From Space.com, December 4:
What might we find: little green men or microbes? How might we find them: radio waves or strange chemicals in the planet's atmosphere? Something no one has even thought of yet?
Over the decades, scientists considering the possibility of life beyond Earth have pondered what such life might look like, how humans might be able to identify it from afar — and whether communication between the two worlds might be possible.
That thinking has included developing classification systems ready to fill with aliens. One such system is called the Kardashev scale, after the Soviet astronomer who proposed it in 1964, and evaluates alien civilizations based on the energy they can harness.
The Kardashev scale is a classification system for hypothetical extraterrestrial civilizations. The scale includes three categories based on how much energy a civilization is using.
Kardashev describes type I as a "technological level close to the level presently attained on the Earth," type II as "a civilization capable of harnessing the energy radiated by its own star" and type III as "a civilization in possession of energy on the scale of its own galaxy."
Each type also includes a numerical cut-off for the energy involved, but those weren't arbitrary cut-offs. "He used things that are easy to visualize," Valentin Ivanov, an astronomer at the European Southern Observatory who has built on Kardashev's work, told Space.com. "I'm almost tempted to say it's a publicity stunt, these comparisons that he uses to make it easier for people to understand."
Kardashev's scale is included in a five-page paper published in 1964 and called "Transmission of information by extraterrestrial civilizations." (The paper was originally published in Russian, but an English translation was published the same year.)
Although the scale is what caught people's imaginations, "Transmission of information by extraterrestrial civilizations" focuses on calculating how powerful a light signal from any point of the universe would need to be for radio scientists at the time to detect it. This value is also the numerical cut-off for the energy use of a type II civilization.....
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And related in an inverse sort of way, a post that almost got lost in the link-vault. What star systems can see earth transiting across the face of the sun?
From Notices of the Royal Astronomical Society, October 2020:
Which stars can see Earth as a transiting exoplanet?
ABSTRACT
Transit observations have found the majority of exoplanets to date. Also spectroscopic observations of transits and eclipses are the most commonly used tool to characterize exoplanet atmospheres and will be used in the search for life. However, an exoplanet’s orbit must be aligned with our line of sight to observe a transit. Here, we ask, from which stellar vantage points would a distant observer be able to search for life on Earth in the same way? We use the TESS Input Catalog and data from Gaia DR2 to identify the closest stars that could see Earth as a transiting exoplanet: We identify 1004 main-sequence stars within 100 parsecs, of which 508 guarantee a minimum 10-h long observation of Earth’s transit. Our star list consists of about 77 percent M-type, 12 percent K-type, 6 percent G-type, 4 percent F-type stars, and 1 percent A-type stars close to the ecliptic. SETI searches like the Breakthrough Listen Initiative are already focusing on this part of the sky. Our catalogue now provides a target list for this search. As part of the extended mission, NASA’s TESS will also search for transiting planets in the ecliptic to find planets that could already have found life on our transiting Earth ....
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Turnabout is fair play, especially at light-year distances. Sometimes I feel like waving at them but then remember a version of the prime directive from my mother when she was feuding with a neighbor:
Ignore them. Do not wave back....
From "In Other News: Possible Evidence of Extraterrestrial Intelligence"
Mom and the neighbor eventually made up but I've seen people smarter than I say something similar:"If we find ET, don’t talk to it, says the man who wants to find ET"
No.
This is a bad, bad, very bad idea.
There is no reason to think space aliens would be favorably disposed toward humans....
