Von Neumann’s Catastrophe


Von Neumann’s Catastrophe

In: Physics

I’m guessing you mean “Von Neumann’s catastrophe of infinite regression” or the problem of infinite regress, which comes from the Von Neumann–Wigner interpretation of quantum mechanics.

Essentially, in many of the standard interpretations of quantum mechanics, quantum systems exist in quantum states until the outside world interacts with them, at which point they collapse down into a specific state. “The cat in the box is a combination of being both alive and dead until you open the box”, or “the electron partially goes through both slits.” Systems act in a wave-like way until interacted with and measured.

Von Neumann’s problem with this is that any tool you use to measure the quantum system can become part of that quantum system, and so gets included in the quantum state. The “collapse” of the wave-like behaviour must happen somewhere between the quantum system (behaving in a quantum way) and the individual observer – but each point seems to be arbitrary.

There are a couple of neat quotes on this concept from [the relevant Wikipedia article](https://en.wikipedia.org/wiki/Von_Neumann%E2%80%93Wigner_interpretation):

> The rules of quantum mechanics are correct but there is only one system which may be treated with quantum mechanics, namely the entire material world. There exist external observers which cannot be treated within quantum mechanics, namely human (and perhaps animal) minds, which perform measurements on the brain causing wave function collapse.

Or to use a more physicsy approach:

> From the point of view of the mathematics of quantum theory it makes no sense to treat a measuring device as intrinsically different from the collection of atomic constituents that make it up. A device is just another part of the physical universe… Moreover, the conscious thoughts of a human observer ought to be causally connected most directly and immediately to what is happening in his brain, not to what is happening out at some measuring device… Our bodies and brains thus become … parts of the quantum mechanically described physical universe. Treating the entire physical universe in this unified way provides a conceptually simple and logically coherent theoretical foundation…

In the chain of things from the quantum system to the measuring device to the photons travelling from that to our eyes, to the cells and chemicals in our eyes, to the signals transmitted to our brains, to the material objects in our brain, there is no obvious point to say “this is quantum” and “this is not quantum.” The “infinite regress” is that you can always break down any of those parts infinitely into smaller, quantum systems. The conclusion reached by Von Neumann (and Wigner, although he later changed his position) was that there must be some kind of non-physical human consciousness that exists outside physics – outside reality.

This is obviously problematic from a physics point of view. One of the cornerstones of physics is that it can model or explain everything – that you can’t have things outside physics.

There are a bunch of other ways of looking at this, though, which don’t require non-physical human consciousness. As noted, Wigner himself came to this conclusion later on. And not all interpretations of quantum mechanics even rely on wavefunction collapse.

That said, there is some interesting research going on at the moment into the “[Wigner’s friend](https://en.wikipedia.org/wiki/Wigner%27s_friend)” thought experiment, that might illuminate some things.