Thanks for a precious interview! I got sidetracked by what Dr Greene said about traveling back in time via a wormhole. So, if ever a time machine is invented, the earliest date it could take you to would be the day the machine was first activated. In other words, the past before the machine is invented will always be inaccessible. That's why we don't see time travelers from the future. People will start to see them once the machine is up and running.