catch = ?

Context:
Still, there are other options. Amos Ori, a physicist at the Technion in Israel, has been working on a time-travel scheme in which all you need is to harness the power of gravity to build a time-travelling path through space-time. It's also conceivable that the universe has done this on its own, so that our 23rd-century travellers could fly through a warped region of space-time and eventually pop out far in the past.


The catch, though, is that they wouldn't be able to control where and when they'd be going. The idea that Santa's friends from the future would be able to even land on Earth is seriously suspect.