Let’s put all of the political kvetching aside for one day and think about something really cool. By way of background, before I got interested in the law I thought I might grow up to be an astronomer. In high school I was a very good physics and calculus student—good enough that when I got to college I was tracked into the introductory math class for potential math majors. It was called “Elementary calculus.” “Elementary” was a word I thought I knew, but evidently I was wrong. I sat through two days of the class before dropping down to a lower-level course, which I finished, and which was the end of my formal career in math. What I didn’t understand at the time was that the math department was diabolical and was actively trying to weed people out. If I had known that the whole point of the “elementary” course was to weed people out, I might have stuck it out! Anyway, I still maintain a geeky interest in all things mathematical and astrophysical. One year for my birthday my wife prevailed upon a friend whose husband was a radio astronomer to give us a tour of his radio telescope. You get the idea.
In 2017, an object named ʻOumuamua was detected by a telescope in Hawaii. ʻOumuamua is a Hawaiian word for “scout.” Its trajectory around the sun was highly eccentric. Indeed, it was a hyperbolic trajectory, which means that its path is so eccentric that once ʻOumuamua passed around the sun, it would travel outward forever. To us on Earth it seemed that ʻOumuamua entered the solar system and flung itself around the Sun at great speed. But if you look at its motion from the perspective of the Galaxy instead of from the rather parochial perspective of our own world, you will see that ʻOumuamua is nearly at rest with respect to the Galaxy, and that we are traveling by it at a high speed!
Astronomers could tell, from looking at the eccentricity of the orbit, that ʻOumuamua had to be an interstellar object rather than one native to our solar system. Astronomers couldn’t get a very clear image of ʻOumuamua, but they were able to make some important observations. First, it didn’t seem to be a comet. It had no tail. It didn’t seem that any gas was escaping from it. Second, astronomers were able to infer, from the way ʻOumuamua got brighter and darker over time, that it was tumbling through space and that it was shaped like a cigar—it is much more elongated than any known object in the solar system. Third, they observed that ʻOumuamua was accelerating, and they could calculate that its acceleration was not due to gravity.
If ʻOumuamua was not throwing off gas like a rocket or a comet, what could cause it to accelerate? One clever hypothesis is that ʻOumuamua is a solar sail. People have considered building solar sails to power spacecraft, though it hasn’t happened yet. The idea is that the Sun gives off radiation, in the form of photons. This solar radiation exerts a pressure on an object that it hits, which varies according to how much light the object reflects and how much it absorbs. Why? Suppose the object is a perfect mirror that reflects everything. Photons hit it and then bounce back. Photons have no mass, but because they have energy, they do have momentum, just like familiar objects, and the total amount of momentum for the sail and the photon before they collide has to be the same as the momentum of the sail and the photon after they collide. So if you have this constant stream of photons hitting the sail and bouncing back, each imparting a tiny bit of momentum to the sail, and if you remember from your high school physics class that momentum is proportional to velocity, you’ll see that the photons bouncing back produce a forward thrust. (If the object is not reflective at all, and it absorbs all photons, then there is no thrust, because the photons do not bounce back).
Let’s run with this. What would ʻOumuamua have to be like, if it were a solar sail? A solar sail has to be very lightweight in order to allow for significant velocity. On the other hand, the greater the surface area for photons to hit, the more momentum the reflected photons can impart. In the case of ʻOumuamua, given what we know about its dimensions, it would need to be made of a thin sheet of material, less than a millimeter thick. Could there be a natural substance that exists in interstellar space but that we don’t see in the solar system that fits the bill? Maybe. Or maybe, one paper by Bialy and Loeb has suggested, it’s artificial—”debris from an advanced technological equipment,” like a piece of flotsam floating in the sea.
Loeb thinks this isn’t that likely, because if ʻOumuamua were just floating randomly in space, it is exceedingly unlikely that we ever would have seen it, unless such spacewrecks are very common. Maybe instead ʻOumuamua is a “reconnaissance” vessel. My own favorite possibility takes account of the fact that ʻOumuamua is nearly at rest with respect to the Galaxy. Maybe, Loeb suggests, it is like a buoy floating on the sea, one of many placed throughout the Galaxy for some purpose. I’d like to have a beer with this Loeb fellow.