One of the most remarkable features of nature is that a Quantum can pursue two different routes, through two different slits, come together and manifest itself as a single quantum. But nothing prevents one from saying that the quantum might be a photon, to speak of quantum of radiation, or it might be an electron, to speak of a particle. A quantum can go both routes or it can go a single route. And it's possible to choose which. After the particle has already made its travel, you choose - after the particle has decided whether it's going both routes or one route, and after it's got through, you yourself decide which it shall have done. You seem to intervene to change the past. But quantum theory says it can be done. And I had the pleasure to spell out some of the features of such an experimental arrangement. My University of Maryland colleague, Carroll Alley, made changes in the experiment, but without changing the principle, and carried it out. And it checked, so that we now know it is indeed true that one can decide, at the quantum level, whether an object shall go two routes to get to its final point or just one route. You can make the decision after it's already made the trip. That sounds like a contradiction, but it works.