Ok, but the effect of the energy output of any civilisation is basically the same, and for the same reasons. It doesn’t matter if it’s a star or TARDIS whose inside is eternally expanding and you’re grabbing energy from its internal dark energy field, if you use that energy, you get hot, and that heat is visible.
Unless you can violate the laws of thermodynamics.
I wonder if advanced civilizations would use a lot of energy?
We always imagine they have very advanced physics and engineering, to do things like take apart planets to build mega-structures and things like that, but usually don't think about their other sciences.
My guess is that by the time they have gotten that far in physics, they have also gotten way ahead of us in biology. They'll have wiped out disease and illness, stopped aging, and only die by choice or accident. They'll have figured out geology and ecology and climatology and psychology.
I suspect that the final steady state for most civilizations that don't end up wiping themselves out by doing something stupid is a relatively small (by our standards) population of essentially immortal beings, living on a world they have restored to a largely pre-civilization state, using less energy by far that we are using but using it way more efficiently.
Maybe collect the heat and release it as a laser towards very sparsely populated parts of the sky? Perhaps that helps stabilize the sphere in the orbit of its star?
Also, a huge laser can certainly direct energy in a direction that nobody will notice. Unfortunately, creating the laser beam also creates waste heat, and that waste heat can be seen. Even collecting waste heat generates waste heat that you cannot collect.
On the other hand, if you want a huge laser for some other purpose (such as vaporizing distant planets), then a Dyson Sphere is the ideal way to create one.
> A Dyson Sphere is not a solid object. Instead, it's many small objects each of which is in a stable orbit.
Actually, we've never seen one so we have no idea how one might be engineered! It could for example be a "fog" of worlds that extends out past several local AU and to us would look nothing more like a dust cloud obscuring their local star. There wouldn't be a telltale signal of a Dyson Sphere, just another star with a big dust cloud.
Imagine a civilization like this, those closer to the star get more power literally, and those on the outskirts and in the shadows of the other worlds become dependent on the more inner worlds to re-radiate their absorbed energy, or to condense and lase the energy outwards to the shadow worlds...at some cost that limitless free energy can't pay for.
Beyond some distance the worlds become so cold that the inhabitants freeze to death and exile of your entire world at the whims of the inners becomes a real punishment. Dead worlds are recycled for mass for the growing population of the inners, or repurposed for other things.
There are billions of such worlds. Perhaps they are customarily shaped as small ringworlds and rotate in a complex manner to produce gravity and a daynight cycle. Dead or frozen worlds may have a reflective sail hoisted along the inner opening and expeditionary generation ships are sent out to nearby stars powered by lasers collected from hundreds of inner worlds.
Successful colonies may start to immediate transform the mass of nearby planetary systems into new "dust" clouds rather than settle on the planetary surfaces. Many adjacent Dyson spheres may look to us like just interstellar gases between several stars containing an unusual amount of organic molecules but could be the exchange of trillions of generation ships moving mass and energy back and forth between stars.
Such a civilization could eventually become nomadic in a way, moving from star to star as they burn out, leaving behind frozen husks of trillions of dead ringworlds.
Unless you can violate the laws of thermodynamics.