As Feynman famously said:
“… all the mystery of quantum mechanics is contained in the double-slit experiment.” (Satinover, 2001), p127.
Quantum theory explains the double-slit experiment as follows:
A photon is a quantum wave that spreads in space by the equations of quantum theory. This wave goes through both slits to interfere with itself as it exits, but if observed immediately collapses to be a particle at a point, 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 goes through both, interferes with itself, then collapses to a point on the screen based on the prior interference.
This explanation doesn’t say what the wave is that goes through both slits, nor why it collapses to a point when observed, hence Wheeler’s question: How come the quantum?
To understand how strange this is, suppose the first photon in a two-slit experiment hits the screen at a 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 first photon goes through the same slit to hit the screen at the same point, to be the first dot of what will never be an interference pattern. The difference between these outcomes must start with their first physical events, but they 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 then can blocking the path that the photon didn’t take cause an interference pattern? How can the slit a photon could have gone through but didn’t decide if there is interference or not? As will be seen, this isn’t the only case of a counterfactual, an event that didn’t physically happen, changing a physical outcome in our world.
In a purely physical world, the double-slit result is impossible, and the unlikely tale of imaginary waves that collapse when viewed makes no physical sense, yet quantum mechanics 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 a recipe without a rationale, so what are quantum waves?