Quantum Mechanics without and with Physical Meaning

Niels Bohr on Confused Understanding.

The crisis of modern physics can be seen to be a consequence of the fact that the foundation of modern physics in the form of Standard Quantum Mechanics StdQM described by Schrödinger's equation from 1926, still 100 years later is viewed as a deep mystery beyond comprehension, as witnessed by all leading physicists including Bohr, Schrödinger, Feynman....

Let me here expose the fundamental mystery as the mystery of the solution to Schrödinger's equation for an atom/molecule with $N$ electrons numbered 1,2,...,N, as a complex-values wave function $\Psi (x_1,x_2,...,x_N)$ depending on $N$ separate three-dimension coordinates $x_1,x_2,...,x_N$ altogether $3N$ spatial coordinates (plus time). 

The wave function $\Psi$ is the crown jewel of StdQM, which theoretical physicists speak about with great pride and conviction: All there is to know about an atom/molecules is carried by its wave function $\Psi$ as it evolves in time according to Schrödinger's equation!

However, because of the many spatial dimensions $\Psi$ cannot be given a direct physical meaning, and instead a probabilistic meaning was assigned by Born in 1926. StdQM thus offers the following meaning of $\vert\Psi (x_1,x_2,...,x_N)\vert^2$ as

•  the probability density for finding electron $i$ at the position $x_i$ for $i=1,...,N$.

To seek to understand, let us simplify to $N=1$ and so consider the Hydrogen atom H with just one electron, with wave function $\Psi (x)$ depending on a 3d space variable $x$:

•  $\vert\Psi (x)\vert^2$ is the probability density of finding the electron at position $x$. (*)

We are thus led to inspect the meaning of "finding the electron at a specific position". What does it mean?

Is it really possible to experimentally "find an electron at a specific position" or "locate an electron to a specific point in space"?

To give a meaning to "finding an electron at a specific point" requires that we view an electron as a particle without extension in space. An electron is thus viewed as a point particle which can be found at different positions $x$ in space with probability density given by $\vert\Psi (x)\vert^2$.

We next note that "finding an electron at $x$" means that somehow the position of an electron as point particle can be measured or observed. This must be the meaning of "finding".

We then recall that measuring the position of an electron precisely is impossible since after all an electron is not a point particle, but rather a wave or charge density extended in space and the extension gives the size of an H atom with its electron "cloud". It is thus impossible to measure the position of an electron as point particle within an H atom and so "finding the electron at position x" has no meaning.

We learn that the meaning given to the wave function by (*) has no meaning. This may seem troublesome, but it has not prevented modern physicists from describing the Schrödinger equations with its wave function $\Psi$ as a scientific triumph surpassing that of Newton's mechanics. As the foundation of modern physics.

The excuse to lack of meaning $\Psi$ is that even if its meaning is hidden to humans, it carries all information there is to find about an atom/molecule. To find this information it is sufficient to compute the wave function $\Psi$, whatever meaning it may have, and then extract meaningful information.

But now comes the next obstacle: Because of its many spatial dimensions, $\Psi$ cannot be computed.

 
To handle this, various compressions of $\Psi$ to computable form have been used in practice like Hartree-Fock and DFT with some success but also many shortcomings. In these compressions electron charge densities play a central role coming with a difficulty of electron density overlap. But if $\Psi$ before compression has no physical meaning, why should it have a physical meaning after compression?

RealQM is an alternative to StdQM based on non-overlapping one-electron densities with direct physical meaning, which is computable for many electrons.

Recall that one troubling contradiction of StdQM (avoided by RealQM) is to (see this post)

• first label identical electrons in the wave function $\Psi (x_1,x_2,...,x_N)$ 
• and then seek to remove the labels. 

Recall that another troubling aspect is the support overlap of the electronic trial functions used in Hartree-Fock and so underlying DFT, an overlap which has to be controled through the Pauli Exclusion Principle introducing Pauli Repulsion as a purely mathematical phenomenon without physics (see this post).