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why don't those probes plant themselves on various planets, mining/harvesting common resources, constructing more of themselves


Hell yeah! Allowing Von Neumann Machines (VNM) would significantly skew the results towards shorter time frames. And, one could argue, the simple shapes and non-contact of purported Alien UFO's would be consistent with a machine that is only entasked with a singular mission and constructed from a simply replicated design.

Bjork v2.0 ought to be very cool if all these other variables are taken into account!