bhobba said:
If QFT is the low energy approximation of just about anything, and QM is a limiting case of QFT, it struck me that could possibly be the 'why' of QM?
Thank you for bringing this up, and hopefully we can return to the more immediate problem of understanding Q(F)T in flat spacetime before drifting into quantum gravity. I´ve been thinking about this for a long time, and I´ve become convinced that the measurement problem will dissolve once we´ve learned to view QFT from the right angle. For many physicists QFT is fundamental, but it is probably better seen as a kind of phenomenological theory: a neat way to describe correlations. Also plain QM is just a machinery for calculating correlation functions (e.g. in Bell-type experiments). Of course it must be explained what it is that is "correlated", for example phenomena in liquid helium or magnetic materials. It´s not sufficient to say it´s "measurements" or "quantum fields". The correlations are features of the real world. Otherwise it would be inexplicable why people at CERN haven´t turned to measuring numbers that are less costly to obtain. :-)
For a homogeneous, tenuous medium it is fairly straightforward to derive a Kubo formula for the photon absorption coefficient: $$
\kappa = \frac {\mu_0 c} {2 \hbar \omega} \sum_{\mu\nu} {\bf e}_\mu^* {\bf e}_\nu
\int dt \int d^3x \ e^{-i (kx - \omega t)}
\langle j_\mu(x,t) j_\nu(0,0) - j_\nu(0,0) j_\mu(x,t) \rangle
$$ It is suggestive to think of the current commutator as absorption minus stimulated emission, and ## j_\mu(x,t_<) j_\nu(0,0_>) ## as signifying absorption, ##
j_\nu(0,0_<) j_\mu(x,t_>) ## as emission events. Currents are
real, and photons can be
counted (at least when their frequencies are well above 60 Hz). One should drop the classical picture of "current (density)" as a continuous field spread out smoothly over spacetime. Quantum theory takes into account the actual graininess of matter, and it is more natural (!) to consider events localized in space and time. Fields retain their meaning only in a statistical sense, as expectation values expressing correlations between events. Since there is only one average, currents cannot flow into two different directions at the same time, but this is a very weak constraint on the microscopic events contributing to the average.
Schrödinger´s equation describes the continuous, deterministic evolution of a "wave function" and clearly doesn´t square with the abruptness and randomness of processes occurring in the real world. Von Neumann patched this up with intermittent "measurements". But this has created the "measurement problem". The problem is the lopsided view of the wave function and Schrödinger´s equation as the essence of quantum theory. There is another Schrödinger equation that has ## i ## relaced by ## -i ## and decribes the (backward) evolution of bras. It is usually considered a redundant mirror image of the normal Schrödinger equation. But it is not. Events on the forward evolving sheet of spacetime are just
tightly correlated with the events on the other sheet evolving backwards. At the classical level (perhaps down close to the Planck scale) they coincide, but in quantum theory we have to consider bras and kets separately. They are combined using Born´s rule, and only the three together give the complete picture of quantum theory. The Schwinger-Keldysh formalism encapsulates all that in a coherent way and doesn´t need the concept of measurement.