[imoverclocked]

I dislike either/or answers in such open-ended scenarios. It points to our lack of humility in the vast unknown.

eg: maybe they exist(ed) but once a civilization gets advanced enough to build FTL-like travel, they invent AI and use it for warfare and then soon cease to exist. This would mean there are potentially many civilizations (and AI?) that are budding and could travel through the universe.

eg: We aren't in an interesting enough place to bother visiting.

eg: they exist and know about us but have "prime directive" (Ala: Star Trek) laws that state they can't make contact until we reach a stable enough civilization to invent warp drive (or some other advancement.)

eg: There is some exotic reason that our pocket of the Milky Way is un-navigable.

[bluGill]

We know enough physics to rule out any ftl travel. Assuming that is correct which seems very likely they can't get here, even radio signals is question able - even if radio signals can get here either they have already passed and their civialization (sun) is dead or ours will be dead by the time they arrive.

[oneshtein]

Cherenkov radiation is a proof, that FTL is possible. We just cannot accelerate enough. However, «burps» from blackholes are proof that blackholes can do that. To achieve singularity, outer layer of blackhole core must spin at a FTL speed anyway.

[uecker]

Cherenkov radiation shows that FTL is possible in a medium where light is slowed down below a the maximum possible speed allowed by special relativity. It does not show that FTL as usually understood is possible.

[oneshtein]

Yep, usually FTL means FTc.