Einstein’s relativity theory and quantum physics, in theory as well as experiment, are extremely concerned with light. This comes directly from the fact that light does not exist as an independent entity – it is plain interaction.

I explained already how relativity makes light’s non-existence obvious. Today, I will tell you why that odd seeming fact only confirms what is known from entirely unrelated quantum mechanics: non-quantum relativity and non-relativistic quantum physics both agree on that light itself does not exist for entirely different reasons!

The quantum aspect which confirms the insight gotten from relativity I hope to discuss best as follows, but I may well fail to reach most readers. Quantum mechanics goes beyond what most people are willing to believe. No didactic can overcome that.

Let me first try to convince you that even your average cat is a quantum system. I will assume that you know and accept quantum physics at least in so far as that the uncertainty in momentum Δp and the uncertainty in position Δx cannot both be arbitrarily small: Their product Δx Δp is larger than a fundamental limit. This is the Heisenberg Uncertainty principle. The following consideration may explain this somewhat more:

We cannot find out the positions of the constituents of bound systems. For example, determining the position x of an electron inside a certain atom means forcing the electron into a situation characterized by a small position uncertainty Δx. This provides it with large uncertainty in momentum Δp. Those constituents that thus acquire large momentum p fly out of the previously bound system.

Put differently: knowing where they are inside of the bound system gives them more energy than their mutual binding energy. Trying to pin down all a cat’s particles’ locations would require us to interact so invasively as to destroy the cat; it would literally explode in a bright flash.

We do not remove the moon from earth’s orbit by observing both celestial bodies’ positions. The difference between the moon-earth system and the electron-nucleus system, i.e. the atom, is the amount of system internal interaction, or their total action A. This will become much clearer further below.

If we try to locate an electron inside an atom, once we succeed to localize the electron in a small, classically meaningful location, it does already, by the very fact alone of it being classically meaningful as an individual particle, not belong to the atom any more.

Loosely speaking in the terminology of quantum modal realism, which is in physics the Everett relative state description, which is in turn also sometimes called ‘multiple worlds (or minds) interpretation' (not "multiverse"): Our measurement of the electron being at x equals the ‘branch’ B_x coming out of its interference with all other branches B_x’ of the quantum universe, in each of which the electron is in another place x’.

An atom is the nucleus interacting with all branches’ electrons at once (in a very loose sense). Take one branch out, and the atom transforms into a nucleus plus an electron. This is related to the fact that all observation is fundamentally interaction. The moon’s own internal interactions are enormously many; it kind of observes itself. The electron has few internal interactions, perhaps none.

The overall action A of a system or process is a measure for its from outside observation independent reality or identity. Technically speaking, if the interaction dA exceeds the total action A of the system, the system has been completely changed; it has become a different thing. Since interaction is quantized, systems with little internal action A basically exist only for whatever observes them. The observation, if it interacts strongly enough, changes them to such a large degree that one may as well speak about the destruction of one system and the construction of another.

Quantum mechanics is all about that the minimum interaction is one interaction quantum dA = ħ (pronounced h-bar, value about 10^-34Js, also called ‘reduced Planck constant’). Why this strange constant? Well, our traditional units are all messy from the historical detours that got us where we are today. That is also why I do not get deeper into either units or numerical values. For all I care, it is equal to 1 bit of ‘fundamental knowledge’ or some such.

The extreme case is the quantized amount of light, called the photon or ‘light particle’. The photon’s action A is exactly one interaction quantum ħ. This defines the photon in the first place. Photons in this sense do not exist at all: they are completely exhausted by a single interaction quantum dA. They are only the interaction, the observation, but nothing by themselves.

Reformulated for clarity: ħ being the smallest interaction means that a system has to change by at least ħ if it is observed. Two successive observations of ‘the same system’ must differ by at least ħ. If the whole system has only action ħ, obviously there is nothing left unchanged at all after a single observation. In other words, it was nothing but the interaction, namely the interaction with the object that ‘emitted the photon’, the observation of it.

That there is nothing but the interaction is precisely what we expect from the discussion about light in relativity.

The next post in this series explains why the obsession of fundamental science with a measure that "does not exist", is unsurprising.