Editing Drake equation (section)

====Lifetime of such a civilization wherein it communicates its signals into space, {{math|''L''}}====
[[Michael Shermer]] estimated {{math|''L''}} as 420 years, based on the duration of sixty historical Earthly civilizations.<ref name="Why ET Hasn’t Called">
{{cite journal
 |last=Shermer |first=M.
 |date=August 2002
 |title=Why ET Hasn't Called
 |url=http://www.michaelshermer.com/2002/08/why-et-hasnt-called/
 |journal=[[Scientific American]]
 |volume=287
 |issue=2
 |page=21
|bibcode=2002SciAm.287b..33S
 |doi=10.1038/scientificamerican0802-33
 }}</ref> Using 28 civilizations more recent than the Roman Empire, he calculates a figure of 304 years for "modern" civilizations. It could also be argued from Michael Shermer's results that the fall of most of these civilizations was followed by later civilizations that carried on the technologies, so it is doubtful that they are separate civilizations in the context of the Drake equation. In the expanded version, including ''reappearance number'', this lack of specificity in defining single civilizations does not matter for the result, since such a civilization turnover could be described as an increase in the ''reappearance number'' rather than increase in {{math|''L''}}, stating that a civilization reappears in the form of the succeeding cultures. Furthermore, since none could communicate over interstellar space, the method of comparing with historical civilizations could be regarded as invalid.

[[David Grinspoon]] has argued that once a civilization has developed enough, it might overcome all threats to its survival. It will then last for an indefinite period of time, making the value for {{math|''L''}} potentially billions of years. If this is the case, then he proposes that the Milky Way Galaxy may have been steadily accumulating advanced civilizations since it formed.<ref name="David Grinspoon 2004">
{{cite book
 |last=Grinspoon |first=D.
 |year=2004
 |title=Lonely Planets
}}</ref> He proposes that the last factor {{math|''L''}} be replaced with {{math|''f''<sub>IC</sub> · ''T''}}, where {{math|''f''<sub>IC</sub>}} is the fraction of communicating civilizations that become "immortal" (in the sense that they simply do not die out), and {{math|''T''}} representing the length of time during which this process has been going on. This has the advantage that {{math|''T''}} would be a relatively easy-to-discover number, as it would simply be some fraction of the age of the universe.

It has also been hypothesized that once a civilization has learned of a more advanced one, its longevity could increase because it can learn from the experiences of the other.<ref name="GoldsmithOwen">
{{Cite book
 |last1=Goldsmith |first1=D.
 |last2=Owen |first2=T.
 |year=1992
 |title=The Search for Life in the Universe
 |edition=2nd |page=415
 |publisher=[[Addison-Wesley]]
 |isbn=1-891389-16-5
}}</ref>

The astronomer [[Carl Sagan]] speculated that all of the terms, except for the lifetime of a civilization, are relatively high and the determining factor in whether there are large or small numbers of civilizations in the universe is the civilization lifetime, or in other words, the ability of technological civilizations to avoid self-destruction. In Sagan's case, the Drake equation was a strong motivating factor for his interest in environmental issues and his efforts to warn against the dangers of [[nuclear warfare]]. Paleobiologist [[Olev Vinn]] suggests that the lifetime of most technological civilizations is brief due to inherited behavior patterns present in all intelligent organisms. These behaviors, incompatible with civilized conditions, inevitably lead to self-destruction soon after the emergence of advanced technologies.<ref name=vinn2024>{{cite journal|last=Vinn|first=O.|date=2024|title=Potential incompatibility of inherited behavior patterns with civilization: Implications for Fermi paradox|journal=Science Progress|volume=107|issue=3|pages=1–6|doi=10.1177/00368504241272491|pmid=  39105260|s2cid=  |doi-access=free|pmc=11307330}}</ref>

An intelligent civilization might not be organic, as some have suggested that [[artificial general intelligence]] may replace humanity.<ref>{{cite news |author=Sulleyman |first=Aatif |date=2 November 2017 |title=Stephen Hawking warns artificial intelligence 'may replace humans altogether' |work=independent.co.uk |url=https://www.independent.co.uk/life-style/gadgets-and-tech/news/stephen-hawking-artificial-intelligence-fears-ai-will-replace-humans-virus-life-a8034341.html}}</ref>