We seem to have a number of folks here who like physics. If my life had worked out differently I'm sure I would have ended up as a physics professor at some mid-size school and be content sitting in my room with my books and my equations. Regardless ...

If you take a sealed box and put a light source within and then place the box in dark room near a screen you have the start of the situation. Next let's make a very small hole in the box. Light will now stream out and be projected on the screen. So far this is obvious. If you make a second very small hole you now have two sources of light projecting on the screen.

Now I suspect most of know that if the device is setup correctly, the holes are the correct size and spacing, etc what actually occurs is one produces a series of light and dark bands known as an interference pattern. Here's a picture -



So far this is pretty much review. What is being observed is the dual nature of light-matter, particularly at the atomic, sub-atomic level.

If we keep the experiment the same but instead of using light we start shooting electrons or neutrons or protons at the two slits (holes) we get the same results. If we go one step farther and start shooting complete atoms at the slits we again get the same results.

These are all examples of the dual-nature of matter-energy (sometimes called a wavicle). One way to think of those, though it is incorrect, is that one wavicle goes through the left slit and another one through the right. However if you use shoot out one atom at a time and put a photographic plate where you have the screen and repeat the experiment you get the same result.

Let me say that again. If I take one atom and throw it though the two slits it produces an interference pattern consistent with two waves having traveled through two slits. How can one particle behave as two waves? Welcome to QM.

So to see what is actually occurring let's take a very precise detector and monitor which one of the two slits the atom actually travels through. We then sit there and take a look. Our detector now tells us the location of the atom as it passes through. As we sit here and monitor the path of the atom all at once the interference pattern disappears! Heisenberg has raised his head!

If we don't know the location precisely the atom will exhibit wave behavior. As we force the atom to tell us its location it stops exhibiting wave behavior.

Pretty amazing.

Here's a nice overview -

Quantum Waves