SETI founder Frank Drake wants to take the search for extraterrestrial intelligence to the next level by implementing a process called gravitational microlensing.
Microlensing is based on the gravitational lens effect: massive objects can bend the light of a bright background object. This can generate multiple distorted, magnified, and brightened images of the background source. More specifically, when a distant star or quasar gets sufficiently aligned with a massive compact foreground object, the bending of light due to its gravitational field leads to two distorted unresolved images resulting in an observable magnification. The time-scale of the transient brightening depends on the mass of the foreground object as well as on the relative proper motion between the background 'source' and the foreground 'lens' object.
In other words, Drake is essentially suggesting that we use our Sun as a 'giant magnifying glass' by positioning an observatory at a distance of around 500AU from it. Theoretically, the resultant microlense would be so powerful that we could see alien planets—and even their continents and oceans.
He contends that advanced extraterrestrial civs may have been doing this for millions of years already and we need to get with the program. Moreover, Drake says this isn't just a one-way system—gravitational lensing could be used to transmit signals to other worlds as well. Considering that our civilization's entire communications schema is about to go digital, he argues that this may be our best bet to communicate with our celestial neighbors.
Okay, now the bad news. The primary problem I have with Drake's suggestion, aside from the fact that it would take over a hundred years to set the crafts into position (which is more an issue of patience than a technical concern), is that the exercise would likely result in failure. Yes, such an observatory would undoubtedly help us discover more exoplanets—even those teeming with life. But it's unlikely that we'd receive any kind of communication by using it.
Among other things, the Fermi Paradox suggests that the timescales in question would not just allow for a civ to set-up and use gravitational microlensing, but to seed every solar system in the Galaxy with Bracewell probes. Sure, extraterrestrials could set microlenses up, but if they're capable of that feat then they're not too far from being able to send out swarms of self-replicating Bracewells.
Again, like I've harped on time and time again, if there are advanced civs out there, and they've wanted to communicate with us, they would have done so by now.
I'm not suggesting that we bail on Drake's project. Quite the contrary. Let's do it. Let's set up this microlense and see what we get. A negative data point can be just as useful as a positive one. And maybe it'll help us discover Dyson Spheres or other megastructures. In addition, the astrological benefits of such an observatory would be incalculable, so it wouldn't be a complete waste by any means.
We just need to temper the expectations of the contact optimists out there, of which Frank Drake is one.