Many Worlds or Many Questions? Schrödinger’s Cat and Interpretations of Quantum Mechanics

Schrödinger’s Cat

   There is no doubt that quantum mechanics is a revolutionary and an exceptionally strange theory. After the establishment of the modern quantum mechanics, physicist divided themselves into several opinion groups. Each of these groups tried to explain the weirdness of the quantum world in a different way, which led to the creation of many interpretations of quantum mechanics. The most acknowledged of these interpretations is the so-called Copenhagen interpretation (this interpretation was used in the previous chapter). Another very interesting interpretation is the so-called many-worlds interpretation. We can demonstrate the difference between these two interpretations on a simple thought experiment.

    Let us say we have a box in which an atom of a radioactive element is located. A radioactive element is an element that undergoes decay to lighter elements in a certain period of time. The problem is that one can never know when the decay occurs, since each radioactive atom is described by a wave function that determines only the probability of the atom decaying over time. The probability of the decay occurring increases with time. Thus, the so-called half-life was defined. Half-life is the amount of time after which the probability of an atom decaying is exactly 50 percent. Each radioactive element has a different half-life (ranging from fractions of a second to millions of years). For instance, if we had 100 atoms of an element with a half-life of one year, 50 atoms would have decayed after one year.

    Let us go back to our atom in a box. For simplicity, suppose that the half-life of our radioactive element is one day, i.e., if we leave the atom in the box for one day, there is a 50 percent probability of it decaying. However, recall that unless a quantum object is observed, it is in a superposition of all possible states. Therefore, the atom is both decayed and not decayed. In other words, our atom isolated inside of the box is in a superposition of two states – decayed / non-decayed. Only when we open the box and observe the atom, does the wave function collapse occur and the atom “decides” whether it is decayed or not decayed based on the probability given by its wave function (after one day, this probability is 50 percent for both decayed and non-decayed state). Now let us consider a situation where we put a vessel full of poisonous gas and a living cat in the box along with the atom. The whole system is set up so that if the decay occurs, the poisonous gas is released, and the cat dies. If the atom does not decay, the gas is not released, and the cat stays alive.

    If this thought experiment seems familiar to you, it is because we are dealing with the most famous “paradox” of quantum mechanics. The author of this thought experiment is a famous physicist Erwin Schrödinger, the experiment is therefore often referred to as Schrödinger’s cat. With this experiment, Schrödinger wanted to demonstrate the vagueness of the Copenhagen interpretation. It bothered him that the Copenhagen interpretation does not clearly define what it means to “observe” a quantum object. According to him, the Copenhagen interpretation basically says that if an atom is in the superposition decayed/non-decayed, the poison is in the superposition released/not released, which implies that the cat is in the superposition alive/dead until the box is opened. The cat obviously cannot be dead and alive simultaneously. That is why Schrödinger considered the Copenhagen interpretation silly.

    However, the authors of the Copenhagen interpretation themselves never saw Schrödinger’s cat as a problem, since they reckoned that the fate of the cat is decided long before the box is opened, since atoms in the air around the radioactive atom “observe” (bump into) it and thereby prevent superposition of the cat. Even the cat herself can observe whether the poisonous gas is released or not, therefore preventing superposition.

    Each interpretation explains Schrödinger’s cat a little differently. For instance, the aforementioned many-worlds interpretation assumes that every time two quantum systems interact, the reality is split into multiple parallel “worlds”. The interaction leads to different results in each of these worlds. In other words, everything that can happen does happen in at least one of the worlds. This means that when the box is opened, the whole universe splits into two universes, one of them containing a living cat, the other one containing a dead one!