And so we get to the notoriously both-dead-and-alive cat, a thought experiment that harms no actual animals, and was actually posed by physicist Erwin Schrödinger in 1935 to illustrate how ridiculous he considerd this interpretation of quantum mechanics to be.
Here’s your basic Wikipedia summary of the situation:
Schrödinger’s cat: a cat, a flask of poison, and a radioactive source are placed in a sealed box. If an internal monitor (e.g. Geiger counter) detects radioactivity (i.e. a single atom decaying), the flask is shattered, releasing the poison, which kills the cat. The Copenhagen interpretation of quantum mechanics implies that after a while, the cat is simultaneously alive and dead. Yet, when one looks in the box, one sees the cat either alive or dead, not both alive and dead. This poses the question of when exactly quantum superposition ends and reality resolves into one possibility or the other. —WIKIPEDIA
Here’s another way to look at the thought experiment:
Quantum mechanics—the study of atoms and even smaller particles, like quarks, gluons, and more—is super weird. It turns out that things at such a tiny scale simply don’t work in the same way that the everyday world works. Some particles can interact at huge distances or be in more than one place at once or tunnel through things—and maybe even time-travel!
The physicist, Erwin Schrödinger, working around the time of Einstein, came up with a thought experiment involving a cat sealed in a box. The box also contains a radioactive substance that could break open a jar of poison that may or may not have killed the cat. According to one interpretation of quantum theory, the cat would be BOTH dead and alive in the box until the box was opened. The act of observing the cat would actually cause it to be EITHER dead or alive, not both. Schrödinger thought the idea was absurd—but it turns out the super-tiny particles of quantum world actually do work something like that.
This thought experiment has to do with a cat in a box, quantum superposition, wave-form collapse, the measurement problem, and the many-worlds theories.
I strongly suggest watching a few videos if you want to get up to speed. But the bottom line is that the quantum world is super-weird and cats in boxes are cute.