should we be looking for a dyson semaphore?

Could an alien civilization set up a way to communicate with us more visually than just sending a laser or microwave broadcast?

Another day, another out of the box SETI concept. This time, the idea is to use telescopes watching for a rocky world like ours passing in front of its parent star to detect alien billboards. While we couldn't directly see the object's shape, we could tell that it's not a planet or a clutch of asteroids based on how the dips in light would register to the telescope's sensors. Extending that thought further, we could assume that since we're actively targeting solar systems we think may have habitable worlds around it, a good, close look at some of them is just bound to produce signs of alien presence, one of which is an astronomically significant light show which an irregularly shaped object zooming around a star would produce to interstellar observers.

Adjust the object in orbit, the concept goes on to elaborate, and you could time its transits to communicate with binary codes. I have to give this idea a thumbs up for effort but a definite thumbs down for thoroughness. As nice as it would be to see alien species communicating with us by using a planet sized object as their canvas, I'm pretty sure that an extraterrestrial civilization certainly has use for the resources required for this kind of endeavor…

Let's put it this way. When last fall, a UN meeting debated whether to outlaw a sun shade intended to block a small percentage of light coming to our world to fight global warming, I crunched the numbers on what it will take to actually build one and the result was in the neighborhood of $840 trillion. And mind you, this is for an orbital assembly that's less than 1,100 kilometers across, definitely less than a planet sized feature which is capable of registering on a space telescope's optical sensor were it floating around an alien star.

To light up an observatory on another world, we'd need to create a structure at least six times as big, and probably more than ten times that expensive. Short of 120 checks made out to a team of several million engineers for all the money currently found on Earth turning up on some major VIPs' office desks, there's no way we can build this kind of beacon to the stars, and even if we did have the money, I'm sure we could certainly use it for a number of pressing problems first. Likewise, it seems only logical to assume than advanced extraterrestrials wouldn't exactly trip over their own feet/tentacles/hover pods housing their cybernetically enhanced bodies to build this sort of astronomical signaling device because of the sheer amount of effort it would take.

Even if we overlook what are likely to be the biggest barriers to this kind of active SETI — the cost and resource scarcity involved — there are other objections to consider. If you build a planet-sized sun shade to let any alien astronomer deduce your existence, what are the chances of it being seen and when will it be spotted? Would all the effort it will take to keep it from becoming a giant sail blown out of the solar system by stellar winds and the constant repairs after meteor and comet impacts be worth it? You can certainly have two alien species in search of other intelligent life and using telescopes to spot each other, but what are the odds that they'll exist at the same time and have the necessary technology to build these planet-sized beacons and detect them?

If we've only recently started looking at exoplanets after more than 100,000 years of existence as a species, can we really rely on a coincidence that'll put an advanced society really interested in talking to us close enough to spot their beacon, at just the right time for us to study the sky with space telescopes and positioning the giant semaphore at just the right angle for us to spot? And if this beacon is supposed to communicate with another species through how it makes sunlight dip on its orbital path, how can we crack the code? As we've seen, the seemingly simple binary signal is actually very tricky to decipher without a shared protocol.

Finally, stretching credulity to the limit and overlooking the issues of cost, resources, time, and difficulties any communication with an alien creature would entail, there's also a compelling military reason not to build vast beacons trumpeting your existence to the rest of the galaxy. Now, as said before, aliens aren't going to track you down for your resources sci-fi movie style, but if you put up an astronomical billboard, who knows what will show up and why? Any space faring species is more than likely to be armed to defend itself from anything unknown in deep space and there's nothing more ominous than two armadas from different worlds locked in an orbital dance, weapons at the ready and very possibly pointed at each other just in case.

Is this scenario a rather unlikely one? Probably yes. But can it happen? We don't know. Maybe there are interstellar marauders out there jumping from world to world in some bizarre attempt to claim a territory as their own. Again, we have no idea, and as of yet, not all that much ability to defend ourselves from just about anything we can encounter when we travel beyond our little planet. Other alien species may feel the same way and decide that the risks and costs far outweigh the benefits of putting up a giant astronomical beacon, sticking to less dangerous and more cost-effective SETI projects that let them find habitable worlds showing signs of civilization.