The last in our series on the four fundamental forces of nature is the weak force — arguably the most unusual of the four. It doesn’t even manifest primarily as a “force” at all — what we usually think of as a push or a pull. Instead, it causes or allows the transformation of certain particles from one type to another. In particular, the weak interaction can transform an up quark into a down quark, and vice versa, which causes a proton to be converted into a neutron, or the other way around.
The type of radioactivity known as beta decay refers to this process, in which an atomic nucleus emits an electron (or an antielectron) and an antineutrino (or a neutrino). In the familiar procedure of carbon-14 dating, the weak force is at work converting one of the neutrons of carbon-14 into a proton, resulting in the formation of stable nitrogen. Although this form of radioactivity is useful for age-dating organic remains, other actions of the weak force play a much more crucial role in sustaining our lives.
Despite its name, the weak force is not the weakest suspect in the line-up of fundamental forces. It’s characterized as about 100 times weaker than the electromagnetic force but is still several trillions of times stronger than gravity.
Ultimately the reason that the weak force is so weak is because the distance over which it operates is so small, rather than the intrinsic weakness of the force itself.1
The Design of Our Universe
The weak force is part of the design of our universe at the most foundational level.
The weak force has few obvious manifestations in our everyday life and, in many ways, is the most intricate and subtle of all the forces. It is intimately tied to the Higgs boson and, through that, the way in which elementary particles get mass.2
Beginning in the 1960s, particle physicists found a way to mathematically unify the weak force with the electromagnetic force. To do so required a process for spontaneously breaking a complete symmetry between the forces that allowed the so-called exchange particle of the electromagnetic force to be massless (the photon), while allowing the exchange particles of the weak force to be massive. The Higgs field provided the solution. Particles that interact with it, such as the W and Z gauge bosons of the weak force, acquire mass. The photon remains massless since it doesn’t interact with the Higgs field.
If you’ve read this far, you’re doing well! Admittedly, there’s a lot of obscure physics packed into the last few paragraphs, and yet, what I’ve described so far barely scratches the surface of the mathematical complexity involved in trying to comprehend these aspects of the forces of nature. Remarkably, however, human minds have derived a successful theory of the weak force and its cousin, the electromagnetic force — even arriving at predictions of the properties of novel fundamental particles, subsequently discovered in the largest particle accelerator experiments ever built. Mildly put, the ability to do theoretical particle physics cannot be legitimately ascribed to an evolutionary process based on Darwinian selection effects.
Another Unexpected Property
Investigations of the weak force revealed another unexpected property — in its interactions with particles, it manifests what is called parity violation. No other fundamental force of nature does this. A broad class of particles known as fermions exhibit “spin,” and if the north pole of their spin axis lines up with their momentum, they’re referred to as right-handed; if their spin direction is opposite, they’re left-handed. The weak force somehow distinguishes between these two cases.
The weak force accomplishes this in the most extreme way possible: only left-handed particles experience the weak force. Right-handed particles do not feel it at all….this is the key property of the weak force and one of the key properties of the Standard Model.3
This unique handedness property of the weak force reflects one of the most significant discoveries in particle physics.
This fact was discovered by Chien-Shiung Wu, on a cold winter’s day in New York City in December 1956. Wu’s experiment was technically challenging but conceptually very simple. She placed a bunch of Cobalt atoms in a magnetic field and watched them die [or radioactively decay]….The whole point of the magnetic field was to make sure that the nucleon spins of the atoms were aligned. Wu discovered that the electrons were preferentially emitted in the opposite direction to the nucleon spin.4
As a consequence of this subtle property of parity violation, experimenters viewing a video of Wu’s experiment would be able to discern whether they were looking at the real thing or if they were viewing the experiment reflected in a mirror (in which case, the electrons would be preferentially emitted in the same direction as the spins of the nucleons).
Particles and Antiparticles
The weak force not only violates parity, but it also affects particles and antiparticles differently (violating what’s referred to as charge conjugation). The three other fundamental forces of nature, in contrast, treat particles and their antiparticles symmetrically. The combined antisymmetric behavior of the weak force is called CP violation.
OK, so this unique property of the weak force is peculiar and interesting, but is it important for life? Consider this: the asymmetry built into the weak force may in fact be necessary to explain why we have a universe filled with stars and planets, rather than an empty void with nothing but gradually cooling radiation energy. As summarized by theoretical physicist David Tong, of Cambridge University:
Why should we care about CP violation? Well, there are two reasons. The first is that our universe is, rather fortunately, full of matter but with very little anti-matter. It’s thought that this imbalance occurred naturally in the early universe, but for this to happen there have to be processes where matter and anti-matter behave differently. This, it turns out, requires CP violation. So although small, it may well have had extraordinarily large consequences.5
More research is indicated, however, since the asymmetry in the weak interaction isn’t enough:
Perplexingly, the amount of CP violation predicted by the Standard Model is many orders of magnitude too small to account for the matter–antimatter asymmetry observed in the Universe. This suggests the existence of new sources of CP violation beyond those predicted by the Standard Model….6
The Impact on Our Lives
Since we don’t experience the weak force directly, it’s easy to dismiss its importance for life. Currently, it’s winter where I live, and we’re facing sub-zero temperatures over the weekend, bringing home the impact on our lives of variations in the amount of solar energy we receive.
Our sun’s energy originates from nuclear fusion reactions deep in its core, where hydrogen is fused into helium. An essential step every time this nuclear reaction occurs is the conversion of two protons into two neutrons — which happens via the weak interaction. As two hydrogen nuclei (protons) fuse, they form a di-proton, which is highly unstable due to the repulsive electric force between them. Fortunately (or, we could say, by good design), one of these protons immediately transforms into a neutron (emitting along the way an anti-electron and a ghost-like neutrino). The weak force mediates this transformation via so-called W gauge bosons. Without this subtle and weak fundamental force of nature, the outlook of our universe would be vastly different, and we would undoubtedly not exist.
Another strategic role of the weak force is in producing radioactivity. We normally want to avoid radioactivity, but we need it within the interior of the Earth.
Without an internal source of heat, the centre of the Earth would have cooled and solidified long ago. However, energy is injected into the rocks through the continual radioactive decay of its elements, particularly isotopes of potassium, uranium and thorium.7
Although the radioactive decay of these isotopes involves two types of decay mechanisms (alpha and beta decay), and only one of them (beta decay) requires the weak force, eliminating this force would certainly reduce the amount of heating in the core of the Earth.
If our planet’s interior cooled and solidified, Earth would lose its magnetic field and the protection it affords against the charged-particle bombardment of our atmosphere by the solar wind and cosmic rays. A cool planetary interior also means a cessation of plate tectonics, meaning no recycling of crustal materials. The crucial CO2 cycle that contributes to long-term climate stability would then shut down, and erosion runoff would deplete continents of soil-based nutrients.8 Without the weak force, the Earth would not support life as we know it.
In our investigation of the weak force, we have arrived on a well-worn path in science — namely that the more we study nature, the more we find that each aspect of the way things are reveals crucial properties for the existence of life. The existence of the weak force wasn’t even suspected until about a hundred years ago, and now we find that we can’t live without it! It seems that there are no non-essential physical properties of nature worth speaking about, and if that isn’t consistent with good design, I don’t know what is.
Notes
- University of Cambridge: https://www.damtp.cam.ac.uk/user/tong/pp/pp4.pdf, p. 25.
- University of Cambridge: https://www.damtp.cam.ac.uk/user/tong/pp/pp4.pdf, p. 1.
- University of Cambridge: https://www.damtp.cam.ac.uk/user/tong/pp/pp4.pdf, p. 4.
- University of Cambridge: https://www.damtp.cam.ac.uk/user/tong/pp/pp4.pdf, p. 2.
- University of Cambridge: https://www.damtp.cam.ac.uk/user/tong/pp/pp4.pdf, p. 37.
- https://home.cern/news/press-release/physics/new-piece-matter-antimatter-puzzle .
- Geraint F. Lewis and Luke A. Barnes, A Fortunate Universe: Life in a Finely-Tuned Cosmos (Cambridge: Cambridge University Press, 2016), p. 83.
- Geraint F. Lewis and Luke A. Barnes, A Fortunate Universe: Life in a Finely-Tuned Cosmos (Cambridge: Cambridge University Press, 2016), p. 85.