the most popular one is The Great Filter Theory.
The Great Filter Theory basically states that at some point in the evolution of life, a filter is encountered, a filter that wipes out the advancing civilization, and that the filter is one that occurs for most, if not all civilizations at the same point in their evolution. Some may get through the filter, but most are destroyed by it. If just 1% of civilizations get through the Great Filter, that would mean there might only be 20 advanced civilizations out there, scattered throughout the 100,000 light-year-wide Milky Way, making it much more likely we wouldn’t have encountered any sign of their presence.
Now, the Great Filter is the leading theory at the moment, and many scientists think if there is such a thing, it must be behind us. We must have been one of the rare civilizations that managed to make it through the filter. They point to the asteroid or comet impact that wiped out the dinosaurs, and they say that that type of event could be the filter, that many civilizations may have been destroyed in that way, never recovering. Our planet did recover from that event, pushing us through the Great Filter.
Others, like renowned British professor and director of the Future of Humanity Institute, Nick Bostrom, think the Great Filter might be Nuclear weapons, that many civilizations, once they achieve the ability to harness the power of the atom, basically destroy themselves in war, something we actually came very close to doing in the 1960s. He thinks we might have made it through the Great Filter when we survived the Cold War.
Others think the Great Filter might be the challenge of life itself. That life doesn’t form as easily as we think it does, or that life only rarely passes beyond the zygote stage. Some think cosmic radiation, or chance, or some other factor stops life in the early, pre-intelligence stage, and that Earth, for some reason…luck, chance, or standard variance, faded that early death. As a gambler, I can appreciate the idea that we dodged the variance bullet, but the thought that Earth and our entire solar system might otherwise be as empty and barren as Mercury is a little disturbing.
Whatever the Great Filter is, we obviously have to hope it’s in our past and not in our future. When Viking 1 landed on Mars on July 20th, 1976, many scientists familiar with the Great Filter concept dreaded the idea that we might find signs of life on the red planet. If life had managed to form on two separate planets in this solar system alone, that would imply that the formation of life in the universe was easy and common, which would make the Great Filter theory the most likely explanation for the Fermi Paradox. If there had been signs of an advanced civilization on Mars – for example, the suspected canals that were big news in 1877, that might have indicated that the Great Filter lay in our future somewhere, a foretelling of our impending doom.
There are many people who believe the Great Filter is indeed in our future, and if that’s the case, it’s scary to contemplate, because it would mean we most likely have less than a .1% chance of avoiding the complete extinction of mankind.
I, unfortunately, am one of those.
Of course, I want to hope that’s not the case, and the optimist in me, and in many who write about this, wants to believe the filter lies in our past. After all, nobody wants to really contemplate the worst-case scenario, and it’s easy to believe we’re special, that we’re the chosen species who won the evolutionary Powerball jackpot, and that we’ll be the ones to eventually colonize and rule the universe.
But there’s certainly no evidence to support that hypothesis. It’s mostly wishful thinking.
Now, I certainly wouldn’t state that the Great Filter lies in our future if I didn’t have an idea of what I thought the Great Filter might be. And I’ll also state that the fact we’ve never found evidence of radio emissions from another civilization is pretty compelling evidence that the Great Filter, if it exists, is indeed in the past. However, that doesn’t mean that there can’t be two Great Filters, or even more than two. There may not be one Great Filter, there may be many Medium Filters. Maybe life is subject to extinction at multiple times throughout its evolution, and we’ve managed to dodge the bullets on several filters already. It’s possible that other civilizations have mostly fallen to one filter or another, with only relatively few surviving to where humans are today. Remember, alien civilizations with human levels of intelligence and evolution as advanced as we were in the 1800s for example, would not have thrown off any sign of their presence into the universe.
If there are many filters, then it’s possible that several of them lie in our future, and it might be worth examining what they could be, and when we’re likely to encounter them.
I think there’s a good chance that one of those filters could be Artificial Super Intelligence, and in part two, I’ll be taking an in-depth look into ASI, including exactly what it is, how far we are from achieving it (it’s almost certainly closer than you think it is), and why achieving ASI could be incredibly dangerous to the human race – dangerous enough even to result in our extinction.