von Neumann probes.

If a civilization could make von Neumann probes, how long would it take them to cover the Milky Way Galaxy?

It depends on how fast the probes travel, how long it takes them to self-replicate, and how many times they can self-replicate. But even if I knew all this, I couldn't make an estimate.

What would be a reasonable travel speed? Humanity's fastest ever spacecraft is the New Horizons probe. It travels at around 16.3 kilometers per second (km/s), with lots of caveats. The speed is relative to the Earth. Voyager I travels faster relative to the Sun. In any case, fast as this is, it's slow when considering interstellar distances. New horizon's travels at a bit over 0.005% the speed of light. At that rate it would reach Proxima Centauri in about 90,000 years.

So we need more speed. New horizons was launched by an Atlas V rocket, and did not, as previous probes, get a boost from gravitational slingshot maneuvers. So no doubt we could have made it go faster, but hardly faster enough to make an appreciable difference in interstellar distances. We may not be at the end of the potential for chemical rockets, but we're nearing it.

Other means of propulsion include solar sails, ion rockets, and nuclear rockets. They all have advantages and disadvantages. The most notable advantage right now belongs to the ion rocket, and that's because a few have been made and at least one actually flew in a NASA probe.

The theoretical speed limit for each is 0.9999999999999999999999999999999...9999999 c, but that's the theoretical maximum speed of anything containing mass. A more reasonable limit, given a probe can't carry unlimited quantities of fuel or propellant and cannot be of unlimited size, is perhaps 0.01 c That gets you to Proxima in 450 years or so. Or maybe as high as 0.2 c for solar sails, assuming a gigantic laser battery to push them along. That gets you to Proxima in just 23 years or so.

Quote: Wizard

There is a term for this but I think that the "survival of the fittest" element of evolution causes us to kill ourselves once technology reaches a certain level. The average man with a little preparation can kill hundreds. In 100 years, it may be millions.

That might be one of the solutions to the Fermi Paradox.

Quote: Pacomartin

That might be one of the solutions to the Fermi Paradox.

I've read Asimov's "Extraterrestrial Civilizations." But that was a long time ago. I think he did touch on the various "filters" which are all the rage these days.