There are four main approaches to the search for extraterrestrial intelligence (SETI), and we tend to channel most of our energy and resources into only one of them: electromagnetic signals.
EM signals are special transmissions of radio and/or light that could have been generated by aliens. But, as Forbes noted recently, we also have the ability to study gravitational wave signals, neutrino signals, and macroscopic probes — and it would be a mistake to ignore these pieces in the hunt for other lifeforms.
We focus on EM signals, in part, because we believe that aliens might be like us: They’ve discovered how to harness the energy of radio waves, and are able to push those signals out into space. Groups like the SETI Institute, and METI International, are dedicated to surveying optical and radio signals and parsing them for signs of alien life (spoiler: pretty much never aliens). They remain the most promising indication we can conceive of — and act on — in terms of locating alien-built civilizations and megastructures. Still, sometimes you want to keep your options open.
Gravitational wave signals
The technology to measure gravitational waves, is helping us to understand the age and origin of the universe. But if aliens have outfitted their home planet with loads and loads of satellites, then we might be able to pick up those satellites with gravitational wave detectors. This is a nascent field, full of nascent technology — we only detected gravitational waves for the first time last year, after a century of searching for them — but it’s absolutely worth keeping an eye on. Unlike EM signals, gravitational signals don’t degrade. As we continue to make technological breakthroughs, this field might increasingly become the one we turn to for our alien-hunting projects.
If you believe nuclear power is the future (for aliens, not just us), then this is the theory for you. Nuclear fusion produces a neutrino signature that would be immediately identifiable as artificial, as opposed to naturally occurring (like what the Large Hadron Collider is capable of doing).
Neutrinos are still the second-most common particle in the universe after photons. If we can calibrate our technology precisely enough to search for that signal, we should be able to pick it up pretty much anywhere.
Macrobiotic space probes
This encompasses everything from small satellites to giant stellar megastructures. As Forbes explained, this one is the most fun to imagine, but probably the least valid from a scientific standpoint. Stars are really far apart, guys! Even if aliens were building probes and launching them directly at Earth, the chances that we’d be able to find them are slim. We’re better off surveying large swaths of sky for signals like the ones above, or, when possible, looking at the surface of promising exoplanets to see if they’ve been altered in some way.