Principles and Concepts
of Quantum Mechanics
Implications of
Quantum Mechanics

6. Particles. Collapse of the Wave Function.

Particle theories and collapse of the wave function are described. They are potential ways to deal with the many versions of reality in quantum mechanics.

Versions of Reality?

In the section on Schrödinger’s cat, we noted that the wave function contains many potential versions of reality. But of course, we don’t perceive a many-versioned reality; we perceive only a single reality. This has led many physicists to conclude that there actually, physically exists only a single, ‘objective’ reality. Two ways have been proposed to bring this about.


The first is to suppose that, in addition to the wave function, there exist particles, point-like objects that possess or carry mass, energy, momentum, spin, and charge (see, however, Mass, Spin, Charge and the Wave Function). It is presumed that the particles ‘ride along’ on or ‘inhabit’ just one version of the wave function. For example, in the Schrödinger’s cat experiment, the particles might ride along on the cat-alive version and it would then be that version of the wave function which we perceive. Or more accurately, it would be the particles themselves that we perceive. The wave functions would ‘guide’ the particles into the ‘live-cat’ configuration but they would never enter our consciousness. The idea that matter is composed of particles is deeply ingrained in our modern psyche. It will be discussed in No Evidence for Particles.

Collapse of the Wave Function.

There is a second way to obtain an objective reality that is also subscribed to by many physicists (although only one method is needed!) and that is to suppose the wave function, for unknown reasons, collapses down to just one version. For example, the Schrödinger’s cat wave function might collapse from the two versions

[nucleus decays]
[Schrödinger’s cat dies]
[version 1 of you perceives a dead cat]

and, simultaneously

[nucleus does not decay]
[Schrödinger’s cat lives]
[version 2 of you perceives a live cat]

to the single version
[nucleus does not decay]
[Schrödinger’s cat lives]
[you perceive a live cat]

in which there is a single, objective, real ‘you,’ consisting of a complex wave function, that perceives an objectively real live cat.

Are Particles or Collapse Needed?

These two approaches—particles and collapse—have one thing in common; they both require amendments to the description of physical reality given by basic quantum mechanics. On the other hand, we know from principle [P3] (Schrödinger’s Cat and Versions of Reality) that quantum mechanics, by itself, does an excellent job of describing physical reality, and so one should be very reluctant to amend this extremely successful theory. So our procedure will be the following: We will define Basic Quantum Mechanics, QMA, with no particles, in a succeeding section. Then we will investigate whether any amendments to QMA, such as particles or collapse, are needed or justifiable.
(We find that neither particles nor collapse are justifiable.)

understanding quantum mechanics
understanding quantum mechanics by casey blood