But it gets weirder. The electron does not always take both paths. Sometimes it takes one path or the other. But this only happens when you try to find out which path it took! In fact, the more you know about the path of the electron, the less likely it is to show an interference pattern. In other words, the electron seems to respond how much we *know* about it! If we set up our experiment to get any "which path" information, it will only take one path. But if we set up our experiment so that we do not know which path it took, it will give an interference pattern showing that it took both paths. So we can decide, based on how we set up our experiment, whether an electron took one path or both paths.
But it gets EVEN weirder! Not only can the experimental physicist decide whether it took one path or both paths by changing how they set up the experiment, but they can do so *retroactively*, i.e., after the electron has made its way through the path(s)!! This is called the delayed-choice experiment. Here is a description from John Wheeler, who came up with the idea:
In the 1970's, I got interested in another way to reveal the strangeness of the quantum world. I called it "delayed choice." You send a quantum of light (a photon) into an apparatus that offers the photon two paths. If you measure the photon that leaves the apparatus in one way, you can tell which path it took.
If you measure the departing photon in a different way (a complementary way), you can tell if it took both paths at once. You can't make both kinds of measurements on the same photon, but you can decide, after the photon has entered the apparatus, which kind of measurement you want to make.
Is the photon already wending its way through the apparatus along the first path? Too bad. You decide to look to see if it took both paths at once, and you find that it did. Or is it progressing along both paths at once? Too bad. You decide to find out if it took just one path, and it did.
The delayed-choice experiment may be weird, but it is not mere science fiction. It was first carried out by physicists at the University of Maryland, where I go to school. Actually one of the physicists who did the experiment works across the hall from the office where I spent most of my time here, Dr. Alley.
Shih & Alley carried out the delayed-choice experiment in the late 1980's using photons, and it has been replicated several times since then. They decided, retroactively, whether the photon took one path or both paths by shifting the experimental arrangement after the photon was well on its way along the path(s), several nanoseconds after it would have had to "choose" one path or the other.
But wait, it gets EVEN WEIRDER!! In principle, as Wheeler has pointed out, this experiment could be carried out using astronomical sources. A photon from a distant quasar, for instance, could have the option of taking two paths towards Earth due to an effect called gravitational lensing. The photon could go straight to Earth or it could be pulled around another path by the strong gravitational force by a galaxy along the way. Since we can still decide whether the photon took one or both paths by how we set up the experimental apparatus on Earth to measure "which path" information or to find an interference pattern, we can decide which path(s) the photon took BILLIONS OF YEARS AFTER it had to have taken them!
Good luck getting to sleep tonight.