They would be visible from Earth because they would be everywhere. Even if some members of a galactic-expansion-capable civilization did not want to expand enough to exploit all available resources, they would be quickly out-competed by the ones who did want to.
The universe is a big and mysterious place. It's doubtful whether the all-pervasive influence of human civilisation on earth would be that evident to a sentient barnacle anchored to an isolated islet far from inhabited land.
Sounds like a Deeply Wise Curiosity Stopper. OK, it's all speculation, but it's still reasonable to expect visible changes, like Dyson spheres.
And the barnacle certainly would detect us. We changed the weather. We use planes all over the place. At the very least, it would detect Iridium Flashes, which are hardly natural. (OK, that would take a scientist sentient barnacle, but humanity does science anyway.)
Depends on the scientist sentient barnacle's frame of reference; I'm sure it could posit naturalistic explanations for weather changes and distant things that move across the horizon and flash light at us. I'm not saying it's impossible for us to observe the activity of extraterrestrial lifeforms (that would be absurd), simply that traces of intergalactic activity might not be immediately obvious; surely one of the side-effects of a Dyson sphere, for instance, would be to render the star almost undetectable to our telescopes.
surely one of the side-effects of a Dyson sphere, for instance, would be to render the star almost undetectable to our telescopes.
No. A Dyson sphere would just emit a lot less radiation in the region that we happen to perceive as "visible". This is a special case of "Stealth is (close to) impossible in Space"
