How do the two people at opposite ends of the barn agree on when to close the doors as a runner approaches at relativistic speeds? Also, relevant xkcd (what-if

How do the two people at opposite ends of the barn agree on when to close the doors as a runner approaches at relativistic speeds? Also, relevant xkcd (what-if) https://what-if.xkcd.com/1/ https://what-if.xkcd.com/1/ Most likely from the runner's perspective the world turns white as they dissolve to plasma prior to reaching the barn.