How Quantum Field Theory Solves the "Measurement Problem".

It is not generally understood that Quantum Field Theory gives an easy solution to the "measurement problem" that was examined on the September letters page of Physics Today. But by QFT I do not mean Feynman's particle-based theory; I mean Schwinger's QFT where "there are no particles, there are only fields".1.

The fields are present in the form of quanta, i.e., chunks or units of field, as Planck envisioned over a century ago. Field quanta evolve in a deterministic way specified by the field equations of QFT, except when a quantum suddenly deposits some or all of its energy or momentum into an absorbing atom. This is called "quantum collapse" and it is not defined by the field equations. In fact there is no theory that describes it. Everything we understand is that the probability of it happening depends upon the field strength at a given position. Or, if it is an interior collapse, like a shift in angular momentum, the likelihood depends on the element of angular momentum in the given direction. In QFT this collapse is a physical event, not a mere change in probabilities as in Quantum Mechanics.

Many physicists are troubled by the non-locality of quantum collapse where a spread-out field (or perhaps 2 correlated quanta) unexpectedly disappears or changes its interior condition. Yet non-locality is required if quanta are to serve as a unit, and it has been experimentally proven. It does not result in inconsistencies or paradoxes. It might not be just what we anticipated, but just as we accepted that the world is round, that the planet orbits the sun, that matter is built from atoms, we ought to be able to acknowledge that quanta can collapse.

Sometimes quantum collapse can cause a macroscopic change or "measurement". However the measurement outcome, i.e., the "decision", was determined at the quantum level. Everything after the collapse follows inevitably. There is no "superposition" or "environment-driven process of decoherence.".

Take Schrödinger's cat as an illustration. If a radiated quantum collapses and transfers its energy into 1 or more atoms of the Geiger counter, that initiates a Townsend discharge that leads inexorably to the demise of the cat. In Schrödinger's words, "the counter tube discharges and through a relay releases a hammer which smashes a little flask of hydrocyanic acid" and the cat dies. Alternatively, if it does not collapse in the Geiger counter then the cat lives.

Of course we don't know the result until we look, but we certainly never know anything until we look, no matter if it's throwing dice or choosing a sock blindfolded. The fate of the cat was actually determined at the moment of quantum collapse, just like the outcome of throwing dice is identified when they hit the table and the color of the sock is figured out when it is taken out of the drawer. After the quantum collapse there is no entanglement, no superposition, no decoherence, only lack of knowledge. What could be easier?

In addition to offering an easy solution to the measurement problem, Quantum Field Theory provides a logical explanation for the paradoxes of Relativity (Lorentz contraction, time dilation, etc.) and Quantum Mechanics (wave-particle duality, etc.). It is regrettable that so few physicists have acknowledged QFT in the Schwinger sense.

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