Found this cool animated gif on the wikipedia page for quantum electrodynamics.
To briefly explain what’s going on: common experience tells you if you shine light on a mirror, light will reflect off at the same angle it reflects on. (In math speak, the angle of incidence equals the angle of reflection). This is true. That’s the result you’ll get if you don’t do any funny business to the mirror. That’s what that final red arrow represents.
But you see, the quantum world doesn’t behave like you might think it does. In fact, to calculate this path, we have to calculate all possible paths the light can take. That’s what all those individual little arrows represent. But because we say a photon (‘light’ = the particle known as a photon) has ‘wave-like’ properties, the arrows NOT in the middle cancel each other out!
"Whatever," you say. "That’s just a bunch of trickery! The light is bouncing from the source, off the center of the mirror, and into the detector (likely your eyeball)."
But it isn’t trickery! These other paths exist! It’s not just a mathematical abstraction.
You see, we could actually, if we wanted to, chop off the center and right of the mirror. If we shone a light on the remaining left from our source, it would not reflect toward point P. I mean, dur right? That’s because the paths are cancelling out. However if we scratch off certain parts of the mirror to avoid this cancelling out, we can actually make the light reflect toward P anyway! In fact, you see this all the time, any time you look at a CD or a DVD for example. This phenomenon (it’s called diffraction) is what causes the light to split into a rainbow.
Just to end, here’s another something to boggle your mind: in QED, a positron is identical to an electron travelling backward in time. In fact, to make accurate calculations, we have to include this possible ‘path’ of an electron going backwards in time. If we fail to do so, our calculations are wrong. So, in the lab, when we view a ‘positron’ what we’re really seeing is an electron travelling backwards in time.