Chapter 2 – Wave functions and uncertainty

Schrödinger’s cat

Erwin Schrödinger’s cat is a thought experiment about superposition.

Imagine a cat in a box with a radioactive particle set to trigger release of poison which will kill the cat when it decays. The particle has a wave function giving the possible states over time.

Superposition says the particle is neither decayed nor not decayed until observed.

Does this mean the cat is both alive and dead until you look in the box?

Wigner’s friend

Wigner’s friend is a thought experiment that extends Schrödinger’s cat. It uses two observers: Alice conducts and experiment in a lab with a quantum state that has a wave function that can be in two states. Her friend Bob observes…

Alice observes the quantum state and causes the wave function to collapse to one of the states

Bob has not yet looked so considerers the wave function to still have two possible states.

Bob asks Alice what state was seen.
Now Bob sees the wave function collapsed.

Paradox: when did the wave function collapse? If it was already collapsed when Bob asked Alice, it means what Bob thought of a superimposed states wasn’t really superposition.

The measurement problem

Schrödinger’s cat and Wigner’s Friend are examples of challenges that push the limits of applying the Copenhagen Interpretation to the full size real world. This is known as the Measurement problem. Specifically, how and why do wave functions collapse? The Copenhagen Interpretation just says it happens without answering why. We will return to these issues later