What the theory of natural selection and quantum physics have in common – 03/22/2023 – Fundamental Science

The concept of reductionism – no matter how complex phenomena are, they can always be described in terms of their individual components – is central to science. For example, to understand the human body, we have to dwell on its organs, an understanding that requires understanding the set of tissues, the functioning of cells and the structures that compose them, such as membranes, cytoplasm and nucleus, and so on.

We are inevitably led to the behavior of molecules, atoms and their indivisible constituents: to the microscopic world ruled by quantum mechanics and its illustrious and strange consequences, such as superpositions, entanglement and cats apparently alive and dead “at the same time”. But then why don’t we experience the world in the same lysergic way in which it manifests itself at the fundamental level of nature? Would that be the limit of scientific reductionism? Two kingdoms, the micro and the macro, governed by completely different rules?

At the heart of this incongruity is the fact that objects governed by quantum mechanics are indecisive in the extreme — they can be in a superposition, that is, even if we know everything there is to know about them, we can still only make probabilistic predictions about them. your behavior. Every time we observe these peculiar creatures, we can find them here or there.

To illustrate the bizarre consequences if we try to extrapolate this to our daily lives, the Austrian physicist Erwin Schrödinger invented his most famous imaginary experiment: a cat is placed in a closed box with an atom, a hammer and a sealed flask with poisonous gas. Initially, the atom is in its excited state, that is, it is not in its maximum resting state, the one with the lowest energy. However, if the atom spontaneously decays to its lowest energy level, it will emit a photon (a particle of light) in the process. This photon causes the hammer to break loose, breaking the vial which releases the gas and kills the cat.

I mean, if the atom remains as it is, the cat is alive. If the atom decays, it is dead. Being a quantum entity, the atom can be in a superposition of having or not decayed, which implies that until we open the box and observe what is going on inside, the cat can be alive and dead. A hypothesis that even the most ardent zombie series fan would have a hard time imagining.

Incredible as it may seem, however, the appearance of the world around us emerges from this vast menu of possibilities and uncertainties. Inside the box we have air and as the atoms that make up the cat interact with the molecules of the environment, the feline becomes entangled in these molecules. This means that the quantum properties are no longer located in the cat, but are delocalized in the whole. The quantum effects are still there, but now diffused into countless particles. Unless we can access not only the cat but also the zillion molecules that interacted with it (something impractical), the superposition of life and death fades away and, as we would expect, each time we look at the cat we will find it in just one of the two possibilities.

In this way, we recover, albeit partially, reductionism. I mean, the characteristics of the macroscopic world are, yes, a direct consequence of the properties of the microscopic quantum world. We call this process decoherence: the loss of superposition of a quantum object by its interaction with the environment, which explains the transition from the fantastic quantum factory to its classical version, the one we are used to, that is, the world as we see it.

This happens because decoherence acts faster the larger the object in question. While for electrons, atoms or even molecules, maintaining their superposition is something feasible and routinely observed in the most varied experiments, performing a similar feat with everyday objects is not feasible. Even a grain of sand superimposed in two positions separated by a centimeter will lose its quantum characteristics in a quadrillionth of a quadrillionth of a second (a number with 30 zeros after the decimal point), an instantaneous time for all practical purposes.

However, by itself, decoherence leaves open the possibility that, depending on the observer, you or me, the results may be different for the same event: when we open the box, one says that the cat died, while the other reaches the opposite conclusion, a paradoxical and subjective reality that we will never be able to experience. It was noted that the properties we are used to are those best adapted to interacting with the environment, not only surviving decoherence but, in the process, making as many copies of themselves as possible. It is a dynamic similar (at least conceptually) to the natural selection of species proposed by Charles Darwin, which is why we call this process quantum Darwinism.

From the Darwinian perspective, we can assign a well-defined state to the cat not because it is in fact alive or dead (after all, according to quantum theory, before we observe it it could be in a superposition): it is only through the proliferation of copies that the information we observe becomes redundant, explaining the emergence of the objective world in which you and I agree about what goes on when you open the box.

To give you an idea, in just one second under the influence of sunlight, a grain of sand imprints its position on more than ten thousand billion photons. It is this incessant proliferation of information that makes different observers, upon receiving the light scattered by the object, able to agree among themselves about what they are witnessing. After all, if I say the moon is in a certain position, all lovers of the night sky will come to a similar conclusion.

Once, upset with the quantum oddities, Einstein asked: “Do you really believe that the moon only exists when we look at it?”. As the British science popularizer Philip Ball put it, the truth is that “the universe is always watching.”

The next time you hear something about quantum healing, quantum coaching or some other baseless nonsense, remember Darwin and his most psychedelic trip to the quantum realm. Yes, the world is quantum. Too bad the way we experience it, no.

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Rafael Chaves is a physicist, researcher at the International Institute of Physics at UFRN and author of the popular book “Uncerteza Quantica”.

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