The world around us certainly appears to be unique, the physical world, upon which we all agree. But in quantum mechanics, the highly successful mathematical description of nature, there is no unique physical world; instead there are many simultaneously existing versions of physical reality. We will illustrate this here using nuclear decay and Schrödinger’s cat. Other examples of simultaneously existing versions of reality, involving the Polarization of Light, the Spin of Particles, the Double-Slit Experiment, Scattering Experiments, and the hydrogen atom spectrum (Measurement Theory), are given in other sections. The first three are particularly important because they are often used to explain the peculiarities of quantum mechanics as well as in actual experiments which test those peculiarities.

We should hasten to add that even though there exist many versions of reality, quantum mechanics implies we perceive only one version (see Classical Perception), in accord with our everyday perceptions.


We first consider the wave function of a single radioactive nucleus. In the mathematics of quantum mechanics, there is both a part of the wave function corresponding to an undecayed nucleus and, simultaneously, a part corresponding to a decayed nucleus. (Typically the ‘decay’ of a nucleus involves a neutron splitting into a proton, an electron and an anti-neutrino). That is, the state of the nucleus at a given time is


Both potential versions of reality, both branches of the wave function (‘branches’ because the wave function branched into two separate versions) exist simultaneously! One cannot get around this; all the successes of quantum mechanics absolutely depend on it.

Note that we are not saying at this point whether ‘the nucleus itself’ is both decayed and undecayed. We are simply saying here that in the mathematics, ‘the nucleus’ is both decayed and undecayed. However, in the section No Evidence for Particles, we will show it is a virtual certainty that there is no such thing as the nucleus; instead there are, in ‘reality,’ several simultaneously existing versions of the nucleus.


The Schrödinger’s cat experiment, devised by Schrödinger in 1935 to illustrate the ‘irrationality’ of quantum mechanics, is a clever and dramatic way of boosting the strange multi-reality situation in nuclear decay from the microscopic level of the nucleus to the macroscopic level of people. A cat is put in a box along with a vial of cyanide. Outside the box are a radioactive source and a detector of the radiation. The detector is turned on for five seconds and then turned off. If it records one or more counts of radiation, an electrical signal is sent to the box, the vial of cyanide is broken, and the cat dies. If it records no counts, nothing happens and the cat lives.

Classically, there is no problem here (unless you are a cat lover). Either a nucleus radioactively decays, the cat dies and you perceive a dead cat when you open the box; or no nucleus decays, the cat lives and you perceive a live cat.

But this is not what happens in the quantum mechanical case. There, the wave function of the nucleus, the cat, and you, as the observer, is


There are now two full-blown, simultaneously existing versions of reality. In one, there is a dead version of the cat, in the other there is a live version. In one, version 1 of you perceives a dead cat, in the other, version 2 of you perceives a live cat.

The consequence is that in quantum mechanics, there is no singular ‘you;’ instead there are two simultaneously existing, equally valid versions of you, the observer, each perceiving something different! (There will also be situations in which there are more than two versions of reality and the observer.) We can summarize these results as

[P2] The wave function contains many versions of reality, with a different version of the observer in each one. Only versions of the observer perceive. No one version is singled out as being ‘special,’ so quantum mechanics does not predict which version will correspond to our perceptions.

Further, there is an astonishing result, a combination of theory and observation:

[P3] In every case where both the calculations and observations can be carried out, our perceptions agree exactly—both qualitatively and quantitatively—with those of one version of the observer.

Property [P3] is, in my opinion, the pivotal observation in deducing the ‘correct’ interpretation of quantum mechanics. Although a good deal of work needs to be done to convincingly show this, it would appear to imply (1) that physical reality is composed of wave functions alone (because the wave function always contains a perfect description of what we perceive), and (2) the wave functions are what ‘we’ perceive.


When we have isolated, non-interacting objects that have no influence each other’s states, such as an atomic nucleus or a cat, the state of one has no correlation to the state of the others. The cat can be alive or dead independently of whether or not the nucleus has decayed. But when some arrangement makes the state of one dependent on the state of another—as when the alive or dead state of the cat depends on the decayed or not decayed state of the nucleus—we say the states of the two are entangled. This entanglement accounts for much of the ‘weirdness’ of quantum mechanics. It is particularly important in the Bell-Aspect experiments on non-locality.