There is a strange effect described correctly by quantum physics whereby if you pass light through two near field slits, two possible behaviors can occur.
If you measure the photons, so that you are aware which of the two slits they pass through, the light behaves as particles and clump up in a distribution of two lines on a film behind the slits.
If you do NOT measure the photons, and as such have no idea what slit they travelled through, the light acts as a wave, and what you observe on the film is an interference pattern like when ripples from two pebbles dropped in water interact with one another.
One explanation for the change in behavior between the two cases has classically been something in the measurement must have affected the photons and thus the outcome of the experiment.
So, an elaborate experiment was set up whereby (through beam splitters and prisms) you actually measure which slit the photons go through long in the future (long in a speed of light sense) from when they actually did and long after they have already formed either a clump of two lines, or an interference pattern. In fact we measure some of them but not others.
What we find is, the ones we measure actually formed two clumps, and the ones we do not formed an interference pattern. This is fascinating, as it would seem impossible that photons, that have already passed through a slit and struck a film, long before we actually measure which slit they passed through, could somehow be influenced by a measurement taken AFTER they struck the film...yet there ya go.
Sounds like the kind of book my father loves.
Did you think the book was easy to understand?