Just thought I'd relay something I saw on TV... (some kind of educational show, don't remember other than that.)
You have a laser, pointing at a wall. (little dot on
wall) Presumably, all the photons are going in the
same direction. You take two objects (I think maybe
razor blades) And start to block the beam with them,
one on each side. (forming a vertical slit, making the
dot smaller) when they get _really close together_
instead of a very slotted vertical dot, you get a
horizontal spread. The explanation was that the fact
that you 'knew the position' of the photons so
accurately (because of the very narrow slit) you
_could not_ know their velocity. (and hence direction)
I figured (I think correctly) that the spread was just
horizontal (it was still as tall as the diameter of
the laser beam) because you only 'knew' the horizontal
position accurately.
I don't know if it was real or not, or if the effect
was actually because of QM, but I decided I didn't
want to think about QM for a few months because I
might hurt something, or scare myself. (Photons should
NOT be that smart)