RE: chaos and uncertainty
Sun, 18 Jul 1999 11:11:15 -0700

James Andrix, <>, writes:
> 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.

This is very easy today, in fact my son did exactly this for his science fair project last month. We just used an ordinary laser pointer and aimed it at the bedroom wall. The razor blades don't have to be that close together; the effect is quite noticeable with them around a millimeter apart. You can actually see it by making a narrow gap with your fingers and shining a laser pointer through. Let it hit a wall 5-10 feet away and someone who looks at the spot up close will see the fringes shifting around as your fingers move.

> 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.

The more conventional explanation uses wave interference. If the distance to a spot on the wall from one side of the slit vs the distance from the other side of the slit has a difference of half a wavelength, then you get destructive interference from the two sides and a dark band. You don't really need to invoke the uncertainty principle.

> 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)

This effect was known long before QM. While it can be fitted into the QM paradigm, there are easier explanations.