“Spooky action at a distance”, as Einstein called it, refers to the experimental fact that particles can impact each other instantly, even when separated by sizable distances. For example, if two photons are produced collectively in what is referred to as an entangled state and the angular momentum of one is altered, then the angular momentum of the other one will adjust in a corresponding manner at the same time, no matter how far away from each other the particles are. This “spooky” behavior has been known for almost a hundred years and still is a source of confusion.
Still there is a theory in which the result is not spooky, but rather a natural consequence. I’m referring to Quantum Field Theory, which describes a world constructed only of fields, with no particles. What we call a particle is really a piece, or quantum, of a field. Quanta are not localized like particles, but are spread out through space. For example, photons are pieces of the electromagnetic field and protons are parts of the matter field. These quanta evolve in a deterministic way as per the basic field equations and there is a term in these equations that restrains the speed of propagation to the velocity of light.
Even so the QFT equations don’t tell the whole story. There are events that are not explained by the field equations– for example, when a field quantum moves energy or momentum to a different object. This event is non-local in the sense that the change in, or even disappearance of, the quantum happens immediately, no matter how spread-out the field may be. It can even happen with two entangled quanta– no matter how much they are separated. In QFT, this is essential if each quanta is to act as a unit, as per the fundamental basis of QFT.
There is a big difference between quantum collapse in QFT and wave-function collapse in QM. The former is a real physical change in the fields while the latter is a change in our knowledge. Even though we don’t have a theory to describe quantum collapse, there is nothing inconsistent about it. To quote from Fields of Color: The theory that escaped Einstein:
In QFT the photon is a spread-out field, and the particle-like behavior takes place because each photon, or quantum of field, is consumed as a unit … It is a spread-out field quantum, but when it is taken in by an atom, the entire field disappears altogether, no matter how spread-out it is, and all its energy is placed into the atom. There is a big “whoosh” and the quantum is gone, like an elephant disappearing from a magician’s stage.
Quantum collapse is not a very simple concept to accept– perhaps more difficult than the concept of a field. Here I have been working hard, trying to persuade you that fields are a real property of space– indeed, the only reality– and now I am seeking you to consider that a quantum of field, spread out as it may be, quickly disappears into a tiny absorbing atom. But still it is a process that can be visualized without inconsistency. In fact, if a quantum is an entity that lives and dies as a unit, which is the very meaning of quantized fields, then quantum collapse must occur. A quantum can not divide and put half its energy in one area and half in another; that would violate the fundamental quantum principle. While QFT does not provide an explanation for when or why collapse occurs, some day we may have a theory that does. In any case, quantum collapse is important and has been confirmed experimentally.
Some physicists, including Einstein, have been bothered by the non-locality of quantum collapse, professing that it goes against a fundamental postulate of Relativity: that nothing can be transferred more quickly than the speed of light. Now Einstein’s postulate (which we must remember was only a guess) is certainly valid in relation to the evolution and propagation of fields as illustrated by the field equations. Having said that quantum collapse is not described by the field equations, so there is no reason to assume or to insist that it falls in the domain of Einstein’s postulate.