What is the Wigner’s Friend thought experiment?

In: Physics

In standard quantum mechanics we have **systems** that can be in various **states**. For example, we might have a “system” which is a “cat-in-a-box” and the “states it can be in are “state where the cat is alive” and “state were the cat is dead.”

Quantum mechanics tells us that quantum systems don’t exist in specific states. Instead, they exist as a combination of all possible states (including contradictory ones – this is called **superposition**) until something outside the system **interacts** with (or “observes”) them. When that interaction occurs, the system **collapses** down into one of the individual states (with a certain probability).

For example, from the outside, our cat-in-a-box system will be in a combination of the cat-is-alive state and cat-is-dead state until we interact with it, at which point we find out which one is true.

Crucially, this isn’t just a mathematical model. The double-slit experiment (among others) shows that this is a “real” effect; in that experiment we get our system into a combination of contradictory states, and those states interfere with each other (we throw an electron at a wall with two slits in it – the electron-wall system gets into a superposition of the state where the electron went through one slit, and the state where it went through the other slit, and those two “bits of electron” interfere with each other on the other side of the wall – kind of like if they had hit each other).

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So what Wigner’s Friend does is take this a step further.

We set up our quantum system as normal, and get it into what is called a “cat state” – where it is in a combination of two contradictory states (call them “1” and “0”). We then get a friend to sit in the lab, interact with the system to measure which state it is in, which collapses the system into only one of the two states (either 1 or 0).

But we sit outside the lab. So from our point of view, the lab is itself a quantum system (with both the experiment and our friend), which means it will be in a cat state until we interact with it; it will be in a combination of the “friend measured 1, experiment was in 1” state and “friend measured 0, experiment was in 0” state until then. When we interact with the lab-system (by asking our friend what they measured) it will collapse down into just one of those states.

The question Wigner’s Friend thought experiment asks is “when did the original experiment system collapse out of a cat-state?” Did it collapse when our friend measured it, or did it collapse when we asked our friend?

It helps if you understand quantum superposition first, and how observing it seems to change how the quantum partical/wave behaves. A thought experiment known as Schrödinger’s cat helps to illustrate this phenomenon. Then Wigner’s Friend expands on this thought experiment by illustrating that the observer of the experiment is caught up in the quantum superposition. The quantum can be a partical or a wave, but the observer might see a partical or a wave, so the observer of the observer notices both superpositions (the experiment AND the observer) in Wigner’s Friend.