[zsayeed]

This is the best explanation ever - from Feynman.

Compare the behavior of a bullet and an electron.
There is a metal plate with a hole in it. You fire a bullet at the plate - broadside. Beyond the plate with holes is another plate with a detector to detect where the bullet lands.

The hole will land about hole A with a pdf (if b is blocked), and hole b with a pdf (if a is blocked) (same pdf but shifted). When the holes are simultaneously open the pdf of landing anywhere on the 2nd plate will be a sum of the two probabilities.

Now use an electron gun to fire at the plates. the pdf of going through either hole is not a straight sum anymore. In fact when one hole is covered you will see a intensity density, when the other covered the same intensity shifted (just like the bullets) but when both uncovered the intensity is not a straight sum - but interfere with each other (the same you observe when light enters through two pin holes - interference patterns). What happens through one hole has an effect on what happens through the other hole. Now if you try to observe the electron in flight, by shedding light between the hole plate and the collector plate, the behavior goes back to bullet behavior. Observation changes the outcome. That is Heisenberg. And that is the duality of wave-particle behavior of minute objects, interference = wave behavior, straight addition of probabilities is particle behavior.

Wonderful isn't it?