The four mythical beasts of physics:

1. Schr?dinger’s cat: beyond life and death.

Of these four mythical beasts, perhaps the most well-known is the most famous thought experiment in quantum mechanics - Schr?dinger's cat.

A century has passed since the birth of quantum theory, and it still confuses people until the 21st century. As Bohr famously said: "Anyone who doesn't get angry when he hears quantum theory for the first time must not understand it."

Schr?dinger's cat is a representative one of many quantum puzzles. Schr?dinger tried to use a thought experiment to test the uncertainties implicit in quantum theory.

First, let’s talk about the idea of ??the experiment. This experiment involves placing a cat in an airtight container containing a small amount of radium and cyanide. There is a probability of the decay of radium. If the radium decays, the mechanism will be triggered to break the bottle containing cyanide, and the cat will die; if the radium does not decay, the cat will survive.

According to the theory of quantum mechanics, since radioactive radium is in a superposition of two states: decay and non-decay, cats should be in a superposition of dead cats and alive cats. This cat that is both dead and alive is the so-called "Schr?dinger's cat".

However, it is impossible for a cat to be both dead and alive, and the result must be known after the container is opened. This experiment attempts to elucidate the issue of the quantum superposition principle at the micro scale from the macro scale, and cleverly connects whether the microscopic material exists in the form of particles or waves after observation and the macroscopic cat, in order to verify the existence form of quantum when the observation is involved.

Second, let’s explain this experiment in plain language: The decay of the atomic nucleus is a random event. What physicists can accurately know is the half-life—the time required for half of the decay. If the half-life of a radioactive element is one day, half of it will be lost after one day, and the remaining half will be lost after another day.

Physicists have no way of knowing when it decayed, in the morning or in the afternoon. Of course, physicists know the chances of it decaying in the morning or in the afternoon—that is, the chances of the cat dying in the morning or in the afternoon.

If we do not open the lid of the secret room, based on our experience in daily life, we can determine that the cat is either dead or alive. These are its two eigenstates. If we use the Schr?dinger equation to describe Schr?dinger's cat, we can only say that it is in a superposition state of alive and inactive.

We can only know for sure whether the cat is dead or alive the moment we lift the lid. At this time, the wave function formed by the cat immediately shrinks from the superposition state to a certain eigenstate.

Quantum theory believes that if we do not lift the lid for observation, we will never know whether the cat is dead or alive. It will always be in a half-dead superposition state, which can turn the microscopic uncertainty principle into macroscopic uncertainty. Determining the principle, objective laws are not subject to human will, and a cat being both alive and dead violates logical thinking.