On 17/09/2018 23:14, RichA wrote:
Radio signals radiate out in a spherical shell. Presuming (because
we have no evidence either way) that advanced civilizations would
have (if they formed like they did on Earth) started broadcasting
200-500 years ago, and given the number of stars in the vicinity,
we'd have picked up something by now. We haven't.

https://www.cloudynights.com/article...-paradox-r3162

An emerging civilisation is only likely to be radio bright and making
obviously non thermal modulated signal emissions for a relatively short
period of time from the point where they have high power transmitters to
the point where they adopt digital techniques and streaming.

That window for the Earth was narrower than 100 years. We occasionally
send powerful beamed emissions in random directions that Arecibo can
point at and illuminate chunks of sky whilst doing TDR imaging of near
Earth asteroids so every now and then the odd pulse still goes out.


There may be someone sat on Trappist-1 40ly away watching the Apollo
missions on our old terrestrial TV transmissions but it will be another
40 years before they could reply even if they decided to do so.



http://earthsky.org/space/3-potentia...ght-years-away


--
Regards,
Martin Brown