We all think we know what gravity is. But where gravity comes from stumped Newton, and 300 years later we are no closer to an explanation. We don't even have a mathematical account of gravity that applies in all situations. Why is a force so central to the universe so elusive in its character?
Theoretical physicist Laura Mersini Houghton, Nobel Prize-winner Frank Wilczek and string theorist Eric Verlinde examine the enigma of gravity.