"Or maybe we'll come to them", you might counter, and maybe, eventually. I'm going to leave that aside for now because, while we have some interesting theoretical ideas of how we might reach another star system, at this point they're just that, and the more realistic ones involve moving maybe a ton of payload to a nearby star in somewhere around a human lifetime, and score around a seven on the scale of difficulty I gave earlier.
The amount of energy involved in doing even that is staggering. By comparison, the New Horizons probe that's currently nearing the Pluto system as I write this was launched on one of the larger rockets ever produced, received a significant gravitational boost from Jupiter, and has been traveling for around ten years. To get to Proxima Centauri in a small number of decades, you'd need to be traveling around 2000 times as fast.
The energy required goes as the square of the velocity (until you get to relativistic speeds, where it gets much, much worse), so we're looking at 4 million of the rocket that launched New Horizons. Put another way, the kinetic energy of a one-ton object moving one-tenth the speed of light is comparable to
- The energy released by the largest nuclear bomb ever detonated
- The energy released by the eruption of Krakatoa
- The electricity consumed by Norway in a year
However, one of the main points of the previous post was that some significant portion of intelligent species would have arisen millions or hundreds of millions of years ago, and another was that there were likely a great many such species scattered throughout the galaxy, not to mention other galaxies. Maybe billions in our galaxy alone, using not-totally-implausible guesses. Put that together, and there might be a large number of species which have had plenty of time to reach us, and plenty of time to develop the technology to do it. The timing that undermined our prospects of spotting other life directly actually works in our favor now.
So let's say that some technological species arose ten million years ago and 50,000 light-years away. If they'd built a craft capable of traveling 1/200 the speed of light, or about 100 times faster than New Horizons, that craft could have reached us by now. And maybe even have stopped.
Or, it could have reached any of a few hundred billion other stars. Even if it had launched, say, ten craft a year for a million years, each equipped with a means of navigating to a distant star and then slowing down, the odds would still be about 10,000 to one against one reaching us.
However, there's a scheme that's been floating around for quite a while that would dramatically increase the chances of something eventually getting to us: send self-replicating craft. This is not something we could do right now but it's quite plausible that we could in the next century or so. It doesn't seem too much to ask that some civilization, somewhere, with a million or more years of head start, could have done this.
It's an interesting scenario to contemplate. Weight is critical at these speeds, so the craft will probably carry only the bare minimum it would need. It would enter a new star system and find a solid planet or moon to set down on. It would then start digging into the surface (assuming there's no one there to run across it and ask "Hey, what's this?") and gradually assemble raw materials. From those it would assemble basic tools, use those to assemble more sophisticated tools, and eventually spacecraft parts, which it would then proceed to put together. Some amount of time later it will have made a copy of itself, which would then take off for a nearby star system, while the original goes on building copies of itself ...
You'd want to be a little careful with this. If you literally sent a copy to every nearby star system, that would include ones you've already visited (even the home world), and some of those would make their way back to where they started, and start making copies of themselves. That is, you would get exponential growth, everywhere the probes visit.
The total number of craft might double every few centuries. If every copy used a ton of raw materials, they would consume the mass of our Moon in about 70 generations of copying, or a few dozen millennia. In a few million years, probes could eat every planet, planetoid and moon in the galaxy. Or, at least, every unpopulated one. Or, at least, the portion of the material of the unpopulated ones that was suited for making the craft.
The polite way to send such probes would be to remember which places you'd already visited and only send copies to new places. In technical terms, this results in a breadth-first search of the galaxy. Instead of exponential growth you get cubic, that is, a steadily expanding radius of exploration. In our scenario of craft traveling 1/200 the speed of light (and not taking too long to build copies), this would cover the whole galaxy in around 20 million years, starting at the edge, or 10 million starting near the center.
In sum, if anywhere in the hundreds of billions of stars in the galaxy 20 million years ago someone had successfully launched a self-replicating interstellar probe, one copy (or more) could have made it to the solar system. A heady thought, to say the least.
One question that always bothered me about this scenario was "why?" Even if the various probes could relay information back to the home world (and this is totally handwaving how one would produce a signal strong enough), the finite speed of light becomes a problem. It's going to take a thousand years to get information back from a system a thousand light years away, and 200,000 from one edge of the galaxy to the other. Why bother? I mean, who am I to say how long a lifespan, or attention span, an alien species might have, or what its motivations might be, but still ... it's hard to see the point.
However, you don't have to set out to cover the whole galaxy in order to end up doing it. Suppose we just wanted to explore our neighborhood of a few thousand stars? A self-replicating probe would be cheaper and easier than trying to send a probe to every star individually. In which case, why stop? If we're interested in data from a star 50 light-years away, why not 60, or 100? The easiest approach is to just let the probes keep copying.
A bit unsettlingly, it's even easier not to bother screening out already visited systems, that is, letting exponential growth continue unchecked.
In a slightly less scary variant, the probes somehow send messages to each other ("heartbeats", effectively) and only send copies to stars that aren't sending anything. This way if a probe goes dead, another will eventually take its place and the galaxy will remain completely covered indefinitely. Among other things, this means that if you're actively trying to get rid of probes, that may turn out to be somewhat difficult -- it only encourages them.
Once we get into galaxy-spanning schemes, we're looking at hundreds of billions of copies. There are bound to be a few mistakes here and there, and those mistakes may continue to propagate. The ones that do will continue to make copies, and some of those copies will be imperfect. "Mistake" here just means "different from the original". Such a mistake might be innocuous, or it might result in a less worthy probe, or it might even result in an improvement. One way or another, the population will evolve over time.
So we have something that can use energy in an organized way, reproduce and evolve. We may as well call that "life". It's most likely not the same life form as the one that first built it on the original home world, but that seems like a minor point. It's quite possible that there is life spreading out through the galaxy, or already occupying every corner of it, even while its creators are still confined to a single world, or no longer around at all. And that life might well have reached us, or even be here now.
This makes the question of "why haven't we noticed" a bit more interesting.
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