[ekianjo]

For 1, Life has more chance to evolve in milliona of other species that dont o serve space, master fire or launch rockets. pretty bad odds once you look around you. plus we have been graced by having fairly mild cosmic conditions for a long time. other planets may not be so lucky

[giantrobot]

Sure, but space is mind-bogglingly large. Our galaxy alone has hundreds of billions of stars. Even one in a billion odds ends up with hundreds of systems that develop just like ours did. And that's just one galaxy out of billions.

That however circles back to space being large. While a galaxy might host hundreds of civilizations just like ours at any given time, the distances between them can be insurmountable. Time is also pretty large so even a civilization that survives thousands or even millions of years might never overlap with another that they can contact.

[roywiggins]

Part of the problem is, you just need one civilization in this galaxy to invent von Neumann probes to expect to see their robot offspring everywhere, even if that civilization is half a billion years dead.

[giantrobot]

The word "just" is doing some pretty heavy lifting in that statement.

It pre-supposes you can event build a bunch of invincible perfectly programmed micro/nanobots in the first place. Then you can identify a target body in a solar system a long ways away. Then predict that system's position with an accuracy your probe could land on it after a trip of hundreds of light years (or launch a probe with a bunch of fuel to do terminal maneuvering). Then those probes would function after millennia of dormancy.

It's all possible. But the concept shouldn't be treated as inevitable.

It also runs right up against panspermia theories. What's the practical difference between a single celled organism and a self-replicating nanobot?

[roywiggins]

Well, yeah. It's really hard to do, so maybe nobody does it, or it's just impossible. But with several billions of years of galactic history it has to be pretty hard if civilizations are common.

Really it's just one example of a technosignature that is longer-lasting than radio waves, that extends the amount of detectable overlap we might expect to have with an older civilization, and the lack of observation either says that sort of technology is either not feasible or there aren't many civilizations in the galactic past or present.