科学美国人60秒:(Probably Not a) Giant Alien Antenna

The Earth is studded with telescopes, listening for electromagnetic radiation from the great beyond. And a decade ago, astronomers stumbled upon a mysterious signal: a powerful pulse of radio waves just a few thousandths of a second long. Mysterious because: 

"What is the nature of the sources?" Avi Loeb, a theoretical astrophysicist at Harvard. Whatever the sources are, he says, "they seem to be brighter by 10 of billions of times more than the brightest radio sources we know about." 

The radio pulses are known as "fast radio bursts"—and Loeb says you'd need something tens of billions of times brighter than a pulsar to produce them. So he and his colleague Manasvi Lingam investigated another possibility: "We know of one simple way to produce very powerful radio waves, and that's using a radio antenna."

A radio antenna built and controlled by extraterrestrials, to be more precise. Loeb and Lingam did the math on how big that stellar-powered radio antenna would have to be, to transmit signals like fast radio bursts. And whether it would even hold up from an engineering standpoint—like would it melt under its own heat?

Using those energy and engineering constraints, they found that the radio beam emitter would have to be twice the diameter of Earth. Pretty big—for us. But at least theoretically possible, he says, for more advanced civilizations. The study is in The Astrophysical Journal Letters. [Manasvi Lingam and Abraham Loeb, Fast Radio Bursts from Extragalactic Light Sails]

To be clear: this is definitely not proof that intelligent aliens exist. It's just proof of concept, that someone, smarter than us, could in theory build such a thing. And why you might build it? "The idea is that an advanced technological civilization could produce a beam of radio waves focused on a sail that is pushing on the sail, such that eventually the sail will reach a fraction of the speed of light." We'll just have to see what arrives first: alien sailors, pushed by radio waves? Or the technological advances to allow us humans to build such a thing ourselves.

—Christopher Intagliata