Al and Bert are identical twins. When Bert is 24 years old he travels to a distant planet at speed 12/13 \(c\), turns around and heads back at the same speed, arriving home at age 44. Al stays at home.

(a) How old is Al when Bert returns?

(b) How far away was the planet in Al's frame?

(c) Why can't Bert reasonably claim that from his point of view it was \(\mathrm{Al}\) who was moving, so that \(\mathrm{Al}\) 's clocks should be time dilated, making \(\mathrm{Al}\) younger than Bert when they reunite?