# InspiringPhilosophy's claim

In the first of my blog series looking at the many claims of apologist InspiringPhilosophy I am going to address his claim that:

Realism is incompatible with quantum mechanics.

Take a second to think what a radical claim that is. According to his definition realism means that "a physical reality exists independent of observation". As Einstein put it, when this view was put to him:

We often discussed his notions on objective reality. I recall that during one walk Einstein suddenly stopped, turned to me and asked whether I really believed that the moon exists only when I look at it. via Mermin

InspiringPhilosophy disagrees with Einstein and thinks things - atoms, the moon - simply don't exist unless something conscious is observing them. He is claiming that there **is no way** in which quantum mechanics can can be compatible with realism.

I hope by writing this post I can there **is a way** of interpreting quantum mechanics, which is compatible with realism. In fact, there's several.

# Interpretations of Quantum Mechanics

There are several different 'realist' 'interpretations of quantum mechanics, which include an objective reality. Some of these are:

- The Many Worlds Interpretation
- DeBroglie-Bohm pilot wave theory
- The Transactional interpretation
- Objective wavefunction collapse theories (such as Penrose's)

Describing any one of these would disprove IP's claim. The most explicitly realist description is DeBroglie-Bohm, so I will describe that one.

# DeBroglie Bohm interpretation

DeBroglie Bohm wave mechanics is a particularly interesting one of these. It was first introduced by Louis DeBroglie at the Solvay conference in 1927 and refined by David Bohm in 1952. The theory has two parts to it:

- The wave function (which evolves according to Schroedinger's equation)
- Particles with a particular position (which are guided by the wavefunction)

The wave function "guides" the particles around, and so this theory is also often called a "pilot wave theory". Once the particles are placed down, they simply follow the paths set for them by the wavefunction. Here is youtube video showing trajectories of particles (according to Bohmian mechanics) in double slit experiment:

Because Bohmian Mechanics depends on the wave function it is a *nonlocal* theory, because the wave function extends over all space. Most of the time we don't know the particle's positions, and our ignorance of the positions of the particles is what leads to uncertainty in where they will strike the screen. Bohmian Mechanics is therefore a *non-local hidden variable* theory.

But notice that the particles exist independently of us measuring them, they even have positions and momentums independently of us measuring them. It is certainly a **realist** theory. You can show you get exactly the same predictions from Bohmian Mechanics as regular Quantum Mechanics for the double slit experiment. So what about other experiments IP mentions?

# Bell's inequalities

Bell's inequalities rule out **local** hidden variable theories. As we just said, Bohmian Mechanics is a **nonlocal** hidden variable theory, so it is not ruled out by this experiment. In fact, Bell himself says so in the first paragraph of the first page of his paper:

A hidden variable theory of elementary quantum theory [Bohmian Mechanics] has been explicitly constructed. That particular interpretation has indeed a grossly non-local structure. This is characteristic, according to the result proved here, of any such theory which reproduces exactly the quantum mechanical predictions.

Far from viewing his inequality as proving that there could be no realist interpretation of quantum mechanics, Bell viewed it as a proof that realistic interpretations had to be nonlocal, in exactly the same way as Bohmian mechanics is. He was (wrongly) told that it was impossible - the claim that IP is making. But then he realised he that was wrong:

But in 1952 I saw the impossible done. It was in papers by David Bohm. Bohm showed explicitly how parameters could indeed be introduced, into nonrelativistic wave mechanics, with the help of which the indeterministic description could be transformed into a deterministic one. More importantly, in my opinion, the subjectivity of the orthodox version, the necessary reference to the ‘observer’ eliminated.

In fact, apart from his inequalities, John Bell had a lot to say on the topic. He published two papers *disproving* claims similar to IP's, claiming the impossibility of local hidden variable theories:

The proof of von Neumann is not merely false but foolish!

# Leggett Inequalities

The Leggett Inequalities rule out a certain class of *non-local* hidden variable theories as well.