That there is life on other worlds is not a matter of faith or wishful thinking. It’s a matter of numbers of stars and planets, and the evidence that life arose on Earth geologically immediately after it formed, together with the principle that physics works there just as it works here.
Depending on what you understand by “intelligence”, that too has arisen many times, again going by the numbers. As to why we haven’t encountered intelligent aliens (publicly anyway), there are plenty of good explanations. The question is not so much a mystery as a matter of inadequate information.
This is all discussed in many places — my favorite was Carl Sagan’s wonderful series Cosmos.
What sort of things are the aliens, and why would they come, if they come?
One thing about the aliens is for sure: they are not much like us, contrary to what we see at the movies. Almost all the sci-fi portrayals of aliens are people with various amounts of latex on their faces. Those shows are space operas, dealing with human issues and history and myths and concerns, such as romance and rape and plunder. This has nothing to do with the reality of space aliens. The real space aliens are not people with latex on their faces.
The real space aliens are not people: on meeting them, romance will not ensue.
Our nearest relatives on this planet are the great apes. They are pretty different from us in a lot of ways, but we share almost the same DNA, because we had a common ancestor not long ago. In a lot of ways, we are almost exactly like them (besides details like the funny growth in our heads.) Most people don’t have romantic aspirations for apes.
The tree nearest you also shares a lot of your DNA, because it too is a very distant relative of yours. Attempts by people to mate with trees are 100% futile (no matter how satisfying they may be for either party.)
The space aliens, on the other hand, are not relatives of ours at all. They might be made of the same atoms, but beyond that, their ancestry will be completely disconnected from ours. We don’t even know if DNA is a necessity.
A space alien is more different from you than an ape, or a tree, or an octopus, or an insect, or a fungus, because all these are your distant relatives, and the space alien is not. Even if the space alien uses DNA, that will have little in common with your DNA, because you and the alien have no common ancestor.
We found humans all over the Earth not because humans have to be everywhere, but because our brethren journeyed all over the planet on their feet and on boats they made. We arose here, along with our other earthly relatives. In the rest of the universe, there are zero humans. We humans are all in the immediate vicinity of Earth, 100%.
The movies portray space aliens as people for business reasons: producers think they will draw in more human viewers by portraying humans doing human things, and it’s way cheaper to put latex on an actor’s face than to produce a really alien (but convincing) alien. Besides, imagination is usually in short supply in the movie business, and is regarded with deep suspicion. In sci-fi literature, imagination is tolerated better, so lots of very odd alien creatures have appeared there. But even Hollywood has its moments: the 2016 movie Arrival is one of the few that does a good job of exploring the question: “What if they come, and they’re very different from us?”
The space aliens will not come to mate with us, or to enslave us, or to eat us or drink our blood, or for our minerals — those are all human motivations, or the motivations of earthling creatures. If they can cross the stars, they can get all the minerals they want, and they sure don’t need puny humans as slaves, and our flesh would certainly be poisonous to them anyway.
But travel between the stars is a bad problem. It’s much worse than the one of navigating oceans, which was resolved by engineering — a matter of making bigger boats. In fact, the bigger difficulty with crossing the oceans was that nobody knew how big they were, or what might lie on the other side — because they couldn’t see the other side.
In contrast, we can see nearby stars quite well (because each is bright like the Sun). But the distance between stars is so great, crossing it by moving would require much more than a human lifetime, unless you allow for travel faster than light. But to move faster than light is a tricky business — what would it look like to an observer? That’s just the tip of the iceberg. It’s much worse than an engineering problem — it’s a conceptual problem. We don’t even know what travel faster than light might mean, let alone how to achieve it.
Even if somehow the unthinkable vastness between stars can be bridged, the motivations for doing so are also hard to imagine. In Star Trek they have the notion of curiosity — which isn’t bad. I guess the space aliens might be curious, too.
Or maybe the aliens have bigger fish to fry. My favorite example is in Arthur C. Clarke’s Childhood’s End, where the aliens come, and they’re nice enough, but they just sit up there in the space ships for generations, because they’re here to watch something … maybe we don’t want to know what. Or the theme of the Brothers Strugatsky’s Roadside Picnic, where aliens arive and then leave, and don’t give a whit about people or their effect on people. And there’s the case of the Vogons in Hitchhiker’s Guide.
Or maybe it is ultimately impossible to sail to the stars, and maybe even communication at the speed of light is just too slow to be practical, so maybe we’ll never get to see our fellow universians. I hope not, surely there is some way. But rocketing around faster than light as in the sci-fi shows is not just a matter of technology. Maybe I’ll write about that elsewhere.
I’ve argued that, for the most part, the biology portrayed in science fiction movies is scientifically silly. So that’s what the biology is not. But what might it be?
Any thinking in this direction is speculation, of course (unless you hope to get compensation for it, in which case it’s science fiction.) I’ll content myself with speculation.
First question: where should our concept of life begin and end? Even among Earth creatures, we have bacteria, which clearly live, reproduce and die, as well as viruses. Viruses do reproduce, but come on — they’re scarcely more than a collection of molecules: outside their interaction with their victims, they do nothing at all — is that “living”?
It isn’t clear that we should restrict ourselves to chemical, corporeal, life forms. People have speculated about life forms consisting of magnetic fields, for instance inhabiting a star, and life forms inhabiting the surface of a neutron star, or somehow living in deep space, away from any astronomical body. And then, there are plenty of stories written about creatures that live somehow outside the physical universe. Is it physically possible? We have no practical means of examining those extreme environments, and any physical theory that covers them is at best an extrapolation from experiments that can be done in a tiny lab. Really nobody knows.
Besides this, it has been proposed that anything that can reproduce and be modified and die ought to be considered a life form — for instance, ideas reproduce themselves among people, and are modified, and can die off. (This is the famous notion of a “meme”.) But even this is just an extension of our experience (in this case, meant to support the mechanism of natural selection). Might there be things we want to call life, that come into being some other way?
Nobody really knows. The field is open for dreaming, but there’s not much to be said definitely.
We know chemicals a little better. It is pretty clear that matter everywhere is made up of the same chemical elements. The most common elements (leaving out helium as a biological component because it’s so inert) are: hydrogen, oxygen, and carbon…
Abundance alone wouldn’t qualify these as the main ingredients for biological organisms, but water, the main compound of hydrogen and oxygen, is a remarkable liquid in many ways, especially in its interactions with other chemicals, and carbon forms an extraordinary variety of compounds with so many elements, but primarily with hydrogen, to make the hydrocarbons, which are, after water, the second-biggest ingredients of our bodies.
Add in a few other elements for fancier molecular behavior: iron, sulfur, nitrogen, magnesium, calcium, sodium, potassium, phosphorus, chlorine, zinc, copper, and you have the ingredients for almost all life on Earth. These elements are also scattered throughout space… the leftovers of supernova explosions.
Does it have to be water and hydrocarbons? Maybe not. Maybe under some conditions, life can arise with different primary chemicals. But these are by far the most common ones overall in space, so they’re a good bet. And for the purpose of speculating about life, they’re what we know best.
So let’s consider just water-plus-hydrocarbon life. What can we expect from that?
Earth is home to a wild variety of biological life forms. They fall into groups: protozoa and plants and animals and several others — but that is because natural selection pruned off most of the other forms that did arise, not because these are the only forms that ever were, or ever could be. Just among Earth animals there are many kinds, including worms, starfish, arthropods, and chordates. But these groups are composed of species that are genetically related to one another, and as I argued earlier, the space aliens are not related to anything on Earth. So what can be said of the forms of alien biological organisms?
Must the reproductive mechanism of extraterrestrial life be based on DNA? It is the only mechanism for organic reproduction on Earth. But for all I know, its ascendancy is dependent on conditions on the planet, or even an early accident of natural history.
Some physical symmetries might be recognizable: some aliens might be star shaped, some tube shaped, some bilaterally symmetric or spherical, or they might be sort of amorphous — we have examples of all those on Earth. Could they be something totally surprising? Sure! Could their form be something very hard to conceptualize? Sure. Might they be horrifying or icky? Oh yeah. Might they be indescribably beautiful? We can hope so.
Their structures might be supported externally, as with insects, or internally, as with vertebrates, or cellularly, as with plants. Or maybe some way that never happened to arise on Earth.
Could we interact with them physically? Touch them? Eat them? Only if we can both live in the same environment: temperature, atmospheric composition, etc. Note that there are creatures living on Earth that can’t really be touched by a human, with both surviving the experience: some deep sea creatures, for example. I’m going to call: “usually not”.
Even if both sides could tolerate the same physical conditions, there is the biological danger. Could we tolerate one another’s microorganisms? Would our bacteria immediately infect an alien? Vice-versa? Or would our respective bacteria find the other’s flesh unpalatable? If their bacteria find our tissues palatable, there will be nothing stopping them from eating us all up, because we will have no immunity against them whatever. Most people don’t spend a lot of time thinking about it, but human bodies are host to myriad microorganisms, many of which are necessary for survival. Would our symbiotes want to eat an alien visitor?
But most of the time, I think, alien tissue would be so different as to be lethal to even our microorganisms.
If they’re intelligent, will we be able to communicate with them? Well, there’s the thrill! Maybe that’s how we’ll end up defining intelligence. (Like a Turing test for aliens.) But maybe they’ll be so different, it will be hard to tell what they’re on about.
Consider that among people, we have a hard time understanding one another when we speak the same language. We recognize a kindred intelligence in apes, but they are in fact our kin. Likewise in other mammals. The intelligence of smart birds, such as crows or parrots, is somehow something other. Still, they too are our cousins, a few tens of millions of years removed. And then some octopuses can solve complex problems… but they’re even more other. And still, they too are remote cousins, by some hundreds of millions of years.
It seems as if affiliation has something to do with recognition of intelligence. And so what happens when there is no direct affiliation whatsoever? Maybe the fact that we’re all star-stuff can bring us together…
The flip-side of this thought is that, even on our own world, there are multiple kinds of creatures whose rudimentary intelligence has developed independently. That would seem to indicate that something like intelligence can arise fairly easily.
Yeah, it’s very academic. If only I still drank beer, I would drink to it. Or maybe our conspiracy theory friends are right about the aliens being here already. (No, not the conspiracy theory friends who think it’s devils and angels.)