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> Eventually probe will find planet with life. May be life is more abundant than we imagine now. May be not. Surely there will be protocols for the probe to investigate life. It's hard to imagine what those protocols would be. May be explore freely until certain technology advances are observed and then hide. So may be our ancestors saw alien probes on a regular basis and just didn't care. But now those probes observe that we've got quick airplanes so they hide better.

Surely the protocol wouldn't be "show a tic tac in the sky and disappear foerver"

The people that believe that these UAP seem to think there is both a Fermi paradox and reverse Fermi Paradox at the same time.

> We already sent probe out of our Solar System.

To give an idea of the scale involved, Voyager 1 is only 0.058% of the way to the nearest star system, and that took 45 years. Granted it wasn't built for speed but given that the best hypothetical plans for a spaceship we have allow travel at ~.1c it'll take 50 years not counting acceleration / deceleration.

> Eventually probe will find planet with life. May be life is more abundant than we imagine now. May be not. Surely there will be protocols for the probe to investigate life. It's hard to imagine what those protocols would be. May be explore freely until certain technology advances are observed and then hide.

Even assuming that some set of automated probes have been travelling our galaxy for millions of years, and reached earth in the last few thousand years, where are they now? By what mechanism could they even be hiding? Every inch of the planet is being watched by satellites.

> My point is that developed civilization inevitably will expand its sphere of influence at speed limited by speed of light. Probably slower than that. But billion of years at 0.1c is still huge chunk of universe.

As an aside, I'm not sure you could meaningfully have one civilization spanning hundreds of millions of light years, the ends are just too spread out for there to be a single cohesive or web of cultures. Imagine if an email took longer than our entire species existence to reach you.

> Are we sure that no civilization born within this radius billion years ago?

I personally am certain that there are many other forms of life throughout our galaxy, and at least some of them must be technological. As much as would like for it to be false, I also believe there is no realistic mechanism for travel between stars and we are all effectively doomed to stay in our little corner of creation, maybe spying on each other through telescopes, assuming we can even spot a neighbor.

> By what mechanism could they even be hiding? Every inch of the planet is being watched by satellites.

As well as ground based cameras, radar systems, infrasonic, particle detectors, scientific instruments, etc. And how would these ships power themselves for so long. To be able to do so while being hidden from xray, visible light, infrared, and radio? Except for maybe a few of these random sightings? Sure, it is possible, but doesn't sound likely.

> Even assuming that some set of automated probes have been travelling our galaxy for millions of years, and reached earth in the last few thousand years, where are they now? By what mechanism could they even be hiding? Every inch of the planet is being watched by satellites.

The most obvious way to hide is stay small. I don't know what physical limits would be to build small robots with advanced technology. Build main base somewhere in asteroid belt. Build some intermediate bases for refilling or something like that in moons. Or may be just rock somewhere in the space with small solar panels which collects electricity and can charge robot of size of sparrow which will travel further to Earth. Can we notice sparrow landing Earth and intelligently hiding e.g. in the sea?

Okay, so they are small and the objects being discussed in this thread aren't those. (there's a lot of complications that come with being small btw, but maybe another user will go into that)

Sure, I actually wrote a longer comment here[0]. My understanding of what you're saying (no FTL) kinda falls under point 2 that I made. While I understand the mechanism you're discussing, is a civilization willing to spend a billion years waiting for the information to travel back home? I'd be under the impression that this would be a pure data collection pov and a quite risky one at that. You can't act on that information in a meaningful way since the life evolves drastically within that billion years (but you can study it!). Even in a thousand things change a lot, but at least not evolutionary. The risk is that if anything goes wrong (glitch, crash, etc) then you've kinda contaminated that life system (this includes the transmission of their data). But maybe they don't care and it is an acceptable risk to them. I'm not an alien, I don't know their ethics. Do also take into account that such a project would be a rather massive undertaking and the idea of mining planets left and right is quite noisy. We're about the at the point where we'd start detecting such stuff since we're looking at exo-planets on the regular now.

I'll bring up another point that I didn't in my longer post though. Maybe these lifeforms solved things like longevity and so such time constraints aren't meaningful to them. What's a few hundred years when you live millions?

> But billion of years at 0.1c is still huge chunk of universe. Are we sure that no civilization born within this radius billion years ago?

That's 1,000,000ly fwiw. That does cover the entire Milkey Way (but doesn't get us half way to Andromeda). But also consider that the Milkey Way is 13.6bn years old and life on Earth is 3.7bn.

I'm not saying that your idea is crazy, it is definitely possible (and has been proposed by others, in quite high detail). I just think there isn't any evidence for this and there's reason to believe that this would be quite difficult to pull off. But we'll see as time goes on.

[0] https://news.ycombinator.com/item?id=34672241

But what kind of evidence could we have?

Sure, if we would find crashed space ship, that would be good evidence. We don't have that (or it's not public knowledge anyway).

Anything less than that would be hard to consider evidence. Especially if those things are trying to minimize encounters.

As to mining planets. I think that it makes more sense to mine asteroids. You don't need to overcome gravitational well and asteroids are quite rich when it comes to resources. Asteroid belt is huge. You can build lots of stuff there without fear to be noticed. Also it's not really clear how far can you go with minituatirization. We think about big ships but if you can operate on molecular level, may be you can stay small. Think about robots size of bacteria, size of ants and stuff like that. Alien anthill in some asteroid pit might be never noticed.

> But what kind of evidence could we have?

The lack of evidence does not support a claim. Otherwise we'd make arguments about ghosts, invisible unicorns, and flying spaghetti monsters. Just to be clear. You're right that it is hard to consider evidence when it doesn't exist. So my believe of aliens visiting earth is in the probability ranges of these other things.