That sounds like it’s really more math and not a physical observation. The paper’s own title is “Observing the Average Trajectories of Single Photons in a Two-Slit Interferometer.”
My question has always been:
If it was possible to shoot one photon at a time through the twin slits, would the photons consistently land at the same spots over and over again? I keep wondering if it’s deterministic. The interference pattern is always the same, isn’t it? So why not the placement of each photon to create it? Kind of like setting up a chess board: each photon has its place.
All the books I’ve read — and they’ve all been for non-physicists like me, the general public — have never addressed that question.
Update: My question has been answered!
Someone pointed me to this section at Wikipedia and the answer is right in the first paragraph:
For any particle small enough for quantum effects to be significant — electron, proton, etc.–, where it will arrive at the screen is highly determinate (in that quantum mechanics predicts accurately the probability that it will arrive at any point on the screen). However, in what sequence members of a series of singly emitted things (e.g., electrons) build up the final distribution pattern is completely unpredictable. The experimental facts are so highly reproducible that there is virtually no argument about them, but the appearance of there being an uncaused event (because of the unpredictability of the sequencing) has aroused a great deal of cognitive dissonance and attempts to account for the sequencing by reference to supposed “additional variables.”
So, in some sense, it is like setting up a chess board, but in “random” order. Sometimes the King is put down first, sometimes a pawn, etc.
Update 2: No, wait. No, it’s not really like a chess board at all, other than there being specific places a photon will land. OK, this is when my head starts to hurt thinking about all this.