The electron (or photon, with light), as you said, is interfering with itself. Yeah, like I said, it's counter-intuitive (though I'm not sure I buy the multiple universe thing).
By using this self-interference property, it is possible to detect the presence of an object without bouncing a single photon (or electron) off the object.
Set up a split-light experiment such that a beam of light is split into two paths, and the two paths are rejoined so that the two paths interfere with each other and product no light at the destination. A single-photon emitter at one end, and a photon detector on the other end, will quickly prove that even a single photon -- even though you would think it would choose one path or the other -- interferes with itself through the two paths so that the photon detector detects nothing.
Now place an object in one of the light paths. (If you want to make it more mysterious, put it in a box with holes in it so the light can pass through, but you can't see if there's an object in the box.) Now send a photon through the apparatus, and see if the photon detector sees anything. Half the time, it will. Half the time, it won't. The times the photon is not detected, the photon follows the path that is blocked by the object. The photon strikes the object, and the photon detector never sees it. The times the photon is detected, the photon follows the path that is NOT blocked by the object, so the photon detector sees it.
So if the photon detector does not register a hit, that can mean one of two things: 1) The object is not present, and the photon interfered with itself, or 2) the object is present, and the photon chose the path with the object, and thus never reached the photon detector.
But if the photon detector ever registers a hit, that means that there's an object in the path that the photon did not take.
You've just detected the presence of an object without interacting with it in any way whatsoever. If you remove the object altogether, the photon will again interfere with itself, and the detector will never see a hit.
So what is it about quantum mechanics that allows us to detect the presence of an object simply by surrounding it with mirrors and prisms that don't even need to allow photons to pass in the object's path?
The photon has a 50/50 chance of going through one path or the other. But we cannot observe a photon in transit. We can observe it only as it interacts with the photon detector (or something else). Therefore, the "unobserved" photon, like Schrodinger's cat, takes both paths, simultaneously. The fact that we observe the photon only after the paths are rejoined means that the photon did not have to collapse these quantum probabilities into a single state of reality by choosing one path or the other. So, against all logic, a single photon will take both paths simultaneously, since neither path can be observed, and the photon won't be caught occupying two places at the same time.
But if you place an object in one of the paths, the possibility exists that the photon can now be observed to strike the object before the paths are rejoined. This means that the photon can be observed to have selected one path or the other, by striking or not striking the object. So the photon must now, in reality, select one path or the other. It cannot pass through both simultaneously, since it "knows" now that it will get caught occupying two places at once by an observer.
But how does the photon know? When it reaches the splitter, before it even reaches the object, it "knows" to go one way or the other, and not to follow both quantum possibilities simultaneously. How does it know the object is there, and that it shouldn't risk getting caught taking both paths at once? |
