The time required for the other to respond is measured. The two photons are, according to quantum mechanics, so entangled with each other that one reacts instantaneously to the other as though they were one single entity. One of the most excellent works of Greek science is Phaenomena, written by Aratus the Cilian circa 276.bc. Aratus took technical material and put into verse. Phaenomena was so admired that Virgil took it as direct inspiration for his Georgics, some of the most fragrant nature poems in all Latin writings.
Nothing is too wonderful to be true.