Wheeler invoked this time-reversal concept to explain the identical properties shared by all electrons, suggesting that " they are all the same electron " with a complex, self-intersecting world line. Yoichiro Nambu later applied it to all production and annihilation of particle-antiparticle pairs, stating that "the eventual creation and annihilation of pairs that may occur now and then is no creation or annihilation, but only a change of direction of moving particles, from past to future, or from future to past." The backwards-in-time point of view is nowadays accepted as completely equivalent to other pictures, but it has nothing to do with the macroscopic terms "cause" and "effect", which do not appear in a microscopic physical description.