The question of whether intelligent alien life exists out there in the broader Milky Way galaxy could be answered by gravitational waves.
According to a new paper penned by an international group of scientists and engineers called Applied Physics, Earth-based detectors such as the Laser Interferometer Gravitational Wave Observatory (LIGO) should – theoretically – be able to detect the gravitational waves generated by extraterrestrial mega-technology.
This, the researchers say, would extend the search for extraterrestrial intelligence beyond Earth's closest neighbors to all the stars in the galaxy.
The paper, yet to be peer-reviewed, will be submitted to the Monthly Notices of the Royal Astronomical Society. It can currently be found on preprint server arXiv.
Whether Earth is alone in the Universe in hosting intelligent life (or even life at all) is one of the biggest open questions deviling humanity. But our ability to search for recognizable technology is limited to just a few tens of thousands of stars. The kinds of electromagnetic waves we use to communicate weakens, or attenuates, as it spreads, making it all but impossible to sift out of the noise beyond a couple hundred light years.
But gravitational wave signals are different. They don't attenuate in the same way electromagnetic signals do, and can be detected across larger distances. So if there was an intelligent alien civilization out there with technology that could generate powerful gravitational waves, well, maybe we could detect it.
Gravitational waves are pretty much the only tool we have to probe the cosmos that doesn't rely on light. They're generated by massive events; here on Earth, the gravitational waves we've detected have been produced by collisions between massive compact objects, black holes, and neutron stars. When these stellar corpses merge, the inspiral and collision send out ripples in the very fabric of space-time – a bit like the way a rock dropped in a pond generates tiny waves, except in all directions and at the speed of light.
Actually, any object with mass that accelerates produces gravitational waves. But the gravitational waves produced by something as powerful as a large rocket would still be too small for our current detection capabilities. So if an alien civilization did have technology capable of producing gravitational waves we could detect, it would have to be pretty freaking impressive.
Led by physicist Luke Sellers of the University of California Los Angeles, the researchers set about calculating the size and speed of an alien spacecraft that would be detectable by LIGO – what they named a Rapid And/or Massive Accelerating spacecraft (RAMAcraft).
They determined that LIGO is capable of detecting a RAMAcraft around the mass of Jupiter (that's 317.8 Earth masses, so we can rule out making one any time soon), with a warp drive that could accelerate to 10 percent of the speed of light. If such a spacecraft were to operate within around 100 kiloparsecs, or 326,000 light-years, of Earth, we could detect it.
Since the Milky Way disk has a diameter of up to around 260,000 light-years, that would comfortably cover the stars of our home galaxy.
"Our study of warp drives has paved the way for gravitational wave detection," says physicist Gianni Martire, CEO of Applied Physics.
"This new method is not limited to the traditional range of electromagnetic signals; thus we already have the ability to probe all 1011 stars in the Milky Way for warp drives, and soon, the ability to probe thousands of other galaxies."
Proposed gravitational wave detectors for future development, such as the DECi-hertz Interferometer Gravitational wave Observatory and Big Bang Observer, should be 100 times more sensitive than LIGO and could therefore expand the search volume by 106, the researchers say.
These are all pretty big ifs. But now that the groundwork has been laid, the team hopes to generalize their methods to search for smaller objects closer to home. These searches would turn gravitational waves into RAMAcraft Detection And Ranging (RAMADAR) units.
"Gravitational wave detection is a sophisticated science, though it is still in its infancy," the researchers write in their paper.
"As the methodology is further developed, the sensitivity of detectors may become such that the detection of these objects is a regular occurrence. In this spirit, it would be interesting to complete a fully-fledged search for these objects… We invite the scientific community to join us."
The research will be submitted to the Monthly Notices of the Royal Astronomical Society, and is available on arXiv.