Thursday, March 22, 2018

Can there be life without the weak interaction?

Beta and neutron decay

On January 30, 2018, the Science News magazine commented on an article recently published in arXiv which states that in a universe without the weak interaction, life would still be possible. When I read the Science News article, I immediately thought of an objection that could ruin both the thesis of the original article and its popularization. The first thing I did was looking up the original article, to see if my objection was mentioned or denied, but there was not a word about it. Next I detailed my objection in a comment in the web version of the Science News article, but so far no one has answered me. However, I think the objection is quite strong, and unless I am answered satisfactorily, in my opinion these articles are discredited.
According to the standard cosmological model and the standard model of particle physics, there are four fundamental interactions or forces in the universe: gravitation, electromagnetic interaction, and the strong and weak interactions, which regulate the work of atoms and elementary particles. In particular, the weak interaction affects all the elementary particles: leptons and hadrons, unlike the strong interaction, which affects only hadrons.

Let us look at some of the things for which the weak interaction is responsible:
Feynman diagram of the beta decay

         The beta decay of atomic nuclei, which is the emission of an electron (beta radiation) and the increase in the atomic number of the nucleus, which at the end has a higher positive charge (one unit more than it had). The reaction takes place according to the Feynman diagram of the attached figure.
         The spontaneous decay of free neutrons into a proton and an electron. This reaction works according to the same diagram as the previous figure.
         The formation of degenerate stars such as pulsars or neutron stars.
         The violation of the P (parity) and CP (charge-parity) symmetries. The first asymmetry implies that, for some elementary particles, their right is not the same as their left (this inspired Isaac Asimov the title of one of his books: The left hand of the electron). The second asymmetry shows that, to restore symmetry, it is not enough to simultaneously exchange parity (right-left and vice versa) and the electric charge (in other words, go from a left-handed electron to a right-handed positron), because this joint symmetry can also be violated by the weak force. The discovery of the violation of these symmetries was the reason for several Nobel Prizes: 1957, awarded to Chen-Ning Yang and Tsung-Dao Lee for the discovery of the P symmetry violation; 1980, awarded to James Cronin and Val Fitch, for discovering the violation of the CP symmetry; and 2008, awarded to Makoto Kobayashi and Toshihide Maskawa, for predicting that the violation of the CP symmetry required the existence of three generations of leptons, as in 1973, when they made this prediction, the third generation had not yet been discovered.
         The violation of CP symmetry is considered the most likely explanation for the asymmetry between matter and antimatter in the universe. Indeed, shortly after the Big Bang, as the universe expanded, different types of particles appeared in succession. Later on, when these particles came into contact, those of matter mutually annihilated with those of antimatter, but as the former were a little more numerous (one part in a billion), when there was no more antimatter, a little matter was left over, from which stars, galaxies and ourselves were formed.
The authors of the article we are commenting seem to assume that the only effect of the weak force, which would disappear if that interaction were absent from a hypothetical universe, is the second one in the above list. That is, free neutrons would not spontaneously disintegrate and there would be many more than there are in our universe. Despite this, they maintain, stars and galaxies could form, and therefore life could arise.
What is my objection? That if there were no weak force there would be no violation of the CP symmetry, and therefore no difference between the number of particles of matter and antimatter at the beginning of the cosmos. All matter would have disintegrated with antimatter, so that a universe without the weak force would have contained neither protons nor neutrons that could form stars and galaxies. Without matter, life would not have been possible.
How is it possible that such a simple argument has not come to the minds of the authors of the article, or the editors of Science News who have popularized it? That they haven’t thought about it is obvious. Otherwise, they would have tried to counteract it with some argumentation. They could argue, for example, that the violation of the CP symmetry is not the only possible explanation for the imbalance between matter and antimatter, that the relationship between both things is not proven. But they haven’t even mentioned it.


The same post in Spanish
Manuel Alfonseca

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