how do we find relativistic alien rockets?

Say that somewhere out there is a species of space-faring aliens which have relativistic rockets or warp drive technology that lets it travel between solar systems. Considering the sheer size of the universe, it’s probably a good bet that at least one exists. And as these aliens are tooling around, their spacecraft will likely leave what we could call a wake in the fabric of space and time, a wake that we could observe under the right conditions, when the stars align. This is the main gist of an arXiv paper which considers that despite the possibilities of a successful detection of an alien craft’s fly-by being almost nil, we could still try just in case we do get lucky.

To start a long term survey, we just need to find star pairs close to each other and aligned with the Earth at about the right angle to give us a good view of the space between them. Then we just look and wait for something to show up, ideally a smear of light magnified by the relativistic wake of the spacecraft we’re trying to detect. It’s a neat idea and the authors readily acknowledge that we may just be too far away to notice alien travelers, or be in a region of space where there are no civilizations capable of interstellar travel, which keeps them grounded when discussing such a lofty SETI approach. But there is one thing they may want to explore a little further…

When we last discussed the Icarus project, did you notice the sheer size of the probe being considered? Go and have a look at that monstrosity and note that the Empire Stare Building does not look all that much bigger by comparison. That’s not because Icarus’ designers have a thing for really large spacecraft, it’s because this craft will have to carry so much fuel and have giant engines to accelerate. Any future interstellar craft designed to support humans, would be even bigger than Icarus to carry all the essentials across trillions and trillions of miles.

Let’s say that at some point, we’ll actually decide to build a ship able to ferry humans between the Sun and Alpha Centauri at relativistic speeds, and equip it with a brand new, state of the art artificial black hole engine which should get us up to relativistic speeds very, nicely, shaving the travel time down to only a couple of years instead of several millennia. We’d need to build something much like the Burj Khalifa tower in Dubai to house all the things necessary to comfortably support and house our crew, then get another pair of similar structures and devote them to being engines and fuel tanks, and at least another one to function as a backup tank and to securely house all the shuttle craft that will let the crew go down to the surface of their target world because that giant assembly is simply never going to be able to land. It’s far too huge and heavy. And keep in mind that these estimates are probably erring on the small side, relying on a radical propulsion system.

Now, our imaginary spaceship which we’ll call something inspiring, say, The Really, Really Huge, would have an approximate mass of 2 million tons empty and without the micro black hole suspended between the giant engines armed with nuclear lasers and fuel. The black hole would add at least another million tons and all of its fuel, all the relevant supplies, and supporting spacecraft would bring the total mass of our interstellar craft to something in the neighborhood of 4 million tons. Depending on its configuration, it could be close to 1,000 or so meters long which is just about two thirds of a mile, and about a quarter of a mile across. Sounds huge and very, very expensive, doesn’t it? And this baby goes from zero to ~0.5c in just 6.3 months!

How could alien astronomers not notice something like that screaming through the voids of space, warping the photons from the sunlight behind it and leaving a high speed smear in the spectrum of our sun on its way out? Well, for the size and speed of this thing, you have to remember that its traveling through space and as such is tiny if we’re going to compare it to the kind of objects telescopes can actually resolve. We have trouble imaging gas giants in other solar systems, gas giants which are 50,000 times bigger than our hypothetical ship. Sure, its wake is going to affect how the spectrum of a star looks but the warping would be so tiny that it may not even be visible as an artifact of the imaging process, the tiniest fraction of a pixel across, smaller than an exomoon.

And that’s the real gotcha in an otherwise interesting plan. Even if you’re lucky enough to catch an alien ship in the middle of crossing between two nearby solar systems and snap that one in a quadrillion shot, how exactly do you prove that this microscopic smudge in the spectrum is the trail of an extraterrestrial spacecraft? What says it wasn’t dust in the air or atmospheric fluctuations at the time of the shot? Even if you take a picture with an orbital telescope to avoid having a stray air particle from blotting out a snapshot of a relativistic craft, there’s still the potential of a microscopic speck of space debris or a wandering electron to mess with the shot.

If the alien species in question built a ship the size of Mercury and flies past our solar system, we’d probably have some chance of catching their relativistic wake by happenstance. Otherwise, the ship will be just too small for a proper identification, if would even register in the image in the first place. Likewise, if we set our sights on a few dozen nearby stars floating close to each other, we wouldn’t necessarily boost our odds of seeing aliens traverse between them since we have no guarantee that they would evolve and thrive in those systems, just a vague estimate of probability that a planet supporting life in general may exist there. It seems that if we’ll ever catch ET mid-flight, it would’ve had to buzz our telescopes on its way to planets unknown…

See: Garcia-Escartin, J.C., et al. (2012). Scouting the spectrum for interstellar travelers arXiv: 1203.3980v1