Season 17, Episode 4: The Copenhagen Interpretation Explained

Hey StarTalkers! Season 17, Episode 4 saw Neil, Chuck and special guest Professor Sean Carroll answering a selection of questions about cosmology and quantum mechanics. As part of this, Sean answered a question from a listener – and Neil – about the “Copenhagen Interpretation” of quantum mechanics:

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Its development saw physicists like Bohr and Heisenberg waxing philosophical about scientific language and the nature of measurement. It’s a mind-bending topic, but luckily, the famous Schrodinger’s cat thought experiment is here to help.

The Basics of Superpositions

We normally think about atoms like a mini solar system: the nucleus sits in the middle like the sun and the electrons orbit around it like planets. The electrons are in a specific place, moving a specific speed, at all times.

But in quantum mechanics, the electrons aren’t really in a definitive location. They’re represented by a wavefunction, which includes all possible locations. It basically says “there’s a 60% probability it’s over here and a 40% probability it’s over there.”

So if you ask, “where is the electron?” This isn’t so easy to answer. The electron is in a superposition of multiple places at once.

Schrodinger’s Cat and Everything Wrong with Quantum Mechanics

The Copenhagen interpretation was the accepted answer to this question for a long time. The electron is considered in both places, until it’s measured. At that point, the wavefunction collapses into one possibility, and that is what we detect in experiments.

As Sean Carroll explains, the most extreme version states that only the outcomes of the measurements are actually real. The wavefunction is just math.

But Erwin Schrodinger had problems with this.

He set up a scenario where a cat was in a box with a cruel device that smashes a bottle of poison if a radioactive atom decays. This atom is also in a superposition prior to measurement, just like the electron’s position: if we don’t measure it, the atom has both decayed and not decayed.

But what would that mean for the cat? Well, according to the Copenhagen interpretation, it would mean the cat was both alive and dead. We would only collapse the wavefunction and find out when we opened the box.

This made a lot of people, including Schrodinger and Einstein, very uneasy.