It’s the ultimate question. Are we alone in the universe? In this issue, respected science journal Parallel Worlds puts the matter to rest, once and for all.
Or not. Aside from us not being a respected science journal, the question remains utterly unanswered. But it’s one that concerns us at this magazine more than most: science fiction is our bread and butter, and a fundamental plank of science fiction is the concept of intelligences from places other than Earth.
I’m not a scientist — so rather than an exhaustive analysis, I’ll simply try to summarise the thinking on the matter as it stands today.
Can anybody hear me?
Although sightings of unidentified flying objects date back at least to the 17th Century, it wasn’t until the 1940s — coinciding with a rapid acceleration of scientific understanding of the cosmos and rocketry — that ‘sightings’ of aliens really took off. The knowledge that space is very large, and suns and planets very numerous, inevitably led to the assumption that we cannot be alone in the universe.
However, there is one fundamental, inescapable, and terrifying fact that flies in the face of the probabilities: we have not, during the time we’ve been looking, seen a shred of evidence that any intelligence beyond our own exists in our universe.
Eerily-regular radio pulses turned out to be spinning neutron stars. Canals on Mars turned out to be an optical illusion. Space is utterly empty of ordered electromagnetic activity, in any direction and of any age.
The explanations for this are many: we’re not looking the right way; aliens are already here, hiding among us; the universe is so old that we’d be very unlikely to coexist with anyone; we’re in an alien zoo; space is so large and life so unlikely that any other civilisation is too far away for light to reach them within the lifetime of the universe; and so on.
There are arguments for and against all of these (some more than others), but they coalesce into two basic suppositions: either we’re alone in the universe, or we’re not.
Intellects vast and cool and unsympathetic
The observable universe is 90 billion light years in diameter (‘observable’ being the area across which light has had time to travel since the Big Bang). It contains somewhere in the region of 100 billion galaxies, each with several hundred billion stars. If 10% of these stars have planets, and 1% of those planets are habitable, and 0.1% of those have life, and 0.01% of those have sentient life, that’s still six trillion civilisations in the universe. And the universe could be very much larger than we can observe.
However, it’s not very useful to think about what life might exist in other galaxies. Galaxies are arranged in clusters; ours includes the Milky Way, Andromeda, the Magellanic Clouds, and some dwarf galaxies. Two forces are at work upon galaxies: gravity, which draws members of a cluster closer together; and the expansion of the universe itself, which draws the clusters themselves further apart from one another. Realistically, any area of the universe not within our local cluster will always be utterly inaccessible to us, because the distance between us and it is increasing so quickly.
More meaningful, then, to look at our own galaxy. The Milky Way contains somewhere between 100 billion and 400 billion star systems (some of which may be binary or trinary systems). The study of ‘exoplanets’, or planets orbiting stars other than our sun, is in its infancy today, but we’ve already discovered more than 2,500 other stars which have planets — and we’ve only just started looking. There are around 20 billion stars like ours in the Milky Way, and it was estimated by the National Academy of Sciences in 2013 that something like 22% of those might have a rocky planet at the right distance to hold liquid water: which would mean about four billion planets a lot like Earth.
Calculations like this were first posited by the astronomer Frank Drake in 1961, collectively known as the Drake Equation. The maths necessarily (for now) relies on much supposition, but it is a useful way to get to grips with the orders of magnitude involved, and the probabilities that very large numbers imply. Even if something is very unlikely, in a very large universe it’s still likely to have happened many, many times.
Trappist 1 is a star just under 40 light years from Earth, which — between 2015 and 2017 — was discovered to have no fewer than seven planets, three of which are within the habitable zone. 40 light years is very close indeed: the Milky Way is around 100,000 light years in diameter.
Ok, there may be many rocky planets with water in our galaxy. But how easy would it be for life to develop? How miraculous is the ‘spark’ that turns inanimate matter into living things?
There is some evidence that life may have developed on Earth several times, independently. Some geothermal vents in very deep parts of the ocean, thought to have been totally isolated from the rest of the Earth’s life, have been discovered to host thriving and completely closed ecosystems, with organisms not found anywhere else on Earth. It is unlikely that these living things migrated to such remote places, which suggests that ‘abiogenesis’, or the process of life beginning, may have occurred on this planet several times. If so, it might suggest that — should the essentials, like a liquid solvent such as water, be present — simple life might reliably emerge on other planets, too. If life emerged on only 0.1% of those four billion rocky planets with liquid water in the Milky Way, that’s still four million planets with life in our galaxy alone.
However, there is a big assumption here: that the only things life needs to come into being are a liquid solvent, complex chemistry, and the absence of an extinction event. Earth could have some other characteristic that we haven’t identified yet that is necessary for the creation of life.
Extinction events are also not rare. In Earth’s history there have been five: most are linked to dramatic climate change, but at least one was the result of an asteroid impact. Supernovae, too, are common and deadly: it is thought that a star explodes somewhere in the Milky Way a couple of times per century, and any planet caught in the 100-light year blast radius would instantly be sterilised. Neither do planets remain unchanged for all of time: there is some evidence that Venus may have once held liquid water before a runaway greenhouse effect turned it into the hell we see today, and Mars appears to once have hosted liquid water when it had more of an atmosphere, and we haven’t yet found any organic matter there.
‘Life’ can come in many flavours, from algae to rocket-designing bipeds. It is possible that simple life might readily evolve on any of our four million planets (and perhaps more exotic ones besides, like gas giants) but that extinction events might just be too regular to allow it to evolve into complexity or sentience. According to this scenario, we exist because simply because there has been a longer lull than usual between extinctions.
Planet Earth is about a third as old as the universe itself, which is also pretty old by now. In terms of dynamic stages, it is thought that the universe is now in its last: and though it will take a bafflingly long time for every star to die, we are on the ‘home stretch’, as it were, in which star and galaxy formation is slowing greatly.
Life on Earth has also had several bites at the cherry to get this far. We think that life emerged on this planet within its first billion years, but multicellular life might only have come about in the last billion years — the latter third of Earth’s history. Interesting things like dinosaurs only turned up in the last 250 million years or so. If the history of Earth was a day, dinosaurs would have turned up at about 10:40pm. We arrive seconds before midnight. The earliest approximation to ‘civilisation’ is the ruins of what we think is a temple at Göbekli Tepe in Turkey, which is around 12,000 years old. We emitted our first electromagnetic whispers to the rest of the universe a mere hundred years ago or so.
So, we have been around and asking the question for a vanishingly short amount of time, and it took a cosmically long time and several false starts for a species like us to emerge. Complex, sentient life could have developed in the Milky Way millions of times in the millennia it has taken us to get our shit together, and many planets are a few billion years older than Earth.
Several parallel studies last year tackled the question of how long it would take a spacefaring species to colonise every habitable planet in the Milky Way with generation ships supporting thousands of humans. Thanks to the magic of exponents, the winning team’s estimate was that it might take 90 million years to colonise every suitable planet in our galaxy. The Milky Way is about 13.5 billion years old, so a spacefaring species could have colonised the whole thing 150 times before we came along to look. If intelligent aliens have ever existed in the Milky Way, and sought (like ourselves) to grow and expand, they would have left a trace of themselves. This mystery was first posited by a physicist called Enrico Fermi, in the question: ‘where is everybody?’
There are several possible resolutions to the ‘Fermi Paradox’, as this question is known. Alien civilisations could all ascend to some higher, undetectable state of being in their first few million years. This is more plausible than it sounds, because video games are fun and space is mostly hostile and boring. Another solution is that life itself could be fiendishly hard to make, and Earth is somehow unique. Perhaps life may be common, given the opportunity, but complex life might be very hard. Or, perhaps most scarily, intelligent life might naturally emerge from a life-bearing planet, given enough time — but something about the nature of intelligence causes its own destruction before it can make many forays into space.
Regardless, some part of the journey from gooey organic chemistry to galaxy-conquering superintelligence appears to be very difficult indeed. One stage of a species’ development appears to stop it in its tracks, before it can conquer the Milky Way — if not, it would have happened already. The key question is: which stage is the hard bit?
This concept is known as the ‘Great Filter’. The possible resolutions to the Fermi Paradox can be summed up into two general scenarios: one, getting to where we are now is very, very hard, and we are the first. Two, getting to where we are now is fairly commonplace, but at some point soon after where we are now (ie the point at which a species starts sending signals out into the cosmos) they are snuffed out by some catastrophic event. Option 1 would be good news: we have overcome the biggest trials life can face, and the universe is our prize. Option 2 would be devastatingly bad: the galaxy is quiet because it’s littered with dead civilisations like ours. We’ve only just started sending our signals into space; if the same forces that killed off all the other civilisations are conspiring to do the same to us, it will necessarily happen soon.
Happily, there are several stages of development between wriggly goo and chartered accountancy that could be the very hard bit, so it’s entirely possible that we are the Milky Way’s first civilisation. One of these is the improbability of mitochondria: little rod-shaped structures that swim around our cells, and those of all animals and plants, enabling respiration with oxygen and creating energy for their hosts. The remarkable thing about mitochondria is that they have their own DNA and seem to reproduce independently of their host cells. This implies that, at some point in life’s very distant past, one species of microorganism consumed another; but rather than breaking it down, struck up a symbiotic relationship with it. The host cell provides the structure and food; the mitochondria provide the energy. This allowed complex life to develop. It is difficult to know how much of a fluke this was, but it seems fantastically improbable (as well as throwing interesting light on the concept of an ‘individual’ organism).
It is also tempting to think of progress from primeval dawn to humanity as a linear, determined thing. It’s not: we ourselves are wildly improbable. Our brains are stupidly, ludicrously expensive in terms of energy. Yes, they have allowed for a degree of social and tool-making intelligence that has set us apart from every other animal on our planet, but come at great cost. They are so large that we must give birth mid-gestation, forcing us to carry around our fetuses for several months, like marsupials. Despite this compromise, fetal brains are still so large that pushing them out of our pelvises (narrowed to allow us to run on two legs) is formidably dangerous and difficult. In nature, modern humans have the highest rate of death in childbirth of any animal on Earth. It is not hard to imagine a prehistory in which this extraordinary gamble on problem-solving ability had not paid off, and we were all eaten by lions.
So it is easy to conclude that we are utterly, totally alone. We are the only living things able to meaningfully form the question or search for an answer. It isn’t correct to say that ‘all the evidence points to this’, but it is true that there is no evidence to refute it. We have been searching the cosmos for signs of someone else out there for several decades now, and have heard nothing.
Tossing pebbles into an ocean
So we’re alone or we’re fucked, right? The Milky Way is like a party we arrived late to. The buffet is still fully loaded — nobody’s eaten a single cocktail sausage. Does that mean there’s nobody else here?
Not necessarily. Perhaps there are loads of other guests here, and they’re just in a different room. Perhaps they don’t like chipolatas. Perhaps the traffic is really bad, and we’re actually early, because we had all green traffic lights on the way in.
If another civilisation sprang up a billion years ago, they would have had the time to conquer the Milky Way ten times over. That they haven’t implies that they were never there, space travel is far harder or less appealing than we think it is, or that life rapidly ascends to a plane of being we can’t yet comprehend.
However, this article is littered with assumptions: assumptions about the distributions of planets and stars, assumptions about the components needed for life to develop, assumptions about the way civilisations behave. As the timeframes we’ve discussed should suggest, we are barely in the embryonic stages of understanding the nature of life and space.
It is useful and mind-expanding to think about the numbers, distances, timescales, and probabilities these questions involve. But perhaps more useful, ultimately, to remember how much we don’t know.