The demon and
the cat:
two quantum

Michele Diego

The demon is able to predict with certainty not only the future of the universe in which it finds itself at that moment, but also of all the universes that will branch off from it.


“When I hear about Schrödinger’s cat, I reach for my gun “, these are Stephen Hawking’s words regarding his thoughts on the famous quantum cat paradox, which can turn a normal evening between physicists into a brawl. The paradox was coined in 1935 by the Austrian physicist Erwin Schrödinger in an attempt to take the predictions of quantum mechanics to the absurd. The sadistic mind experiment is as follows: we lock a cat in a box with a tiny portion of radioactive substance, a Geiger counter, a hammer, and a vial filled with cyanide. Once an atom of the radioactive substance decays, the Geiger counter registers the decay and activates the hammer which breaks the vial with cyanide, resulting in the cat’s death. We cannot know for sure when the first atom of the radioactive substance will decay; the only thing we can be sure of, is that if it does, the cat will die from poisoning. Therefore, without opening the box, there is no way of knowing whether the cat is dead or alive. So far, nothing so extravagant. The point is that, in quantum mechanics, a system can be in a superposition of different states, even ones that are incompatible with each other. This means that the function associated with the atom of the radioactive substance can describe the atom as simultaneously ‘decayed’ and ‘undecayed’. When a measurement is then made on the atom, its function is said to ‘collapse’ to only one of the possible states (in this case ‘decayed’ or ‘undecayed’) and thus, the result of the measurement records a well-defined and non-contradictory condition. In practice, according to quantum mechanics, as long as a system is not observed, it exists simultaneously in all possible states; it is only with its measurement that the system collapses into a definite state. This, going back to Schrödinger’s cat, would lead to the paradoxical conclusion that it is both alive and dead until we open the box, and only then will it collapse to either the ‘alive’ or ‘dead’ state.

We could imagine that in reality the atom (and hence the cat) is in a precise ‘decayed/not decayed’ (‘alive/dead’) state at all times, and that, in our ignorance, we can only find this out by taking a direct reading. Until this moment of measurement, we somehow mask our ignorance by pretending that the system is simultaneously in two distinguished states, though in reality we know that this is not really the case. The function describing the state of the system during the observation process would, therefore, not collapse to one of the possible states, but simply reveal explicitly the state in which the system was, prior to measurement. Albert Einstein thought something similar – a thought encapsulated in his dictum “God does not play dice with the universe”.
In conventional physics, this is indeed the case. The results of a given experiment are always certain, at most it is we who are unable to define the parameters of the system well enough. In the latter case, we conceal our ignorance with the concept of probability. Let us consider, for instance, the tossing of a coin. We all know that the probability of getting ‘heads’ is fifty percent, but if we could know exactly from what height the coin is tossed, with what force, at what exact point this force is applied, etc. we could predict the result with great certainty. Pierre-Simon, marquis de Laplace imagined the most disturbing consequence of such a world: Laplace’s demon, a being who knows the position and speed of all the particles in the universe. Through the laws of conventional physics, it would be able to predict precisely and decisively the entire past and future of the world; nothing could surprise it or escape its calculations.

In truth, the matter is much more complex, and there are experiments in which an interpretation in these terms of quantum mechanics leads to erroneous predictions. Without going into too much detail, we can simply say that the experiments show interference effects between the different coexisting states prior to the measurement, so we have to accept the fact that, prior to observation, the system is not in one precise and well-defined state. This implies that there is no certain prediction of what the state of the system will be once measured. In some ways, God actually does play dice with the universe, implying that probability is inherent in quantum systems. Probabilistic statements in the quantum field are ‘irreducible’, they do not reflect our ignorance. Laplace’s demon seems to be vanquished by an unpredictability inherent in nature itself.

And yet, is the atom really simultaneously decayed and undecayed before we measure it? There are different answers to this question, corresponding to different interpretations of quantum mechanics. According to the ‘Copenhagen’ interpretation – one of the most widely accepted among scientists – the question is ill-posed because science can only study what is detectable and observable, thus the question of what happens before measurement is meaningless.
However, there is another interpretation, equally legitimate, known as the ‘many-worlds’ interpretation. According to the proponents of this interpretation, there are two universes: one in which the decay of the atom with the consequent death of the cat takes place, and another in which the opposite scenario takes place. Basically, whenever a quantum system is at a crossroads, it takes both possible paths, and the universe somehow splits into two universes with opposite scenarios. This is where a more modern version of Laplace’s demon returns. When faced with a quantum experiment, it knows exactly all the possible outcomes and knows that they will all come true, each in a different universe. Therefore, the demon is able to predict with certainty not only the future of the universe in which it finds itself at that moment, but also of all the universes that will branch off from it.


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