Judging from the posts I've been seeing on certain threads recently, it
looks like it is time to revisit this topic. Old-timers who've had the
debate a dozen times already may want to skip this thread.
The question at hand is this: "Given what we know about human history,
human technology, and the nature of reality, how do we decide whether some
novel idea about possible alien civilizations is plausible?" Or, to put it
another way, "What is and is not predictable about alien civilizations?"
The question at hand is this: "Given what we know about human history, human technology, and the nature of reality, how do we decide whether some novel idea about possible alien civilizations is plausible?" Or, to put it another way, "What is and is not predictable about alien civilizations?"
There are several big mental pitfalls that you have to avoid when approaching this subject. Most people manage to get stuck in at least one of them, and consequently never say anything that is even remotely plausible. They are, in no particular order:
Unrealistic Technology Expectations
This idea comes in two closely related forms: "Future technology will be just like current technology" and "Future technology will be just like old science fiction." Wrong. This kind of thinking leads you to ignore even old, modest technologies like robotics and genetic engineering. If you want a realistic picture, you need to deal with the prospects for nanotechnology, AI, and superintelligence in a realistic manner. If you are going to dismiss them, you need to explicity say so (and supplying a justification for doing so would also be a good idea).
Short Time Horizons
We aren't talking about a few hundred years of event here. Or even a few thousand, or a few tens of thousands. Unless you think there is some special reason why technological civilizations have just now become possible, you need to deal with time spans of at least tens of millions of years. This is one of the reasons why most of us don't find scenarios that include technological roadblocks or galactic Prime Directive treaties to be very plausible.
Insufficent Appreciation of the Effects of Diversity AKA "maybe not everyone will want to invent nanotechnology". It doesn't matter if 99.9999999% of all sentients spend all of eternity contemplating their navels. If I had the technology of 100,000 years hence, you can be sure my personal projects would be visible from distant galaxies. Simple evolutionary processes will give you a universe dominated by active explorers, even if almost everyone starts out being a homebody. This means that if you want to hypothesize a universe full of advanced civilizations you need to think really, really hard about explaining why they haven't set up shop here.
Exaggerated Respect for the Immensity of Space The galaxy is big, but it isn't infinite. A single expansionist species with sublight travel could easily colonize every single system in it in less <10^6 years. Suggesting that we are surrounded by aliens, but they don't happen to have visited us, is equivalent to sugggesting that a particular lot on Manhatten Island might remain vacant for the next million years.
Exaggerated Sense of Mortality
AKA "maybe the aliens did thus-and-such, then died off..." What exactly do you think could have killed them? Most of the ideas I've heard wouldn't kill of modern-day humanity, let alone spacefaring aliens. Remember, even without nanotechnology, a single surviving colony could repopulate an entire species in a very short amount of time (instantly, form a geological perspective). If you use a realistic technology projection, and keep in mind that some small fraction of any population is going to be prepared for even fairly extreme emergencies, it is hard to imagine any event that could kill of a spacefaring species. Certainly, any disaster that big isn't likely to leave any intact planets in the galaxy.
Billy Brown, MCSE+I