In his original thought experiment, Schrödinger imagined that a cat is locked in a box, along with a radioactive atom with a half-life of one hour that is connected to a vial containing a deadly poison. If the atom decays, then the Geiger counter will detect the radiation and it causes the vial to smash and the cat to be killed. If the atom does not decay, then the vial will be intact and the cat will be alive. After the one-hour period, the atom is in a state where it is both decayed and not-decayed, this means that the vial is both broken and not-broken and, ultimately, according to the Copenhagen interpretation of quantum physics the cat is both dead and alive at the same time – which clearly does not happen in classical physics.
With this, Schrödinger was attempting to demonstrate the limitations of quantum mechanics. Quantum particles can be in two or more different quantum states at the same time but surely, he argued, a classical object made of a large number of atoms, such as a cat, could not be in two different states.
The biggest problem with this analogy is that quantum physics typically only operates on the microscopic scales of atoms and subatomic particles, not on the macroscopic scale of cats and poison vials.
The quantum wave function portrays all physical quantities as a series of quantum states along with a probability of a system being in a given state. Consider a single radioactive atom with a half-life of one hour. According to the quantum physics wave function, after one hour the radioactive atom will be in a state where it is both decayed and not-decayed. Once a measurement of the atom is made, the wave function will collapse into one state, but until then, it will remain as a superposition of the two quantum states.
This is a key aspect of the Copenhagen interpretation of quantum physics – the physical reality is not determined until the act of measurement takes place. In some unknown way, the very act of observation is what solidifies the situation into one state or another . Until that observation takes place, the physical reality is split between all possibilities.
In some strict views of the Copenhagen interpretation, it is actually an observation by a conscious entity which is required.
Another interpretation, the Many Worlds Interpretation of quantum physics, proposes that the situation actually branches off into many worlds, universes. In some of these worlds the cat will be dead upon opening the box, in others the cat will be alive.