Editing Fermi paradox (section)

==== Lack of resources needed to physically spread throughout the galaxy ====
{{See also|Project Daedalus|Project Orion (nuclear propulsion)|Project Longshot}}
The ability of an alien culture to colonize other star systems is based on the idea that interstellar travel is technologically feasible. While the existing understanding of physics rules out the possibility of [[faster-than-light]] travel, it appears that there are no major theoretical barriers to the construction of "slow" interstellar ships, even though the engineering required is considerably beyond existing human capabilities. This idea underlies the concept of the Von Neumann probe and the Bracewell probe as a potential evidence of extraterrestrial intelligence.

It is possible, however, that scientific knowledge cannot properly gauge the feasibility and costs of such interstellar colonization. Theoretical barriers may not yet be understood, and the resources needed may be so great as to make it unlikely that any civilization could afford to attempt it. Even if interstellar travel and colonization are possible, they may be difficult, leading to a colonization model based on [[percolation theory]].<ref name=landis>{{cite journal|author-link=Geoffrey A. Landis|author=Landis, Geoffrey|url=http://www.sff.net/people/Geoffrey.Landis/percolation.htp|title=The Fermi Paradox: An Approach Based on Percolation Theory|journal=Journal of the British Interplanetary Society|volume=51|pages=163–166|date=1998|issue=5|bibcode=1998JBIS...51..163L|access-date=June 6, 2004|archive-url=https://web.archive.org/web/20060927154115/http://www.sff.net/people/Geoffrey.Landis/percolation.htp|archive-date=September 27, 2006|url-status=live}}</ref><ref name=kinouchi>{{Cite journal| last1=Galera| first1=E.| last2=Galanti| first2=G. R.| last3=Kinouchi| first3= O.|title = Invasion Percolation Solves Fermi Paradox but Challenges SETI Projects|journal = International Journal of Astrobiology |volume= *| issue=4|pages= 316–322|date=2018| doi=10.1017/S1473550418000101| s2cid=126238563}}</ref>

Colonization efforts may not occur as an unstoppable rush, but rather as an uneven tendency to "percolate" outwards, within an eventual slowing and termination of the effort given the enormous costs involved and the expectation that colonies will inevitably develop a culture and civilization of their own. Colonization may thus occur in "clusters", with large areas remaining uncolonized at any one time.<ref name="landis" /><ref name="kinouchi" />