On Tuesday, 18 September 2018 05:05:39 UTC-4, Martin Brown wrote:
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.

That's assuming they don't purposefully continue to radiate to make contact. There was a radio broadcaster using 500,000 watts in the 1930's at about 900KHz. Not sure what the power and frequency would be 4-20LY out.