[mrpopo]

"The Great Filter" is the only non-egotistical, and therefore IMO the only valid explanation. Even then, people still believe in egotistical great filters such as global thermonuclear war.

We already have evidence of an extinction-level event (Chicxulub impact) 66 million years ago. A strong enough solar flare would eradicate all life in extra-terrestrial colonies. We are just lucky to be alive.

[A_D_E_P_T]

> "The Great Filter" is the only non-egotistical, and therefore IMO the only valid explanation.

That's not how I see it. The Great Filter implies, and quite strongly, that the Filter is in our past -- and, as such, it's extremely egotistical. For, ultimately, the implication is that we may quite literally be unique.

It's also not at all clear that something like the Chicxulub impact would result in the extinction of humanity. Unlike dinosaurs, we're able to adapt with technological solutions, such that at least a breeding population would survive such a catastrophic event. (This applies broadly, to other technological societies.)

Granted, a supernova could go off somewhere in the neighborhood, but this is a low-likelihood event, and thus cannot be a Filter. The Filter, after all, has to apply universally.

I think that the most parsimonious explanation is new physics. There's nothing egotistical in the notion that we don't understand everything there is to understand about how science and technology might progress, and looking for infrared waste heat and the technosignatures we're already familiar with, at our present level of development... this might, for obvious reasons, be totally futile.