Why Call Diffraction Uncertainty?

I did the Heisenberg's uncertainty principle exp at home and it worked out but isnt it just the diffraction of light? my question is what is the reason behind choosing uncertainty over diffraction of light to reason out the spreading of light ?
- Varun (age 14)
Mumbai,maharashtra,india

This is a great question on a core issue. In your experiment, you saw that light was spread out and showed diffraction patterns due to the sort of interference that waves show. In order for a wave in 3D to have a sharply defined travel direction, is has to be spread out in space. Since the wave really does have some spread in positions and some spread in momenta (a measure of where it's going), why say there's anything uncertain? When you look at water wave ripples on a pond, you don't say there's "uncertainty", you just say there's a range of travel directions for a wave that's spread out in space. What's different here?

The key difference comes when you look in closer detail. Say that instead of using some crude light detector, you used an array of sensitive ones (good photomultiplier tubes or good photodiodes). Instead of showing steady outputs at different positions, they'd give a bunch of little output blips. Although the average pattern of where the blips show up is just the pattern you saw, where each individual blip appears is random, unpredictable. That's why we call the spread here "uncertainty". Instead of knowing that the wave sloshes in in a spread out way, we see these blips and are uncertain where the next one will be.

Mike W.

p.s. We say the blips come from "photons".

(published on 11/09/2015)