As Feynman famously said:
“… all the mystery of quantum mechanics is contained in the double-slit experiment.” (Satinover, 2001) p127.
Quantum theory explains Young’s results as follows:
A photon wave function spreads in space by the equations of quantum theory. This ghostly wave goes through both slits to interfere with itself as it exits but if observed immediately “collapses” to be a particle in one place, as if it had always been so. If we put detectors in the slits, it collapses to one or the other with equal probability. If we put a screen behind the slits, it interferes with itself, then collapses on the screen due to the prior interference.
The mathematics doesn’t say what this wave is that goes through both slits, nor why it shrinks to a point particle when observed, hence Wheeler’s question: How come the quantum?
To see how strange this is, suppose the initial photon in a two-slit experiment hits a screen at some point to become the first dot of what will always turn into an interference pattern. Now suppose that in another experiment with a detector blocking the other slit, the initial photon goes through the same slit to hit the screen at the same point to become the first dot of what will never be an interference pattern. The difference between these outcomes must exist from the start but the physical events are identical – a photon goes through the same slit to hit the same screen point. The only difference is whether the slit the photon didn’t go through was blocked or not.
How can blocking the path that the photon didn’t take be part of the later result of an interference pattern or not? How can the slit a photon could have gone through but didn’t decide if there is interference or not? How can a counterfactual, an event that didn’t physically happen, change a physical outcome?
In a purely physical world, such a thing is impossible. Quantum theory’s unlikely tale of imaginary waves that collapse when viewed makes no physical sense, yet it is the most fertile theory in the history of science. This leaves two key issues unresolved:
1. What are quantum waves? What exactly is it that spreads through space as a wave? The current answer, that the waves that predict physical events don’t exist, is unsatisfactory.
2. What is quantum collapse? Why do quantum waves restart at a point when viewed? The current answer, that quantum waves collapse “because they do”, is equally unsatisfactory.
Until it answers these questions, quantum mechanics is just a recipe without a rationale.