IS...
Speaking purely philosophically, IF it is ever possible to invent a machine that can travel to this time and place, then time travel ALREADY is POSSIBLE for this time and place.
If it is NOT, then it is not.
Now, it is another matter whether it is or will ever be PRACTICABLE...
(Consider the discussion of civilisation-ending crazy physics experiments under the LHC thread. Maybe the invention of a true time machine would be a Universe-ending experiment by breaking the boundaries of space-time and unravelling the threads that support our existence?)
