Editing Drake equation (section)

====Fraction of the above revealing their existence via signal release into space, {{math|''f''<sub>c</sub>}}====
For deliberate communication, the one example we have (the Earth) does not do much explicit communication, though there are [[Active SETI|some efforts]] covering only a tiny fraction of the stars that might look for human presence. (See [[Arecibo message]], for example). There is [[Fermi paradox#They choose not to interact with us|considerable speculation]] why an extraterrestrial civilization might exist but choose not to communicate. However, deliberate communication is not required, and calculations indicate that current or near-future Earth-level technology might well be detectable to civilizations not too much more advanced than present day humans.<ref>
{{cite journal
 |last1=Forgan |first1=D.
 |last2=Elvis |first2=M.
 |year=2011
 |title=Extrasolar Asteroid Mining as Forensic Evidence for Extraterrestrial Intelligence
 |journal=[[International Journal of Astrobiology]]
 |volume=10 |issue=4 |pages=307–313
 |arxiv=1103.5369
 |bibcode=2011IJAsB..10..307F
 |doi=10.1017/S1473550411000127
|s2cid=119111392
 }}</ref> By this standard, the Earth is a communicating civilization.

Another question is what percentage of civilizations in the galaxy are close enough for us to detect, assuming that they send out signals. For example, existing Earth radio telescopes could only detect Earth radio transmissions from roughly a light year away.<ref>{{cite journal |title=The Search for Extraterrestrial Intelligence (SETI) |journal=Annual Review of Astronomy and Astrophysics |first=Jill C. |last=Tarter |author-link=Jill Tarter |volume=39 |pages=511–548 |date=September 2001 |doi=10.1146/annurev.astro.39.1.511 |bibcode=2001ARA&A..39..511T|s2cid=261531924 }}</ref>