Experiments with photons. Are they isolated or entangled?
Various experiments have been devised to elucidate the behavior of photons, particles of light: whether they act as particles or exhibit wave functions, or perhaps both. Examples include the renowned ‘double-slit experiment’ and the ‘delayed-choice experiment’. See below.

These experiments reveal either a particle effect or a wave effect, but never both simultaneously. Each precise replication of the setup consistently produces the same result. However, if an interim observation is made to covertly examine how the behavior unfolds during the process, it leads to a different outcome in the final result. The clandestine observation has altered the ultimate outcome. How is this possible?

Wikipedia provides a comprehensive description of the iconic double-slit experiment.

Wikipedia also provides an informative animation that illustrates what happens when we add an observer to the double-slit experiment.

source: wikimedia

The paradox of the double-slit experiment lies in the peculiarity that, for a portion of the experiment, the behavior of light/photons must be described as waves, while during another part of the same experiment, they behave as particles. This paradoxical behavior is not only applicable to electromagnetic radiation – a continuous stream of photons – but also to photons passing through the experiment one at a time. Now, photons may still be viewed as a special category, a kind of hybrid blend of particles and waves. However, the same apparent contradiction is observed with electrons, recognized as elementary particles. Even buckyballs, relatively large molecules composed of 60 carbon atoms, exhibit dual behavior in the experiment, acting as both particles and waves.

The Copenhagen interpretation posits that the photon (particle) should be regarded, from the moment of creation, as a probability distribution describing a wave motion. This persists until the moment of measurement. The measurement leads to the collapse of the wave function and yields a concrete value, such as a location. Between creation and collapse, interferences of wave functions can occur. Richard Feynman introduced a path integral formulation. By this, he meant – in an extremely simplified manner – that a photon, between creation and measurement, follows all conceivable routes in spacetime on its way from A to B. Thus, the photon exists in many places and in many forms throughout the universe simultaneously.

In relation physics, the various elements of the experiment (photons, electrons, laser cannon, slits, and detector) are not perceived as isolated components. The so-called one-by-one shooting of photons also doesn’t align with this perspective because everything is entangled. Each photon is entangled with its source. During measurement, there is a redistribution of information, leading to the emergence of new entanglements. Between creation and measurement, the information is entangled not only through its connection with the source but also through other relations with the universe. This perspective of superposition of information can be regarded as a variant of Feynman’s path integral formulation

Classic view of the double-slit experiment compared to a relation perspective
The following steps apply to both perspectives:

• When the majority of the probability distribution/wave function of the photon hits the plate with the two slits, the photon will be absorbed by this plate (In terms of relation physics: When most of the photon’s information hits the plate, collapse is most likely. Any other option has become so improbable that it can be neglected.)
• When the majority of the probability distribution/wave function of the photon goes through the two slits, everything will pass through these slits (In terms of relation physics: When most of the information goes through the slits, the information will continue to be shared with all possible neighbors until collapse on the detection plate becomes inevitable. Then all the information from the photon will be absorbed by the detector.)

The difference between both interpretations becomes apparent when placing an observer in front of one of the slits. In the classical interpretation, the photon passes the observer unaltered. However, it is then a mystery why the photon only goes through that one slit and not both, eliminating interference. In a relation interpretation, the photon will collapse with the observer. Following this collapse, a new photon is created with similar information to what the first photon arrived with. However, the new photon is no longer connected to its source, the laser machine, but to the observer. While a route through both slits is not impossible for this new photon (as the information disperses throughout the entire universe), it is so unlikely that it can be neglected. The probability (‘retrospectively’ and in macroscopic terms) determines that the photon went through one slit because the majority of the probability distribution went through one slit.

Compare this to photons traveling through a medium
Keep in mind that light propagates in a vacuum (a classical term) at the speed of light. In a medium (air, water, glass), the speed is lower. This is because photons constantly interact with the electron clouds around the atoms composing the medium. This leads to absorption and emission of photons, resulting in a loss of time.

In terms of relation physics: New photons are continually formed. The new photon has a different source entanglement.