The Twin Slits Experiment

The Twin Slits experiment is mentioned in the text several times. 
I had intended to provide an overview of the experiment here, however there are so many other excellent sites the exercise seemed pointless.
A Google search using the string ‘Twin Slits experiment’ will provide a number of explanations at every level.
A fascinating and  very readable account of the Twin Slits experiment and its implications is given in John Gribben’s book “Schrodinger’s Kittens”