Why is Schrödinger's cat experiment weird? Isn't anything in the macro world in a state of superp… by Viktor T. Toth

Answer by Viktor T. Toth:

No. Nothing in the macroscopic world is in a state of superposition, except perhaps for rare systems like superfluids.

First of all, what makes a quantum system “quantum” is not size, but number of degrees of freedom. An elementary particle is small, granted, but more importantly, it only has a few degrees of freedom. In contrast, a typical macroscopic object, which consists of a gigantic number of particles, has an enormous number of degrees of freedom. As a result, a macroscopic object is “almost in an eigenstate almost all the time” (my expression), but the “almost” part here really means exponentially vanishing probability of there being anything else.

When a quantum system interacts with a macroscopic system, it is forced into an eigenstate (this is what we call wavefunction collapse). Since a cat, a vial of poison, a hammer are all macroscopic systems… wavefunction collapse occurs long before anyone opens the box that contains Schrödinger’s cat.

Here is another way of looking at this problem: compare the cat of Schrödinger to the classic two-slit experiment. In the two-slit experiment, electrons go through the slits and hit the screen. The important lesson of this experiment is that even after you observe the electron’s impact on the screen, you cannot reconstruct its path. That’s because it did not have any. Between its emission and its impact on the screen, the electron had no classical path.

In contrast, when you open the box with Schrödinger’s cat inside, if there is a live kitty, you can easily reconstruct its history; it was alive all along, the radioactive atom not having decayed. And if the kitty is dead, not only do you observe that it’s dead, you can reconstruct the moment of its death with the help of a qualified veterinarian (or better yet, just include a camera in the box).

This is a crucial difference: the cat had a classical history. We were not able to predict it, but we can reconstruct it unambiguously. There was no superposition at any time. The electron, on the other hand, really went through both slits at once. It was in a superposition between its emission and its impact on the screen. It had no classical path that could be reconstructed.

Why is Schrödinger's cat experiment weird? Isn't anything in the macro world in a state of superposition until observed?

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