Really well-written article and some really well-thought responses. Kudos all.

Regarding your scenario 1 (technological great filter), yes, that’s the scariest and most difficult to refute.

Regarding your scenario 2 (they’re among us), it is possible, especially if they’re watching from afar (say the Oort Cloud, or even a nearby solar system), which would make it quite unlikely that we’d already see them.

Regarding your scenario 3 (technological civilizations are highly unlikely), that may end up the easiest scenario to believe and be with. You say that, “If any step in the evolution to intelligence was vanishingly unlikely, that step would most likely have taken a disproportionately long time on Earth.” Yes, that would most likely be true, but if only one occurrence has happened, it could have happened in a relatively unlikely manner.

Adding to that from Scott McGregor’s excellent analysis, technological civilizations may need a long time (>200 mega-years) to develop before they can leave their planet of origin. This may make it more probable for an extinction event to occur too early in dense stellar neighborhoods.

Additional “filters” include:

• Have to be within the Goldilocks zone around the exoplanet’s star
• Older stars tend to be metal-poor, which may make it more difficult for a star-faring technology to evolve, even if intelligent life manages to evolve
• Very young stars may not have planets mature enough for technological civilizations to have developed
• Massive stars may pour out so much radiation that they may make their Goldilocks zone inhospitable for complex life forms
• Planets without a magnetic field lose their atmospheres due to stellar winds (a la Mars), possibly before technological societies can develop from whatever life may have evolved on those planets
• Rocky planets that are significantly larger than the Earth may make it impossible for even a technological society to reach space due to the energy density required to reach escape velocity

Given all these filters, and possibly others, it may be that the probability of a technological society developing anywhere isn’t very high, and since the density of stars in our galactic neighborhood is relatively low, we may be the first to develop within a distance across which we can detect others with technologies we’ve already deployed.

Alternatively, it’s possible that such societies develop more frequently, but that their mean lifetime before collapsing to the point that they’re no longer detectable by us is much shorter than the mean time between such civilizations developing. For example, if once such a civilization reaches detectibility it survives on average 10 mega-years, but such civilizations only arise about once every 100 mega-years in our galaxy, then the likelihood is that we’d never see anybody else.