Quantum physics is the highly successful theory of the physical universe. It accurately describes the properties of elementary particles, nuclei, atoms, the semiconductors used in electronic devices and so on. And it has never given a result that disagrees with experiment. But this theory has one most peculiar property; it gives several simultaneously existing versions of reality! For example, Schrödinger’s cat is both dead an alive at the same time.

This many-versions-of-reality property is the underlying source of all the confusing aspects of quantum physics. One cannot get around this property; it is essential to all the successes of the theory. This leaves us with the problem of understanding how the many versions in the theory give rise to the single-version, ‘objective’ world we perceive. If we can understand that, then all the other seemingly nonsensical aspects of quantum physics should fall into place.

How are we to proceed? It will not do to simply take our naïve, face-value view of the world and try to fit quantum physics into it (see Quantum Physics and Reality). Instead, we will take three ‘facts’ that we can presumably trust and base our reasoning on them. These will lead to various principles that severely limit the possible structures for reality. So our first and most basic principle is:

[P1] Our reasoning will be based primarily on four trustworthy facts:
(1) Our everyday perceptions of the world around us.
(2) The highly verified mathematical structure of quantum physics.
(3) The highly verified Probability Law of quantum physics.
(4) We must also rely on the results of experimental searches for particles, hidden variables, and collapse of the wave function.

Hopefully the use of [P1] will reduce or eliminate pre-judgments (such as assuming there is an objective reality made up, perhaps, of particles) on the nature of reality.

Using [P1] with the current results of experiments, our conclusions are that (1) only the multi-version wave function physically exists (no particles, no collapse); and (2) our awareness of the physical world and our brain-based thoughts must originate ‘outside’ our physical brain, ‘outside’ the physical universe.


There are three parts to Understanding Quantum Physics—principles, evaluation of interpretations, and the explanation of different phenomena such as wave-particle duality, Schrödinger’s cat and the double-slit experiment. The explanations of the phenomena depend critically on which interpretation is chosen. And the evaluations of the various interpretations—views on the structure of existence, most of which turn out to be invalid—depend on the principles. So the most logical way to proceed would be to first, understand the principles, then the pluses and minuses of the various interpretations, and finally the phenomena.

Many of the readers, however, will be more interested in the explanations of the phenomena and/or the interpretations than the principles. To accommodate this, we reference the relevant principles in each section of the ‘phenomena’ and ‘interpretations’ divisions of the site so the reader can, for the most part, read the sections in any order.

But there is one pivotal idea that is needed for understanding every section, which is that quantum physics contains many versions of reality. This is explained in section 2, on Schrödinger’s cat. After reading that, you can go to any section in Physics you wish. To put everything together, and to see a list of the principles, you may wish to browse through Overview of Understanding Quantum Physics, section 3, at some point.