The quantum wavefunction of a photon in the double-slit experiment is always depicted as emerging from the source and collapsing at the screen, which only reflects our biased perception of time and our preference for the emitter's reference frame, since that’s the frame that naturally represents the possibilities as evolving forward in time.
However, we can consider at least four different perspectives for the same experiment, which can clarify the issue of when or how the wavefunction collapses:
- The emitter only knows where and when emission took place, and it could only map possible landing spots forward in time.
- The absorber only knows where and when absorption took place, and it could only map possible emission sites backward in time.
- As material observers of both the emission and absorption events, we can combine both mappings retrospectively to infer the most likely paths and intervals for a possible propagation process between emitter and absorber. As we always experience that emission precedes absorption, we usually take the emitter's perspective to define the wavefunction as evolving forward in time (because that is how we experience the passage of time), but once we know that detection has occurred, we suddenly switch to the detector's perspective. So, as you can see, the wavefuction collapse is just an artifact, a sudden change of reference frame in order to keep our description of what happened in line with our experience of time. Collapse is not a real physical process, but an update of the information at hand. Born's rule in this case (the square of the wavefunction amplitudes to get the real probabilities) would arise from the confrontation of the emitter’s forward mapping and the absorber’s backward mapping, since they must match to correctly describe the real physical outcome of the experiment, much like in the transactional interpretation of quantum mechanics.
- For the emitted photon, and due to its zero proper time, both mappings would coexist simultaneously, but with no spacetime considerations. This is the true reality of every photon's propagation process: A point-like and static confrontation of both emitter and absorber mappings at once, in a single moment of existence that forbids any causal order or evolution, yet somehow describes a propagation process from emitter to absorber at the speed of light through spacetime.
- If we use a material particle instead of a photon, all the interactions composing its internal structure actually comply with the speed of light, so particles and objects are just more or less complex collections of these emitter-absorber mappings we talked about. This explains why the quantum tunneling and self-interference capabilities of material particles decrease with increasing size or mass, due to the growing number of constituent emitter-absorber mappings that have to fulfill the geometrical constraints of the experimental setup at the same time.
