What decides where a photon hits a screen when it arrives? In quantum theory, the quantum wave defines the probability it will hit at any point but where it actually hits is a random choice from those probabilities. The probabilities are exact but the actual hit point varies with no known physical cause.
Quantum theory calculates the probability a photon will hit a screen point as follows:
1. The wave equation describes how the photon cloud spreads through both slits.
2. Given two paths to a screen point, positive and negative wave values add to a net result.
3. The net amplitude squared is the probability the photon will physically exist at that point.
Quantum theory then explains Young’s experiment as follows:
The photon quantum wave spreads through both slits, then its positive and negative values add or cancel at the screen to give interference that affects the probability of where it hits.
All this quantum activity is seen as entirely imaginary so it doesn’t really happen but in quantum realism, there really is a quantum wave that really does generate physical events. If a quantum wave is a processing wave and a physical event is a node overload that restarts the server, what decides that? Servers have many clients so a quantum server response to a client node reboot request could be:
1. Access. The server restarts its processing at that node, which denies all other nodes access to it and collapses the quantum wave. This then is a physical event.
2. No access. The server doesn’t respond as it is busy elsewhere so the node drops the process and carries on. This then was a potential physical event that didn’t happen.
Quantum collapse is random to us because it is a winner takes all lottery run by a quantum server we can’t observe. When many nodes reboot, the first to initiate a server restart locks out the others and wins the prize of being the photon, leaving other instances to wither on the grid. It follows that screen nodes with more server access are more likely to reboot successfully.
Quantum theory defines its probabilities based on the square of the quantum wave amplitude because a quantum wave is a sine wave and the power of a sine wave is its amplitude squared. This power defines the processing demand that determines access to the photon server. That positive and negative quantum amplitudes cancel locally is an expected efficiency. Nodes that access the server more often have a greater probability to successfully reboot and host a physical event.
When many screen nodes overload at once, where a photon actually hits depends on server activity that is to us random, as quantum theory says. But quantum theory can deduce the probability of where a photon hits from the square of the quantum wave amplitude at each point because the power of the quantum wave at a node defines its server access. Quantum realism derives what quantum theory declared based on known data, so it describes Young’s experiment in server access terms as follows:
a. The photon processing wave spreads instances through both slits.
b. If they reach the same node by different paths, positive/negative values cancel or add.
c. When many screen nodes overload and reboot, the net quantum amplitude squared defines the probability of server access that results in a physical event.
In Young’s experiment, the photon server supports client instances that pass through both slits then interfere as they leave, even for a single photon. This interference alters the server access that decides the probability a node overload will succeed. The first screen node to overload and restart the server is where the photon “hits”. If detectors are in both slits, both fire equally because both have equal server access. If a detector is in one slit, it only fires half the time because the server is attending to instances going through the other slit half the time. Table 3.1 below interprets Feynman’s summary of quantum mechanics (Feynman et al., 1977) p3710 as a calculation of server access.
This model now answers questions like:
a. Does the photon go through both slits at once? Yes, photon instances go through both slits.
b. Does it arrive at one screen point? Yes, photon processing restarts at one screen node (point).
c. Did it take a particular path? Yes, the instance that caused the node reboot took a specific path.
d. Did it also take all other possible paths? Yes, other instances, now disbanded, took every path.
If quantum theory is literally true, a photon really is a “wave” that goes through both Young’s slits but it arrives at a screen point because a physical event is a server restart triggered by one node. A photon as server processing never dies because it can be born again from any of its legion of instances. Quantum realism explains what physical realism cannot: how one photon can go through both Young’s slits at once, interfere with itself, but still arrive at a single point on a screen. It can explain a mystery of light that has baffled scientists for centuries.
Table 3.1. Quantum theory as server access 

Quantum theory 
Server access 
1. Existence. The probability a quantum entity exists is the absolute square of its complex quantum amplitude value at a point in space 
1. Restart.The probability a quantum entity restarts a server in a physical event depends on node access, which is the absolute quantum amplitude squared 
2. Interference. If a quantum event can occur in two alternate ways, the positive and negative amplitudes combine, so they interfere 
2. Combination.If quantum processing can arrive at a node by alternate network paths, the positive and negative values combine, so they interfere 
3. Observation.Observing one path lets the other occur without interference, so the outcome probability is the simple sum of the alternatives, so the interference is lost 
3. Interaction. Interacting with a quantum wave on one path lets the other occur without interference, so the probability of either path occurring is the simple sum of the alternatives, so the interference is lost 