That's true, and stealth is an interesting proposition. However, there are probably easier methods of preventing detection, such as not building a giant structure in the first place. Also, even if they manage to radiate away most of their waste heat, they'd have to assume eventual detection by an advanced civilization. Sure, with exponential effort you can always get infinitesimally closer to being virtually undetectable, but is that really feasible?
In the end, it's certainly not impossible that a civilization would go to extreme lengths in order to hide its enormous artificial structures. But the real issue is: why would all civilizations hide in this way? And if it turns out they do all have elaborate stealth tech in operation, that means there must be an incomprehensibly huge threat out there - which in itself might be the answer to why nobody is calling us up.
Revelation space is a speculative fiction novel set in the far future during the golden age of space colonization. Spoilers ahead.
Throughout the novel there's alien civilizations that use different kinds of way to cloak constructs and settlements. Examples include devices like artificial moons and holographic, multi-spectral holograms that emulate gas giants to hide stuff inside.
The one I think andrewflnr may be referring to is a device that one race used to cloak entire regions of space to hide their civilization from technologically superior aggressors. Essentially they set up variable-gravity devices at strategic points around their region of space, and had the devices resonate to build 'waves' of gravity that would constructively interfere at a massive scale at the borders of their space, which would essentially tear any intruders apart using the amplified+alternating gravity waves. Several million years in the future, observers/scientists of other races on the outside would dismiss these chaotic space regions as an unexplained anomaly.
Mostly, I was thinking of that whole theme in relation to the
"incomprehensibly huge threat" thing Udo mentioned. If you're talking about Shrouders, I don't remember that method of using gravitational waves being mentioned. Did you hear that somewhere else?
In the end, it's certainly not impossible that a civilization would go to extreme lengths in order to hide its enormous artificial structures. But the real issue is: why would all civilizations hide in this way? And if it turns out they do all have elaborate stealth tech in operation, that means there must be an incomprehensibly huge threat out there - which in itself might be the answer to why nobody is calling us up.