Time Traveler Convention
A time traveler convention will be held at MIT on May 7. Apparently the organizers have in mind a branching universe model of time travel, otherwise this makes no sense:
Can't the time travelers just hear about it from the attendees, and travel back in time to attend?
Yes, they can! In fact, we think this will happen, and the small number of adventurous time travelers who do attend will go back to their "home times" and tell all their friends to come, causing the convention to become a Woodstock-like event that defines humanity forever.
Anyway, suppose no time travelers from the future show up at the convention. Does that decrease your credence in the physical possibility of time travel? If so, would your credence decrease by the same amount if the convention was (now) set to take place in the past, say on May 7, 2004? After all, there's little point announcing a time traveler conference in due time before the event.
Perhaps the time travelers will travel back in time to the convention, find it painfully boring, travel back to the future and then travel back in time again to make sure that they don't travel back to the convention in the first place. Thus, their absence will in fact confirm the possibility of time travel.