Active SETI, or messaging to extraterrestrial intelligence (METI), has taken many forms, from encoded radio signals to artifacts attached to space probes sent out of this solar system.
Critics of METI point out that alerting intelligent aliens to our existence may not be a good idea. Stephen Hawking has noted that meetings between advanced and less advanced human populations in the past have often not worked out well for the less advanced population.
Another means of detecting advanced alien civilizations would be to spot evidence of artificially modified environments. Giant structures such as Dyson spheres (as seen in “Star Trek: The Next Generation”) or habitable rings surrounding a star (as described in Larry Niven's novel “Ringworld,” 1970) would reveal great technological power to reshape a solar system. The lights of cities on a planet surface would indicate a civilization at or above our own level of development.
Some scientists estimate that life could have evolved very quickly after the Big Bang, perhaps when the universe was only 15 million years old. Life that evolved intelligence rapidly could therefore have a civilization that is now more than 13 billion years old.
Russian astronomer Nikolai Kardashev proposed in 1964 the Kardashev scale, a means of estimating possible levels of technology and resource usage:
The most basic technological civilization might be capable of searching radio waves for signs of intelligent transmissions. Carl Sagan wrote in “The Cosmic Connection” (1973) that Earth at that time was about 0.7 on the Kardashev scale (adjusted to 0.724 in 2012).
A civilization with the ability to utilize all the energy generated by a planet’s atmosphere, or about 1016 power watts.
A civilization capable of stellar engineering, including the building of a Dyson sphere surrounding a solar system. Energy utilization of about 1026 power watts.
A civilization capable of utilizing the energy of an entire galaxy. About 1036 power watts.
Beings who utilize the resources of the entire universe.
Physicist Enrico Fermi asked in 1950, if most stars have planets, and alien civilizations could be billions of years older than humans’, and if the galaxy could be explored and colonized within only about a million Earth years (even at sub-light speed) ... where is everybody?
Some possible refutations of the Fermi Paradox:
IT’S NOT A PARADOX: The fact that we don’t see evidence of aliens in space does not mean they do not exist. Aliens could be all around us, but we don’t know how to recognize them. Perhaps radio is not the best means of interstellar communications and nobody else uses it.
ALIENS DON’T BEHAVE HOW WE EXPECT: The assumption that alien civilizations would expand continually through the galaxy, and that their colonies would persist for millions or billions of years, could be wrong. Perhaps space travel is so difficult and expensive that nobody does it.
TECHNOLOGICAL CIVILIZATIONS COMMIT SUICIDE: Carl Sagan observed that even 20th-century humans had the ability to destroy all life on Earth with nuclear weapons. Perhaps all civilizations, once they reach a certain stage of development, wipe themselves out.
STARS GO BOOM: Gamma-ray bursts could flood the galaxy with deadly radiation from time to time. This would destroy all life that had managed to evolve.
THE WORLD ISN’T WHAT WE THINK IT IS: It has been proposed that the Earth’s solar system is in a “quarantine zone,” a protected zoo set up by aliens. Or perhaps we are living in a computer simulation a la “The Matrix.”
WHACK-A-MOLE: Vicious predators could roam the galaxy, wiping out any civilization that reveals itself. That would also be why we don’t hear any radio signals: Nobody’s sending them.
NOBODY’S HOME: Perhaps advanced technological beings move from normal space-time to some safer plane of existence, or implant their consciousness into machines that roam the space between galaxies.
WE’RE No. 1: Maybe humans are the first technological species to evolve in this galaxy.