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 should be in their physical events, but they are identical – a photon goes through the same slit to hit the same screen point.
The only difference is that the slit the photon didn’t go through is blocked, but how can blocking a path the photon didn’t take stop the interference pattern? Equally, how can leaving open a slit the photon could have gone through but didn’t cause the interference? It seems that in our world, a counter-factual, an event that didn’t physically happen, can change a physical outcome.
In a purely physical world, the double-slit result is impossible. And the unlikely tale of quantum 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 questions unresolved:
1. What are quantum waves? What exactly is proposed to spread through space as a wave? The current answer, that quantum waves don’t exist so it doesn’t matter, 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 theory is a recipe without a rationale, not a real theory. Theories describe real causes, not imaginary ones, so quantum theory isn’t actually a theory at all, but at best a Pinocchio theory, a puppet of physics that wants to be what it isn’t. Yet it could be, if only there was some rationale to quantum waves.