Science & Technology

Adding a Second Cat to Schrödinger's Cat Experiment Might Break Quantum Physics

Even if you might not fully understand it, you've probably heard of Schrödinger's cat. It's that old thought experiment where a cat inside a box is both alive and dead, and it's supposed to illustrate the absurdity of quantum mechanics. Recently, a pair of physicists gave it a reboot: They added a second cat in a second box. What happens when two observers open both of those boxes is very bad news for quantum mechanics.

Related Video: The Quantum Experiment That Broke Reality

Schrödinger II: Inception

The original, canonical, before-they-added-all-the-CGI version of Schrödinger's cat goes like this: You put a cat in a box with a vial of poison that releases at random. Because you don't know whether the poison released, the state of the cat is uncertain — it's both alive and dead until you open the box. This metaphor from 1935 is an illustration of a few basic principles of what's known as the Copenhagen interpretation of quantum mechanics. When the cat is both alive and dead, it's in a state of "superposition"; when you open the box and see its state, you've "collapsed the wave function." That all might sound outlandish, but something similar happens to quantum particles. You can never measure them exactly; they're more of a cloud of probabilities than a concrete "thing." As a result, they exist in a state of superposition until they're measured, at which point their wave function collapses into one state.

Ok, so if opening the box and looking at the cat collapses the wavefunction, what if you change the story a little? Instead of a cat inside the box, let's put a tiny physicist capable of making her own observations. That's what Hungarian physicist Eugene Wigner proposed in 1967: A physicist, let's call her Alice, gets inside of a box with a measuring device that produces one of two results at random — say, it flips a coin to show either heads or tails. Does the wave function collapse when Alice sees the result? It seems likely. But hold on — what if Wigner comes along and opens the box? Alice and her coin should be in a state of superposition until that moment. Weird, right?

Alice in Quantumland

In 2016, Daniela Frauchiger and Renato Renner of the Swiss Federal Institute of Technology proposed an even more complex setup: take Wigner's setup and double it. Now you have two tiny physicists (Alice and Bob) inside two boxes with Wigner A and Wigner B looking on from the outside (they're in separate rooms, so they can't see each other's boxes). Alice flips a coin, then sends a quantum message about the result — say, a quantum particle spinning clockwise if it's heads and counterclockwise if it's tails — to Bob. Bob uses his knowledge of quantum mechanics to figure out the result Alice is trying to communicate to him. Finally, both Wigners open their boxes and determine the result of the coin toss. What happens?

According to Frauchiger and Renner, in certain scenarios, it's possible for both Wigners to be certain of the result but come up with different answers. That's a big deal: It basically says that the Copenhagen interpretation of quantum mechanics contradicts itself. Understandably, that has a lot of physicists in a frenzy. "Some get emotional," Renner told Nature.

There may be a few ways out. One is going with another interpretation of quantum mechanics, although Frauchiger and Renner ran the same scenario through a few other interpretations and they also had issues. Another possibility is running the actual experiment using quantum computers in place of imaginary physicists. That way, they could stop the program along the way and see how things were shaking out. Unfortunately, no quantum computer powerful enough to do this exists quite yet. But it may someday, and it's exciting to think about the ways it could help us understand the topsy-turvy quantum world.

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Written by Ashley Hamer October 4, 2018

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