A few weeks ago, someone told me they don’t believe there are “aliens”, by which they meant sentient extraterrestrial life*, because “if they did exist, we would have met them by now.”
Time, of course, is the answer to this. The statement shows a misunderstanding of how much time has elapsed since the beginning of the universe in relation to how long humanity has been thinking about ETs.
(For now, let’s ignore various harebrained assertions that we have already met aliens. I might deal with those in a separate entry.)
Time factors into this question in four ways:
1) The age of the universe and its planets
2) When each civilization developed.
3) The duration of each civilization.
4) Travel time between stars.
◊ The Age Of The Universe And Its Planets
As far as physicists know, the observable universe is just under 14 billion years old, while our Earth has been here for about 4.5 billion years. It is known that other stars are older and younger than our sun, and therefore that their planets’ ages do not match up with our own.
A billion years is a very long time, when seen through the lens of our 80-year lives. Stacking 80-year lives end to end, you would need 175 million of them to span the existence of our cosmos. Even compared to the total duration of human civilization (about 10,000 years), the universe’s age is 1.4 million times longer. A million such civilizations could rise and fall without ever existing at the same time.
◊ Time Of Development
Given the age of the universe and the varying ages of planets, the major time factor that explains why we haven’t met ETs is when each civilization developed.
Even if planets all popped into being at the same time (they didn’t), there is no reason to assume abiogenesis would have happened at the same time everywhere, nor any reason to assume evolution — which depends largely on environmental factors — would have followed the same timeline on every planet. Further, mass extinction events — such as the one that wiped out all non-avian dinosaurs 66 million years ago (mya) — are unique to each planet. On another planet, even if we incorrectly stipulate that evolution had followed the exact same path as Earth up to that point, civilization might have arisen much later (or much earlier) due to differences in the timing of extinction events.
So it is extremely probable — to the point of certainty — that intelligent life elsewhere, if it ever existed or will exist, arose long before we did or will arise long after we are gone.
For Earth’s first 700 million years there was no life here at all, and for quite a while after that, there was only microscopic life. Primates have existed only in the last two percent of the Earth’s existence. (The earliest known primate fossils on Earth date back to 55 mya or so, and genetic studies show primates diverged from other mammals as far back as 85 mya.)
The first hominids didn’t arise until 14 mya, having inhabited the Earth for less than a third of a percent of its existence. Homo sapiens came much later, and did not develop what we would recognize as civilization until about ten thousand years ago.
Any ETs visiting our planet during the past 4.5 billion years would have seen no civilization whatsoever, except in the extremely improbable coincidence that they came during the past few thousand years. They would have been much more likely to see no life at all, or very small bits of life. Even in the unlikely event they came in the past 200 million years, it’s much more probable that they would have seen the dinosaurs than us — the age of the dinosaurs lasted 135 million years, while the age of humanity as the dominant species has been very short indeed.
The ETs would have detected no radio signals, seen no roads or buildings — nothing that ET scientists on a starship would recognize as civilization. They might have mapped the solar system and moved on. Maybe they landed and took samples. Maybe they attempted to colonize but it didn’t work out, perhaps due to a natural disaster or their lack of defenses against Earthly infections. Any signs of such a colony would have long disappeared from our planet.
No matter how we define civilization, humans have only had it for a mere blink of cosmic time. It would be an extreme improbability for other civilizations’ blinks to match up with ours.
ETs visiting Earth just 20,000 years ago would have seen only small gatherings of humans in caves or wandering the jungle. The ETs would have no reason to suppose these creatures would someday build a world-spanning technological civilization. Any ETs flying by 20,000 years in the future might see only the sad remains of our society. This relates to the next factor:
◊ Duration Of Civilization
There is also no reason to assume that any civilization — even one capable of interstellar travel — will last interminably. (However, I will assume that colonizing multiple habitable planets would greatly increase a species’ changes of survival.)
We know of only one civilization: ours. We don’t know how long it will last or when or how it will end. It could have easily ended 30 years ago with a nuclear war. It could easily end a few hundred years in the future due to massive sea level rise and scarcity of clean drinking water, or from some new plague. Or it could end next week from an asteroid strike — because we have no way of knowing whether one is approaching.
Even if we somehow mitigate climate change, avoid nuclear war, stay one step ahead of new diseases, and luckily avoid a giant asteroid, there is still no reason to expect the civilization will last forever, especially with only a few thousand years behind us.
When civilization ends, evidence of it will disappear. Our buildings, our canals, our billions of electronic devices and plastic toys, our tons of garbage floating or sinking in the oceans, and all other traces of our civilization will — with no fanfare — disappear sooner than we might think. Natural forces will erode our buildings and bury our trash. As put forth in The World Without Us (Weisman, 2007), the artifacts that will last the longest are our ceramics, bronze statues, and the carvings on Mount Rushmore, not counting a few items we’ve sent into space like the Pioneer plaques, the Voyager Golden Record, and radio waves. The likelihood of those space-bound items being found is slim-to-none, and almost anything left on Earth will be buried under vegetation within a few centuries, and/or eventually obscured by volcanic activity, glaciation, sea level changes, or tectonic shifts.
The same will be true of any hypothetical sentient species in our universe. Even if they advanced much further than we have, to the point of sending interstellar probes or colonizing other worlds, and even if their civilization endured a million years, we have every reason to suspect that there would be an end point. And if that end point came 50 mya, we would never have heard of them, nor ever discover any evidence of them in the future.
◊ Required Travel Time
Perhaps the most obvious time factor is how long it would take a starship to traverse the vast distances between stars.
Even the closest stars require four years for light to travel here. A civilization would have to be orders of magnitude more advanced than our own to get anywhere close to the speed of light, possibly so advanced that they would want nothing to do with us.
With humanity’s current ability to travel in space, it takes months just to reach the nearest planet, and years just to leave the solar system (New Horizons required nine years to get to Pluto). Even if we wanted to, and cobbled together the massive resources for a starship, it would require hundreds of years to reach the nearest star, and thousands to reach the closest extrasolar planet.
Despite our desire to constantly pat ourselves on the back for being so advanced, our space travel accomplishments are very primitive compared to what is required. The longest any human has been in space is 438 days. So far, no one’s given birth in space, which would be required in a starship that took a thousand years to get to the nearest star system with a planet. The people entering the ship would do so knowing that they would die on the ship, as would their children, grandchildren, and so on. The generation actually in charge upon reaching the destination would have very little connection to Earth. Even if they found evidence of an ET civilization, I’m skeptical about their ability to communicate with Earth from that distance, and about whether they would want to.
Simply put, any ET civilization with abilities similar to ours would be stuck on their planet just like we’re stuck on ours.
◊ Combination Of Factors
Considering only one or two of these factors in isolation, the chance of meeting or finding ETs rises significantly, but when taken all together, there is almost zero chance the human race will ever make contact, even if there have been millions of civilizations in the history of the universe.
1) The age of the universe is 14 billion years
2) Civilizations would have arisen on different planets at different times
3) Each civilization has a finite duration
4) Thousands of years are required to traverse the distance between civilizations
The first two are known and unchangeable. The third is unknown, but assumed. It is conceivable that a civilization could last long enough that its existence would overlap with others. Perhaps multiple civilizations exist now, but because of the fourth factor they will never know about each other. The fourth factor, I assume, will change with technology. Already in barely more than a century, we’ve increased humankind’s top speed from “horse and buggy” to “Space Shuttle”.
Scientists and engineers may someday develop new propulsion methods for space travel. The degree to which our travel speeds can be increased will determine the possibility of visiting planets outside our solar system.
Even if close-to-light speeds become possible someday, we could still arrive at another Earth-like planet only to discover that a civilization ended a million years ago. Or a billion years ago. Or that they are still huddling in caves and wondering about fire.
Perhaps they will be somewhere in between, building computers and highways but still fighting wars amongst themselves based on arbitrary and unnecessary borders, torturing prisoners, polluting their own water supplies, hoarding and wasting food in rich nations while millions starve elsewhere. In that case, we might choose to study them from orbit and not announce ourselves.
* I use ET in this entry, since “alien” depends on where you’re from and where you’re going. Humans are aliens from the perspective of ETs. Extraterrestrial simply means “outside of Earth” — life from somewhere other than this planet.
Further note: Nothing in this entry is intended as an assertion that ETs certainly exist. My only point is that there’s no reason to expect we would have seen them by now, or even that they would have seen us. In other words, “we haven’t seen them” is not a reason to presume they have never existed or don’t currently exist.