Put a cat in a box, he proposed, and rig up a Rube Goldberg contraption involving a hammer, a vial of poison and a quantum triggering device. If an electron is in one position, the hammer will remain safely cocked. But if the electron moves into the opposite location, the hammer will drop, smashing the vial and killing the cat. The laws of quantum mechanics hold that as long as the electron remains undisturbed, it hangs in limbo, occupying both its possible states. The cat, by extension, is both dead and alive.This is always presented as some profound quantum mechanical truth, but the pre-quantum explanation is not any different from the quantum explanation. There is not a shred of empirical evidence that a cat can be half dead and half alive. For all we know, the above ket (wave function) is just a mathematical shorthand for our lack of knowledge about whether the cat is dead or alive. Some people say that the cat is dead in one world, and alive in another. But that explains nothing. In quantum mechanics, a state is often written as a superposition of possibilities, even tho a measurement would constrain the state to one of those possibilities. The Schroedinger cat is just an analogy for thinking about such superpositions. But it is not to be taken too seriously.
Cosmologist Sean M. Carroll has put his Quantum Mechanics Made Easy online. It is not bad. He is interested in the arrow of time, and he notes that collapse of the wave function is irreversible. There is no agreement on what the collapse means, or whether the wave function really collapses. He also encourages describing quantum mechanics in 5 words:
Interfering probability theory.I like the "waves until measured".
Everything is waves, until measured.
representation theory of Poincaré group
unintuitive doesn't mean not comprehendable
You understand it? Me, neither.
multiple outcomes to single measurement
Probabilities on orthomodular proposition lattices
Imperfectly predictable and unpredictably perfect.
Schizophrenic states waiting for measurement.
nonrelativistic particles with discrete energies.
They're everywhere. Until you look.
Everything not forbidden is mandatory.
All knowledge is precisely uncertain.
Stop dictating to God, Einstein!