For the accompanying Trash Skunk Podcast episode to this article, listen here.
Welcome back to the Trash Skunk Guide to Aliens. In Part One, we discussed the pop culture view of aliens, and had some fun with the sillier aspects of it. I covered alleged abductions, UFOs, proposed alien species, the Roswell Incident, Ancient Alien Theory, and made five or six anal probe jokes. A tremendous time was had by all. By the end of Part One, I concluded that much of pop culture's take on aliens was probably false, a mixture of hoaxes, conspiracy theories, and psychological phenomena that painted an unlikely picture of weird humanoid beings visiting Earth to confuse airline pilots and play with our butts.
This is Part Two: The Science of Extraterrestrials. We're going to cover some more realistic views on aliens, and discuss what science thinks about the possibility of extraterrestrial life. If the core assumption of pop culture's take on aliens is that we have already made contact, the scientific view is that we have not. Therefore this article addresses the questions "do aliens exist?" and "if so, where are they?"
Before we dive in, let me briefly re-introduce my list of possible attitudes towards alien life.
Aliens do not exist
Aliens are unlikely to exist
Aliens might exist, but we have no evidence of them
Aliens are likely to exist, and we might even have some evidence
Aliens do exist, but they haven't been to Earth
Aliens do exist, and they visited earth in the past
Aliens exist and are currently visiting Earth, but we haven't made official contact
Aliens are visiting Earth, we've made official contact, and it's a government cover-up
Aliens exist and one of them stuck something up my ass
Part One of this guide chiefly concerned itself with attitudes 5-9. Part Two will ground us in the more measured realms of 1-4. Let's begin.
I can see my house from here.
The Fermi Paradox
Perhaps the best place to start is the so-called "Fermi Paradox", named after Italian-American physicist Enrico Fermi, who first casually brought this idea up at a lunch with fellow scientists in 1950. It was later elaborated upon by others, but the gist of it is as follows (from Wikipedia):
There are billions of stars in the Milky Way similar to the Sun.
With high probability, some of these stars have Earth-like planets.
Many of these stars, and hence their planets, are much older than the Sun. If the Earth is typical, some may have developed intelligent life long ago.
Some of these civilizations may have developed interstellar travel, a step humans are investigating now.
Even at the slow pace of currently envisioned interstellar travel, the Milky Way galaxy could be completely traversed in a few million years.
And since many of the stars similar to the Sun are billions of years older, the Earth should have already been visited by extraterrestrial civilizations, or at least their probes.
However, there is no convincing evidence that this has happened.
To sum it up, Fermi's paradox is saying "statistically speaking we should be seeing aliens, but we're not - why?". Keep in mind we're discounting the people who share fantastic and unprovable stories about abductions and UFOs, because these are not testable, observable, hard-concrete evidence of anything. As far as we can say for certain, we have no evidence that aliens have visited Earth or contacted humanity.
So Fermi's Paradox brings up a great question: if the universe is filled with life, where is everyone?
It leaves us in the "alien attitudes" realm of 1 and 3, either "aliens do not exist" or "aliens might exist but we have no evidence of them (yet)". As far as observable evidence is concerned, this is where mainstream science stands today.
There are several theories that could explain the Fermi Paradox and answer the question "where is everyone?". The first that comes to mind is the Drake Equation.
My time to shine
The Drake Equation
The "Drake Equation" is the name given to a mathematical formula conceived by an astrophysicist named Frank Drake. This formula is meant to estimate the number of intelligent extraterrestrial civilizations that may exist in our galaxy, and figure out our odds of ever receiving communication from them. It looks like this:
N = R* • fp • ne • fl • fi • fc • L
Where:
N = the number of civilizations in our galaxy with which communication might be possible
R∗ = the average rate of star formation in our galaxy
fp = the fraction of those stars that have planets
ne = the average number of planets that can potentially support life per star that has planets
fl = the fraction of planets that could support life that actually develop life at some point
fi = the fraction of planets with life that actually go on to develop intelligent life (civilizations)
fc = the fraction of civilizations that develop a technology that releases detectable signs of their existence into space
L = the length of time for which such civilizations release detectable signals into space
This is a fun thought experiment, but the problem with the Drake Equation is that we just don't have the values for some of those variables. Especially the last four - "the fraction of planets with life that go on to develop intelligent life", etc. We only have Earth to work with on that one, so it's anyone's guess. The same for "L" - the length of time for which civilizations release detectable signals into space. Who knows the answer to that? We can't even answer how long we'll be broadcasting signals from our own planet.
If we could get some better answers to those questions, Drake's equation might provide us with a decent ballpark calculation of the number of civilizations in our galaxy. Without answers, however, it's just a fun talking point.
For now, Drake's equation doesn't fully answer the fundamental question of the Fermi Paradox: where is everyone? There are a few other proposed solutions.
The Great Filter / Rare Earth Hypothesis
Maybe one reason we're not seeing intelligent life outside of Earth is because there is some sort of natural "filter" that prevents civilizations from existing long enough to colonize space. It could be something as simple as "intelligent civilizations are inherently self-destructive", either for social reasons, the inevitable problem of resource-depletion any large-scale civilization will eventually face, or some other unknown factor. If this is the case, our own Great Filter preventing us from colonizing the stars may lay ahead of us - nuclear war, a new dark age, a pandemic, an asteroid that obliterates life on Earth, etc.
However, it could also be that our Great Filter is in the past, not the future. Perhaps life arising to begin with was our Great Filter, and we are simply the winners of an astronomical lottery, looking around an empty universe for other lottery winners, wondering why the place is so empty. This idea is called the Rare Earth Hypothesis, which suggests that the reason we're not seeing extraterrestrial life is because it's incredibly rare for life to exist at all.
Below is a list of evolutionary steps that must take place in order for non-living matter to undergo abiogenesis and evolve into intelligent living beings that colonize space. Keep in mind this takes place over billions of years.
The right star system must exist (including organics and potentially habitable planets)
Reproductive molecules form (e.g. RNA)
Simple (prokaryotic) single-cell life forms
Complex (eukaryotic) single-cell life forms
Sexual reproduction begins
Multi-cell life evolves
Tool-using animals with intelligence evolve
A civilization of intelligent beings starts advancing toward the potential for a colonization explosion (where humans are now)
Colonization of the stars
The Great Filter hypothesis assumes that at least one of these steps is improbable, we just don't know which one it is. Somewhere along this evolutionary chain of events there must be a bottleneck that most planets simply can't get around. As you can see, we've made it through eight of them here on Earth. Will we make it to the ninth and final step? If so, how many other planets can do the same? Is it rare to come this far? Or is it normal, meaning our filter is ahead of us in the form of some catastrophe that will destroy human civilization and prevent us from becoming space colonizers?
My hunch is that humanity's Great Filter, if it exists, is likely in our past. Think about all of the animals and multi-celled organisms that have lived on Earth for billions of years, and realize that only one of them has ever become "intelligent" in the way we use that word for the sake of this conversation: humans. The Great Filter could actually just be intelligence itself, which for whatever reason humans developed while other animals get along fine without. Maybe intelligent, self-aware brains are the rare exception and not the inevitable outcome of evolution.
In other words, the universe may be teeming with life... but most planets could simply be populated by creatures with the intelligence of, say, rats. This would certainly explain why we expect there to be life everywhere but are puzzled not to be receiving messages from aliens. Maybe they are everywhere, but are just too dumb to invent spaceships or radios.
Until we come across intelligent life from another planet, we really have no idea. For now, The Great Filter and Rare Earth Hypothesis are just two possible solutions to the Fermi Paradox, but they are not the only ones.
Earth As A Zoo
Another interesting hypothesis is that perhaps intelligent extraterrestrials are aware of Earth, but are staying away because they see us as some sort of primitive nature preserve or zoo. It could well be that intergalactic empires exist, and Earth's most intelligent creatures (humans) are such pathetic piss-ants in comparison that we're not even worth contacting. Instead, they see us as a curious little planet only suitable for watching to see how we develop.
This is certainly possible, but for me it assumes a little too much about extraterrestrial psychology (that aliens would have an interest in conservation and preservation of the natural world, etc.). Of all the solutions, this one is probably my least favorite - although it would fit right in with the pop culture "they're watching and abducting us" line of thought.
The Dark Forest
A slightly terrifying theory is that alien civilizations exist, but are staying quiet for a reason: they're hiding from something. It could be that some sort of interstellar super-predator, be it a hostile alien civilization or some errant world-eating space organism, is prowling the cosmos looking for prey. Alien civilizations, aware of this danger, are quiet because they are trying not to be found.
In other words, the universe is like a dark forest with a monster in it, and all the animals are keeping silent to protect themselves. Earth, meanwhile, is like a dumb hiker strolling through the woods whistling a tune. Are we drawing undue attention to ourselves by broadcasting radio signals all over the place, hoping that whoever contacts us will be friendly? It's a pretty big gamble, given the fact that we know nothing about who's out there.
But one thing we do know is that things typically don't go well for a less-developed civilization that gets contacted by a technologically advanced one. See the Aztec and Incan empires, the Polynesian cultures, and Native Americans for a reference on how such interactions typically play out (at least on Earth). In general, whoever has the better technology is the one who survives the encounter, even if relations start off friendly.
Do we really think our tech is that great? I wouldn't bet on it.
I like the Dark Forest Hypothesis, but not because I think it's the most likely solution to the Fermi Paradox. I like it because it's really fucking scary. Humans treat space like we're desperately single people trying to get laid - we are really putting ourselves out there to meet someone. And even if no cosmic super-predators exist, it could simply be that extraterrestrial civilizations aren't such desperate horndogs, and have no interest in new friends.
This Hubble Ultra-Deep Field image shows some of the most remote galaxies visible with present technology, each one consisting of billions of stars. - Wikipedia
Everything Is Just Too Big / The Timing Is Off
Another possibility for why we haven't made contact with an extraterrestrial civilizations is that the sheer size of the universe might be preventing it. The place is huge - some even think it could be infinite - and the space between Earth-like planets is vast. It's entirely possible that our radio signals just haven't reached anyone capable of listening. Similarly, it's possible that no one else's signals have yet reached Earth, given the distance they might have to travel.
It's conceivable that alien radio signals came our way at some time in the past before we were capable of receiving them, which has only been for a little over a century. On a cosmic timeline, that's nothing at all, a window of time that's easy to miss. And speaking of time, it might be that nearby cosmic civilizations rose and fell before humans were even a species, and we missed out on contact with life in our neighborhood through bad timing alone.
Think about the speed of light, which is 299,792,458 miles per second. Now realize that some of the stars we observe through telescopes are so far away that we are observing them as they were 65 million years ago. That means light from these stars traveling at 299,792,458 miles per second took 65 million years to reach us. That is a really, really distant star - and it might have an Earth orbiting it.
This means that if an alien civilization in a galaxy 65 million light-years away sent a message to Earth at light speed when the dinosaurs were walking around, we'd only just be receiving it now. And if we beamed a message back at the speed of light it would be another 65 million years before they got it. 130 million years will have passed on that alien planet before they received a response to their initial message. The odds of the aliens civilization who sent the first message still being around when our reply arrives are basically zero. Look how much life on Earth has changed since T-Rex was walking around, and that's only half the time we're talking about for these poor aliens to get their reply.
So, while Earth is broadcasting endless amounts of shit into the cosmos, humanity may well be - and indeed are likely to be - long gone before we get an answer - if we get an answer at all.
The Odds Of Life In The Universe
One popular view is that, even if we never wind up contacting alien civilizations, they must exist simply because of the statistics - the universe is enormous, and there's just no way there aren't other worlds out there that will spawn life. We already know that Earth itself is hardly unique, there's other Earth-like planets all over the place. These planets have water, carbon, nitrogen, are a perfect distance from their suns, etc... so simple math would suggest that the universe must be teeming with life, right?
Maybe, but maybe not. There is an opposing argument to the theory that "aliens must exist because of statistical likelihood". Paul C. W. Davies, a professor of natural philosophy at the Australian Center for Astrobiology at Macquarie University, writes in an essay for Edge:
Simple statistics show this argument to be bogus. If life is indeed a freak chemical event, it is so unlikely to occur that it wouldn't happen twice in a trillion, trillion, trillion planets.
So betting on sheer numbers alone doesn't necessarily win the game here. But Prof. Davies is specifically talking about the idea that life "accidentally" pops up with frequency. He goes on to state that he does believe the universe harbors life, just for different reasons. He explains:
... I believe we are not alone because life seems to be a fundamental, and not merely an incidental, property of nature. It is built into the great cosmic scheme at the deepest level, and is therefore likely to be pervasive.
At the end of the day, even this statistical skeptic believes life is out there somewhere. Davies just doesn't think it's a statistical, anomalous byproduct of nature - he thinks it's an essential and fundamental element of it.
We're knit-picking here, but Davies's argument brings up a great point. Most scientists agree the odds are heavily in favor of alien life existing, although their reasons for arriving at this conclusion sometimes differ. It seems quite improbable that we are the only sentient beings in the universe, because looking around the cosmic neighborhood we can see that there's really nothing all that special about us. If life can happen here, why not elsewhere?
Which leads us to dolphins.
Alien, All Too Alien
One possible reason the universe seems quiet is that alien life might exist but be too alien for us to recognize. We tend to assume that life elsewhere would look like it does on Earth, but maybe there are extraterrestrials that aren't carbon-based, water-drinking, oxygen-breathers like we are. And even if we meet aliens who do share these biological traits, their brains might be so incredibly, well, alien to us that communication would be impossible.
Think about how intelligent dolphins are. They have language, operate in social groups, and possess cognitive abilities that are second only to humans on planet Earth. And yet dolphins live in the ocean, which is as alien an environment to humans as a Starbucks would be to a dolphin. Because of this, their intelligence has taken on a vastly different form than ours through mere accident of evolution. They never needed hands, for example, and so never wound up building anything. Why would they need to?
And while we've trained dolphins to do backflips and put on aquatic shows at SeaWorld, we've never figured out how to communicate with them in any meaningful way. We hear their vocalizations, we know that they speak to one another, yet we haven't been able to translate their chatter or have them understand ours. The gulf between these two species is huge, even though humans and dolphins live on the same planet and are distant genetic relatives.
Communicating with aliens might very well have this problem to the extreme. Alien technology may not be electronics based, and their language may not be vocalized or even visual. They could be sentient gas clouds who communicate with one another via electrical pulses or chemical processes. How the hell would we talk to such beings if we can't even understand a goddamned dolphin, an Earth-native intelligent mammal that uses speech?
To summarize this issue, Carl Sagan once theorized that an extraterrestrial intelligence could think much slower or faster than we do, meaning any message it might send would simply be lost on us because we wouldn't recognize it as intelligent communication at all. You can follow this logic almost infinitely - the numerous ways alien intelligence could be different from our own gives rise to an unfathomable number of reasons we may never make contact.
The Future
Although it's been slow-going so far, humanity continues to search for intelligent life in the universe. The flagship organization helming this effort is SETI, the Search for Extraterrestrial Intelligence. These are the people with the enormous satellite dishes in the desert, perpetually scanning the heavens for some sign of life. If an alien civilization ever beams a message to us, SETI are the ones who will pick it up.
Which would lead to another problem: what would we do if we received such a message? The famous question "Who Speaks For Earth?" comes into play here. Who has the authority to beam a message back to a mysterious alien intelligence on behalf of humanity? Would international coalitions meet and vote on it? If so, how could a simple majority of votes be allowed to make an existential decision for all of us?
It presents a real conundrum, and one that we may indeed face at some point in the future. For now, perhaps blessedly, we don't have to make this decision. The skies are quiet.
Conclusion
So, after going over both pop culture's take on aliens and the opinions of the scientific community... how do I feel about the matter? Are we being visited? Is life out there at all?
The official Trash Skunk stance on aliens is that I do believe life exists in the cosmos, probably in great abundance. Whether it's because of statistics - the universe is potentially infinite, meaning anything that can happen eventually will - or because, like Professor Davies, I believe life seems to be a fundamental part of nature, I just can't help but think aliens are out there somewhere.
If you read Part One of this guide, you'll know that I don't subscribe to the UFO / abduction / government conspiracy take on extraterrestrial life (although I confess to being greatly amused by the anal probing trope). So when I say I "believe" in aliens, I'm not talking about this crap.
Rather, I believe life exists in the cosmos and doesn't seem to be in touch with us for one or more of the reasons I've listed in this article. If I had to choose, I'd wager on these two:
Life is abundant, but intelligent life is rare
The distances and timing involved in successfully transmitting a message from one civilization to another are too great a hurdle to overcome.
I think we can look at the odds of "intelligent" life on our own planet and apply them to the universe at large. Of the more than five billion species that have lived and died on Earth, humans are the only ones to rise to a civilizational level of intelligence. We're the only sample we have, but this data points to intelligence being more or less rare - 1 in 5 billion. Couple that with the odds of life developing in the first place, surviving existential threats like disease, asteroids, climate change, etc... well, you get the picture.
In regard to the second point, even if intelligent life does arise on some other planet, we run into the hurdle of distance and time. Civilizations will have to arrive at the right time, send messages in the right directions, and sustain themselves long enough to get a reply. It's a lot to ask, and it seems unlikely to happen.
Between these two points, my hunch is that we may already know why the universe seems quiet. Then again, I could be wrong - I'm not a scientist, just a casual fan.
No matter how you look at the topic of extraterrestrial life, it's a fascinating, thought-provoking subject that will probably never go away. And why should it? Finding out who's out there would help us better understand nature, the universe, and ourselves. It's a quest worth continuing on.
This concludes the Trash Skunk Guide to Aliens. Live long and prosper.
コメント