This illustration shows what the activities of a Kardashev Type III civilisation might look like - encapsulating the energy of stars by so-called Dyson spheres is one way to harness enormous energies on galactic scales; the resulting waste heat products should be detectable by telescopes. Image credit: Danielle Futselaar / ASTRON.
Michio Kaku 3 types of Civilizations
In 1964, Soviet astronomer Nikolai Kardashev proposed that a civilization's level of technological advancement could be measured based on the amount of energy that this civilization is able to utilize. He identified three types of civilizations called Type I, II, and III.
A Type I civilization can manage the entire energy and material resources of a planet. A Type II civilization is capable of harnessing the energy and material resources of a star and its planetary system. A Type III civilization is able to marshal the energy and material resources of an entire galaxy.
Theoretical physicist Freeman Dyson proposed in the 1960s that such advanced civilizations could be detected by the telltale evidence of their mid-infrared (IR) emissions.
Earlier this year, Roger Griffith of Penn State University and co-authors compiled a catalogue of 93 candidate galaxies - culled from a total population of 100,000 objects - where unusually extreme mid-IR emission is observed. One problem is that although rare, this kind of emission can also be generated by natural astrophysical processes related to thermal emission from warm dust.
Prof Garrett has used radio measurements of the very best candidate galaxies and found that the vast majority of these systems have emission that is best explained by natural astrophysical processes.
"The original research at Penn State has already told us that such systems are very rare but the new analysis suggests that this is probably an understatement, and that advanced Kardashev Type III civilizations basically don't exist in the local Universe," said Prof Garrett, author of a paper published in the journal Astronomy & Astrophysics (arXiv.org preprint).
"In my view, it means we can all sleep safely in our beds tonight - an alien invasion doesn't seem at all likely."
“In particular, the galaxies in the sample follow a well-known global relation that holds for almost all galaxies - the so-called 'mid-IR radio correlation."
"The presence of radio emission at the levels expected from the correlation suggests that the mid-IR emission is not heat from alien factories but more likely emission from dust - for example, dust generated and heated by regions of massive star formation."
According to Prof Garrett, his method could also be used to help identify less advanced, Kardashev Type II civilizations.
"It's a bit worrying that Type III civilizations don't seem to exist. It's not what we would predict from the physical laws that explain so well the rest of the physical Universe," Prof Garrett said.
"We're missing an important part of the jigsaw puzzle here. Perhaps advanced civilizations are so energy efficient that they produce very low waste heat emission products - our current understanding of physics makes that a difficult thing to do."
"What's important is to keep on searching for the signatures of extraterrestrial intelligence until we fully understand just what is going on."