The Two-Slit Experiment
Shining light on a screen produces basically
uniform illumination.
Shining light which has passed thru a pair of slits
onto a screen produces an interference pattern.
If we turn down the intensity of the light so that
only one quantum is passing from source to
screen at a time, we still get the pattern!
Which slit did the quantum go through?
Closing either slit destroys the pattern!
How does the quantum know about both slits?