View Full Version : Particle Tennis

2010-Sep-20, 01:32 PM
I saw some documentary on TV a while back, and it showed results on some test where they fired particles through two slits and measured where they hit. It showed that most of them hit where they should, but some were in the middle. This somehow was interpreted as the particle being in two places at once.
They likened it to tennis balls being hit through two slits, and that the ones that got through should hit the wall behind, not in the middle.
I couldn't help think that the ones that hit in the middle were just deflected off the sides of the slit. Could this not be the case, rather than them existing in two places at once?
Also, what was the experiment?

2010-Sep-20, 03:23 PM
This is the famous double slit experiment, also called Young's experiment. Particles can be sent one at a time through a double slit arrangement. The slits are separated by about the particle's wavelength. The resulting pattern will either be 2 bars (corresponding to the 2 slits) or a series of bars of varying intensities - the interference pattern. Which one you get is dependent on one factor: was it possible to determine the path to the screen or not? If it was possible, you get a classical pattern with the 2 bars (as if you shot very small tennis balls through the slits).

If it is not possible to determine the path, you get interference between the possible paths (from each slit). It is as if the particle went through both slits! And no, the resulting pattern cannot possibly be a result of bouncing off the sides of the slit. That activity, if it happened, would always occur. Yet when you block one of the slits, the other shows a single bar.

2010-Sep-20, 03:38 PM
The experiment is normally known as the Double-slit experiment (http://en.wikipedia.org/wiki/Double-slit_experiment), note that the page can get confusing for a beginner because it looks at the full description from the start rather than using a simpler explanation first.

It's fairly easy to test for the bouncing off the sides idea, by trying with only one slit.
In that case the particles hit at the point straight through the slit as if they do act like tennis balls with only a few affected by the edges.

And with two slits it isn't just in the middle they hit, some of them also hit outside where you'd expect.