The oft-repeated criticism of the official Search for Extraterrestrial Intelligence (SETI) is that they are searching for alien civilizations by scanning the sky for a single, very-20th-century-Earth-specific form of communication. As the late Terence McKenna once said, “To search expectantly for a radio signal from an extraterrestrial source is probably as culture bound a presumption as to search the galaxy for a good Italian restaurant.”
So, here’s a novel idea – search instead for their spaceships. No, I’m not talking about scanning the sky for UFOs – though some would say there’s merit in that too – I mean looking for radiation signatures from the energies that might just be powering inter-stellar spaceships. It’s actually not such a new idea though, as scientists were discussing this topic back in 1995. In a paper titled “Detection of Extraterrestrial Civilizations Via The Spectral Signature of Advanced Interstellar Spacecraft“, author Robert Zubrin put forward his suggestion that such a search may pay dividends, at least in certain cases:
This paper examines the possibility of detecting extraterrestrial civilizations by means of searching for the spectral signature of their interstellar transportation systems. Four methods of interstellar propulsion are considered: antimatter rockets, fusion rockets, fission rockets, and magnetic sails. The types of radiation emitted by each of these propulsion systems are described, and the signal strength for starships of a characteristic mass of 1 million tons traveling at speeds and acceleration levels characteristic of the various propulsion systems is estimated. It is shown that for the power level of ships considered, the high energy gamma radiation emitted by the antimatter, fusion and fission propulsion systems would be undetectable at interstellar distances. Bremsstrahlung radiation from the plasma confinement systems of fusion devices might be detectable at distances of about 1 light-year. Visible light emitted from the radiators of an antimatter-driven photon rocket might be detectable by the Hubble Space Telescope at a distance of several hundred light-years provided the rocket nozzle is oriented towards the Earth. The most detectable form of starship radiation is found to be the low frequency radio emissions of cyclotron radiation caused by interaction of the interstellar medium with a magnetic sail. A space-based antenna with a 6km effective diameter could detect the magsail emission of a characteristic starship at distances of up to several thousand light-years. Both photon rockets and magnetic sails would emit a signal that could easily be distinguished from natural sources. We conclude that the detection of extraterrestrial civilizations via the spectral signature of their spacecraft is possible in principle.
Okay, so there’s still a fair bit of 21st-century anthropic baggage involved regarding methods of propulsion, and building a space antenna with an effective diameter of 6km may not be high on the risk-reward ladder for investors. But still some fun, out-of-the-box thinking on how to find evidence that we’re not alone.