A single classical photon exists as a pulse of light with both a location and a spectral superposition of frequencies and phases called a spectrum. A single quantum photon location in a beamsplitter device can be in a superposition of locations and any measurement will affect the single photon spectrum since the measurement becomes part of the device. Any simultaneous knowledge of both photon location and spectrum is necessarily limited by the uncertainty principle and the fact that it is a single photon.
Even very smart people can ask the absurd question about a quantum photon location in a beamsplitter resonator and location has no meaning since single photon has no classical meaning. A photon is a superposition of all frequencies and phases and all locations in the entire universe that happen to make up what we call a pulse of light that shows up on one path with one spectrum. This is how the universe works and yet, these same very smart people seem forever confused by the discrete nature of quantum matter and action.
Why is the universe full of things that happen? Why do things happen at all? Why do things happen to one person and not to someone else? These are questions that people ask and answer all the time, but there are no single precise answers to such questions. The things that happen to us are simply how the universe is and there is no further explanation needed, just belief in the way the universe is.
However, all things that happen are outcomes that have matter action precursors and so we can find out a lot about the matter and action that causes something to happen, but we cannot know everything. Even though there are answers to all questions about the matter and action precursors that cause something to happen within the universe, there are limits to the precision of any answer. Classically, however, there is no limit to the precision of knowledge besides the complexity of chaos. However, in quantum space and time, precise knowledge of both location and momentum is not possible. In fact, a more precise measurement of location results in more uncertainty in the spectrum of a photon. Thus, there is a discrete quantum limit to simultaneous knowledge of both the matter and action that causes something to happen.
Both double slit and beamsplitter resonators as well as any laser resonators are all examples of photon resonators and there are many ways to fabricate a single photon resonator at light wavelengths. Such a light resonator includes a source, confinement of some sort with mirrors and beamsplitters, lenses and apertures, and a detector. Depending on dephasing time and frequency of the source and detector, there are any number of semiclassical approximations or simplifications for the quantum phase superposition and entanglement that are a part of even a single photon resonator. However, since quantum superposition and entanglement of a single photon with itself has no completely classical meaning, and the many semiclassical questions will necessarily result in absurd semiclassical answers.
For example, a 1996 sciAm article reported a photon resonator that detects objects with a photon that never hits those objects. The underlying assumption is that it is only by photon absorption or emission that we detect objects, but of course this is not true. A shadow is a perfect example of detecting an object with the photons that do not hit the object instead of those that do. Since there are two or any number of paths in superposition within this single photon resonator, this resonator recorded an object shadow by blocking one path and thereby changing the photon output along the other path. Therefore, the photon that passed through the resonator recorded a change without ever hitting the object. The authors then implied that the photon was identical before and after, but that was really not true either. The photon spectrum did change and in particular, the phase and polarization of the photon changed and that recorded change showed the blockage of one path.
In other words, a single photon carries both frequency and phase information and so the photon did change even though it did not hit the inserted object. This single photon resonator is analogous to the hydrogen atom, since hydrogen is also a single photon resonator where photon exchange between the electron and proton binds the hydrogen orbits. Thus, a hydrogen atom resonator is an electron source, a proton detector, and photon confinement due to exchange. Note that a hydrogen atom is completely symmetric (actually, not quite because of spin polarization) and therefore also equivalent to a proton source and electron detector. Creation formed each hydrogen atom in the universe by emitting a photon of light complementary in frequency and phase to the photon exchange that binds hydrogen. In fact, this complementary photon pair is the biphoton that we call gravity force.
Thus, just as there is no way to really explain the bonding of a hydrogen atom without quantum phase or to locate the photon being exchanged, there is likewise really no way to precisely locate a single photon in a quantum resonator, either. Hydrogen is made up of two opposite semiclassical charged particles, but what bonds the electron and proton of hydrogen is photon exchange, which makes no classical sense at all. The semiclassical observer can then imagine the photon as a free particle independent of its source and detector traveling independently in space and time. While this is often a very useful semiclassical approximation, the neglect of source and detector quantum phase entanglement can lead the observer to many absurd semiclassical conclusions.
One absurd conclusion is that a semiclassical electron falling into a proton eventually exceeds the speed of light. Another absurd conclusion is that since a semiclassical electron moves through space and time in its orbit around a proton, there is instantaneous communication across the diameter of the orbit. In fact, these same absurd semiclassical conclusions result from any single photon resonator given semiclassical approximations.
A second very common semiclassical approximation that a single photon behaves in a similar manner to a large collection of photons. However, while a large number of uncorrelated photons give a classical statistical average classical behavior, a single photon will necessarily show only a quantum outcome just as a large number of highly correlated photons become a laser. Therefore, a single photon does not have a single classical determinate outcome because even a single photon represents a quantum superposition of many possible outcomes and not just a single classical outcome like a classical cannonball.
Unlike a classical cannonball, which only has a semiclassical mass, a photon and indeed all quantum matter, even cannonballs, have both masses and a spectrum of frequencies and phases and so no two photons or particles are ever exactly alike. Even though two photons may come from the same source and end up at the same detector, they never have exactly the same spectrum. Therefore, ignoring quantum phase entanglement for any single photon resonator like a double slit or a beam splitter can lead to absurd semiclassical determinate answers instead of uncertain quantum answers.
Including quantum phase entanglement and decay in a single photon resonator resolves all of these semiclassical paradoxes with probabilistic quantum answers. Quantum nonlocality and action at a distance are both the direct outcomes of semiclassical and determinate assumptions that completely ignore quantum phase entanglement and decay. Classically, there is no limit to the precision of the simultaneous knowledge of the mass and action of a particle or body like a cannonball. However, there is a discrete quantum limit to the precision of the simultaneous knowledge of matter and action, the uncertainty principle, because quantum matter and action have quantum phase and entanglement.
A classical cannonball has a classical mass measurable to an arbitrary classical and relativistic precision as long as the cannonball is at rest. However, the electrons, protons, and neutrons of the quantum cannonball are never at rest since they are all in perpetual motion, even at absolute zero temperature. Therefore, the cannonball mass actually depends on its temperature as well as on the atmosphere it is in contact with and so on. Thus, there are a large number of semiclassical approximations that we make when we measure a classical cannonball rest mass. When we measure the quantum mass of a cannonball, its action is always a part of that quantum measurement and the relativistic rest mass is then just a semiclassical approximation that has no quantum meaning. And there is a semiclassical assumption that the universe does not change during the course of the measurement, but the universe does in fact change all the time and those changes do actually affect the measurement, if only very slightly.
Quantum phase entanglement and decay can lead to very complex analyses called two-dimensional photon spectroscopy. The more complex the photon resonator, the more complex the spectral analysis and even very smart people can end up with absurd semiclassical answers given semiclassical approximations. There are really two outcomes for source and detector phases and pure decay to heat is just one outcome while pure dephasing is a second outcome that results in no heat. Semiclassical approximations usually assume pure decay and completely neglect the pure dephasing of quantum phase, but many of the absurd semiclassical conclusions of the double slit and beamsplitter resonators result from the neglect of pure dephasing and entanglement.
Classically, atom excitation energy decays only to heat and results in a classical emission spectrum after quantum phase decay. However, it is possible for quantum phase to diffuse to other matter and couple source and detector even though the excitation energy does not decay to heat. Rather, quantum phase entanglement persists and the emission spectrum evolves and can result in photon echoes and other pure phase entanglements that have no classical meaning at all.
Thus there is no Wittgenstein sense to the many absurd questions about semiclassical single photon resonators. Single photons as well as large numbers of highly phase correlated photons in resonators have only quantum and not really classical answers. Thus, the determinism of gravity relativity is a very misleading semiclassical approximation for the biphoton phase correlation of quantum gravity. It is the biphoton phase entanglement and correlation of the emitted and exchanged photons of hydrogen and all matter in the universe that is gravity force. In other words, gravity force is due to a persistent biphoton quantum phase correlation and so gravity relativity is a very good approximation that neglects the fundamental role of phase for the biphoton of quantum gravity.
The penultimate photon resonator is a black hole where only photon phase exchange binds matter into a pure phase gravity photon resonator while the ultimate photon resonator is the universe itself. A black hole outcome represents a pure quantum phase matter action that really has no meaning in classical space and time. A black hole quantum phase or spin outcome preserves all of its precursor matter action information as both matter and pure phase and there is no meaning for black hole space and time. Our notions of space and time as well as black holes then all emerge from things that happen and really space and time and black holes do not therefore have meaning without things that happen. Space, time, and black holes all emerge from the causal set of the precursors and outcomes of discrete matter action.