According to Einstein's picture of the universe, space and time are curved and very closely related to each other. This means that traveling through time would be much like traveling through a tunnel in space—in which case you'd need both an entrance and an exit. As a time traveler, you can't visit an era unless there's already a time machine when you get there—an off-ramp. This helps explain why we're not visited by time-traveling tourists from our own future. Futuristic humans don't drop in for dinner because we haven't yet invented time travel.The main I thing I take from this is that as a historical device, time machines actually are crap. If you want to go to the future, great. If you want to see dinosaurs, go to the Field Museum.
The whole article is interesting. Free will takes a bit of a beating, but your grandpa should feel good about it. Unfortunately, time traveling dudes that will someday be leaders of the world may not actually be able to marry most excellent medieval princesses. Heinous.