The nuclear force: one of the four fundamental forces in physics, but what is it?

Despite the many mysteries still surrounding the weak force, the hope is that by better understanding it, we can gain a deeper appreciation of the complex and interconnected nature of the universe and the fundamental forces that drive the behavior of the smallest particles and the largest structures in the cosmos.

What is the weak nuclear force?

As a fundamental force in the universe, the weak nuclear force plays an essential role in some of the most vital functions that make our universe what it is. Without it, the universe as we know it would not exist, and it’s hard to imagine what some alternate universe without the weak nuclear force would look like (though some have definitely tried).

Its most defining characteristic or role in physics is its importance to critical nuclear reactions like those that power stars and produce radioactivity. It is responsible for the production of important subatomic particles like neutrinos, as well as producing elements heavier than iron, which cannot be produced by nuclear fusion alone.

As a fundamental force, it is also responsible for the evolution of the early universe at a cosmological scale, with evidence suggesting that it might have played an important guiding role in the composition of the particle soup that our universe eventually grew from. It is also deeply tied to the other fundamental forces in ways that we are only now beginning to understand, with a great many mysteries still to be uncovered.

Why is it called the weak nuclear force?

The weak nuclear force is called "weak" because it is about a million times weaker than the strong nuclear force, which binds together atomic nuclei. Still, despite its name, it’s nothing to scoff at since it is actually considerably stronger than gravity when compared side-by-side at the scale of an atomic nucleus within which the weak nuclear force operates. 

The other three forces also influence our universe in far more obvious — and visible — ways, so these often take up more attention in physics and in the popular imagination. It is easier to understand electromagnetic radiation and gravity since we can literally see electromagnetic radiation as light, and we can see gravity’s influence on matter, like when a leaf falls to the ground.

Even the actions of the strong nuclear force are visible when you look at the model of an atomic nucleus, even if it needs a bit more explanation and intuition. How exactly does one “see” beta decay, a concept that is already hard for most people to wrap their heads around?

In order to see the weak nuclear force in action, you need to work with some of the smallest particles in the universe, like quarks and antineutrinos. What’s more, as we’ll get into in a bit, the weak nuclear force also works with some very (relatively) heavy carrier subatomic particles that severely limits how far that force can actually be carried. 

So even though the weak nuclear force can be extremely active, as it is inside of a star, it is doing so in such a confined, limited space that you can only “see” the accumulated effect of an uncountable number of weak force interactions. This helps to obscure the role the weak nuclear force plays in some very important and well-known phenomena.

What evidence do we have for the weak nuclear force?

There are several lines of evidence for the existence and properties of the weak nuclear force, all of which are visible by the effect the force has on the matter and energy we see.

The weak nuclear force can mostly be observed in certain nuclear reactions, such as the fusion of protons to form deuterium (a hydrogen isotope with a proton-neutron nucleus), which occurs in the Sun and other stars. The rates at which these reactions occur depend on the properties of the weak nuclear force.

The most important of the nuclear reactions the weak force governs, though, is beta decay. This is the process in which a neutron in the nucleus of an atom decays into a proton, an electron, and an antineutrino. This process can also occur in reverse, with a proton in the nucleus decaying into a neutron, a positron, and a neutrino. The fact that beta decay occurs and follows observable patterns is strong evidence for the existence of the weak nuclear force.

Another related effect of the weak nuclear force comes from neutrino interactions. Neutrinos are electrically neutral particles that are produced in various nuclear processes (including beta decay), and the weak force is responsible for the interactions between neutrinos and other particles, including electrons and protons. The fact that neutrinos are observed to interact weakly with matter is further evidence of the weak force’s existence.

We’ve also seen strong, direct evidence of the weak nuclear force from high-energy particle physics experiments, such as those conducted at CERN and Fermilab. These experiments involve colliding particles at high speeds and observing the particles that are produced in the collisions. By analyzing the properties of these particles, physicists can infer the properties of the weak nuclear force.

Overall, the combination of nuclear reactions (especially beta decay), neutrino interactions, and particle physics experiments provide strong evidence for the existence and properties of the weak nuclear force.

What role does the weak nuclear force play in nuclear decay?

The weak nuclear force is essential to nuclear decay, and since this process is fundamental to how matter exists in our universe, the weak nuclear force plays a keystone role in the composition of elements in the universe. 

Nuclear decay is the process by which the nucleus of an unstable atom breaks down and emits particles and energy. This process can occur through several different types of radioactive decay, including beta decay, alpha decay, and gamma decay.

Beta decay, in particular, is directly influenced by the weak force. There are two types of beta decay, positive and negative, with negative beta decay being far more common. This type of decay occurs when one of the down quarks in a neutron in an atomic nucleus is converted to an up quark by the action of the weak force. 

Since a neutron is made of two down quarks and one up quark, converting a down quark to an up quark produces two up quarks and a down quark in the particle, which makes the particle a proton. This conversion does not alter the atomic mass of the atom, but it does alter its energy and its charge. A deuterium atom has a proton and a neutron in its nucleus, so converting a neutron to a proton produces two protons in the nucleus, forming a helium atom. 

And, since a neutral-charged particle became a positive-charged one, the process forces the nucleus to emit an electron to balance the conversion. The conversion also produces an antineutrino, which carries away the difference in energy between a deuterium atom and a helium atom since the total energy of the universe must be conserved, even at such a minute scale. 

The same process can turn a proton into a neutron (positive beta decay), which results in the emission of a positron to carry away the positive charge and a neutrino to carry away the change in energy, 

The exchange of these particles is what makes the weak nuclear force so unique compared to other fundamental forces. Unlike the strong nuclear force, which is mediated by gluons and binds protons and neutrons together in the nucleus, or electromagnetism, which is mediated by photons and governs the interactions of charged particles, the weak nuclear force interacts with both particles and anti-particles freely and interchangeably.

Furthermore, since the weak nuclear force is involved in beta decay, it plays a critical role in determining the stability of atomic nuclei. Unstable nuclei, which contain too many or too few neutrons, can undergo beta decay in order to reach a more stable configuration. By emitting particles and energy in the process, these unstable nuclei can transform into new elements and ultimately contribute to the diversity of matter in the universe.

How does the weak nuclear force help nuclear fusion in the Sun?

The role of beta decay, and thus the weak nuclear force, is crucial to the fusion of hydrogen atoms into helium, which is the process that powers the Sun and other stars. In this process, two hydrogen atoms combine to form a helium atom, releasing energy in the process.

The fusion process also involves the strong nuclear force, which binds the protons and neutrons together in the nucleus of the atoms, and the electromagnetic force, which governs the interactions of charged particles. However, the weak nuclear force really is the key ingredient to making the Sun’s fusion process the life-giving energy source that it is.

Specifically, the weak nuclear force is involved in the conversion of a proton into a neutron, which is necessary for the fusion process to occur. During fusion, two protons come together and often break apart just as fast, but sometimes beta decay occurs to stabilize the nucleus by turning one of the protons into a neutron, producing a stable deuterium atom. 

In the process, this conversion produces a free positron, which will annihilate any electron it comes into contact with to produce a gamma ray, which will eventually make its way to the Sun's surface and be released as sunlight. 

In order to form helium, another proton must be added to this nucleus, and once a free proton is fused into the deuterium atom, it may or may not decay into another neutron in the process to form tritium, which is a proton-neutron-neutron isotope of hydrogen. Deuterium and tritium atoms will ultimately fuse to form a stable helium atom, though, with two protons and two neutrons, with a free neutron ejected from the atomic nucleus along with a considerable amount of energy.

The constant conversion of neutrons to protons and vice versa that takes place at the extremely high temperature and pressure of the Sun’s core (or that of any star) produces energy far more efficiently and sustainably than would otherwise result from fusing “pure” elements together, allowing stars like our Sun to shine brighter and hotter, and creating conditions on Earth where life can thrive.

Without the weak nuclear force, then, fusion reactions would be much less efficient, and the energy output of stars would be significantly lower. Understanding the role of the weak nuclear force in fusion is therefore critical for understanding the behavior of stars and for developing new technologies that harness the power of fusion for energy production on Earth.

How is the weak nuclear force involved in the supernova of a star?

Just as the weak nuclear force plays an essential part in the life of a star, it has a role to play in the spectacular demise of a star as well: a supernova. 

A supernova is a catastrophic event in which a star undergoes a rapid and massive explosion, releasing an enormous amount of energy and producing heavy elements that are essential for life. 

During a supernova, the fusion within the core of a sufficiently large star slows to the point where it doesn’t have the energy to hold up the weight of its own mass, leading the star to collapse under its own gravity. As this is happening, new particles are created through various processes, including the weak nuclear force.

Specifically, the weak nuclear force is responsible for the creation of neutrinos, which are tiny, elusive particles that are produced in vast numbers during a supernova. Neutrinos are electrically neutral and interact only very weakly with matter, which makes them difficult to detect. However, they are important because they carry away a significant amount of the energy released during a supernova, helping to power the explosion.

In addition to producing neutrinos, the weak nuclear force is also involved in the creation of heavy elements during a supernova. As the core of the star collapses, the intense pressure and temperature cause the nuclei to fuse together, creating heavier and heavier elements. However, this process can only proceed up to a certain point.

Normally, a star can only fuse elements up to iron since it takes more energy to fuse elements after iron than is created by the fusion itself. That means that iron is as high on the periodic table as a star will go in its active phase.

This is where the weak nuclear force comes in. During a supernova, the energy being produced by the star’s collapse is more than enough to fuse elements even heavier than iron, but those elements wouldn’t be stable without a whole lot of additional neutrons.

The weak nuclear force provides a way for these heavier elements to stabilize by converting excess protons into neutrons. In addition, with the pressure of the star's collapse fusing electrons and protons together into neutrons all throughout the star's core, lighter elements can transform into heavier elements as atoms with neutron-heavy nuclei convert excess neutrons into protons through beta decay and the weak nuclear force. 

One other critical process during a supernova is neutron capture, where the increasing amount of free neutrons allows quickly growing atomic nuclei of heavy elements to absorb additional neutrons. When this happens, beta decay ensures that the final resulting nucleus will ultimately reach a stable state.

This role for the weak nuclear force during a supernova is critical for the production of heavy elements such as gold, silver, and uranium, which would otherwise be impossible for a star to produce, but which are essential for life as we know it.

As these stars violently explode, they disperse these newly created heavy elements into the wider universe, and they eventually go on to form part of new stars and planets like Earth.

Who discovered the weak nuclear force?

The weak nuclear force wasn’t discovered by a single individual, but its existence and properties have been determined by many scientists contributing experimental observations and theoretical developments for the better part of a century.

In the 1930s, nuclear physicists such as Enrico Fermi and Chen Ning Yang proposed theoretical models to explain the nature of beta decay, specifically when a neutron decays into a proton, an electron, and an antineutrino. This decay process suggested the existence of a new type of particle that mediated this process.

Experimental evidence for the weak nuclear force developed in the 1950s and 1960s through studies of beta decay and other nuclear interactions. The biggest movement forward came in the 1960s and 1970s with the development of the electroweak theory and the proposal of weak bosons by Sheldon Glashow, Abdus Salam, and Steven Weinberg. The confirmation of these bosons carrying the weak nuclear force in 1983 at CERN's Super Proton Synchrotron cemented the weak nuclear force as a distinct fundamental force in nature.

What is the electroweak theory?

The electroweak theory is a unification of the electromagnetic and weak nuclear forces. The electromagnetic force is responsible for interactions between electrically charged particles, while the weak nuclear force is responsible for certain types of radioactive decay, but — according to the electroweak theory — at sufficiently high energies, these forces become indistinguishable.

The electroweak theory was developed in the 1960s and 1970s by Glashow, Salam, and Weinberg and is based on the idea that the electromagnetic and weak nuclear forces are different aspects of a single, unified force. This theory proposes that at very high energies, the electromagnetic and weak nuclear forces are indistinguishable, but at lower energies, they become distinct.

The importance of the electroweak theory is that it predicted the existence of a then-unknown set of particles called intermediate vector bosons (or weak bosons), which mediate the weak nuclear force. More importantly, though, this theory also directly implied the existence of the Higgs field (a field that permeates all of space and gives mass to particles), which was confirmed in 2012 by the discovery of the Higgs boson.

The electroweak theory has been confirmed by a wide range of experimental observations and is a cornerstone of the standard model of particle physics, which describes the behavior of particles and fundamental forces at the smallest scales.

What particles carry the weak nuclear force?

The weak nuclear force is mediated by three particles: the W+, W-, and Z bosons. These particles are collectively known as intermediate vector bosons or weak bosons.

The W bosons have an electric charge of +1 and -1, while the Z boson is electrically neutral. Unlike the photons that carry the electromagnetic force or the gluons that carry the strong nuclear force, the weak bosons have a relatively large mass as far as particles go, which makes the weak force much weaker than the other fundamental forces.

What is weak interaction?

When talking about the weak nuclear force as it is actually being mediated by the weak bosons, the weak force is often described as weak interaction.

Particles interact through the weak interaction by exchanging weak bosons. And, since these bosons are thought to have a mass 100 times greater than a proton, their weight severely restricts the weak force’s range, making the weak interaction look pathetically weak at low energies, like those tied to radioactivity.

This interaction is also notable in that it acts upon left-handed fermions — i.e., quarks, leptons, and most composite particles like protons, with half-integer values (1/2, 3/2, etc.) of intrinsic angular momentum, or spin — and right-handed antifermions.

Essentially, it acts asymmetrically: it treats right and left differently, and this asymmetry might actually produce real-world effects in everything from the composition of the universe to the nature of our biology.

How did the weak nuclear force form?

The weak nuclear force is thought to have emerged during the first moments of the universe's existence. According to the Big Bang model, the universe began as a hot, dense, and highly energetic state, and as it expanded and cooled, it underwent a series of phase transitions. During these transitions, the various fundamental forces of nature became distinct.

The weak nuclear force is believed to have originated from a process known as electroweak symmetry breaking. In the early universe, the electromagnetic and weak nuclear forces were unified and indistinguishable from one another. However, as the universe cooled and expanded, this symmetry was broken, and the two forces became distinct.

This process is thought to have been triggered by the Higgs field, which is a field that permeates all of space and gives particles mass. It is thought that, as the universe cooled to about 1015 Kelvin, the Higgs field underwent a phase transition, and this caused the electromagnetic and weak nuclear forces to become distinct at about 10^-10 seconds after the Big Bang.

What theories predated the weak nuclear force?

Before the discovery of the weak nuclear force, physicists had a limited understanding of the nature of radioactivity and nuclear decay. In the early 20th century, the prevailing theory was that atoms were composed of protons and electrons and that radioactive decay was the result of the spontaneous disintegration of atomic nuclei.

In the 1930s, Enrico Fermi developed the theory of beta decay, which proposed that in this process, a neutron within an atomic nucleus could transform into a proton, emitting an electron and a neutrino. However, Fermi's theory did not explain how the neutron could transform into a proton since the neutron and the proton had the same mass and charge.

It wasn't until the 1950s and 1960s that physicists developed a more complete understanding of the weak nuclear force. The theory of weak interactions, developed in the 1950s by Richard Feynman and others, proposed that the weak force was mediated by intermediate vector bosons, later named W and Z bosons. This theory provided a framework for understanding the processes of beta decay and other weak interactions. 

The weak force was later incorporated into the electroweak theory. 

Overall, the development of the theory of weak interactions and the discovery of the W and Z bosons represented a major advance in our understanding of the fundamental forces of nature and provided a framework for explaining many previously puzzling phenomena in nuclear physics.

What technology relies on the weak nuclear force?

The weak nuclear force is not directly used in any technology; however, our understanding of this force has produced some notable developments. 

One such technology is Positron Emission Tomography (PET) imaging, which is a medical imaging technique used for diagnosis and research. PET works by detecting pairs of gamma rays emitted indirectly by a positron-emitting radionuclide (tracer), which is introduced into the body on a biologically active molecule.

The tracer accumulates in the region of interest, and the emitted gamma rays are detected by a PET scanner. The decay of the tracer is mediated by the weak nuclear force, as the decay involves the conversion of a neutron to a proton and the emission of an electron and an anti-neutrino.

Another technology that relies on our understanding of the weak nuclear force is the detection of neutrinos. Neutrinos are fundamental particles that are produced in nuclear reactions and during the fusion of stars. They are electrically neutral and interact very weakly with matter, which makes them very hard to spot. 

However, since the weak nuclear force governs the interaction of neutrinos with matter, scientists can use our knowledge of how the weak nuclear force works to detect when a neutrino is interacting with a detector. The detection of neutrinos has led to significant advancements in our understanding of particle physics and astrophysics.

Where will the study of the weak nuclear force take us next?

Of all the fundamental forces in nature, we know the least about the weak nuclear force, which is part of the reason why studying it is so exciting.

It was only in the last five years that the actual strength of the weak nuclear force was measured, but that measurement, produced by the asymmetrical nature of the weak force, might provide scientists a way to probe other mysterious phenomena like dark matter and dark energy.

And, in physicists' never-ending quest for a quantum mechanics-compatible theory of gravity, one of the most natural places to start looking for answers is the fundamental force that still has plenty of mysteries to uncover, like why it breaks the universe's strong preference for symmetry or what exactly is going on with neutrinos.

It will likely be a very long time before we get to resolve any of those questions — and this will probably just lead to even more questions — but a force so essential to the universe as we know it is as good a place as any to look for answers.