Raise your hand if you have never heard of Schrödinger’s dead cat, live cat paradox. More or less, we all know the facts: a cat, some poison, a radioactive particle, and a box. If I put the cat in the box with the poison, how do I know if it is dead or alive? It will always be in a state of being alive and dead at the same time until I lift the lid. But isn't that obvious? Actually, no, because if we follow the principle that the state of a quantum system is not determinable, then we have to think of a superposition of states. It’s about changing the way we look at reality.
Superposition of states
For quantum mechanics (QM), many of the properties of an object are generated by observation; they are not "properties" that can be determined prior to observation. Consider a small electron. In classical physics it is possible to assign it a "position" and a "velocity" (momentum), but in QM velocity and momentum are not "properties" but " possibilities" that we measure and discover only at the time of measurement, before which they are only probabilities, and probabilities can coexist and superpose. Until we observe the electron, it is in all possible positions. The cat experiment can be related precisely to the "problem" of measurement.
The Copenhagen School
At the beginning of the twentieth century, currents of theoretical thought in quantum physics found their main point of reference in the "Copenhagen School," formed by a group of physicists around Niels Bohr. In short they say:
- Talking about objectively existing reality is pointless;
- Reality is not intelligible;
- Reality does not follow the laws of causality;
- Determinism does not exist.
The Copenhagen interpretation says that if a quantum system is in a superposition of the form A + B, measuring it forces it to go permanently into state A or state B. When we measure, the superposition disappears and the wave function, which contained all the possibilities of states, collapses (imagine the wave function as a probability cloud around the nucleus of an atom, instead of the classical orbit we are used to). The Copenhagen interpretation never explains how this collapse occurs. This led Einstein to doubt the theory.
Schrödinger theorized the cat experiment precisely in the context of the discussion of the measurement paradox as put forward by Einstein-Podolsky-Rosen in a 1935 paper (called EPR). The EPR paradox is a critique of quantum entanglement, the idea that two interacting physical systems should be treated as one system. Schrödinger agreed with these doubts. He also pointed out that superposition, one of the cornerstones of QM, had some other problems. For him, entangled and superposed states were paradoxical. To make the paradox more obvious, he decided to leave the subatomic dimension and apply the concept to something macroscopic, like a cat.
Here is the Cat
This is the conceptual experiment. Consider a perfectly insulated box ("perfectly isolated" is an indispensable condition in QM, because it prevents observation). Put two elements in the box: a cat and a radioactive particle. The particle must be radioactive, because radioactive particles can transform into other kinds of particles, and thus have multiple potential states. The box has a mechanism that triggers the opening of a poison bottle when the particle transforms. So when the particle transforms, the mechanism opens the poison bottle and kills the cat. The half-life of the chosen radioactive substance is 1 hour, so in 1 hour the particle has an equal chance of transforming or not transforming. No one can "observe" the transformation of the particle or predict whether it will transform or not because the box is isolated. What happens to the cat then?
Schrödinger’s cat experiment is based on the principle that the "measurement" of a quantum system is the entanglement between the (macroscopic) measurement instrument and the quantum system under investigation. So, the cat is the measuring instrument, because he "observes" the state of the radioactive particle. In other words, the presence of the cat causes the superposition in which the particle is (transformed or not transformed) to evolve into a system of the cat + the particle. It is not the cat that is in a superposition of two states. The superposition affects the entire system of cat + radioactive particle, which are in a state of entanglement.
Cats and paradoxes
The paradox arises from the fact that in QM it is not possible to describe objects classically. Instead, a probabilistic representation is used, which is partly incompatible with "classical physics" and the way we interpret reality. Since every particle, once observed at a position, definitely takes this position and is therefore no longer in a "superposition of states", the operation of observation (measurement) irrevocably changes the system. Thus, by opening the box, the wave function of the cat-particle-system collapses. And the cat will probably be live, because cats are known to have nine lives. Maybe that is why Schrödinger chose it to go down in QM paradox history.
I hope that I have been able to fascinate you. Well, I'm not saying I'm sure I understand Schrödinger’s cat. But that's what makes it so cool. I like quantum physics ‘cause every time you think you've got it, it eludes you.