A theoretical physicist hid his dream of time travel for decades. Now, his time machine theory is forcing scientists to rethink what’s really possible.

Prof Mallett has wanted to build a time machine for most of his life. His passion, he explains, can be traced to a tragic event early in his life.

But according to Albert Einstein's general theory of relativity, which explains how gravity operates in the universe, real-life time travel isn't just a vague fantasy.