When I last (new tab) talked about Schrödinger's Cat, I left one thing out because I didn't know the answer: when and where did the misinterpretation come from? I think I know the answer.

When Schrödinger and Einstein made up the thought experiment they were trying to answer one question: can quantum effects be seen in the real world. By "real world" they meant the macroscopic, non-atomic-level, world. 

In the early 1970's Stephen Hawking was trying to take Einstein's General relativity back in time to the origin of the universe, and was failing to get there. He took a post-doctoral fellowship in the lab of Roger Penrose, an expert in quantum theory, and at some stage of the very early universe switched form general relativity to quantum theory to get all the way back. He published, became famous, wrote a book, A Brief History of Time, and it's about then I remember hearing of the other interpretation of Schrödinger's Cat.

In this version the Cat experiment wasn't a question, it was a demonstration. Hawking presented the Cat as a way of explaining a longer-range quantum effect, one large enough to encompass the universe. Hawking used the Cat to explain how there are multiverses, how entanglement shows that particles at a great distance are still part of the same wavefunction, so that he could get large-scale quantum effects. He needed to do this, because his theory had a huge problem: he was dealing with a universe, not an atom.

In 1924 Louis de Broiglie put in his dissertation a derivation for a simple but perplexing equation, λ = h/mv. It calculated the wavelength, λ, of any matter of mass m traveling velocity v, and h is Plank's constant, 6.626 x 10^-34 J-s, a very small number. The mass of the universe is about 10⁷⁷ kg, and the velocity is constrained by the speed of light to below 3 x 10⁸ m/s. So put those numbers together and you see that the biggest wavelength possible of a quantum universe is 10^-92 m. And since quantum effects are exceedingly rare more than about 3 wavelengths distant, the early quantum universe falls apart before it has even formed. And Hawking I think knew he needed to get around this, so he misinterpreted the Cat. 

Because A Brief History was popular, his version of the Cat became the most-stated version. The popular version. The version you've heard. The wrong version.

When Schrödinger and Einstein asked, "Can quantum effects be seen in the real world?" the clear answer was a resounding No! We see no effect in the real world that in any way appears to be directed by atomic-level quantum effects. That's why understanding it is so difficult; it describes behaviors none of us will ever see. We can see these effects in atoms, and sometimes molecules, and even in solids up to ten nanometer (10^-8 m) scale. We've accepted the quantum explanation of larger-scale phenomena, like the Bose-Einstein condensates, supercritical fluidity and superconductivity, but (as far as I'm aware) those examples have evaded quantum calculability, and quantum theory falls short of explaining them as well as it does atomic and molecular properties.

So the next time someone tells you how weird the universe is because of Schrödinger's Cat, remember that the Cat really only shows you how normal the universe is.

UPDATE June 13 2024: I just read a Physics World opinion piece by Robert P Crease on the popularization of the Cat, attributing it to a short SciFi story by Ursula Le Guin in 1974. He's right. Hawking published about the Cat scientifically during the 70's, but didn't write A Brief History until 1998, long after the Cat entered the public imagination. Interestingly, Le Guin's short story includes the reader caught up in the experiment, which makes it sound more like Wigner's Friend than the Cat.