[hermitcrab]

So a probe travelling between galaxies at 99%c can't hit anything bigger than 1.46×10−12 kg? It is ~2.5 million light-years between us and Andromeda. I don't know how much matter there is between the galaxies (does anyone?). But travelling that sort of distance without hitting anything above that tiny mass seems very unlikely. Also how is your tiny robot probe supposed to power any guidance or maintenance systems for that sort of duration of trip?

[zubspace]

The paper uses a minimal weight of 30g and surface area of 1cm² for a replicator. On page 13 it says that you need at least 40 of them to travel through intergalactic space, but also states that there is more mass floating around in interstellar space. The probes should create muliple staging areas.

I assume if such a replicating probe would really be possible, it should be straightforward to just send of a million of them and hope for the best.

In my opinion, a huge limiting factor is communication. How do you know if those probes reached their target?

There's also an ethical aspect to it. Should we really fill the universe with self replicating paperclips? Because once you start the process, when and how would you stop it?

[hermitcrab]

>The probes should create muliple staging areas.

How do you create a staging area outside of a galaxy? There wouldn't be enough matter or energy locally available to do anything useful.

>In my opinion, a huge limiting factor is communication. How do you know if those probes reached their target?

Presumably you wait millions years for a return probe.

>Because once you start the process, when and how would you stop it?

Indeed. These probes are going to be so far apart they are unlikely to be able to communicate with each other in any meaningful way.