A major threshold concept

Over coffee this morning, Peter, Juliet and I were discussing last night's TV; so of course we talked about Horizon on gravity. Peter pointed out that "spacetime" (clearly all one word and sometimes referred to as "the spacetime"), is clearly a threshold concept—and troublesome knowledge—for understanding post-Newtonian physics.

How did the programme do in getting over this critical idea? Do respond!

You can watch the whole programme again for the next six days, or download it and keep it for a month, from the link.