[Grishnakh]

Yes, but that doesn't prove anything. You're not going to see alien life on other planets when your technology is so primitive that you've never even send manned missions beyond your nearest moon, and you've only recently even begun to detect other planets (outside your own star system), and even there you don't have the capability of detecting planets as small as your own.

If you don't bother to actually observe planets similar to your own, then of course you're not going to observe any alien life similar to your own.

[pklausler]

We do have the ability to detect signals: none found. We have also not detected any von Neumann - Bracewell probes. These are significant negative results, since any civilization slightly more advanced than ours could quickly (a few M years) seed the galaxy with vN-B probes -- and would eventually do so with probability approaching unity.

[Grishnakh]

The other responder is correct.

The signal detection thing is just dumb. Signals quickly fall in strength to background noise levels, so you need to broadcast a huge amount of power in a tight beam to overcome that over any significant distance. So basically, you're arguing that just because no ETs have bothered to pour a lot of resources into 1) finding us and learning of our existence, and 2) building a gigantic, power-consuming radio transmitter and directing it at us, that they must not exist.

The probes thing assumes that the ETs actually want to talk to us. That's a pretty huge assumption. We've had civilizations here on Earth which had no desire for outside contact (namely the Chinese during some of their dynasties). What makes you think the ETs are so intent on pouring resources into making artificially-intelligent probes to establish communications?

By that logic, WE don't exist, because we haven't bothered to do these things either.

[pklausler]

All it takes to have a galaxy forever full of self-replicating vN-B probes is the launch of one probe from one civilization sometime in galactic history. The odds that we would observe no probe here are (1 - P(a civ will launch a probe)) ^ N(tech civilizations). I assert that no probe has been observed, and I contend that N(tech civs) must be low, since P(launch) must be pretty high -- it just takes one E.T. script kiddie with a Stephenson matter compiler to make it happen sometime during the lifetime of a tech civilization.

[Grishnakh]

You're making a lot of assumptions. You're assuming that it's that easy to make a self-replicating probe that's that intelligent, for one. Any civilization that advanced may have put a lot of thought into what such probes should do, and how they should interact with other civilizations. They may very well have come up with the Prime Directive and only observed, without becoming detected by primitive cultures such as ours.

You're assuming that there aren't other spacefaring civilizations out there who are opposed to these probes, and that don't actively seek them out and destroy them.

You're assuming that the galaxy and nearby vicinity is old enough for one of these civilizations to become this advanced and to build these probes and for one of them to reach us. Who knows, maybe there just aren't that many really old civilizations around yet. Just look at how long it took us to evolve to this point.

[pklausler]

My original point stands. We haven't observed vN-B probes. This negative result is useful in a Bayesian way; it makes it less likely that the galaxy is full of tech civilizations, and makes other possibilities more likely, including several that you mentioned in a needlessly belligerent fashion.

[fergussimpson]

Ah, but take a look at this chart:

https://xkcd.com/1633/

For anything more than 10^6 km away and less than 10 metres in size, we just wouldn't see it. We have only just noticed some enormous lumps on Ceres, and it seems a whole planet has evaded detection until now.

I don't think it's particularly likely we will find a vN probe. But to consider their non-detection to be of statistical significance is, I think, an overestimation of our technological capabilities.

[zahmahkibo]

Assuming they actually want to talk to us.

[pklausler]

Assuming at least one of them might want to talk to us, yes.