“…The newly discovered BCJ relations have been understood both from string [4,5,6] and field theory [7,8,9,10] perspectives. BCJ relations also serve as the key to the understanding of KLT relations [11], which express gravity tree amplitude in terms of products of two color-ordered Yang-Mills tree amplitudes (See [12,13,14,15]). Although a proof at loop-levels is currently absent, explicit calculations show that the duality (1.2) is also satisfied at the first few loops [16,17,18,19,20,21,22,23,24,25].…”