Two-slit Interference

Most recent answer: 01/17/2013

Q:
Ok my question is about the well known two slit experiment with atomic particles. I don't understand the mathematics of quantum mechanics but this has been in my mind and it's annoying me so i thought I would ask a physicist. So if you fire atomic particles at two slits you get an inteference pattern on the panel behind the two slits. If you place a measuring device behind the slits the interference pattern disappears but you can still determine if the atom went through a particular slit. So if this is the case why do physicists insist on saying the particle went through both slits at once when clearly it doesn't? Or even better place detectors behind both slits then you can see which slit the particle goes through. Ok you lose the interference pattern but at least you know the atom only goes through one slit at a time and that the atom is always a particle and not a wave. Hope someone replies. Many thanks
- Jonathan (age 35)
Northern Ireland
A:
So here's the situation. If there's some sort of detector around that makes a record of any sort of which slit the object went through, it records that the object went through one slit or the other, not both. Then there's no two-slit interference. If there is no such record, then there is two-slit interference, just like any wave would show.

When there's a record of which slit the object went through we don't insist that the object went through both slits. Why do we say that when the record doesn't exist the object actually went through both slits? Couldn't you imagine some sort of tricky behavior where objects that go through only one slit somehow mimic the arrival pattern of waves which slosh through both slits at once?  You might, but you'll run into problems.

What you're looking for is some sort of "local realist" picture in which particles have trajectories, even if we happen not to measure them. John Bell showed that any local realist model of the world must obey certain rules, called Bell's Inequalities. The sorts of quantum events you're talking about have been tested many times in slightly more complicated experiments to which these inequalities apply. (They're typically done with two "entangled" objects.) The local realist models are always violated, in just the ways that quantum mechanic predicts. This rules out not only the simple pictures you're thinking of but also a much broader class. When nothing is around to record the trajectories, the trajectories don't exist. Again, if they existed, the Bell Inequalities would be obeyed, but in our world they aren't.

Given that quantum mechanics works, and all local realist pictures fail, the last thing we would want to say is that "the atom is always a particle and not a wave". The quantum equations describe the propagation of waves. Depending on the circumstances, sometimes these waves can lump into small regions and sometimes they are more spread out. There's no indication that the objects can ever exist as particle-like things at specific points in space. Some of the things people say about particles are left from the early days of quantum, when things were even less well understood.

Mike W.

(published on 01/17/2013)