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In the early 1900s, Albert Einstein developed an idea - called Relativity - that changed our understanding of reality. It explained how both space and time were flexible - and how the Universe was made of a four-dimensional fabric called space-time. This single idea gave us a new way to understand the force of gravity, explained how the stars were born and introduced us to the concept of the big bang. And, in the hands of Stephen Hawking, it allowed us to understand the most extreme monsters in the Universe - black holes. This two-part BBC documentary explores how two of the most famous scientists of the 20th Century transformed our understanding of the Universe - thus changing the world.