The Hart–Tipler conjecture is the idea that an absence of detectable Von Neumann probes is contrapositive evidence that no intelligent life exists outside of the Solar System.
[1][2] This idea was first proposed in opposition to the Drake equation in a 1975 paper by Michael H. Hart titled "Explanation for the Absence of Extraterrestrials on Earth".
[3] Assuming that the probes traveled at 1/10 the speed of light and that no time was lost in building new ships upon arriving at the destination, Hart surmised that a wave of Von Neumann probes could cross the galaxy in approximately 650,000 years, a comparatively minimal span of time relative to the estimated age of the universe at 13.7 billion years.
Hart’s argument was extended by cosmologist Frank Tipler in his 1981 paper entitled "Extraterrestrial intelligent beings do not exist".
[4] The conjecture is the first of many proposed solutions to the Fermi paradox (the conflict between the lack of obvious evidence for alien life and various high probability estimates for its existence).