"The greatest challenge for physics is to combine its two most successful theories: general relativity and quantum mechanics. The resulting quantum theory of gravity would explain the universe across all scales. Much has been said about the approach based on string theory. Here, Jim Baggott describes its powerful rival: loop quantum gravity. Today we are blessed with two extraordinarily successful theories of physics. The first is Albert Einstein's general theory of relativity, which describes the large-scale behaviour of matter in a curved spacetime. This theory is the basis for the standard model of big bang cosmology. The discovery of gravitational waves at the LIGO observatory in the US (and then Virgo, in Italy) is only the most recent of this theory's many triumphs. The second is quantum mechanics. This theory describes the properties and behaviour of matter and radiation at their smallest scales." - Copac