“Sufficiently advanced” tech could also plausibly identify all those hidden civilizations. For example, an underground civilization would produce unusual seismic activity, and taking up some inner portion of a gas giant or star would alter their outward behavior. Ultimately, civilizations use mass-energy in unnatural ways, and I don’t see a fundamental physical principle that could protect that from all possible sensing.
More importantly, I don’t think your suggestions address my point that hostile civilizations would get you before you even evolve.
But, let’s grant that you’re the first civilization to evolve in your galaxy, or at the least among the first before someone starts sending out probes to prevent any new civilizations from arising and threatening them. And let’s grant that they will never find you. That is a victory, in that you survive. But the costs are astronomical: you only get to use the mass-energy of a single planet, or star, or Oort Cloud, while someone else gets the entire galaxy.
To put it another way: mass-energy is required for your civilization to exist and fulfill its preferences, so far as we understand the universe. If you redirect any substantial amount of mass-energy away from its natural uses (stars, planets, asteroids), that’s going to be proportionally detectable. So, you can only hide by strangling your own civilization in its crib. Not everyone is going to do that; I seriously doubt that humanity (or any artificial descendant) will, for one.
(This comes back to my link about “no stealth in space”—the phrase is most commonly invoked when referring to starships. If your starship is at CMB temperatures and never moves, then yeah, it’d be hard to detect. But also you couldn’t live in it, and it couldn’t go anywhere! You want your starship—your civilization—to actually do something, and doing work (in a physics sense) is detectable.)
“Sufficiently advanced” tech could also plausibly identify all those hidden civilizations. For example, an underground civilization would produce unusual seismic activity, and taking up some inner portion of a gas giant or star would alter their outward behavior. Ultimately, civilizations use mass-energy in unnatural ways, and I don’t see a fundamental physical principle that could protect that from all possible sensing.
More importantly, I don’t think your suggestions address my point that hostile civilizations would get you before you even evolve.
But, let’s grant that you’re the first civilization to evolve in your galaxy, or at the least among the first before someone starts sending out probes to prevent any new civilizations from arising and threatening them. And let’s grant that they will never find you. That is a victory, in that you survive. But the costs are astronomical: you only get to use the mass-energy of a single planet, or star, or Oort Cloud, while someone else gets the entire galaxy.
To put it another way: mass-energy is required for your civilization to exist and fulfill its preferences, so far as we understand the universe. If you redirect any substantial amount of mass-energy away from its natural uses (stars, planets, asteroids), that’s going to be proportionally detectable. So, you can only hide by strangling your own civilization in its crib. Not everyone is going to do that; I seriously doubt that humanity (or any artificial descendant) will, for one.
(This comes back to my link about “no stealth in space”—the phrase is most commonly invoked when referring to starships. If your starship is at CMB temperatures and never moves, then yeah, it’d be hard to detect. But also you couldn’t live in it, and it couldn’t go anywhere! You want your starship—your civilization—to actually do something, and doing work (in a physics sense) is detectable.)