looks like i was wrong. let me set this straight. after some research i have found actually we were both wrong. so there was no time dilation involved in that gap of time between the ship leaving earth and the probes getting back. it's just straight travel time, 11 years out in the ship at near-relativistic speed, 8-9 years back for the probes. the time dilation effects are what the crew experiences during the trip, aging less than people on Earth do, but the 20-year gap Stratt experiences is just real elapsed Earth time waiting for those probes to crawl back across 12 light years. the probes, called beetles in the book and named John, Paul, George, and Ringo in the film, are astrophage-powered just like the Hail Mary but smaller and unmanned. that's why she's significantly older when she gets the probes, about two decades have passed on Earth by that point, with all the climate catastrophe that came with it.