Interesting point that at some point your observing apparatus gets good enough that you can 'see' the structures built by sufficiently advanced civilizations (sure they cloak their ships in orbit but you can see how they make their home world comfy!)
At one of the SETI seminars there was a discussion about when would be the "right" time to alert a newly discovered intelligent species that they aren't alone in the universe. There was a lot of back and forth about indigenous tribes in the Amazon, some of who learned of other tribes by the arrival of missionaries, some by loggers, and some who were out walkabout and came upon the strangers. How you meet outsiders has a different impact on how it affects you.
So if you were aliens and you didn't want to 'alarm' or 'damage' humans, what would you use as a signal that it was probably a good time to say "Hello" ? I've always felt that once you could detect they were having conversations on other planets you would now "know" we weren't alone and someone could appear in orbit and say Hi. Others felt it would only be safe if humans felt reasonably confident in their own ability to meet them at their level (so perhaps at least colonies on other solar system bodies). One person at our table was firmly in the only when it is unavoidable, which is to say they are about to send a probe to an inhabited planet or come across a construction like a station that is not easily concealed or moved.
Which is kind of in between of the positions you described in the comment.
It's one of those Fermi paradox answers that requires all of the supposedly millions or billions of intelligent species to all happen to have adopted one particular answer uniformly and for all time. Those answers aren't very compelling without a rock-hard explanation as for why it's absolutely mandatory that all star-faring civilizations will A: adopt that ideal immediately and B: keep that ideal intact for millions or billions of years despite all the changes they may go through.
The Prime Directive was a tool for creating drama, not a well-thought-out philosophical statement on the inevitably of certain ethics. Heck even in Star Trek itself, only the Federation has this policy, which it violates accidentally and otherwise fairly often anyhow. The other major civilizations show no sign of having this policy at all.
It is also an easy answer to try to explain the Prime Directive in a dramatic setting in such a way that our real world might still be in a space full of aliens, despite the fact that if space was really crawling with aliens there shouldn't be any such thing as virgin territory anymore and everywhere in the galaxy should be showing millions of years of proof of alien interactions, just as it's increasingly hard to find a square meter of Earth's surface that doesn't have some sort of unambiguous evidence of the presence of a technological civilization, if examined with powerful enough tools.
It's the 'figure out there are others' bit that made something like the Prime Directive problematic. Given our advances in sensing and observing technology we are much more likely to 'see' an advanced civilization before we can actually fly out to meet them. If you know that there is intelligent star faring people living on a 'nearby' star, and they happen to have been observing your progress, do they introduce themselves or wait for you to figure out how to reach them?
I don't think this is really true (though I could be wrong, I am not an astronomer). Sure, we may be able to "see" evidence of an advanced civilization, but without warp drive, we won't have real proof. All we'll have is some observational data showing that, for instance, the light from their host star dims in a really weird way that's unlike anything we've seen before in our observations, or that there's some object sitting in their L1 spot, etc. We can hypothesize that this data indicates artificial structures, because what natural phenomenon could possibly cause such things? But that's not proof; there really could be some natural thing causing those readings.
So the ETs don't really have to establish contact until we have warp drive and the actual ability to go see these artificial structures up-close. They can just leave us to sit around our planet wondering what these observations mean, and slowly come to grips with the idea that there may very well be some intelligent ETs out there with starshades, and argue over it for a few centuries as we'd surely do. They have no obligation to come visit us and clear up the matter. And considering how backwards, irrational, and violent a species we are, there's no good argument I see for the ETs to rush first contact with us, and every reason to delay it as long as possible.
Going to them with warp drive without landing and talking to them would be equivalent. What would you be able to do? Basically sense EM radiation from there with better resolution, but it's essentially the same thing, "seeing".
Finally, even if we could conclusively detect a civilization on a distant planet, that still doesn't explain why they have any obligation to contact us. They can easily just ignore us, and I for one can see why they would. The only time they'd really need to strongly consider establishing contact is when we have sufficient capability to visit them, and that's never going to happen the way we're going. It'd probably by prudent on their part to spy on us though, since we're a potential threat.
Pretty sure it only ever came up when it was being violated. A rule literally made to be broken, lol.
Prime-Directive-related conflicts, meanwhile, seem to revolve around cases where the Directive itself is flawed (or at the very least not ideal for a given situation).
It could be that we go 100,000 years without ever being able to reach other stars, but spend the entire time passing messages, like prisoners in individual jail cell passing notes to friends in nearby cells.
... Damn, that would be a really great setup for a sci-fi story...
(Those who have read the novel(s) in question will probably find my description of the Qeng Ho network incomplete at best, but I don't want to write too many spoilers.)
You don't say Hello if you are at 10 000 lights years away and have no means of traveling faster than light, because 10 000 years ago humans had no means of Radio communication anyway.
And for all we know, even if intelligent life arises everywhere in the cosmos, there is absolutely no model that says that intelligent life is supposed to evolve in the direction of mankind. After all, the Earth is full of intelligent life, well spread among numerous species (dolphins, crows, mammals, apes, etc...) yet only one species among them has been able to move to the next level. There could be many things happening in the meantime to prevent such a single species domination, and indeed for most of the time mankind was just another ape fighting for survival.
So, for me that's the big missing factor in Fermi's equation: species may not end up evolving in the same way that we did, and I can well imagine planets swarming with life yet having no mankind-like species that would be reaching out in the first place.
Especially if it involves anything more than a beeping solar sail.
(To be fair, I've only read The Three Body Problem, and I'm not sure how it evolves from there.)
(Of course the more general idea, that it might be smart not to be noticed until we know what's out there, goes back much farther.)
So yes, the goal here would indeed be to detect life capable of creating large structures and keeping them actively in orbit. There is some tongue-in-cheekness to this sort of research, but as the abstract notes, also some seriousness. The tongue-in-cheekness is that we're nowhere near being able to build something this large between Earth and the Sun; as the abstract notes, it has to be around as large as the Earth in order to usefully eclipse the Sun; that's a very very large structure, and we're certainly able to get into space right now: and yet we have no prospects even in the next many thousand years for engineering anything at that scale, unless folks are right about an impending technological singularity.
The seriousness, on the other hand, comes from one consideration: blotting out the Sun is probably easier than fleeing to nearby star-systems. It's very hard to say exactly how much energy it takes to flee to a nearby star-system, but a lower limit for the technology that I can presently envision is maybe 10^20 J of kinetic energy; my imagination might not be very good though, but I think that's reasonably conservative. Now the solar power irradiating a disk-that-blots-out-the-Sun is known to be 10^17 W or so. So the claim is, to even harvest the level of energy that is required to try and scout out neighboring star systems, one needs Sun-blotting scales of energy-harvesting technology; the length scale could maybe be smaller by a couple orders of magnitude to scout, but one can imagine that the energy needs of billions of entities fleeing their home planet would be several orders of magnitude larger, too.
If we want to later use the sun-blotter as a solar panel to power our interstellar exploits, then yeah.
If we just want to make the Earth a little cooler, we'd be far better off seeding the stratosphere with some dust, creating a mini 'nuclear winter' / 'Krakatoa effect' in a controllable way.
As with a lot of stuff in space, we assume a lot to be unlikely until we find out that it's not :)
I still estimate that by the time a civilization has planetary-scale engineering capability, that they won't need to make things like starshades.
If you have molecular nanotechnology, you can either adapt yourselves to whatever location you find, or just skip the biological body business, and directly upload your consciousness to a computer network.
The 2nd option is far more mass and energy efficient to support large numbers of sophonts, and I expect that any civilization to endure long enough will have the majority of its population living online instead of offline. If that even ends up being a thing, and they all don't just merge into a single entity (going in the direction of Star Trek's borg).
How do we even know what life looks like out there? Or what it has turned itself into in order to survive, travel and spread? Or even what it started out as?
We can't even talk with dolphins, even though we have a very recent evolutionary common ancestor. They're sentient life, tool-users, do we think they'd build a starshade to solve this problem? Shame we can't ask them, or understand their answer.
We know trees are alive, we know they communicate with each other. We don't think they're sentient. But how would we know? What would a sentient tree do differently from a non-sentient tree? What would a sentient tree do about the sun getting hotter?
Even if they started out like us, the evolutionary responses to living on other planets will change them. We're adapted to the conditions on Earth. If we live on other planets with different conditions then we will adapt, because that's what we do. We don't know how far those changes will go, because all the life we've ever seen has been adapted to Earth. Maybe high-gravity humans are less like sci-fi dwarves and more like Pratchett trolls.
We seem to be expecting sentient life out there to resemble us and have similar solutions for their problems. This seems dumb to me.
You'd think that the aliens might not be too keen to create huge beacon advertising their presence to all and sundry...
Humans have the ability to imagine a possibility for which no supporting evidence currently exists--that species alien to our solar system might exist. That ability might not be shared by the species we can imagine. That is, humans can imagine a species that cannot imagine humans. Such aliens would not even realize that building a planet-sized object that cannot be explained by natural processes might present an existential risk.
We, of course, are afraid of hypervelocity impact weaponry, even while having no evidence whatsoever that any such weapon exists anywhere in the universe.
Of course, that just means that any species that did fear annihilation by near-c rocks smashing into their planet would simply disguise their star shade so well that we could not detect it. Meaning that any solar shade we can detect was not likely built by any species able to think like us.
We could clean up by selling them insurance.
Therefore, instead of hiding, the aliens are going to pro-actively fry every competitor civilization by using their solar shades as a Nicoll Dyson Beam... :-)
Maybe we should be building ourselves one of these...
For our problems shades in Earth orbit would be just fine, and are probably the best solution overall, really, for dealing with climate change. Not just "man-made" climate change, but all climate change, full-stop.
Though I can't help but enjoy the dark humor of refusing to fix one somewhat hypothetical problem with a hypothetical solution because it might create a different hypothetical problem. ("Hypothetical" here simply in the sense that none of these things have actually happened yet.)
And before anyone chants "Ringworld is unstable" of course my rings's orbits would be constantly adjusted by using solar radiation pressure, electrodynamic tether thrusting and/or ion propulsion.
To be able to shade most of the planet would require ~100X the surface area of the L1 proposal. But using recent nanomaterials and the much smaller transportation distances for LEO might make up the difference in cost. In addition a much smaller coverage system might still be enough to keep critical areas of glacier and tundra below freezing for more of the year and keep the most critical tropical storm spawning areas of ocean cooler.
The general idea was to make lightweight thin films by metal vapour deposition onto wax (or something, iirc), followed by heating the wax to remove it. These films can be far thinner than what could survive launch from Earth and unfolding.
I'm not gonna reread it now, but say 50nm thick (which is above the knee of the opacity curve) times the cross-sectional area of the Earth gets you about 6 million cubic meters -- you wouldn't make one giant shade, but a lot of small ones; there's overhead for each one's control/power/cooling. Aluminum masses 2700kg/m^3, so we need to manufacture 17 million tons of solar sails (+ the overhead) somewhere up in space, I guess at a convenient asteroid that supplies the materials? And then navigate them to the Lagrange point, but solar sails are great for that.
Easy for me to say, but this sounds doable in this century if we wanted to enough. If you just want to reduce insolation by a few percent, then scale down the requirements to a few percent.
Added: I found the R. Angel paper the OP referenced, with the sunshade design: http://www.pnas.org/content/103/46/17184.full -- also a thin-film swarm, though not as thin, launched from Earth, and 20 million tons for 1.8% reduction.
Also, my sketch above left out stationkeeping overhead, which I was vaguely thinking could be small: rocking back and forth to alternately spiral out and in from your orbit. But that's wrong, it's not even in orbit. I'm not sure how well you can do.
Maybe because by the time a planet is able to send such signals, they have discovered better means, faster than light speed, communications.
So, the "Hello" communications have been coming to us for maybe millions of years, but like our planet before we understood radio, we can't detect their communications.