Hoping Not to Find Life in Space

Walk outside on any clear night, away from the city lights, and you can see a thousand stars or more. A thousand suns, each perhaps with its own collection of planets. Since the 1990s, over 300 planets have been detected around the nearby stars. They are mostly gas giants like Jupiter (these are the easiest to detect), but we have no reason to believe that small, rocky planets like the Earth are not common as well.

The thousand stars you can see are only a tiny fraction of what's there. Telescopes have mapped the Milky Way for decades, and it is estimated to contain 200 to 400 billion stars, all within a disk perhaps 100,000 light years across. Since the 1920s, other galaxies have been identified, and there are currently thought to be perhaps 100 billion galaxies of varying sizes in the "observable universe".

That's a lot of stars.

All of those stars and their planets have been around for a very long time. The Earth and Sun are around 4.5 billion years old, but the universe is thought to be 13.7 billion years old. In other words, the whole history of the Sun, the Earth and all life could have come and gone twice before the Earth ever existed.

Ever since people speculated that the stars were distant suns, or that the planets in our own solar system were worlds like the Earth, we have wondered if there was life elsewhere, even intelligent life like our own. Given all those stars and all that time, it seems like a sure bet. Yet we don't see any evidence of life elsewhere in our solar system, or any evidence of intelligent life out among the stars. This puzzle is called the Fermi Paradox, named after physicist Enrico Fermi, who is supposed to have said something like "if aliens exist, why aren't they here? There's been plenty of time."

Great Filters

In a Technology Review article, Nick Bostrom digs into this problem more deeply. If you have billions of planets where life could begin, and only few (perhaps just one) planets with intelligent life, there must be one or more "Great Filters" that prevent intelligent life from developing, or lasting very long if it does develop.

The development of intelligent, tool-using life shouldn't be considered some inevitable sequence that will always happen given enough time. Instead, it's a series of transitions from one type of life to another. Each of those transitions has a certain probability. The transition may never happen, or may take an extraordinarily long time before it happens, just depending on the odds. We think we know some of the steps, from looking at the history of the Earth, but we don't know much about the odds. We can guess that something which took a long time to happen on Earth has low odds of happening at all, but we can't really know. We don't know if there are completely different routes to intelligent life -- routes never taken by life on Earth.

Here are some of the possible Great Filters, taken from the Bostrom article, or from other reading I've done:

  • According to the Rare Earth hypothesis, we wouldn't get Earth-like life if the Sun were too near the center of the Galaxy or too far away, if the Sun didn't have enough metals to form rocky planets with complex chemistry, or if the Sun were the wrong size. These considerations rule out the majority of all stars in the Galaxy.

  • The Sun and Earth orbit through the galaxy every 220 million years or so. We have been fortunate that the Earth has not come near any really violent astronomical events such as supernovas, which would have sterilized the planet. In fact, we are fortunate not to live in a galaxy which is colliding with another galaxy, which presumably produces a more dangerous environment.

  • The Earth has a peculiar history. According to the Giant Impact theory, the Moon formed when a Mars-sized planet hit the Earth, late in its formation. No other planet in our solar system has such a large moon relative to its size. This gives us our tides, and regulates the tilt of the Earth's axis. The Earth also has plate tectonics (perhaps necessary to life) and a strong magnetic field, which neither Mars or Venus have.

  • No one knows when life originated on Earth. Most of the discussions I've read center around when conditions might have settled down enough to allow life to form, and where that formation might have occurred. The implication is that life formed as soon as it was possible for it to form. On the other hand, Bostrom speculates that life took perhaps hundreds of millions of years to form, and was not a likely event at all.

  • Assuming life formed by around four billion years ago, several important stages in the development of complex, modern cells take a very long time. The creation of "eukaryotic" cells doesn't happen until two billion years ago. Photosynthesis, sexual reproduction and multicellular organisms require additional hundreds of millions of years to evolve. These transitions must be either very low probability, or require very specific environmental conditions that took a long time to develop. For example, there's the theory that the Earth's oceans completely froze over, resulting in a Snowball Earth and that the end of this period might have provoked the Cambrian Explosion, which saw a great increase in the diversity of life on Earth, 530 million years ago.

  • Mass extinctions of life have occurred more than once during the history of the Earth. Perhaps 95% of life on Earth became extinct 250 million years ago, for example. If the dinosaurs were wiped out by an impact event 65 million years ago, this could easily have been a larger impact, resulting in more damage. Whatever the causes, we're lucky that these "mass extinctions" were not "complete extinctions."

  • By 200 million years ago, creatures our size and level of activity were present on the Earth. As far as we know, during all the time since, no other animal using language and tools developed, despite what we think of as obvious advantages for that behavior. By some calculations, the Earth may be habitable for only another few hundred million years. If the Earth could go this long without producing an intelligent species, a similar planet might never produce one.

  • By 200,000 years ago, anatomically modern humans had appeared. We have spent nearly all the time since living in small bands of hunter-gatherers. According to a recent article, the human race had only 2,000 members as recently as 70,000 years ago. How long would it have taken another similar species to evolve if we had died out?

  • Agriculture and civilization did not appear until 8000 years ago. Many civilizations have come and gone without inventing anything like what we consider science and technology. Most humans seem to have basic mechanical skills, but the kind of interest in mathematics and logic it takes to develop science seems much rarer. It's easy to imagine a species that lives a civilized life but never develops any high technology. Until recently, we were that species.

  • There may also be "Great Filters" yet to come. We could kill ourselves off with the disasters relating to biotechnology, nanotechnology or artificial intelligence. Although nuclear war or environmental damage probably couldn't lead to our extinction, they could certainly reduce our numbers dramatically, and perhaps crash our current civilization.

What we should expect

So if you were to use the Earth's history as your guide (which we have to, since we don't know of any other examples of life), you'd expect a universe that looks like this:

  • Huge regions of the Galaxy, and of other galaxies, would be sterile. Only a minority of planets (a few percent) would have life. On other worlds, life might start, but it would be wiped out by later events.

  • Of the planets that have life, only a small number would have complex life. We would assume this because it took so long for complex life to develop on Earth (billions of years.) By complex, I mean anything with multiple cells or sexual reproduction. A worm would be complex life.

  • Of those with complex life, only a few planets would have anything we'd call intelligent life. Earth's biosphere seems to have produced a large variety of creatures without creating anything that used language before us.

  • Of those with intelligent creatures, few will have the interest in mathematics and logic that it takes to produce science and high technology. They will live for awhile (millions of years, perhaps) but then evolve into something else, or perhaps die out.

  • Of those that produce technology, many may wipe themselves out with that technology. You could make the argument that any high tech which can modify life and intelligence itself is a runaway feedback loop, and inherently unstable.

In his article, Bostrom's point is that it matters whether the Great Filter that prevents intelligent life from developing has already happened or is still to come. Finding life on Mars, for example, changes the odds. If there's simple life on Mars as well as Earth, then it probably means the filter is after the origin of life. If there is complex life, or fossils of complex life, then again, it means many planets probably have complex life and something later kills it off or prevents intelligence from developing. And if we were to hear radio signals in one of the SETI (Search for Extraterrestrial Intelligence) projects, that would be very bad news. It would mean that other planets reached our state of development and still failed.

That sounds surprising -- after all, isn't development of technology a success? And just because we hear radio signals, why does that imply that intelligent species everywhere die out after they invent technology? The answer is the Fermi Paradox again. If a species invents technology and doesn't die out, it should spread everywhere. It should already be here by now.

The Spread of Technological Life

Science doesn't take reports of flying saucers and alien abductions seriously, but interstellar travel is possible. Two human spacecraft (Pioneer 10 and 11) are on their way out of the solar system even now. True, it will be millions of years before they reach another star, but they have left. Designs for faster ships have been created (see Project Daedalus.)

If a technological species wanted to spread through the stars, it would probably build what's called a von Neumann probe. This is a machine which can reproduce itself. In the original version, it would be launched towards a nearby star, where it would find a rocky planet. Then it would set up a factory and build copies of itself. These would be launched towards other nearby stars. Eventually, the probes would visit every star in the galaxy. What they do there depends on their programming. Given a powerful enough technology, they could do all sorts of things, including starting new colonies of the creatures that built them.

I prefer a more dramatic version. If you assume a more advanced technology, the probe is not simply a machine reporting back to its builders, but instead an artificial organism, with an artificial intelligence. It is the technological species, which isn't exploring the galaxy, but colonizing it. On top of that, assume the technology allows the creature to be the size of a seed, and live in or near a star. Let's call them the Appleseed species.

The Appleseed species originates on a planet like Earth. Being artificial, it has to be created by a biological species like ours. It is then released into the sun of that planet. It reproduces there, building copies from material in the solar wind, or mined from the star. Given how small it is, and how large the sun is, it can build up huge populations (in the billions of billions.) Gradually it uses up its available resources and wants more territory. What you might call the "logic of life" is irresistible. Spread first, or else lose out to competition that spreads before you do.

The community of Appleseeds around the sun build a launcher. They have all the power of a solar flare to work with. The spacecraft is the size of a sugar cube. It just needs to hold an Appleseed and whatever it needs to reproduce. The ship is fired out of the sun at 1000s of gravities, propelled by a shaft of LASER light. By the time it leaves its home solar system, it is traveling a decent fraction of the speed of light.

After just a few years, the Appleseed plunges into (or orbits near) a new star, where it is at home. It starts to reproduce. Doubling in population every generation, it doesn't take many generations for it to cover the new star as well. As soon as the infrastructure is there, it can send out more tiny seed ships. The entire galaxy is colonized in just a few hundred thousand years.

This sounds like science fiction (and there is some along these lines), but I'm not describing anything impossible. No "warp drives" or time travel, no transporters or magic mental powers. Every cell on planet Earth knows how to reproduce. The Appleseeds don't have some unique capability. Living in the Sun is a bit of a stretch, but if you don't like that, just make them orbit the sun in a huge shell, at whatever distance seems reasonable to you. If you don't buy the idea of human-equivalent artificial intelligence, just assume something more mechanical, programmed to reproduce. It doesn't change the overall pattern.

The Fermi Paradox Revisited

In science fiction TV and movies, we envision the galaxy populated by many different alien species, each around its own home planet. They can all get together in that bar in Star Wars, or shoot at each other from spaceships on Star Trek. The inspiration here is from Earth, comparing the spread of species to the spread of nations, with colonies and wars. Or you could compare the galaxy to the ecology of Earth, with many intelligent species, each in its own niche or territory. Both of these analogies are wrong.

The timing of the situation guarantees that is not the case. The first technological life that occurs can spread everywhere in the galaxy, inhabiting all the stars and planets, before the next species can arise. The galaxy is only 100,000 light years across. At 10% of the speed of light, that's only a million years for complete settlement. I don't see any reason to think that a really advanced technology couldn't manage the spread faster than that. Even with 100 billion stars in the galaxy, intelligent life would have to be arising constantly for two species to be in competition at the same time, back at the beginning.

This sounds odd, but I'm just making an analogy with life on Earth, but at a different level. At some point in the past, life got started somewhere (I favor undersea volcanic vents, myself.) Soon after, it spread to all parts of the Earth. Life cannot arise again. If those some conditions were occurring today (and they probably are), life would already be occupying that niche. In other words, any new life is going to evolve in the most suitable place, but that's where you'd be most likely to find life already. Also, any new form of life is going to just barely manage to reproduce. It's not going to have defenses or senses or mobility. It would be easy pickings for the advanced life that is already here.

I expect the same rules to hold on the interstellar scale. The first life spreads to all available niches. No other life can arise without encountering advanced competition already in place. The only difference is that intelligent life could decide to give new life a chance (or create it deliberately!)

If we don't see evidence of intelligent life, the simplest explanation is that there isn't any. People have suggested that aliens can't travel easily, or doesn't use radio to communicate, or are avoiding us for some reason. The kind of artificial life I've described isn't that subtle. It spreads everywhere, takes over the stars themselves, and is very obvious. The fact that we see stars at all, that there are not huge areas of blacked out suns covering the night sky, says that this has not happened.

This all seems horribly aggressive and not at all what we expect from the "wise, old aliens" of popular culture and SF. The point is, it's possible, and it's the logical end result of a species that wants to use all the available resources, just as every form of life on Earth does. And it only has to happen once. If intelligent life is common, the slow, cautious, wise ones are going to find their star fading out, populated by the aggressive, shallow, greedy ones. Again, this is the "logic of life."

The Future

I assume we are the first technological species. The odds of getting this far must be horrible. Perhaps the odds of successfully managing technology are horrible too. At this point, we just don't know.

I assume that we will create a new form of intelligence. Either we'll alter our genes eventually to create a new species, or we'll merge our brains with our computers, or (most likely) we'll create a true artificial intelligence. Either we'll create it from studying brains or by just simulating a human brain.

Once technology has created a new intelligence, which can use technology to improve itself, we're in new territory. Biological evolution is done, and all future progress is cultural and technological. Estimates have been made of how fast the fastest computers allowed by physics could be. The limits are astronomical -- minds trillions of times faster than our own should be possible. There's no way that plain old humanity is going to compete with that.

I assume those technological children of ours will spread to the stars. That's where all the energy is, after all. There's no reason they can't colonize it all. If that doesn't happen, it will probably be because something even weirder is possible.

After that first species gets control, it makes all the rules. If it shells over all the stars, no other life can even develop, since all the planets are frozen solid. If it wants to let biological evolution continue, it can do that, by avoiding stars with fertile planets. It can prevent any other technology from arising (by monitoring all the planets where life is evolving.) It can guide or change any life that it does find.

This may seem horrible to you -- little robots putting all the stars out! Spreading like a weed and killing or preventing any new life from developing. But you're looking at it the wrong way.

The galaxy now is like the Earth was at the moment when life first began. At the beginning, only one tentative little speck was alive, in a whole lifeless world. So precarious! The whole future depended on it surviving and spreading. And it did spread, and changed the entire planet.

Intelligence may be limited to the Earth now -- just a single thinking speck in an empty galaxy. From the viewpoint of our creations, from the viewpoint of the future, all they can do is hope that it survives and spreads, to colonize all those mindless stars.

The first species out there gets to decide the future, for every species that follows. For lack of any other evidence, let's hope it's us.

An Analogy

Let me finish with an analogy, an image that occurred to me many years ago.

I imagine a prehuman ancestor of mine, from perhaps half a million years ago. I imagine him sitting at a fire, working on his stone tools. His children chase each other around the fire, laughing. It's a warm night, the stars are bright, and everyone has had a good meal. It is the best night of his life.

I imagine him having a vision of the far future. He sees the vast buildings and bridges. He sees a people who race across the ground in cars, fly through the air in planes. Some of them even walk on the Moon. More immediately, they are rarely too warm or too cold, rarely starving or dying of thirst. Disease does not cut them down, and nearly all their children live to adulthood. They live a long time, and they even keep their teeth! I imagine he would have great satisfaction at all of the things he sees.

But then the vision clears and he sees the people themselves. They are too tall, some of them hairless and pale, their faces are shaped wrongly. He knows they are his children, but they are not his race. And he realizes that they would pity him. They would respect that he has taken the first steps on a long road, but still, his condition would seem only slightly better than that of an animal. And he understands that all his works, all his people, his language and his lands, are forgotten. Nothing left but a few skeletons and a few chips of stone.

I wonder what he would make of this? It would be natural for him to reject it all, for him to wish that his people last forever, and not change into something alien, no matter how accomplished. Yet, if he got his wish, we would not exist. And so I feel that it would be evil to wish for that -- evil to wish that change would stop.

This image has a lot of power for me, because I feel we are in exactly that position. Unlike our distant ancestor, we know that our race will not last. And unlike him, we have some power over the shape our successor will take. I wonder what we will decide?

by Michael Goodfellow.
For more, see Free The Memes!

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