In the last years of the eighteenth century, Thomas Young began to think about the properties of light, and in 1801, he presented a paper to the Royal Society on what has come to be known as Young's Double-Slit Experiment.
What he discovered was that light from a single source shining through a card with two parallel slits in it at an appropriate distance apart would be diffracted by the edges of the slits and merge to show an interference pattern of alternating light and dark areas on a surface behind the card. No such pattern was observed when one slit was closed. What appeared with one slit closed was a single lighted area with the same shape as the open slit but with the edges blurred by diffraction.
This interference pattern seemed to be the result of waves bending around the portion of the card between the slits because waves in media on a larger, directly observable scale show a similar interference pattern when the waves bend around a single barrier (the way that waves bending around a post in water do).
Fast forward now to the late twentieth century when the technology to emit and aim a stream of single photons was developed.
What scientists discovered in working with the apparatus for Young's experiment was that a single stream of photons going through one slit to trace out its whole open area of that one slit avoided the dark areas of the predicted interference pattern in Young's experiment when both slits in the card were open but traced out the whole area of the slit when the other slit was closed.
In other words, it was just as if the photons knew when the slit that they were not going through was open and when it was closed and behaved differently under the two completely different, completely external conditions.
The obvious, naïve interpretation of these results is that the universe tracks the state of gross objects (like cards with double slits) and controls photons to behave one way when the slit with no photons going through it is open and to behave another way when it is closed.
Things being the way they are, I am reasonably certain that the scientific community has not seriously considered this obvious, naïve interpretation. But this interpretation is consistent with the obvious, naïve explanation (as suggested in an earlier post) of the way that every entangled particle, etc., seems to know the state of all the other objects that it is entangled with and to keep its state in sync with its fellows.
That naïve explanation is just the idea that the universe is a living entity with an extremely sophisticated nervous system that tracks the state of everything on every level and responds to its evaluation of those states with signals to keep the system at an optimum level of performance – which should be an especially interesting possibility for you, dear reader, because you (like the card with the slits in Young's experiment) are also a gross object of the universe.
I am, of course, open to any other explanation of the phenomena in the modern forms of Young's experiment that anyone has to offer.
And I should mention, as was also true in my last post, that my understanding of such matters as Young's experiment is strongly dependent on Brian Silver's The Ascent of Science (Oxford University Press, 2000).