So IO9 ran a piece by George Dvorsky on ways we could wreck the solar system. And then Anders Sandberg responded in depth on the subject of existential risks, asking what conceivable threats have big enough spatial reach to threaten an interplanetary or star-faring civilization.
This, as you know, is basically catnip for a certain species of SF author. And while I've been trying to detox in recent years, the temptation to fall off the wagon is overwhelming.
The key issue here is the nature of the Great Filter, something we talk about when we discuss the Fermi Paradox.
The Fermi Paradox: loosely put, we live in a monstrously huge cosmos that is rather old. We only evolved relatively recently -- our planet is ~4.6GYa old, in a galaxy containing stars up to 10GYa old in a universe around 13.7GYa old. Loosely stated, the Fermin Paradox asks, if life has evolved elsewhere, then where is it? We would expect someone to have come calling by now: a five billion year head start is certainly time enough to explore and/or colonize a galaxy only 100K light years across, even using sluggish chemical rockets.
We don't see evidence of extraterrestrial life, so, as economist Robin Hanson pointed out, there must be some sort of cosmic filter function (The Great Filter) which stops life, if it develops, from leaving its star system of origin and going walkabout. Hanson described two possibilities for the filter. One is that it lies in our past (pGF): in this scenario, intelligent tool-using life is vanishingly rare because the pGF almost invariably exterminates planetary biospheres before they can develop it. (One example: gamma ray bursts may repeatedly wipe out life. If this case is true, then we can expect to not find evidence of active biospheres on other planets. A few bacteria or archaea living below the Martian surface aren't a problem, but if our telescopes start showing us lots of earthlike planets with chlorophyll absorption lines in their reflected light spectrum (and oxygen-rich atmospheres) that would be bad news because it would imply that the GF lies in our future (an fGF).
The implication of an fGF is that it doesn't specifically work against life, it works against interplanetary colonization. The fGF in this context might be an emergent property of space exploration, or it might be an external threat -- or some combination: something so trivial that it happens almost by default when the technology for interstellar travel emerges, and shuts it down for everyone thereafter, much as Kessler syndrome could effectively block all access to low Earth orbit as a side-effect of carelessly launching too much space junk. Here are some example scenarios:
Simplistic warfare: As Larry Niven pointed out, any space drive that obeys the law of conservation of energy is a weapon of efficiency proportional to its efficiency as a propulsion system. Today's boringly old-hat chemical rockets, even in the absence of nuclear warheads, are formidably destructive weapons: if you can boost a payload up to relativistic speed, well, the kinetic energy of a 1Kg projectile traveling at just under 90% of c (τ of 0.5) is on the order of 20 megatons. Slowing down doesn't help much: even at 1% of c that 1 kilogram bullet packs the energy of a kiloton-range nuke. War, or other resource conflicts, within a polity capable of rapid interplanetary or even slow interstellar flight, is a horrible prospect.
Irreducible complexity: I take issue with one of Anders' assumptions, which is that a multi-planet civilization is largely immune to the local risks. It will not just be distributed, but it will almost by necessity have fairly self-sufficient habitats that could act as seeds for a new civilization if they survive. I've rabbited on about this in previous years: briefly, I doubt that we could make a self-sufficient habitat that was capable of maintaining its infrastructure and perpetuating and refreshing its human culture with a population any smaller than high-single-digit millions; lest we forget, our current high-tech infrastructure is the climax product of on the order of 1-2 billion developed world citizens, and even if we reduce that by an order of magnitude (because who really needs telephone sanitizer salesmen, per Douglas Adams?) we're still going to need a huge population to raise, train, look after, feed, educate, and house the various specialists. Worse: we don't have any real idea how many commensal microbial species we depend on living in our own guts to help digest our food and prime our immune systems, never mind how many organisms a self-sustaining human-supporting biosphere needs (not just sheep to eat, but grass for the sheep to graze on, fungi to break down the sheep droppings, gut bacteria in the sheep to break down the lignin and cellulose, and so on).
I don't rule out the possibility of building robust self-sufficient off-world habitats. The problem I see is that it's vastly more expensive than building an off-world outpost and shipping rations there, as we do with Antarctica -- and our economic cost/benefit framework wouldn't show any obvious return on investment for self-sufficiency.
So our future-GF need not be a solar-system-wide disaster: it might simply be one that takes out our home world before the rest of the solar system is able to survive without it. For example, if the resource extraction and energy demands of establishing self-sufficient off-world habitats exceed some critical threshold that topples Earth's biosphere into a runaway Greenhouse effect or pancakes some low-level but essential chunk of the biosphere (a The Death of Grass scenario) that might explain the silence.
Griefers: suppose some first-mover in the interstellar travel stakes decides to take out the opposition before they become a threat. What is the cheapest, most cost-effective way to do this?
Both the IO9 think-piece and Anders' response get somewhat speculative, so I'm going to be speculative as well. I'm going to take as axiomatic the impossibility of FTL travel and the difficulty of transplanting sapient species to other worlds (the latter because terraforming is a lot harder than many SF fans seem to believe, and us squishy meatsacks simply aren't constructed with interplanetary travel in mind). I'm also going to tap-dance around the question of a singularity, or hostile AI. But suppose we can make self-replicating robots that can build a variety of sub-assemblies from a canned library of schematics, building them out of asteroidal debris? It's a tall order with a lot of path dependencies along the way, but suppose we can do that, and among the assemblies they can build are photovoltaic cells, lasers, photodetectors, mirrors, structural trusses, and their own brains.
What we have is a Von Neumann probe -- a self-replicating spacecraft that can migrate slowly between star systems, repair bits of itself that break, and where resources permit, clone itself. Call this the mobile stage of the life-cycle. Now, when it arrives in a suitable star system, have it go into a different life-cycle stage: the sessile stage. Here it starts to spawn copies of itself, and they go to work building a Matrioshka Brain. However, contra the usual purpose of a Matrioshka Brain (which is to turn an entire star system's mass into computronium plus energy supply, the better to think with) the purpose of this Matrioshka Brain is rather less brainy: its free-flying nodes act as a very long baseline interferometer, mapping nearby star systems for planets, and scanning each exoplanet for signs of life.
Then, once it detects a promising candidate -- within a couple of hundred light years, oxygen atmosphere, signs of complex molecules, begins shouting at radio wavelengths then falls suspiciously quiet -- it says "hello" with a Nicoll-Dyson Beam.
(It's not expecting a reply: to echo Auric Goldfinger: "no Mr Bond, I expect you to die.")
A Dyson sphere or Matrioshka Brain collects most or all of the radiated energy of a star using photovoltaic collectors on the free-flying elements of the Dyson swarm. Assuming they're equipped with lasers for direct line-of-sight communication with one another isn't much of a reach. Building bigger lasers, able to re-radiate all the usable power they're taking in, isn't much more of one. A Nicoll-Dyson beam is what you get when the entire emitted energy of a star is used to pump a myriad of high powered lasers, all pointing in the same direction. You could use it to boost a light sail with a large payload up to a very significant fraction of light-speed in a short time ... and you could use it to vapourize an Earth-mass planet in under an hour, at a range of hundreds of light years.
Here's the point: all it takes is one civilization of alien ass-hat griefers who send out just one Von Neumann Probe programmed to replicate, build N-D lasers, and zap any planet showing signs of technological civilization, and the result is a galaxy sterile of interplanetary civilizations until the end of the stelliferous era (at which point, stars able to power an N-D laser will presumably become rare).
We have plenty of griefers who like destroying things, even things they've never seen and can't see the point of. I think the N-D laser/Von Neumann Probe option is a worryingly plausible solution to the identity of a near-future Great Filter: it only has to happen once, and it fucks everybody.
What other fGF scenarios can you think of that don't require magical technology or unknown physics and that could effectively sterilize a galaxy, starting from a one-time trigger event?