Feynman once 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 works, but it doesn’t say what the wave is that goes through both slits, or 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 interference or not should be in the first 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 second photon didn’t go through is blocked, but how can blocking a path a 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? This implies that in our world, a counter-factual event that didn’t physically happen can change a physical outcome.
In a purely physical world, counter-factual effects are impossible, as is quantum theory’s unlikely tale of waves that collapse to a point when observed. Quantum theory may be the most successful theory in the history of science, but why it works is a mystery to physics, and it breaks down to two key questions:
1. What are quantum waves? What exactly spreads 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? How do quantum waves collapse to a point when viewed? The current answer, that they collapse because they do, is equally unsatisfactory.
Until it answers these questions, quantum theory is a recipe with no rationale, not a theory of reality. In the story of Pinocchio, a man wanted a son but couldn’t have one so he made a puppet instead, but it wasn’t a real boy. Likewise, physics today wants a physical theory of sub-atomic events but can’t have one so it made quantum theory instead, but it isn’t a real theory. Quantum theory is then just a puppet of physics that is used but not considered real. However if it becomes a real theory, then the answer to the above questions can be discovered.